Level III Biomechanics - Unit A__formatted_raw__v1

{{PAGE_1}} LEVEL III · BIOMECHANICS

Unit A

Essentials of Orthodontic Diagnosis · Concepts of Orthodontic Treatment Planning · Biologic Response to Orthodontic Force · Mechanical Principles in Controlling Orthodontic Force · Orthodontic Anchorage and Controlled Tooth Movement

Proffit Instruction — generated for offline reference

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Contents

1. Essentials of Orthodontic Diagnosis
2. Concepts of Orthodontic Treatment Planning
3. Biologic Response to Orthodontic Force
4. Mechanical Principles in Controlling Orthodontic Force
5. Orthodontic Anchorage and Controlled Tooth Movement

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1. Essentials of Orthodontic Diagnosis

Overview

Definition of Orthodontic Problem

In modern health care, diagnosis and treatment planning is built around the problem-oriented approach. The question in diagnosis is: “What is the problem?” Its answer is a description of the problem and its cause. The question in treatment planning, of course, is what to do about it.

The problem-oriented approach to orthodontics is particularly valuable because it forces the doctor to focus on what difference it makes to the patient if the teeth aren’t correctly aligned or if the occlusion is imperfect. The evolution of thoughts about the goal of orthodontic treatment is covered in some detail in the module titled “Why Orthodontics?”

What is an orthodontic problem? The simplest definition: A condition characterized by alterations from normal alignment and occlusion of the teeth which causes problems for the patient. Does this girl have an orthodontic problem? To evaluate that, you will have to examine her teeth and facial appearance, and you’ll have to ask her what she thinks.

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Definition of Orthodontic Problem (cont.)

In fact, Katherine and her mother agreed that they didn’t like the appearance of her teeth. This caused some problems for her now and had the potential to affect social interactions in the future.

It’s obvious that she has a malocclusion, that is, her teeth don’t fit properly. But saying she has a problem isn’t a diagnosis—it’s a reason for doing a diagnostic evaluation, to obtain a complete list of the problems that constitutes a diagnosis.

An important concept: The goal of diagnosis is the truth about the patient. So pulling together the facts about the patient is a scientific procedure. At this stage the doctor’s opinion needs to be kept out of the picture as much as possible—don’t jump to conclusions.

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{{PAGE_5}} Diagnostic Process What’s needed is a systematic approach to gathering and analyzing the necessary diagnostic data. The process calls for, first:

• development of a diagnostic data base, derived from interview clinical examination analysis of records then
• systematic description, often called classification, to produce the problem list that is the diagnosis

Orthodontic Diagnosis

Interview

Interview Process Let’s follow the diagnostic process with Anna, who was 12 years, 6 months of age when she was evaluated for orthodontic problems. Her problems aren’t quite so obvious, but she illustrates the general approach reasonably well.

The diagnostic work-up begins with collection of data from an interview, which typically includes review of a form that the patient filled out in advance and some follow-up questions.

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*Image 1, smile: Anna, age 12-6, initial evaluation.* </td> <td> *Image 2, dentition: Anna, age 12-6, initial evaluation.* </td>

*Image 3, combined: Anna, age 12-6, initial evaluation.* </td> <td></td>

Interview Data

The goal of the interview is to answer four major questions:

1. Why is this patient seeking treatment, and why now?

The intent is to understand her concerns and motivation. For a child, this often is straightforward, but children occasionally and adults often tell the doctor what they think he or she wants to hear. If

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{{PAGE_7}} Bold text: Interview Bold text: Interview Data (cont.)

you don’t understand why the patient is seeking treatment, you have no chance of determining what the real problem is.

For Anna, she and her family simply want to have her dental problems corrected. The motivation is to detect anything that is abnormal and get it corrected in the optimal way.

Anna’s mother had several congenitally missing teeth, and she is concerned that Anna may have a similar problem.

• Why is this patient seeking treatment, now? chief complaint, motivation

2. How did things get to be the way they are? That, of course, involves understanding the pertinent medical and dental history, and (to the extent possible) the etiology of any abnormalities.

Anna has had normal physical development, with no pertinent medical history of any serious illnesses.

3. What, if anything, is likely to change in the near future? That requires considering two important things: (1) the progression of any medical condition, which is not pertinent for Anna; and (2) probable growth changes, which is highly pertinent.

Anna has not yet reached sexual maturity but recently has begun to grow rapidly. Since the adolescent growth spurt is the ideal time for most orthodontic treatment, it is important to establish

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{{PAGE_8}} where she is in development.

Interview

• Why is this patient seeking treatment, now? chief complaint, motivation
• How did things get to be the way they are? medical / dental history, etiology
• What is likely to change in the near future? medical condition, growth status

Interview Data (cont.)

4. What do the patient and parents expect as a result?

The important thing here is to establish whether what they expect is realistic or not. This relates to motivation for treatment but isn’t quite the same thing. Sometimes patients correctly identify problems but have unrealistic expectations. For example, if Anna’s mother is worried about a possible congenitally missing tooth and thinks the dentist can stimulate one to grow, it is important to know about this unrealistic expectation from the beginning.

In fact, the expectations are realistic. Anna and her parents simply think that if there is a problem, the dentist will find it and offer appropriate treatment.

It only takes a minute or two to review the history form and ask the questions. The bottom line from this interview: No special problems with this patient. But sometimes a red flag goes up—it can be critically important to get the answers to these questions.

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Interview

• Why is this patient seeking treatment, now? chief complaint, motivation
• How did things get to be the way they are? medical / dental history, etiology
• What is likely to change in the near future? medical condition, growth status
• What does he or she expect as a result? internal / external motivation, expectation

Clinical Exam

Steps in Clinical Examination

The second step in the diagnostic evaluation is to take a good look at the patient. This includes examining the mouth but certainly is not limited to that.

The clinical exam has four goals:

1. Evaluate facial proportions and tooth-lip relationships.
2. Evaluate the health of oral hard and soft tissues.
3. Evaluate jaw function.
4. Determine what diagnostic records are required.

Let’s take these one at time, starting with facial proportions and tooth-lip relationships. This evaluation, of course, is the facial form analysis with which you already have had some experience.

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Clinical Examination

• Facial proportions / tooth-lip relationships facial form analysis
• Health of intra-oral hard and soft tissues caries, periodontal disease, other lesions
• Jaw function chewing ability, TM joint function
• What diagnostic records are needed?

Facial Examination

In the facial examination, it is important to look at three things:

• First, in the full face view, symmetry and proportion. Anna has a symmetric face, with no right-left or up-down disproportions, so there is no skeletal problem in these planes of space.
• Second, in the profile view, a-p and vertical jaw relationships. Anna has mild mandibular deficiency, but the jaw relationships are within normal limits.
• Third, in the smile and profile views, lip-tooth relationships and lip support. Anna’s display of incisor teeth on smile is normal. In the profile view, her lip profile is normal. She has no evidence of lip strain to close her lips over protruding teeth and does not have excessive lip separation at rest.

The bottom line: No skeletal or dentofacial problems.

{{PAGE_11}} Image 1, frontal relaxed: Anna, age 12, frontal view with lips relaxed. Image 2, profile relaxed: Anna, age 12, profile view with lips relaxed. Image 3, smile: Anna, age 12, smile view; note normal display of all the incisors but no gingiva.

Intraoral Exam, Health of Tissues

Now we can look at the teeth—but the focus should be first on the health of the tissues and on jaw function, not the details of the occlusion.

Anna has healthy-appearing teeth, with all primary teeth exfoliated except the lower second primary molars, and with permanent teeth erupting.

In the examination of the soft tissues, it is important to examine the amount of attached gingival tissue, in addition to looking for areas of gingivitis/bleeding on probing.

Anna has mild gingivitis around upper central incisors but has normal attached gingiva, no evidence of periodontitis, and no clinical caries or restorations.

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{{PAGE_12}} Image 1, frontal view: Anna, age 12-6. Image 2, right lateral view: Anna, age 12-6. Image 3, left lateral view: Anna, age 12-6. Image 4, maxillary occlusal: Anna, age 12-6. Image 5, mandibular occlusal: Anna, age 12-6.

Intraoral Exam, Jaw Function

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{{PAGE_13}} The third important step in clinical examination is evaluation of jaw function. If a malocclusion is so severe that the patient has difficulty eating, of course that would be noted. Anna is the other extreme. But TM joint function/dysfunction must be considered. The best single indicator of whether the TM joint is normal is whether the patient can open normally and move the jaw laterally and forward. The general guideline is simple: If a joint moves normally, almost surely there isn’t a problem with function. If it doesn’t move normally, at least to some extent, there is a problem. Anna had normal opening for a girl her age of 42 mm, with normal lateral and forward movements, and no deviation on opening. There were no joint sounds or pain on palpation. This girl, unlike Anna, deviates to the left on opening, although she can open almost the normal distance. She moves laterally more on one side than the other. Both these findings are consistent with a problem with the left TM joint.

Clinical Exam: What Diagnostic Records? The final part of the clinical examination is to determine what diagnostic records are needed. For a patient with a potential orthodontic problem, a panoramic radiograph always is indicated. It should be supplemented with bitewings if caries is noted on clinical examination or if restorations are present, and with periapical radiographs in specific areas if there is evidence of pulpal pathology or periodontal disease. Anna’s panoramic radiograph shows that formation of one mandibular second primary molar is very delayed, and the other is missing.

{{PAGE_14}} Diagnostic Records (cont.)

Other essential orthodontic diagnostic records are:

• Dental casts These are necessary to allow the measurements in space analysis and to provide a record of the pretreatment alignment and occlusion.
• Intraoral and facial photographs These are primarily a record to allow evaluation of change. It is important to photograph areas of soft tissue problems, for instance, lack of gingival attachment in the lower incisor area.
• Lateral cephalometric radiograph/tracing The ceph is essential to allow evaluation of the response to treatment, and it allows greater precision in evaluating jaw and tooth-jaw relationships. The cephalometric radiograph and tracing for Anna confirm mild mandibular deficiency and otherwise normal relationships.

{{PAGE_15}} Diagnostic Records (cont.)

Three important questions about typical orthodontic diagnostic records:

1. Is there any advantage in having the dental casts mounted on an articulator? That depends on the individual case. If extensive restorative treatment or maxillary surgery must be planned as part of comprehensive evaluation in an adult, then yes, articulator mounting is indicated. But as a general rule for orthodontics and especially for children, the answer is no. In a growing child, because the relationship of the dentition to the TM joint changes rapidly, the articulated casts quickly become only a historical artifact.

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{{PAGE_16}} 2. Is a cephalometric radiograph necessary for orthodontic diagnosis? The cephalometric radiograph makes it easier to define skeletal and dental relationships, but it is not strictly necessary for diagnosis. Problems in jaw relationships and lip support can be detected on careful clinical examination. Yet modern orthodontic treatment almost always requires cephalometric analysis. It’s malpractice to do comprehensive treatment without cephalometrics. Why? Because it is impossible to determine the progress of treatment without being able to superimpose serial cephalometric tracings. You can easily be fooled on clinical examination as to what really is happening. If you didn’t take a pretreatment ceph, a progress ceph is of minimal value.

3. What is the indication for a P-A cephalometric radiograph in addition to the lateral ceph? The primary indication for a P-A ceph is jaw asymmetry noted on clinical examination. A P-A ceph is not taken routinely for two reasons: (a) symmetric transverse relationships can be evaluated from clinical records and dental casts, and (b) in contrast to the lateral ceph, evaluating growth and treatment response from serial superimpositions is difficult and inaccurate.

The bottom line:

• Articulator-mounted casts are indicated primarily as an aid in planning treatment that involves extensive restorative dentistry and/or orthognathic surgery.
• Lateral cephalometric radiographs are essential for evaluating treatment and quite helpful (but not essential) for diagnosis.
• P-A cephs are indicated primarily for asymmetry and not obtained routinely.

3D Imaging?

The ability of obtain 3D images and use them in evaluation of orthodontic patients has developed rapidly in the last few years. In fact, 3D images of three types now can obtained if desired:

1. 3D photographs or 3D video
2. MRI (magnetic resonance imaging)
3. CT (computed tomography)

Let’s consider these in turn.

3D Photography

Several systems for obtaining 3D facial photographs that allow precise evaluation of facial soft tissue proportions and dimensions now are available. This can be a valuable research tool, particularly when the dynamics of facial soft tissue change are evaluated (which requires video, not just still images). This type of evaluation is valuable, for instance, when the effects of plastic surgery to correct a cleft lip are being considered, or when the amount of lip support provided by a fixed orthodontic appliance is to be determined.

In these superimposed images from a 3dMD camera system, one can precisely measure the amount of change in lip support created by removing a fixed orthodontic appliance (image 1). The color map shows the amount of change: the blue color indicates a change in lip posture moving into the background, green is minimal change, and red indicates forward change—so for this patient, the lips

{{PAGE_17}} moved back about 2 mm in most areas immediately after braces were removed. In a series of 9 patients (image 2), the superimposed images show that a 2 mm change occurred in most of them, with an extension of change beyond the corners of the mouth in 3 of the group. With a 4-camera 3dMD version that allows video recording, the rapid pattern of change in facial expressions can be detected.

It is unlikely that this high-definition 3D photography will be widely used in evaluation of most orthodontic patients, however, simply because it is expensive and does not add valuable diagnostic information beyond what can obtained from the type of facial form analysis that you have already learned how to do.

An interesting research question: What’s the effect of removing braces on lip support?

3dMD research image: Using special software to evaluate changes between superimposed 3D camera images, it is possible to precisely measure change in lip position. The color shows the direction of change (blue moving back, into the screen; red moving forward), the color intensity shows the magnitude.

3dMD research image sequence: Note that for most of these patients, the lips moved back about 2 mm, with a decrease in soft tissue prominence extending into the cheeks for three of them.

Magnetic Resonance Imaging

Magnetic resonance imaging, like photography, has the advantage that no ionizing radiation is used to obtain the images. In contrast to radiographs, internal soft tissues are revealed more clearly than hard tissues, and in dentistry MRI can be quite valuable in evaluation of TM joint problems like disk displacement (image 1) or soft tissue pathologic changes in or near the joint (image 2).

For an individual in whom a problem internal to the TM joint has been detected during the clinical examination, MRI, not radiography, now is the diagnostic procedure of choice. Fortunately, most orthodontic patients do not have TM joint pproblems—but referral for MRI of those in those who do is important.

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{{PAGE_18}} MRI image of TM joint: The MRI image shows clearly that disk has been displaced from its normal position over the head of the condyle. MRI image of TM joint: In this image, pathologic changes in the disc can be visualized.

Computed Tomography

Computed tomography (CT) imaging has become an important part of medical diagnosis despite its relatively high radiation dose and the expensive equipment. In recent years cone-beam CT (CBCT) has been developed as a method for obtaining 3D images of the head and face, at both lower radiation levels and lower economic cost. It is rapidly becoming an important part of orthodontic evaluation.

There are two concerns about the use of CBCT in dental and orthodontic practice. The first is that its ability to reveal unexpected pathology. Does one need additional expertise to evaluate pathologic changes revealed by these images? The answer to that is a clear-cut yes—which means that a radiologist needs to view them to screen for pathology.

The other concern is the amount of radiation exposure with CBCT, which is a factor in deciding whether to use it. The larger the field of view with CBCT, the higher the radiation dose. The table in image 1 shows the relative radiation dose with modern technology for periapical, panographic and cephalometric images in comparison to the radiation from medium- and large-field CBCT. It is apparent that the radiation to image a small area, such as you would find around an unerupted maxillary canine, is similar to what would be generated from a panographic radiograph, while the radiation from a large-field CBCT image of the face and jaws is considerably larger than the radiation from a cephalometric radiograph. Logically, one would choose the higher cost and higher radiation exposure when, and only when, the value of the additional diagnostic information it would provide would benefit the patient. As a general guideline, CBCT of the area of impacted teeth now is indicated for most patients, and full-face CBCT is indicated for skeletal asymmetry. Other potential reasons for obtaining CBCT are not (yet?) supported by evidence.

The analysis of CBCT images is discussed in the following section of this module.

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{{PAGE_19}} DOSES AND RISKS WITH RADIOGRAPHS AT UNC

Technique	Effective dose (µ sv x 5500)	Days of background (3000 µ sv)	Probability of Fatal Cancer (x / million)
Single PA/BW digital, rect.	2	6 hrs	0.1
Single PA/BW digital, round	9	0.1	0.5
Panoramic, digital	16	2	0.9
Cephalometric, digital	5.5	0.7	0.3
Newtom 3G (large FOV)	68	8	4
Kodak 9000 (average FOV)	21	3	1

source: Dr. John Ludlow, UNC

Digital pan vs. small FOV CBCT: The comparative radiation dose for a digital panoramic radiograph vs. small field-of-view CBCT.

DOSES AND RISKS WITH RADIOGRAPHS AT UNC

Technique	Effective dose (µ sv x 5500)	Days of background (3000 µ sv)	Probability of Fatal Cancer (x / million)
Single PA/BW digital, rect.	2	6 hrs	0.1
Single PA/BW digital, round	9	0.1	0.5
Panoramic, digital	16	2	0.9
Cephalometric, digital	5.5	0.7	0.3
Newtom 3G (large FOV)	68	8	4
Kodak 9000 (average FOV)	21	3	1

source: Dr. John Ludlow, UNC

Digital ceph vs. large FOV CBCT: The comparative radiation dose for a digital cephalometic radiograph vs. large field-of-view CBCT.

Analysis of Records

Photo/Video Records

The diagnostic records for an orthodontic patient typically consist of:

• photographs (and/or digital video clips)
• dental casts
• radiographs

Occasionally, articulator-mounted casts, CT scans, MRI images, or other data are added. Almost always, this occurs in children with severe (syndromic) problems or in adults with multiple dental problems, including missing teeth (see the modules on Adult Orthodontics and Adjunctive Treatment).

Photographs primarily provide confirmation and documentation of what was observed clinically. Facial animation, especially on smile, is an important part of evaluating esthetics. Short video clips (as seen in the accompanying image) can be obtained with almost any modern digital camera and incorporated into digital records, and are likely to become a routine part of orthodontic evaluations in the future. The video clips can provide facial views in multiple dimensions as well as a record of lip-tooth relationships on smiling. Careful observation, not frame-by-frame measurement, is the primary method of analysis.

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{{PAGE_20}} Digital video clip: A digital video that shows both soft tissue animation and views from multiple orientations provides a record of facial proportions that cannot be obtained from a series of still images. https://proffit-instruction.netlify.app/Modules/essentialsdx/video/18.mp4

Cast Analysis

Analysis of dental casts is primarily space analysis, the calculation of space available versus space required for alignment of the permanent teeth (image 1). The use of this form is shown in detail in the Level 2 module on space analysis. Space analysis now can be carried out using a computerized version of this form (image 2), and in your future practice you probably will have that available—but doing it manually now so that you understand the steps in the procedure is a better learning experience.

If the patient is in the mixed dentition, careful analysis to indicate the extent of potential crowding is important, but quantification of actual crowding also is important for patients in the permanent dentition.

If a posterior crossbite exists, measuring arch widths can provide insight into whether the upper arch is unusually narrow because of a skeletal problem (narrow palatal vault) or a dental problem (alveolar processes tipped lingually), or the lower arch is unusually wide.

These measurements become part of the diagnostic database. They provide an important guideline for treatment planning.

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{{PAGE_21}} Image 1, space analysis: Mixed dentition space analysis form. Space analysis, virtual models: Space analysis using digitized casts can be done with accuracy equal to manual measurements of space available vs. space required.

Cast Analysis (cont.) Space analysis for Anna was done using maxillary and mandibular casts (image 1) and the UNC form for this purpose. Measurements for space available in the upper and lower arches are shown in images 2 and 3. The results of space analysis for the two arches are shown in image 4, and the other things that must be taken into consideration when space analysis is done are shown in image 5. The data show that in the mandibular arch, if normal 2nd premolars were present in the arch, she would have enough space. With missing 2nd premolars, of course, there is a large excess of space. Space analysis predicts mild crowding in the upper arch.

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{{PAGE_23}} Table:

<table>
    <tr>
        <th>SECTION 7</th>
        <td>SKELETAL JAW RELATIONSHIP<br>(from Facial Profile Analysis)<br><br>□ CLASS I: ☑ CLASS II: □ CLASS III</td>
    </tr>
    <tr>
        <th>SECTION 8</th>
        <td>OCCLUSION OF PERMANENT FIRST MOLARS<br><br>RIGHT SIDE □ ANGLE CLASS I □ LEFT SIDE<br>□ END-TO-END ☑<br>☑ ANGLE CLASS II □<br>□ ANGLE CLASS III □</td>
    </tr>
    <tr>
        <th>SECTION 9</th>
        <td>MOLAR SHIFT (From end-to-end to Class I)<br>For Skeletal Class I only<br><br>RIGHT SIDE + LEFT SIDE = TOTAL SHIFT<br>____ mm + ____ mm = ____ mm TOTAL</td>
    </tr>
    <tr>
        <th>SECTION 10</th>
        <td>LIP POSTURE (from Facial Profile Analysis)<br>☑ ACCEPTABLE: □ PROTRUSIVE: □ RETRUSIVE<br><br>MANDIBULAR INCISOR POSITION<br>(from Facial Profile Analysis and casts)<br>☑ ACCEPTABLE: □ PROTRUSIVE: □ RETRUSIVE</td>
    </tr>
</table>

Image 5, other considerations: Anna, other considerations in space analysis.

Cephalometric Analysis can be complex and difficult to understand. All the measurements and calculations, however, are indicators of critical relationships that were already evaluated less precisely on clinical examination.

The critical relationships are:

• How the jaws relate to the cranial base
• How the jaws relate to each other
• How the teeth of each jaw relate to the supporting bone of the jaw itself

These relationships must be judged in both the anteroposterior and vertical planes of space. The key is to understand the relationships. Any set of measurements is a means to an end, not the end in itself. For example, the ANB angle often is used to evaluate the relationship of the maxilla to the mandible—but the value of the angle must be interpreted. No individual measurement or calculation can give the big picture of the important relationships.

{{PAGE_24}} Cephalometric Analysis, Anna

For Anna, we can see the important things just from inspection of the tracing, with a true vertical line dropped from nasion and the important horizontal planes drawn in.

First, look at the relationship of the anterior maxilla and mandible to the nasion vertical line. Note that point A is almost on the line, while the chin is about 6 mm behind it. So the a-p relationship of the maxilla is normal. Then look at the relationship of the S-N (cranial base), true horizontal and palatal planes. The palatal plane is slightly tipped down posteriorly, indicating slightly excessive maxillary posterior vertical growth.

Then look similarly at the mandible. The chin is about 6 mm behind the true vertical line, and the mandibular plane angle is a little large.

Then look at the teeth relative to each jaw, using the true vertical line moved to points A and B, respectively.

The bottom line: Anna has mild skeletal deviations from ideal, compensated by mild displacement of the position of the teeth so that dental relationships are normal.

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{{PAGE_25}} CBCT for Impacted Teeth

Although Anna has one missing lower second premolar and one slowly-developing one, she does not have impacted teeth and so had no indication for CBCT as part of her diagnostic records. This, however, is the logical place to briefly discuss analysis of CBCT images to evaluate impacted teeth.

As we have noted above, impacted teeth—usually but certainly not always maxillary canines—are an indication now for CBCT of that area, but not CBCT of the whole head. CBCT is indicated simply because it is likely to change the treatment plan over what probably would have been done using 2D images. The relationship of the impacted tooth to the bone and other teeth around it can be seen clearly in the 3D image, which can be rotated so that a view from any direction is possible (image 1), and sequential slices through any area at any orientation are possible (image 2).

The 3D images often show that the impacted tooth needs to moved away from the root of another permanent tooth (for an impacted canine, away from the root of a lateral or central incisor) before then being brought toward the oral cavity. Otherwise, the impacted tooth may be pulled directly into the adjacent tooth root, further damaging it. Analysis is just a matter of observing how the impacted tooth should be moved in order to minimize damage to other teeth.

{{PAGE_26}} Image 1, Composite view: In this oblique view, the tip of the crown of the impacted canine can be seen to be behind the root of the central incisor—which may already have been damaged, and would need to be avoided as the impacted tooth was brought toward the oral cavity. A view of this type can be rotated to be seen at any angle on the computer screen.

Image 2, Series of slices: A series of slices, moving along the dental arch, provides more detail. This impacted canine is behind and below the root of the central and lateral, both of which could be damaged by moving the canine directly facially.

Skeletal Asymmetry

The second major indication for CBCT, and the major reason to use a large field of view, is skeletal facial asymmetry. Planning treatment for these patients using information from CBCT is much more straightforward than using a combination of frontal and lateral cephalometric radiographs, and of course the CBCT is increasingly valuable as the degree of asymmetry increases. A composite rendering of the CBCT (image 1) can be rotated on the computer screen so that the extent and location of the asymmetry can be viewed from any perspective. Slices (images 2-4) at any plane reveal details of the normal and affected sides. This allows both a more precise evaluation of the patient’s condition and more precise planning of both the orthodontic and surgical treatment that is likely to be needed.

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{{PAGE_27}} Image 1, Jaw asymmetry, frontal composite Image 2, Same patient, sagittal midline slice Image 3, Same patient, coronal slice, molar region Image 4, Same patient, axial slice, molar region

Detailed Evaluation of Treatment Outcomes

The third major use of CBCT is the use of superimposed sequential images to obtain a more detailed evaluation of treatment outcomes, which makes it a valuable research tool. Cephalometric radiographs are important in orthodontic diagnosis, but their most important use is to determine the

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{{PAGE_28}} Effect of treatment on both the teeth and jaw relationships, which is difficult to detect just from clinical examination. Superimposition of CBCT images is difficult but provides much more information about the location and amount of change.

A good illustration of this use is the evaluation of changes following orthognathic surgery. It is possible to superimpose CBCT images on the surface of the cranial base, and then to display changes created by surgically repositioning the jaws as color maps. Image 1 shows changes in the position of the maxilla and mandible created by 2-jaw surgery for correction of a severe Class III problem that included a mandibular asymmetry. The red color shows the forward movement of the maxilla that included a forward rotation, so that the paranasal area was brought forward more than the incisor teeth. The blue color shows the backward movement of the mandible, with more setback on the right than the left side. It is impossible to get a similar sense of the surgical movements from lateral and PA cephalograms. Movements can be seen from other views, as shown in image 2.

It also is possible to view changes in the position and orientation of the mandibular condyles, as the ramus segments are rotated several degrees as the mandible is set back (image 3). Color maps of the condyles from immediate postsurgery to follow-up allow a better understanding of the adaptive changes to the surgical rotation.

{{PAGE_29}} Frontal view

Image 2, Same patient, multiple views: CBCT superimposition (on the cranial base for all views) allows changes to be evaluated from any perspective.

• Frontal view
• Superior view
• Lateral view
• Inferior view
  • -4mm … 0 … 4mm

Condylar displacement and remodeling A. Pre-surgery (transparent) and 1 week post-surgery B. 1 week post-surgery (transparent) and 1 year post-surgery

Image 3, Surgical changes, condyle/ramus close-up: Close-up views of changes in the orientation / position of the ramus segment and condyles allow observation of the amount of surgical change, and remodeling can be observed directly with postsurgery to follow-up superimpositions.

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{{PAGE_30}} Classification, Problem List

Classification = Systematic Description

At this point, with the diagnostic database assembled, the next step is to generate the problem list that constitutes diagnosis. The approach:

• First, note any pathologic problems separately. These receive priority for treatment, not because they are more important but because they must be brought under control before other treatment starts.
• Then, classify the diagnostic findings of developmental problems to develop the rest of the problem list using systematic description.

The systematic description approach (Ackerman-Proffit classification) is designed to ensure that important aspects of the patient’s orthodontic problems are not overlooked, while minimizing the size and complexity of the resulting problem list. You already have learned how to use this method—now let’s review it in the context of applying it to Anna’s diagnosis and treatment plan.

Systematic description has 5 steps:

1. Alignment/symmetry of the dental arches
2. Evaluation of dental protrusion/esthetics
3. Transverse skeletal/dental relationships
4. A-P skeletal/dental relationships
5. Vertical skeletal/dental relationships

The classification scheme can be expressed as a Venn diagram, as shown in image 1. This emphasizes the possibility for different sets of problems and for interactions in multiple planes of space—but the key is simply to look at the 5 characteristics in order and note any problems related to each characteristic.

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{{PAGE_31}} 1. Dentofacial Appearance Asymmetric ←> Symmetric Disproportionate ←> Proportionate

2. Teeth / Arch Form Alignment, Symmetry

3. Transverse Wide ←> Narrow Skeletal Dental

4. Sagittal (A-P) Class II ←> III Skeletal Dental

5. Vertical Deep ←> Open Bite Skeletal Dental

Yaw Roll Pitch Position / Orientation

Spacing Symmetry Crowding Asymmetry

Profile: Concave Straight Convex

Lips: Protrusive Normal Retrusive

Incisor Display: Excessive Normal Inadequate

Step 1. Alignment/Symmetry The first step is to look at the alignment and symmetry within the dental arches, considered separately, with the results of space analysis and other measurements available, and with the panoramic radiograph and any other dental radiographs available. Only positive findings (problems) will be carried forward to the problem list.

For Anna, the arches are symmetric and there is adequate space in the maxillary arch. The mandibular arch is mildly crowded, one second premolar is missing, and the other is severely delayed. These items go to the problem list.

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{{PAGE_32}} Step 2. Esthetic Impact of Malocclusion The second step in classification is to evaluate the impact on facial esthetics of the tooth and jaw positions. It requires looking at the notes from clinical examination of tooth-lip relationships and facial proportions, photographs, and cephalometric analysis.

Anna has normal tooth-lip relationships as seen clinically, in the photos, and on the cephalometric radiograph. Her tooth-jaw relationships and soft tissue profile are normal.

Bottom line: no esthetic problem.

{{PAGE_33}} Step 3. Transverse Relationships The next step is to put the dental casts together and evaluate skeletal and dental relationships in all 3 planes of space, beginning with the transverse plane.

Anna has normal transverse relationships of the teeth (no posterior crossbite), and there is no evidence of any skeletal transverse problem. If an asymmetry had been noted on clinical examination, a P-A cephalometric radiograph or (and better) CBCT would have been included in the diagnostic records.

{{PAGE_34}} Step 4. A-P Relationships

Now the dental and skeletal relationships in the antero-posterior plane of space are evaluated, using the dental casts and the cephalometric analysis.

The Angle classification is used to describe the molar relationship, and its extended version is used to describe the skeletal relationship.

It is important to note that although the dental and skeletal relationships usually match, they may not. For individual patients, an accurate description might be “skeletal Class II with Class I molars” or “Class II molars with Class I skeletal relationship.”

For Anna, the description is “mild skeletal Class II with Class I molars,” and that’s not really a problem.

{{PAGE_35}} Step 5. Vertical Relationships The final step is to look at the dental casts and the cephalometric analysis to evaluate dental and skeletal relationships in the vertical plane of space.

Dental vertical problems are excessive overbite, anterior open bite, or posterior open bite. Skeletal vertical problems are excessive or deficient mid- or lower face height, often related to rotation of either or both jaws. Excessive or deficient eruption of teeth also can be noted on the ceph.

Cephalometric analysis often puts more emphasis on A-P than vertical relationships. Particularly, many published analyses that are a set of prescribed measurements often do not reveal skeletal vertical problems.

For Anna, there is a mild tendency toward skeletal open bite, shown by the rotated palatal plane and increased mandibular plane angle. This is compensated by slightly increased eruption of the lower incisors, and overbite is normal.

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Problem List

Anna’s orthodontic diagnosis is the list of problems derived from this procedure.

Should the missing/delayed mandibular second premolars be considered a pathologic or developmental problem? In this case, the structure of the problem list would not really be affected whichever way you did it: The missing teeth are the major problem. As a rule, however, missing teeth are dealt with most effectively if they’re considered a developmental problem, and pathology is reserved for disease entities.

The final problem list:

• missing mandibular right 2nd premolar, severely delayed mandibular left 2nd premolar
• mild crowding of mandibular incisors

A major advantage of having the diagnosis in this form is that attention is immediately directed toward the treatment that will be needed—which is a giant step toward developing an appropriate treatment plan. In Anna’s case, the orthodontic diagnosis has served largely to rule out problems other than the missing teeth that would have complicated treatment. But you have to know whether other problems exist before you can develop the correct treatment plan. The problem list for a patient with a more severe malocclusion would be longer. Note, however, that there would be a maximum of 5 developmental problems (each of which might have several characteristics).

Experience has shown that this grouping of problems makes it easier to keep up with the treatment possibilities for patients with severe problems—and the same diagnostic method is applicable to all potential orthodontic patients, however simple or complex their problems may be. In the apparently simple cases, it’s important not to overlook something important. In the complex ones, it’s important to be thorough and pick up all the problems, not just the most obvious ones.

The development of a treatment plan for Anna, and the outcome of that treatment, are shown in the Concepts of Treatment Planning module.

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{{PAGE_37}} Self-Test Referral

The self-test section of this program is designed to help you be sure you have understood the material.

Now that you have gone through the module, do the assigned reading in Contemporary Orthodontics(pages 147-184 in 5th ed.; pages 163-201, 4th ed.) Then take the self-test, and use it as a guide for further study and review.

Copyright 2013, UNC Dept. of Orthodontics

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{{PAGE_38}} 2. Concepts of Orthodontic Treatment Planning

Treatment Planning Scheme

Diagnostic/Treatment Planning Process

Diagnosis and treatment planning in modern orthodontics is built around the problem-oriented approach. The question in diagnosis is: “What are the problems?” The answer is a description of the problems and their cause. The question in treatment planning, of course, is what to do about the problem(s) (image 1).

The diagnostic process is summarized in image 2 (see the module Essentials of Orthodontic Diagnosis for details). The diagnosis is a list of problems that represent the truth about this individual patient. The list is derived from an objective and scientific evaluation of the patient.

This module focuses on a structured approach to treatment planning based on the problem list. As in the previous module, we will use Anna as our illustrative patient.

It is important to understand that treatment planning is fundamentally different from diagnosis. Its goal is to establish what a wise and prudent clinician would do to maximize benefit to this particular patient. That requires the application of judgment. The goal of diagnosis is truth—the goal of treatment planning is wisdom. So diagnosis is a scientific procedure; treatment planning is not and cannot be. But, it can and must be carried out in a systematic manner (image 3) so that snap judgments do not compromise the quality of patient care.

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{{PAGE_39}} Image 1, questions: Anna, diagnosis/treatment planning questions. Image 2, diagnostic process: Steps in orthodontic diagnosis. Image 3, tx planning process: Steps in orthodontic treatment planning.

Orthodontic Diagnosis

Interview History Reason for Tx

Clinical Exam Health of tissues Jaw function Facial proportions

Analysis of Dx Records Photos / video Dental casts Radiographs

DATA BASE

Classification

Problem List = Diagnosis

Orthodontic Treatment Planning

Pathology (caries, perio, etc.) Control before orthodontic treatment

Problem List = Diagnosis

Orthodontic (developmental) Problems Priority Order

A → A B → B C → C etc → etc

Possible Solutions

Evaluate Advantages/Disadvantages/Costs/Risk

Patient Parent Consent Informed consent

Tx Plan Concept Effective time Efficiency

Tx Plan Details

Treatment Planning Method The first step in treatment planning is to separate out the patient’s pathologic problems, which will require other types of treatment, from the developmental problems that are treated with orthodontics (image 1). The guideline is that pathologic problems are treated first, not because they are more important, but because they must be brought under control before orthodontic treatment begins. If they are not under control, they can—and quite likely will—become worse while the relatively prolonged orthodontic treatment is carried out. Then the developmental problems are placed in priority order, and the treatment plan is developed by evaluating the possible treatment procedures relative to the patient’s prioritized problem list (image 2).

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{{PAGE_40}} Orthodontic Treatment Planning

Problem List = Diagnosis Pathology (caries, perio, etc.) Control before orthodontic treatment

Pathologic problems must be under control before treatment of developmental problems begins

Image 1, pathologic problems: Pathologic problems must be brought under control before orthodontic treatment starts.

Orthodontic Treatment Planning

Problem List = Diagnosis Pathology (caries, perio, etc.) Control before orthodontic treatment

Orthodontic development Problems A → B Possible Solutions Evaluate Patient/Parent Consult Tx Plan Concept Effective-ness/Efficiency Tx Plan Details Priority Order C → B Solutions etc → etc Informed Consent

Image 2, tx planning process: The other steps in planning orthodontic treatment must wait for control of pathology.

Treatment Planning Process Let’s examine the treatment planning approach to developmental problems in more detail, starting with prioritization of the developmental problem list. It is at that point that judgment must be introduced, and pure science is left behind. If the patient has multiple problems, which is the most important? That depends…on some combination of what the patient thinks is most important and what the doctor might think is most important based on his or her training and experience. What’s the importance of prioritizing the problem list? Very simply, the same problem list prioritized differently will result in different treatment plans. Then the approach is to: consider the possible solutions to each problem, starting with the most important one examine the “practical considerations” of interactions among the possible solutions, cost-benefit, necessary compromises, and other factors and meet with the patient/parent to review alternative treatment possibilities, seek their input, and obtain informed consent Level III Biomechanics — Unit A · 40 / 170

{{PAGE_41}} Flowchart illustrating the treatment planning process, starting with a “Problem List = Diagnosis” box at the top. An arrow points down to a large box containing “Orthodontic (developmental) Problems” and “Priority Order” on the left side, and “Possible Solutions” (labeled A→A, B→B, C→C, etc.) on the right side. From this central box, an arrow labeled “Evaluate” (with subtext: Interaction, Compromise, Cost/Benefit, Other factors) points to the final box on the right, titled “Patient-Parent Consult,” which lists “Alternative plans” and “Patient input.”

{{PAGE_42}} Patient-Parent Consult Alternative plans Patient input

Informed Consent

Tx Plan Concept

Effectiveness Efficiency

Tx Plan Details

Anna: Treatment Plan

Anna, Diagnostic Summary Let’s continue with Anna, the 12-year-old girl whose diagnostic evaluation was illustrated in the previous module, Essentials of Orthodontic Diagnosis. Her key diagnostic records are shown in those images. She had no skeletal problems, but a rather severe problem of dental development. The detailed evaluation is in the other module. Her problem list:

• missing lower right 2nd premolar, severely delayed lower left 2nd premolar
• mildly crowded mandibular incisors

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Anna: Prioritization, Possible Solutions

For Anna, there were no pathologic problems, and there was no doubt that the missing teeth were more important than the mild crowding. So her prioritized problem list was the same as the initial one (which is often not the way it works out).

For her most important problem, the missing/delayed mandibular 2nd premolars, there were two possible solutions:

1. Maintain the 2nd primary molars as long as possible, hope the delayed premolar would eventually complete its formation and erupt, and replace the missing 2nd premolar with implants or bridges when the primary molars were lost.
2. Extract the 2nd primary molars and the delayed 2nd premolar, and close the space orthodontically by bringing the permanent molars mesially, hoping the 3rd molars would erupt into what would have been the space of the 2nd molars.

For the second problem, the mandibular incisor crowding, the solution would be orthodontic alignment, but alignment would require space. If the 2nd primary molars were retained, it would be difficult to align the incisors unless the width of the primary teeth was reduced. There would be plenty of space if the 2nd primary molars were removed.

For Anna, the choices involve the family dentist directly. Parents and patients often seek their dentist’s opinion about the choice between alternative treatment approaches. If you were the family dentist, they almost certainly would want to talk to you about a choice like this.

So who makes the decision? Remember, the doctor(s) advise, the patient decides.

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Treatment planning for Anna:

Prioritized problem list

1. missing / delayed mandibular 2nd premolars
2. mildly crowded mand incisors

Possible solutions replace with implants or bridges orthodontic space closure

availability of space?

Patient / Parent Conference

Patient/Parent Consultation

At a meeting with the patient and parents, the doctor’s role is to evaluate the alternative treatment possibilities, and present them as clearly and objectively as possible (image 1). The goal is to get both the patient and the parents to understand the alternatives.

When the patient is an adolescent, it is a serious error to discuss treatment only with the parents. It’s the child, after all, who has to cooperate during treatment. She’s more likely to do that if she understands the plan and recognizes that treatment is being done for her, not to her.

The first thing to discuss is the probable fate of the mandibular premolar whose development is so severely delayed (image 2).

What’s the chance that it will develop into a normal tooth and erupt into the space of the primary molar? Not zero, but small. Orthodontics might be required to get it into the arch, some years in the future, if it did continue to develop. If it becomes hopelessly impacted, as it might, it would have to be extracted and replaced.

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{{PAGE_45}} Image 1, patient-parent conference: The doctor’s role in the patient-parent conference is to evaluate the alternative treatment possibilities and help the patient and parents understand.

Image 2, panoramic radiograph: A severely delayed premolar like the mandibular left one has only a small chance of completing normal formation and erupting into the correct place in the dental arch.

Patient/Parent Conference (cont.) What if nothing is done now? That amounts to selecting the plan to eventually replace the missing premolars.

What are the advantages of the “wait and eventually replace” plan?

• Perhaps only one side will need an implant or bridge.
• Some of the cost of treatment will be deferred.

What are the disadvantages?

• Orthodontic treatment to prepare for the implant(s) or bridge(s) eventually will be needed, and it will be more difficult for the patient to tolerate as an adult than now.
• If the primary molars are lost before growth is complete, temporary bonded bridges would be needed before implants could be placed—implants should be delayed until vertical growth is completed.
• The prosthetic replacements will require lifetime maintenance.
• Ultimately, costs are likely to be greater.

Patient/Parent Conference (cont.) What are the advantages of the “extract and close the space now” plan?

• This is the ideal time for comprehensive orthodontics, which should provide excellent occlusion.
• There will be no need for prosthetic replacements requiring long-term maintenance.
• The lower third molars appear to be well formed, and bringing the second molars mesially should provide space for them to erupt.

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{{PAGE_46}} What are the disadvantages?

• Comprehensive treatment over an 18- to 24-month period would be needed, with retainers afterward until growth was essentially completed.
• The space would have to be closed almost entirely by bringing the lower permanent molars forward, otherwise there is the risk of flattening the profile too much and adversely affecting facial esthetics, so good patient cooperation would be required.

Is orthodontic treatment to correct the mild malocclusion indicated now, even if the plan is to retain the second primary molars? Perhaps, but excellent occlusion will not be possible as long as the wide second primary molars are retained.

What do you think the patient and parents would choose?

Treatment Plan Concept, Details Anna wanted to go ahead with orthodontic treatment now, and her parents felt that avoiding the long-term maintenance of replacement teeth was a positive factor.

They endorsed the treatment plan concept:

• extraction of primary second molars and delayed right second premolar
• comprehensive orthodontics with space closure
• oral health maintenance within the family practice

The doctors’ treatment plan required more details:

• oral surgery: consultation appointment, then extractions
• complete orthodontic appliance 2-3 weeks later
• space closure with NiTi springs and light Class II elastics to bring second molars mesially
• maxillary and mandibular removable retainers, full-time except eating for first 2-3 months, 12 hours/day another 3 months, just at night until growth completed

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{{PAGE_47}} Patient / Parent Conference maintain 2nd primary molars, wait? extract, close space?

ADVANTAGES / DISADVANTAGES

INFORMED CONSENT

Treatment planning for Anna:

Tx Plan Concept extract 2nd primary molars, 2nd premolar close space, bring molars mesial retainers until growth ends

DOCTOR-STAFF COMMUNICATION

Tx Plan Details apply braces 2-3 weeks after extractions

space closure: NiTi springs, Class II elastics

retention plan

Anna: Treatment Outcome

Treatment Progress As planned, Anna’s orthodontic appliance was placed 3 weeks after the teeth were extracted, the teeth were aligned, and space closure was accomplished with superelastic nickel-titanium springs (image 1). She wore light Class II elastics (lower molar to upper incisors) to further assist in closure of the lower extraction space without creating overjet.

The treatment was completed and the orthodontic appliances removed at age 14-7, with an active treatment time of 22 months.

She matured rapidly during treatment, and facial proportions were maintained reasonably well (images 2, 3). There was no esthetic problem from the extractions.

Forward movement of the lower molars created a Class III molar relationship (image 4), but normal overjet/overbite and good interdigitation and alignment of the teeth (image 5) were maintained.

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Treatment Outcome

Image 1, progress: Anna, age 13-1, during space closure.

Image 2, change in smile: Age 12-6 to age 14-7, change in treatment.

Image 3, change in profile: Age 12-6 to age 14-7, change in treatment.

Image 4, post-tx occlusion: Posttreatment occlusion—note the Class III molar relationship.

Image 5, post-tx alignment: Posttreatment alignment, space closure in the lower arch.

The panoramic radiograph shows the extraction space closure, with the lower molars brought mesially (image 1).

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{{PAGE_49}} Note the tendency of the upper second molars to elongate as their occlusal contact is removed by bringing the lower molars mesially. One of the goals of retention will be to control the vertical position of the upper 2nd molars until the lower 3rd molars erupt into function with them.

The posttreatment cephalometric radiograph (image 2) allows an evaluation of the changes produced by treatment. Note in the cranial base superimposition tracing (image 3) that Anna grew vertically during treatment. Her upper incisors were tipped lingually somewhat, and the lower extraction space was closed almost totally by bring her lower molars mesially.

The maxillary and mandibular superimpositions (image 4) show that the lower molars and upper incisors were slightly elongated, in addition to mild retraction of the upper incisors and considerable mesial movement of the lower molars. Both the retraction of the upper incisors and the changes in vertical tooth positions undoubtedly were created by the Class II elastics used to bring the lower molars forward.

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{{PAGE_50}} Image 1, post-treatment pan: Panoramic radiograph, age 14-7. Image 2, post-treatment ceph: Cephalometric radiograph, age 14-7. Image 3, ceph superimposition, overall: Ceph superimposition on cranial base, 12-6 to 14-7. Image 4, ceph superimposition, max/mand: Ceph superimposition on maxilla and mandible.

Two-Year Recall

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{{PAGE_51}} At age 17-2, 2 1/2 years after the completion of treatment, Anna was entering her senior year in high school. Facial esthetics were entirely satisfactory (image 1). The extractions and space closure had no deleterious effects.

Despite the unusual molar relationship, her functional occlusion was good (image 2), and the extraction space remained closed (image 3). As planned, she was still wearing removable retainers at night.

On the panoramic radiograph (image 4), continued development of the mandibular third molars can be seen. There is a good chance that they will erupt in the previous position of the second molars and will function against the upper second molars.

Summary, Diagnosis & Treatment Planning

The approach to problem-oriented orthodontic diagnosis and treatment planning is outlined in the attached image. It looks complex, but it is an organized and time-efficient way to meet three important criteria:

• gather the necessary diagnostic information as objectively as possible

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{{PAGE_52}}

• introduce judgment at the appropriate point, as treatment planning begins
• produce the treatment plan with the best chance of maximizing benefit to the patient

Consider it the equivalent of the pilot’s checklist at the end of the runway before beginning take-off. Going through the list of prescribed procedures, no matter how many times you have done it, is the best way to be sure that something important was not overlooked.

Orthodontic Diagnosis/Treatment Plan

Self-Test Referral The self-test section of this program is designed to help you be sure you have understood the material. Do the reading in Contemporary Orthodontics (5th ed., pages 220-250 and 395-403; 4th ed., pages 234-267). Then take the test, and use it as a guide for further study and review.

Copyright 2013, UNC Dept. of Orthodontics

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{{PAGE_53}} 3. Biologic Response to Orthodontic Force Periodontal and Bone Response to Normal Function

Learning Objectives

It is easy to think about moving teeth as a mechanical problem that requires an engineering solution, and in fact an orthodontic appliance can be analyzed like any mechanical device—but the engineering solution doesn’t predict what will happen when the orthodontic appliance is used.

Why not? Because tooth movement requires remodeling of the bone that supports the teeth, and the objective of the appliance is not to move the teeth mechanically, but to use gentle force to produce the biologic response that allows tooth movement and/or affects growth of the jaws and face. That concept can be a bit difficult to really accept, but it is the key to understanding clinical orthodontic treatment.

In this module, the goal is to explore the principles that underlie the biology of treatment, looking at both the local (dentoalveolar) and distant (jaw / face) responses to orthodontic force.

In addition to viewing this module, read Chapter 8, pages 278-311 in the 5th edition of Contemporary Orthodontics (or Chapter 9, pages 331-358 in the 4th edition).

Biting Force and Tooth Movement

You certainly have already noticed, perhaps without thinking much about it, that your teeth are repeatedly subjected to heavy loads during mastication but are stable in their position. How can we explain that a tooth stays in position quite nicely despite the forces of occlusion, which normally range up to 50 kg and can be much higher at maximum effort—and yet the same tooth can be made to move with light force from an orthodontic appliance?

The first thing to remember is that the periodontal ligament space (image 1) functions very nicely as a shock absorber. In between the tooth and the alveolar bone is the periodontal ligament (PDL), which of course consists of cellular elements in a fluid-filled chamber. For the shock absorber effect, the cellular elements are irrelevant—it’s the fluid that’s important. When you bite down, the fluid is incompressible, and the first thing that happens is that the alveolar bone bends. The tooth (or teeth) moves relative to the jaw, but not relative to the alveolar bone. After one second or so, the fluid begins to be squeezed out, and at that point the cellular elements begin to feel pressure. If you bite down on what you thought was a peanut and it doesn’t break apart, you probably maintain the force and bite a little harder. If it still doesn’t shatter, you begin to feel pain because the cellular elements are being loaded more as the fluid continues to be squeezed out—and you stop biting. Maybe they put one of the finest little Virginia rocks in the bag of Virginia peanuts!

Note in the attached chart (image 2) the time course of heavy pressure against a tooth. Over the time of a very few seconds, the physiologic response is bone bending, then pain if the heavy pressure is

{{PAGE_54}} maintained.

Image 1, Periodontal structures: Diagrammatic representation of periodontal structures (which are shown in pale red). The collagen fibers in the PDL are oriented so that they resist force that pushes the tooth into its socket.

Bone Bending as a Stimulus to Bone Maintenance

Bone bending turns out to be important for more than just allowing us to withstand heavy forces during mastication. Have you thought about why the alveolar bone resorbs and all but disappears over time after a tooth is extracted? We know now that bone bending is necessary to maintain normal calcification and remodeling. Somewhere in orbit at this moment an astronaut is pedaling away on an exercise bike or standing on a vibrating platform, trying to prevent decalcification of his skeleton by causing the bending of arm, leg and other bones that happens as you function against the pull of gravity. It’s not just alveolar bone that bends under normal function. The mandible bends every time you open wide, and some bending occurs in all bones that are subjected to external forces.

Bone bending produces an interesting electrical effect, the generation of a piezo-electric current (image 1). Force against a crystalline structure (many nonbiologic crystals, but also bone and collagen) mechanically distorts the crystals. This produces a rapid current flow as electrons move to a different location within the crystal lattice, which quickly declines as they reach their new position.

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{{PAGE_55}} When the force that produced bending is removed, the electrons move back to their original position, and a reverse current flow is observed. As you walk down the street, there is a rhythmic current flow with each step. When you chew, the same rhythmic current flow occurs in your alveolar bone and throughout your jaws.

So why is alveolar bone lost when a tooth is extracted? After the tooth is gone, it doesn’t get the rhythmic loading necessary to generate the piezo-electric currents, and the result is decalcification and resorption.

Piezo-electricity and Orthodontic Tooth Movement

When biologic piezo-electricity was discovered in the 1960s, it was immediately thought that perhaps orthodontic tooth movement would be more effective if pulsed rather than continuous forces were used. Fortunately for the complexity of orthodontic appliances, experiments showed that this was not the case.

The bottom line: piezo-electric currents are important for maintenance of calcification of bones that are loaded during function, which very much includes alveolar bone, but are irrelevant for orthodontic tooth movement.

Does that mean electrical signals have no role in the bone remodeling that makes orthodontic tooth movement possible? No, because it has been demonstrated that electrical fields can alter cell membrane potentials and the ability of molecules to enter cells. Will the time come that a patient is to

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{{PAGE_56}} Bioelectricity: Bone Health and Tooth Movement

• Tooth displacement in function → bone bending
• Piezo-electricity: critical for bone health, irrelevant for orthodontic tooth movement
• Other electrical signals?

Electrical fields can alter cell membrane potentials, thereby affect cell activity

Response to Sustained Orthodontic Force

Sustained versus Short-Duration Force

What happens when force against a tooth, which creates pressure in the PDL, is maintained? If the force is heavy, as with the multi-kilogram force generated during chewing or clenching the teeth tightly, severe pain is felt immediately. If the force is lighter, perhaps 50 or 100 grams, immediate pain does not occur.

Suppose you activate a spring that presses against a tooth with a force of 100 grams? What’s the first thing that happens? That’s right, the tooth is displaced and the alveolar bone bends. But this time the force is not released. Within a few seconds the bone begins to spring back, creating pressure in the PDL. Then fluid begins to be expressed from the pressure side, and although the tooth stays in the same position relative to the head or other external references, now it is displaced relative to the alveolar bone. This displacement maintains pressure in the PDL on one side and creates tension on the other.

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{{PAGE_57}} Physiologic Response to Sustained Pressure Against a Tooth

Time	Event (with light pressure)
<1 sec	PDL* fluid incompressible, alveolar bone bends, piezoelectric signal generated
1-2 sec	PDL fluid expressed, tooth moves within PDL space
3-5 sec	Blood vessels within PDL partially compressed on pressure side, dilated on tension side; PDL fibers and cells mechanically distorted
Minutes	Blood flow altered, oxygen tension begins to change; prostaglandins and cytokines released

*PDL, periodontal ligament

Pressure-Tension Effects

The pressure-tension explanation for orthodontic tooth movement relies on chemical rather than electrical signals as the stimulus for cell differentiation and ultimately tooth movement. There is no doubt that chemical messengers are important in the cascade of events that lead to remodeling of alveolar bone. Because this theory explains the course of events reasonably well, it is the basis of our current understanding.

From the perspective of pressure-tension and release of chemical messengers, application of a light but sustained force to a tooth has two effects:

1. mechanical distortion of cells in the periodontal ligament, which causes release of the contents of some cells, and
2. a decrease in blood flow in the PDL on the side opposite the direction of force application, where the PDL is compressed, and an increase in blood flow on the other side, where the PDL is under tension. This leads to a change in oxygen and carbon dioxide levels in the PDL. Both effects lead to the release of chemical messengers from affected cells.

Classic animal experiments by David Khouw in Goldhaber’s lab at Harvard in the 1960s allow visualization of the blood flow change. He infused India ink into experimental animals while the animal was being sacrificed, so that blood vessels in the periodontal ligament could be seen easily. Note in images 1,2 and 3 the effect on blood flow of compressing the PDL, while image 4 shows the effect on the tension side.

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Pressure-Tension Effects (cont.)

The same effects on vessels in the PDL are shown diagrammatically in the attached drawings. As pressure increases, the PDL is increasingly compressed, and if the pressure is high enough, blood flow is totally cut off.

What is the result of totally cutting off blood flow? Necrosis of the tissue in that area, of course. In early studies of the effects of sustained force, someone thought that the PDL tissue where blood flow was cut off resembled hyaline cartilage, and still sometimes these areas are called hyalinized. Perhaps that’s an acceptable euphemism—it sounds better than necrotic, but a better description is sterile necrosis.

{{PAGE_59}} Cascade of Events Leading to Tooth Movement

As shown in the slide with the first screen in this module, within minutes after pressure and tension are created within the PDL by sustained light force, oxygen / carbon dioxide levels are altered, and prostaglandins and cytokines are released.

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{{PAGE_60}} Physiologic Response to Sustained Pressure Against a Tooth

Time	Event (with light pressure)
Hours	Metabolic changes occurring: Chemical messengers affect cellular activity, enzyme levels change
~4 hours	Increased cAMP levels detectable, cellular differentiation begins within PDL
~2 days	Tooth movement beginning as osteoclast/osteoblast remodel bony socket

*PDL, periodontal ligament

Cascade of Events with Heavier Sustained Pressure

Now let’s look at the sequence and timing of events leading to tooth movement if heavier force is used, so that there is enough pressure in the PDL to totally occlude blood vessels in some areas on the compression side.

What’s the first thing that happens with heavier sustained force? The same thing that happens with any application of force to a tooth: bone bending. With sustained force the alveolar bone springs back and the PDL is placed under compression in some areas and tension in others.

Let’s look at the sequence after that. In 3-5 seconds blood vessels are totally occluded in some compressed areas and mechanically-distorted cells are leaking cytokines and prostaglandins. Within a few minutes, blood flow ceases in those areas, and cell death begins. After some hours, there are no

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{{PAGE_61}} cells in the PDL to differentiate into the osteoclasts and osteoblasts needed for remodeling of the socket. Where are the cells going to come from? There are two answers to that question: (1) from adjacent PDL areas that are not necrotic, and (2) from bone marrow spaces outside the lamina dura. If the necrotic area is tiny, cells from adjacent areas would be the major source. If it’s larger, the bone marrow is the more likely source via a process called undermining resorption. Take a careful look at the sequence of events in this chart, and then we’ll examine undermining resorption more closely.

Physiologic Response to Sustained Pressure Against a Tooth

Time	Event (with heavy pressure)
3-5 sec	Blood vessels within PDL* occluded on pressure side
Minutes	Blood flow cut off to compressed PDL area
Hours	Cell death in compressed area
3-5 days	Cell differentiation in adjacent marrow spaces, undermining resorption begins
7-14 days	Undermining resorption removes lamina dura adjacent to compressed PDL, tooth movement occurs

*PDL, periodontal ligament

Undermining Resorption

Undermining resorption has that name because when there are large necrotic areas in the PDL, it is necessary for osteoclasts to resorb the lamina dura from its underside. The necrotic area sends a chemical signal to stimulate the formation of osteoclasts, but it takes a few days for this to penetrate through the lamina dura into the bone marrow. So instead of 2 days for remodeling to begin, it’s 3-5 days before an osteoclastic attack on the underside of the lamina dura begins. Note the typical Howship’s lacunae on the underside of the lamina dura in this image from Dr. Khouw’s research (image 1).

It takes 7-14 days for the lamina dura to be removed, and at that point tooth movement begins with a sudden jump in the direction of the force. If heavy force still is present, there’s another delay while further undermining resorption occurs, so light force actually produces more tooth movement early in treatment than heavy force (image 2). One indicator that force is too heavy is that the tooth gets

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{{PAGE_62}} quite loose. The periodontal ligament space enlarges during orthodontic tooth movement, so there’s always some increase in mobility while tooth movement is occuring, but a really loose tooth reflects loss of a lot of the lamina dura.

Force Distribution and Types of Tooth Movement

Tipping: the Simplest Form of Tooth Movement

Tipping of a tooth occurs when a single force is applied to the crown of a tooth. Why does the tooth tip? Because the resistance to movement of a tooth is created almost entirely by its root, the force against the crown is at a distance to the “center of resistance”, which is at the center of the root, about halfway down.

You should remember from the physics course you once had to take that a force at a distance produces a moment, which rotates the tooth around the center of resistance (as a force at a distance would rotate any other object). Because of the rotation, the heaviest pressure / tension in the periodontal ligament is felt at the apex of the root and at the crest of the alveolar bone, and both decrease progressively toward the center of resistance. In this circumstance, the center of resistance also is the center of rotation. We will look in some detail at the physics in a later module. For now, concentrate on the biologic effect.

Both human and animal experiments indicate that tipping forces should not exceed approximately 50 grams, and for small teeth it should be less than that. Heavier force creates pressure high enough to create necrotic areas in the PDL that require undermining resorption.

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Bodily Movement

If you want to move a tooth bodily (create as much root as crown movement in the same direction), it is necessary to apply two forces simultaneously to the crown of the tooth, so that there is both a force to move the crown and a couple (another pair of forces applied at a distance) to prevent tipping. This loads the entire PDL area, so that there is equal compression all along one side of the root.

If twice as much of the PDL is loaded in bodily movement as in tipping, you would think that twice as much force would be needed. That’s correct, about 100 grams is the ideal amount to produce bodily movement.

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Rotation, Extrusion and Torque

Other types of tooth movement are rotation, extrusion, torque and intrusion. Intrusion is a special case we will discuss in the next screen.

You would think that rotation of a tooth around its long axis would potentially load the entire volume of the PDL, not just an area on one side of the tooth. Extrusion, pulling the tooth gently out of the socket (gently, we don’t want to extract it!) also could load the entire PDL volume. Would that mean that much higher force would be needed for either of these movements? The answer is no. In fact, it is impossible to rotate or extrude a tooth without compressing areas similar to those that are loaded in tipping. Why? Because the roots are irregularly shaped, not conical, as soon as a tooth responds to either of these directions of force, it tips as well as rotating or extruding. For that reason, the ideal force for rotation and extrusion is the same as for tipping, ~50 grams.

Torque is the type of tooth movement in which the root apex is moved further than the crown of the tooth in the desired direction. To accomplish this, a relatively larger moment to move the root is needed than the moment to produce bodily movement. This produces a loading diagram intermediate between what is needed for bodily movement and tipping, and therefore an intermediate force of ~75 grams is needed.

Intrusion: A Special Case

{{PAGE_65}} Until recently, orthodontic intrusion was so difficult that many orthodontists considered it impossible. Now we know that successful intrusion is possible only if exceptionally light force is applied down the long axis of the tooth, so that the PDL is compressed just at the apex of the root.

That makes sense, of course, when you remember that force against the crown becomes pressure in the PDL. Intrusion compresses only a small area at the root apex, and so a small force produces the same pressure that a larger force would if it were distributed over a larger area of the PDL. The optimum force for intrusion is ~10 grams, less for smaller teeth, perhaps a bit more for larger ones like canines, definitely more (but still small) for multi-rooted teeth.

Force Magnitudes for Tooth Movement

This table summarizes the amount of force needed to produce different types of tooth movement. The way to think about it is to realize that there is only one optimum pressure in the PDL, and different forces are needed to obtain that optimum pressure depending on the type of tooth movement that is desired.

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{{PAGE_66}} Optimum Forces for Orthodontic Tooth Movement

Type of movement	Force* (gm)
Tipping	35-70
Bodily Movement	70-120
Root Uprighting	50-100
Rotation	35-60
Extrusion	35-60
Intrusion	10-20

*Values depend in part on the size of the tooth; smaller values appropriate for incisors, higher values for multi-rooted posterior teeth

Force Duration

Does the force to move a tooth have to be there every minute in order to get the tooth to move? If so, that would require a major effort in cooperation by the patient. Wearing a removable appliance all the time is impossible. You have to take it out to clean it and brush your teeth, and it’s so difficult to eat while wearing it that you almost have to take it out for meals. With a fixed appliance, if you’re wearing rubber bands, you’d have to remove them at least briefly to brush your teeth and put on new ones, even if you wore them during eating.

Orthodontists have run many inadvertent experiments on how many hours an appliance has to be worn, simply by using removable appliances and observing the results. Anything that can be removed by a patient will be—the only question is how many hours it will be in the mouth instead of out.

With removable appliances there is a threshold somewhere between 4 and 8 hours per day, below which no tooth movement happens. Above the threshold, the amount of tooth movement increases with increasing hours per day, with decreasing returns toward the top of the curve. A theoretic plot of tooth movement efficiency versus duration of force is shown in the attached image. The concept it illustrates is correct, but the details simply aren’t known.

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{{PAGE_67}} Chart showing tooth movement % efficiency versus duration of force (hrs/day), with a threshold marker at 6 hrs/day. The curve indicates that tooth movement efficiency increases with longer duration of force, starting from zero below the threshold and approaching 100% near 24 hrs/day.

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Continuous vs Interrupted Force

In contrast, a spring with poorer force decay characteristics might produce a force that dropped to zero before the patient returned for reactivation. This is defined as interrupted force. If the initial force level is light enough, the tooth will move a small amount by frontal resorption and then stop until the spring is activated again. If the initial force level is heavy but decays to zero after the first movement of the tooth, the tooth will move only when undermining resorption has occurred—and stop there, since the force has now dropped to zero. This provides an opportunity for regeneration and repair of the PDL before force is applied again.

It is difficult to keep orthodontic force low enough to totally avoid necrotic PDL areas, so some areas of undermining resorption probably are generated in almost every patient. The heavier forces that produce this response are physiologically acceptable only if the force declines to zero before the next reactivation appointment.

That leads to an interesting paradox: the more perfect the spring (the slower its force decays), the more careful the clinician must be to apply only light force. Springs with poor force decay characteristics produce only interrupted force. They are not capable of causing the biologic damage that can occur with heavy continuous force. Heavy continuous force must be avoided; heavy interrupted force can be clinically acceptable even though this is less efficient and more likely to produce pain.

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{{PAGE_69}} Interrupted force B (Green arrow pointing up) Force Time (Blue arrow pointing right)

Intermittent Force Continuous force is impossible with a removable appliance, since the force falls to zero as soon as it is removed and is reestablished when the appliance is inserted again. The result is intermittent force, which also shows force decay as tooth movement occurs. Intermittent force would also become interrupted force if after some tooth movement, there was no reestablishment of force when the removable appliance was reinserted.

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Reactivation Intervals

A clinical rule is that orthodontic appliances should not be reactivated more frequently than every 3 weeks, and a 4- to 6-week interval between appointments is more typical. Undermining resorption takes 7 to 14 days. When this is the mode of tooth movement and when force levels decay rapidly, tooth movement is essentially completed in that amount of time. Another 2 weeks or so is needed for repair before the appliance is reactivated.

So with a springy appliance that produces light force, there is no need for frequent reactivation at short intervals, while with a less springy appliance that produces heavy force initially, a repair period is needed. You might think that if patients were seen every week to reactivate the orthodontic appliance, faster tooth movement would have to result. Bad idea—the result might be heavy continuous force and significant tissue damage.

Deleterious Effects of Orthodontic Force

Pain

If heavy sustained pressure is applied to a tooth, pain develops immediately as the PDL is literally crushed. If you activate an orthodontic appliance and there is immediate pain, it’s obvious that too much force is being used, and action to reduce the force is required. There is no immediate pain with appropriate force magnitudes.

{{PAGE_71}} Several hours later, however, the patient usually feels a mild aching sensation, and the teeth are sensitive to pressure so that biting something hard hurts. The pain typically lasts 2-4 days, then disappears until the next activation of the appliance. At that point the same cycle may recur, but almost always the greatest pain follows the first activation. There is a great deal of variation: some patients report no pain, others have considerable pain with even very light force.

The pain is related to the development of ischemic areas in the PDL that are on the way to sterile necrosis. During the first 24 hours, having the patient bite repeatedly on a plastic wafer or chew gum decreases pain—probably because this displaces the tooth temporarily and allows spurts of blood flow through compressed areas.

Effects on the Dental Pulp

You would expect a light sustained force against the crown of a tooth to produce a PDL response, but not an effect on the pulp. The increased sensitivity to biting force after initial activation of an orthodontic appliance, however, suggests inflammation at the root apex, and this probably is related to mild pulpitis that occurs during the first few days.

The mild pulpitis has no long term significance. Loss of tooth vitality during orthodontic treatment almost never occurs. When this does happen, usually there is a history of previous trauma to that tooth, but poor control of heavy orthodontic force also can be the culprit. If heavy continuous force is used, a large abrupt movement of the tooth after undermining resorption can result in severance of the blood vessels entering the pulp. It also is possible to sever these vessels by moving the root apex through the labial cortical plate. Fortunately, this is hard to do, but it has occurred.

Because tooth movement is a PDL phenomenon, not a pulpal one, moving endodontically-treated teeth should be possible, and this is correct. There is no contraindication to orthodontic repositioning of a non-vital but endodontically treated tooth. This is most likely to be needed in an adult before definitive restorative or prosthodontic treatment is done.

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Effects on Root Structure

The goal of orthodontic force is to generate remodeling of alveolar bone so a tooth can move. For many years it was thought that osteoclasts did not attack tooth roots. Now we know that roots often are attacked by clast cells, but that repair of the root also occurs.

In the absence of necrotic (hyalinized) PDL areas, uncalcified cementum on the root surface protects against osteoclastic attack, but cementum adjacent to a necrotic area is “marked” or stained by adjacent necrotic tissue in the PDL, and clast cells attack this area when the PDL is repaired. Cementum (and dentin if the attack penetrates all the way through the cementum) is removed, and then new cementum is formed to fill in the defect in the tooth root.

In this figure, which is a coronal section of a premolar being moved to the right before it was extracted for study (which is ethical if premolar extraction was required anyway), note the area of compression of the PDL on the right side and the tension area on the left side. Osteoblasts and bone formation can be seen in the tension area; osteoclastic activity is apparent on the compression side; and areas of root resorption with penetration into the dentin also can be seen on that side. These areas would be filled in with new cementum under normal conditions. So root remodeling is a feature of orthodontic tooth movement—but there would be a loss of root structure only if repair did not replace the resorbed cementum.

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Apical Root Resorption

Repair of the damaged root does restore its original contours along the side of the root and at the apex—unless the attack on the root surface at the apex leads to coalescence of craters. This can create islands of root structure that are separated from the root surface. Once that happens, the island resorbs and is lost, as image 1 shows diagrammatically. This is why permanent loss of root structure related to orthodontic treatment occurs primarily at the apex.

Several studies have shown that in general, root length for most teeth decreases about 1 mm for each year of complete fixed appliance treatment, but of course there is considerable individual variation. The data presented in image 2 come from a study comparing treatment changes in patients treated for Class I crowding at the Univ. of Washington. One group, who had had serial extraction earlier, had an average of 15 months in a fixed appliance. The other group, who had extractions just before their fixed appliance started, had an average of 24 months. Note that some loss of root length occurred in the 15 months group, but a bit more occurred for most teeth as the treatment time increased. All other things being equal, the longer the treatment time, the greater the root resorption.

Resorption of this magnitude, 1 or 2 mm (usually referred to as moderate generalized resorption) is a finding, not a problem (images 3 and 4). It is best considered as an inevitable part of comprehensive orthodontic treatment.

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{{PAGE_74}} Image 1: diagrammatic representation of apical resorption Image 2 Image 3: mild blunting of root apices Image 4: moderate apical root resorption (loss of less than 1/4 of root length)

Severe Generalized Resorption

There are three types of apical root resorption that can be seen after orthodontic treatment: (1) mild to moderate generalized resorption, which we have already discussed; (2) severe generalized resorption, and (3) severe localized resorption. Now let’s look at the severe varieties.

Severe generalized resorption describes the loss of most of the roots of most of the teeth. It is quite rare—the average dentist is likely to encounter only one or two such cases in a career. But if you see one, you won’t forget it.

Why a few individuals have resorption of this type remains unknown. We know a lot of things that don’t cause it. Among those is orthodontic treatment. Severe generalized resorption is as likely to occur in people who never had orthodontic treatment as in those who did.

{{PAGE_75}} Severe Localized Resorption

Severe localized resorption, as seen in this image, is defined as loss of more than ¼ of the root of some teeth. It also is uncommon but occurs in 2-3% of orthodontic patients, and is related to the treatment. The maxillary incisors are by far the most likely teeth to be affected. Other teeth can be affected, but this is uncommon.

Why does this happen to some patients but not to the great majority? There seem to be two major factors: (1) some individuals clearly are more susceptible to root resorption than others, and (2) one aspect of tooth movement, bringing the root apices into contact with cortical bone, is a definite risk factor.

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{{PAGE_76}} Severe Localized Resorption (cont.) At this point it is not known why some individuals are susceptible, but we do know that in them resorption starts earlier and advances more rapidly. As a result, as part of informed consent, orthodontic patients and parents should be told that significant root resorption is unlikely but possible. Root contact with cortical bone is most likely to occur when the root apices of maxillary incisors are moved lingually, either as the crown is tipped facially (which leads to root apices moving lingually) or as the roots of the incisors are torqued lingually. A study at North Carolina reported that the chance of severe resorption was increased 20-fold when the incisor roots contacted the lingual cortical plate. This becomes a limitation on how much maxillary incisors can be moved during treatment. The bottom line: It is good practice to take a panoramic radiograph 6-9 months after orthodontic treatment begins to check the status of roots. If obvious resorption is occurring at that point, keeping treatment as short as possible and compromising treatment goals may be necessary.

{{PAGE_77}} Distant Effects of Orthodontic Force: Growth Modification

Growth Modification? Because orthodontic force applied to the teeth also is felt by the facial skeleton, it has the potential to modify growth of the jaws. If this were successful, it would provide a way to treat skeletal problems. It seems reasonable that pressures resisting the normal downward and forward growth of either jaw could diminish the amount of growth, while adding to the pressures that pull the jaws downward and forward could increase the growth. The clinical effectiveness of some treatment procedures aimed at modifying growth has been demonstrated in recent years, but it is important to understand that some things are more possible than others and that the amount of skeletal change is relatively small.

The goal of this section is to help you understand how growth modification works, and to put its effectiveness for various possible changes into perspective.

Principles in Growth Modification When growth modification is considered, at least three principles should be kept in mind:

1. You can’t modify growth that isn’t happening. That means treatment at a period of rapid growth usually is the best plan, and for practical purposes, it means that the adolescent growth spurt is the preferred time for most growth modification treatment (but not all, as we will discuss below).

{{PAGE_78}} 2. When growth modification is desired, tooth movement almost always is undesirable. Correction of a malocclusion is not a primary goal of growth modification treatment, correction of an improper jaw relationship is. The less a malocclusion is corrected by tooth movement, and the more it is corrected by favorable jaw growth, the more successful the growth modification treatment is.

3. The hours of the day are not created equal relative to growth. In fact, almost all skeletal growth (and tooth eruption) occurs in a critical time period between early evening and midnight. Whatever the growth modification device is, it’s important for it to be worn during these hours—but wearing it all the time may be neither necessary nor desirable.

Restraint of Maxillary Growth

Excessive maxillary growth contributes to both Class II and long face problems. Since formation of new bone as the sutures above and behind the maxilla are pulled apart by soft tissue growth is a major mechanism of growth, a force to restrain growth at the sutures ought to be effective.

Extra-oral force to restrain maxillary growth that is produced by a headgear device (image 1) has been used in orthodontics for many years. Because of the large area of the sutures, a heavier force than is recommended for tooth movement is generally considered necessary for effective restraint. A force of about 250 grams per side (500 gm total) is thought to be about the minimum. Because this force often is applied just to the maxillary first molars, its effect on the teeth is a matter of concern. As we have discussed, heavy continuous force against the teeth is undesirable. Intermittent force is less effective for tooth movement, and it makes sense to suggest that the headgear should be worn during the early evening (don’t wait until you go to bed!) and night, but perhaps not during the day. The force duration should be at least 12 hours per day. Whether this really limits tooth movement without compromising the skeletal effect has not been clearly demonstrated.

A change in the direction of maxillary growth often is observed with headgear treatment (image 2). Note that the maxilla actually moved slightly back in this individual as he grew while wearing his headgear consistently. Not all treatment results are so favorable, however. Clinical trials indicate that there is about a 75% chance of an improvement in a skeletal Class II jaw relationship with headgear treatment during adolescence. Although this surely is affected by the degree of the patient’s cooperation, it appears that cooperation is not the whole story. For whatever reason, some children respond better than others.

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{{PAGE_79}} Image 1: Schematic of a skull showing the application of an upward-backward force via high-pull headgear to restrain maxillary growth. Image 2: Cephalometric superimposition comparing two facial outlines of a boy, demonstrating downward and slight backward maxillary growth.

Augmentation of Maxillary Growth

Increasing the amount of forward growth of the maxilla by pulling forward with a face mask (reverse headgear) (image 1) has not been as successful as restraint of maxillary growth, especially when the treatment is done during adolescence. Part of the problem is getting enough force at the maxillary sutures to pull them apart, but a bigger problem is the anatomy of the sutures.

Human autopsy material has shown the morphology of the mid-palatal suture at different ages, and the other maxillary sutures are similar. In the images with this screen, the two halves of the maxilla are shown below, with the nasal septum above. Note in image 2 that the mid-palatal suture is an open straight-line structure in infants and young children. It shows some convolutions in childhood (early mixed dentition) (image 3), and by late childhood / early adolescence the suture is so interdigitated (image 4) that opening it requires micro-fractures of the bone spicules along the suture line. After the adolescent growth spurt opening the suture is possible only with surgical assistance.

{{PAGE_80}} If you want to move the maxilla forward you have to do so at an early age, before the sutures become so locked up. So maxillary advancement to augment growth is an exception to the rule about treatment during adolescence. It has to be done before adolescence, during the elementary school years.

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{{PAGE_81}} Image 1 Image 2: Mid-palatal suture in the early preschool years Image 3: Mid-palatal suture in the mixed dentition Image 4: Mid-palatal suture in late childhood

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Restraint of Mandibular Growth

You have learned already that the mandible grows downward and forward as it is pulled in that direction by the growth of the soft tissues in which it is embedded—so a force opposing that direction of growth should effectively restrain growth. Unfortunately this just doesn’t work very well. A chin cup or other restraining device attached to the chin (image 1) may rotate the mandible down and back, so that growth is more down and less forward, but it’s remarkably ineffective in preventing the mandible from growing.

There are several explanations for this ineffectiveness, none totally satisfying: children are not willing to wear it enough hours of the day, it’s difficult to load the whole TM joint area instead of just a part of it (image 2); a force large enough to be effective may be painful; a chin cup is uncomfortable to wear even if it isn’t painful; and so on.

For patients with excessive mandibular growth, the only really successful treatment has been surgery to reduce the size of the mandible after adolescent growth has stopped.

Image 1: A typical chin-cup appliance, attempting to control excessive mandibular—which unfortunately is not very effective.

Image 2: Force against the chin tends to be concentrated on part of the growing condyle, not on all of it.

TADs to Modify Class III Growth?

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{{PAGE_83}} A new and potentially important way to augment maxillary growth and perhaps restrain mandibular growth, which was developed by Dr. Hugo DeClerck, is the use of Class III elastics from miniplates at the base of the zygomatic arch to miniplates mesial to the mandibular canines (image 1). The optimal time for this treatment is later than that for facemask. It is done as early as possible, but now the bone must be mature enough to retain bone screws, which means that age 10 1/2 or 11 (early adolescence) is about as early as it can begin, and age 13 or so is too old for a good response. Maturation, not age, is the determinant—early adolescence is OK, late adolescence is not.

Heavy force is not necessary—about 150 gm on each side is sufficient—but the elastics need to be worn essentially full-time. For most orthodontic treatment, light force of long duration is optimal, and this result suggests that this also is true for growth modification.

Most of the patients show increased maxillary growth (images 2 and 3 show maximum change, not the average), but in some, the effect is largely restraint of forward growth of the mandible. The average amount of maxillary change with this method is twice as much forward movement of the maxilla than is achieved with facemask treatment.

{{PAGE_84}} Image 1: Class III elastics to miniplates: Class III elastics to skeletal anchors in children in early adolescence have the potential to modify maxillary and mandibular growth.

Image 2: Before / after treatment: Note the increased prominence of the mid-face after treatment for one year.

Image 3: Before / after treatment: The changes for this patient included forward movement of the zygomatic arch.

TADs to Modify Class IIIs Growth? (cont.) The images with this screen are color maps showing the change from before to after treatment for six patients treated with Class III elastics to bone plates. The 3D images from CBCT were superimposed on the cranial base. The patient whose clinical photos you saw in the previous screen is image 1. Note the variation in outcomes seen individually in images 1-6,and collectively in image 7. In this view of pre- and post-treatment superimpositions, movement out of the screen (forward) is shown by the red color—the darker the red, the more the area moved forward. Movement into the screen

{{PAGE_85}} (backward) is shown by blue—the darker the blue, the more the mandible moved back. As you can see, the outcomes ranged from major advancement of the midface (images 1 and 3) to smaller changes in the maxilla (images 2 and 4) to very little change at all (image 5) to significant (and surprising) backward movement of the mandible, which could occur only with repositioning or remodeling of the condyle and/or TM joint.

Data for long-term outcomes of this treatment approach are not yet available, and there is no way at present to predict whether the major effect will be on the maxilla or mandible—but Class III elastics to TADs seems likely to largely replace face mask treatment, because the average effect is greater and it is much easier for patients to wear elastics. Extra-oral force to the chin is rarely used now because it is ineffective, and the effect on the mandible of elastics to TADs is less than the effect on the maxilla, so growth modification for true mandibular prognathism is less likely to be a major use of the new method.

The effects of this type of treatment are discussed further in later sections of this module.

Augmentation of Mandibular Growth

The soft tissues pull the mandible forward, and the condylar process grows upward and backward in response, so a device to further move the mandible forward ought to make it grow larger. Mandibular deficiency is the major cause of skeletal Class II problems. Not surprisingly, appliances to try to make the mandible grow have played a prominent role in growth modification treatment.

There are dozens if not hundreds of devices to hold the mandible forward and encourage it to grow (images 1, 2 and 3). They can be removable (images 1 and 2) or fixed (image 3), but cooperation by

{{PAGE_86}} the patient is important even with a fixed appliance. They’re referred to generically as “functional appliances”, but function has little to do with their effect. The name of the game is holding the mandible forward so that the condyle is down and forward from the glenoid fossa.

The typical outcome of functional appliance treatment is encouraging on one hand, and discouraging on the other. It often happens that in the short term, an acceleration of mandibular growth is observed, but in a longer term the growth rate tends to decline so that the mandible no longer is growing as much as might have been expected without the appliance (image 4). Does the mandible end up larger than it would have been without treatment? Perhaps a little bit, only a millimeter or two.

The interesting result is that although the theory behind their use is totally different, the long-term outcome of headgear and functional appliance treatment is remarkably similar. More restraint of maxillary growth is seen with headgear, but the reaction to holding the mandible forward is a restraining force against the maxilla, so maxillary restraint usually occurs with any type of functional appliance. How much skeletal change can you get? On the average, 4-5 mm of forward movement of the mandible relative to the maxilla, perhaps somewhat more in the short term—but post-treatment growth tends to decrease the long-term skeletal change.

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{{PAGE_87}} Image 1: Bionator appliance: a block of plastic between the teeth guides the patient into advancement of the mandible Image 2: The Frankel appliance: shields hold the lower lip and cheeks away from the teeth while the mandible is held forward by a wire framework and lingual pad Image 3: Herbst appliance: The Herbst appliance, a fixed functional appliance, holds the mandible forward all the time and forces the patient to function in that jaw relationship.

Image 4: The usual response to a functional appliance is short-term acceleration of mandibular growth, followed by slow growth later so that there is little or no long-term increase in size

Retention

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{{PAGE_88}} Why Is Retention Necessary? If you have had orthodontic treatment, you know that when the braces come off, some type of retainer follows, and the orthodontist tells you quite severely that you’d better wear it or your teeth won’t stay straight. Perhaps you didn’t believe that, didn’t wear the retainer, and found out for yourself that the teeth really will move spontaneously for a while after treatment. Why is that?

There are three major reasons for retention:

1. the gingival and periodontal tissues require time for reorganization after the orthodontic appliances are removed;
2. the teeth may be in an inherently unstable position, so that soft tissue pressures constantly produce a relapse tendency; and
3. changes produced by growth may alter the treatment result.

Let’s consider these reasons one at a time.

PDL / Gingival Reorganization Disruption of the collagen fiber bundles in the PDL and widening of the PDL space are necessary to allow tooth movement to occur. Even if active tooth movement stops, reorganization of the PDL does not occur as long as each tooth is tightly splinted to the one next to it by a heavy archwire. After the braces are removed, reorganization of the PDL takes 3-4 months, and only at that point does the slight mobility that is normal after treatment disappear. During this time the teeth will be unstable against occlusal and soft tissue pressures that can be resisted later. For this reason, every patient needs retainers for at least a few months.

Gingival fibers are stretched when a tooth is moved a considerable distance or is rotated significantly. These fibers remodel quite slowly, and are still capable of rotating a tooth back towards its original position even a year after treatment. This becomes a reason for more prolonged retention after correction of severe crowding. For a tooth or teeth that had correction of severe rotations, like the maxillary incisors shown in these images, it is wise to sever the gingival fiber network around them (which must be done carefully to maintain the interdental papillae) before the braces are removed. Without this it can be almost impossible to maintain the rotation correction.

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{{PAGE_89}} Image 1: Sectioning the supra-crestal gingival elastic fibers is important if correction of rotation is to be maintained

Image 2: Completion of fiber section surgery - an important adjunctive procedure in orthodontic treatment

Unstable Tooth Positions

How far can you move teeth before the pressures of the lips/cheeks or tongue change enough to be a long-term factor in their stability? The question is asked correctly if you put it that way, because it’s how far the teeth were moved, not how their position after treatment compares to “standards” derived from population averages, that determines long-term stability. As a general rule (to which there are exceptions), the lower arch can be considered a foundation on which the upper arch rests, so how far the lower teeth were moved is important.

For a typical patient, the magnitude of change that is likely to be stable is shown in the attached figure. Note that the incisors can be advanced a little but not a lot, that stable expansion across the canines is extremely limited, and that the amount of stable transverse expansion is greater as you go posteriorly.

If the teeth are in an unstable position (for instance, after major arch expansion), the only possibilities are permanent retention or accepting relapse after retainers are discontinued.

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Growth Effects

Continued growth after treatment creates a problem in two circumstances:

1. Patients who received growth modification treatment for a skeletal problem. You’ve already learned that the pattern of growth tends to be constant. If treatment changed it, growth after treatment still will be in the same pattern as it was originally. So the more a Class II, Class III, long face or asymmetric patient grows after treatment, the more likely it is that some of the correction will be lost. In these patients, the retainer may need to be a modification of a functional appliance, and it will need to be worn at night until the patient’s late teens or early twenties.

2. Patients without a skeletal problem whose incisors were aligned with some expansion (image 1) or who had space closure in the maxillary incisor area (image 2). You will remember that there’s normally a little late mandibular growth in the late teens that occurs after maxillary growth has essentially ceased. This has the potential to create lower incisor crowding, even in an individual who never had orthodontic treatment. The late mandibular growth carries the lower incisors forward, which tends to increase pressure from the lip against the lower incisors, and also creates pressure against the upper incisors that can re-open space in that area. For that reason it is a good idea to bond a retainer in the lower incisor segment that remains there at least until the early adult years, or to continue night-time wear of a removable mandibular retainer, and also to maintain a maxillary retainer, at least until the late growth has occurred. If significant forward movement of the lower incisors occurred during treatment, a retainer will be needed indefinitely.

{{PAGE_91}} Do you still have a retainer even though you’re now in dental school? If you’re in the approximately 50% of dental students who had orthodontic treatment, maybe you should.

Summary To summarize this discussion of the biology of orthodontics:

• Orthodontic tooth movement requires sustained light force.
• The optimal force depends on the type of tooth movement. Force against a tooth creates pressure in the PDL. There is only one optimal pressure, but varying forces are needed to obtain it, depending on how much of the PDL is loaded.
• Light continuous force (which may decline as teeth move, but not to zero) is the most effective and biologically benign way to move teeth. Heavy intermittent force, which declines to zero so that there is a repair period before the force is resumed, can be acceptable. Heavy continuous force must be avoided.
• Moderate pain for 2-4 days is to be expected when orthodontic treatment is initiated. The magnitude of the pain is related to the magnitude of the force being used, because heavier force creates larger ischemic (and eventually necrotic) areas in the PDL.

Summary (cont.)

• Modest apical root resorption (1-2 mm) accompanies almost all orthodontic tooth movement. More severe root resorption is seen in 2-3% of orthodontic patients.
• Growth modification via forces to the teeth can produce a few millimeters of improvement in jaw relationships, with restraint of mandibular growth being the least effective.
• Retention after orthodontic treatment is needed for three reasons: it takes 3-4 months for PDL reorganization to occur teeth may be in an unstable position, particularly after rotation correction or excessive expansion growth after treatment can affect stability, especially in patients who had a

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{{PAGE_92}} skeletal problem, but also in those who experience late mandibular growth that often causes crowding of lower incisors.

Referral to Self-Test The self-test section of this program is designed to help you be sure you have understood the material.

Now that you have gone through the module, do the assigned reading in Contemporary Orthodontics(pages 331-347 in 5th ed.; pages 329-347, 4th ed.) Then take the self-test, and use it as a guide for further study and review.

Copyright 2013, UNC Dept. of Orthodontics

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4. Mechanical Principles in Controlling Orthodontic Force

Elastic Materials

Learning Objectives

As you learned from your study of the biology of orthodontics, the trick in moving teeth is to use the right amount of force—and almost always the problem is to avoid using too much force. Teeth respond best to light continuous force. In an orthodontic appliance, how are things arranged so that the force is light and at least reasonably continuous?

The objective of this module is to help you answer that question, from both a theoretical and a practical clinical perspective.

In addition to going through the module, read Chapter 9, pp. 312-328 and Chapter 10, pp. 347-389 in the 5th edition of Contemporary Orthodontics (or pp. 359-377 and 395-429 in the 4th edition).

Precious Metals in Orthodontics

If you want to move a tooth, it’s necessary to use a spring that places a light but prolonged force against it—and that spring must function in the mouth, which is accurately described as a hostile environment. The better the spring, the less the force it delivers will decrease as a tooth moves. One possibility is a rubber band, and if you had orthodontic treatment yourself, you may have worn rubber bands connecting the upper and lower arches. Why did you have to change them every day? Because even the best latex materials lose their elasticity (spring quality) rapidly in the mouth.

The orthodontic appliances of a century ago almost had to be made with gold that was alloyed to be as springy as possible, because nothing else would work for more than a few days in the mouth—but gold is not known for its spring qualities. An interesting appliance from that era is the Crozat appliance shown in this image, which had a gold framework and gold springs that were slightly activated every time the patient was seen. It’s interesting because it still is used occasionally, by dentists who believe that if it’s made of precious metal, it must be better. Like many other antique devices, it works by tipping teeth to a new position—but more slowly than its modern counterparts that use steel springs.

Gold wires also were used with the first fixed appliances, and the edgewise appliance that is still the basis of modern orthodontics (which we will discuss in more detail below), was engineered for use with gold wires.

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Stainless Steel and Titanium

Stainless steel, which has excellent spring qualities, rapidly replaced gold when it became available for orthodontic applications. Fortunately, that transition was complete before the price of gold increased 10-fold in the 1960s. More recently, nickel-titanium and beta-titanium materials have come into wide use. It is likely that composite plastics will replace them before long. Orthodontic technology trails aerospace technology by 10-20 years. As passenger planes made largely with composite plastics (like the Boeing 787) now are replacing their aluminum and titanium predecessors, it’s a safe bet that use of composite plastics in orthodontics will follow. Like the materials in high-performance aircraft, orthodontic wires and attachments are extremely stressed, and it makes sense to use the best materials available.

Whatever the material, however, the same physical laws apply. Different materials differ in three principal properties: strength, stiffness / springiness (which are different terms for the same property), and range. These can be illustrated by a stress / strain plot, as shown here.

On the graph, four measures of strength are shown. The proportional limit is the point at which the wire begins to deviate from elastic behavior, but it is very difficult to establish. Yield strength, the point at which a measurable deviation (like 0.1%) is observed, is taken as the point at which the elastic limit of the material is reached and it begins to bend. This value is the one typically cited in reports on strength of an orthodontic wire. The ultimate tensile strength is the point at which the material begins to fail, and the failure point is where it breaks.

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{{PAGE_95}} The graph also shows springiness and stiffness, which are measures of the same thing, as the slope of the line. Springiness = 1 / stiffness and vice versa. The stiffer the wire, the more vertical the line will be; the springier it is, the more horizontal the line will be.

Force / Deflection Curves

Stress and strain are internal properties of a material. In laboratory experiments, the force against a material produces a deflection, and stress and strain can be calculated from that interaction. Force / deflection curves like the one shown here are what you usually will see in reports of material properties. These curves, the original laboratory data, provide the same information as a stress / strain curve.

What properties would you want in an elastic material to be used as an orthodontic spring? Think about that from the perspective of the three key properties seen in a force / deflection plot. You’d want…

…enough strength that the spring didn’t get bent out of shape. That would mean that the distance along the y axis to the yield point should be as large as possible. Note that a spring still works beyond the yield point, but it doesn’t totally spring back to its original position if it’s loaded beyond its yield point.

…the best possible springiness. That would mean that the slope of the line should be tilted to the right (i.e., more horizontal), so that the amount of force delivered by the spring would be as constant as possible.

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{{PAGE_96}} …the best possible range. That would mean that the distance along the x axis at the point of permanent deformation should be as large as possible.

Obviously, you can’t have the maximum of all these things. A very strong wire would tend to be stiff and to have little range, while one with maximum range would tend to be quite springy but not very strong.

Force / Deflection Curve (cont.)

Another property that you are likely to see in descriptions of wire materials is resilience. This is the area under the curve up to the proportional limit. It gives a measure of a wire’s combination of the principal properties, and so it is likely to be used (especially in advertisements) as a way to emphasize the balance that the wire has among strength, springiness and range.

As we have noted, a spring that is deflected beyond its elastic (proportional) limit still has spring properties, it just won’t completely return to its original shape. It’s easy to bend a wire beyond its elastic limit, and when you form the wire into some shape (bend a loop in it, for instance, as you form a spring to move a tooth), you’d also like some distance between the elastic limit and the point at which the wire breaks. In fact, to bend wires you must have reasonable formability, which is the area under the curve between ultimate tensile strength and failure.

Some very springy steel wires allow almost no bending beyond their elastic limit—they break, as one student said, if you look at them cross-eyed. They’re not very useful when you’re trying to form a clasp or finger spring for a removable appliance.

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Stress-Strain Curve Diagram

The diagram illustrates the relationship between stress and strain for a material. The x-axis represents Strain, and the y-axis represents Stress. A curve is plotted showing three key points: “Proportional limit” at the beginning of the linear region, “Yield strength” where the curve begins to deviate from linearity, and an open circle representing the ultimate point on the curve. Two shaded regions are highlighted: “Resilience,” which is the area under the curve up to the proportional limit (shaded with dots), and “Formability,” indicated by a double-headed arrow spanning the horizontal distance between the yield strength and the final point on the curve.

Material Effects on Wire Size Text Content

Material Effects on Wire Size The size of a wire obviously would affect its basic properties. The bigger it gets, the stronger but the less springy it will be, and the less range it will have. This means that for orthodontic purposes there are a range of useful wire sizes. You don’t have to memorize those wire sizes—you can always look them up—but you do need to understand that concept. Steel is stronger than gold or titanium, so all other things being equal, smaller steel wires would be used for orthodontic purposes.

Orthodontic springs come in two forms: cantilever beams, which are attached only at one end (image 1), and supported beams that are attached at both ends (image 2). A finger spring from a removable appliance is an excellent example of a cantilever beam, but cantilever beams also can be used with a fixed appliance. The section of an arch wire between the brackets on adjacent teeth is a supported beam.

As we have said, whatever the material (with the exception of the superelastic materials that we will discuss next), the same laws of elastic behavior apply. For any orthodontic cantilever beam, the effects of changing its size might surprise you (image 3). Doubling the size of wire used to make a finger spring:

• increases its strength 8 times: $x\ast 2^3=8x$ strength
• decreases its springiness 16 times: $x\ast 2^{-4}=1/16x$ springiness
• decreases its range by half: $x\ast 2^{-1}=1/2x$ range

Doubling the size of a supported beam produces similar but even more pronounced change—it puts a coefficient in front of the multiplier.

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{{PAGE_98}} Image 1: Cantilever beam, supported on only one end. Image 2: Supported beam, free to move at abutments on each end, which is analogous to an orthodontic arch wire. Image 3: B and C are the two types of supported beams. C is a fixed bridge, B could be an arch wire. Both are stronger, stiffer and have less range than a cantilever beam.

For A: Strength d → 2d = 8 (2d/d)^3 Springiness d → 2d = 1/16 (d/2d)^4 Range d → 2d = 1/2 (d/2d)

Effects of Beam Length

A second important variable in determining the principal properties of an orthodontic spring, whether it’s a cantilever or supported beam, is its length. The influence of beam length is much greater than you probably would have thought—unless you really paid attention in that pre-dental physics course or studied engineering before dental school. For a cantilever beam (finger spring), doubling its length:

• cuts its strength in half: x * 2x = ½ strength
• increases its springiness 8 times: x * 2x³ = 8x springiness
• increases its range 4 times: x * 2x² = 4x range

It is apparent that increasing beam length produces significantly more increase in springiness and range than it costs in lower strength. For a spring, length is measured along the wire from which it is

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{{PAGE_99}} formed, not from where it is supported to where it ends up. So bending a loop or helix in the wire is a common way to gain springiness and range without reducing strength too much.

That has a very practical application. In fabricating an orthodontic spring, it would be smart to choose a relatively large wire size to gain strength, and then obtain springiness and range by making the length of wire longer between its attachment and its contact with a tooth. For that reason, it is common practice to lengthen finger springs with a helix (image 1), and to lengthen the effective distance between two teeth with brackets with loops in an arch wire (image 2).

Image 1: To improve its springiness and increase its range, this spring has been lengthened by placing a loop in the wire.

Image 2: The loops in this steel arch wire also add springiness where it is needed in the relatively stiff steel wire.

Elastic vs. Superelastic Materials

Superelasticity

Hooke’s law, which applies to all elastic materials, is the basis for the information that we just discussed. It was almost shocking when materials that violated Hooke’s law became available for orthodontic purposes in the 1980s, not so long ago. These superelastic materials have a different force/deflection curve from elastic materials, as shown in this image. Note that there is an almost flat portion in the center of the force/ deflection curve, with an area at the beginning and end that looks like the curve for an elastic material.

You can see immediately that a superelastic spring that delivered the correct force would be quite valuable in orthodontics, because how far you deflected it would make little difference in the force it delivered over quite a wide range.

{{PAGE_100}} Stress δD δB δF A H G F C C1 D E ε C1 ΔT=O

Nature of Superelasticity

How is superelasticity possible? Because superelastic materials undergo a phase transition with changes in temperature or internal stress. For a material with a transition temperature close to mouth temperature, stressing it by tying it to irregular teeth literally turns it into a different material than the one you picked up off the counter near the dental chair. The word “super” is overused, often to describe trivial things. This time the term super is warranted. You don’t often get to see physical laws apparently overturned.

For orthodontic purposes, a nickel-titanium (NiTi) alloy with a temperature transition near mouth temperature is used. It is delivered in an austenitic phase that bends elastically when it’s first deflected (distance A-B on this plot), and transitions to a martensitic phase (B-C) as internal stress builds up. That’s the upper almost flat line in a superelastic force/deflection curve. Once all the austenite has changed to martensite, there’s again an elastic curve (C-D-E).

Interestingly, the force delivered by the spring differs from the force used to deflect it initially. If you deflect the superelastic material to point C1 on the graph, the recovery curve is elastic from C1 to F, then almost flat from F to G as martensite transitions to austenite, and elastic again from G to H. It’s important to realize that the force that’s delivered to a tooth is shown by the unloading curve, the lower (green) line in the plot.

The phase transition that makes superelastic wires so useful in modern orthodontics produces an exception to Hooke’s law, but doesn’t really repeal it. Both the austenitic and martensitic phases follow the law, as this plot shows, and their properties are affected by wire size and beam length. But that superelastic plateau offers a great advantage when your goal is to align crowded and irregular teeth.

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{{PAGE_101}} Stress δD δB δF A H B C C1 D E G F ΔT=O εC1

Superelastic Wires

There’s another interesting aspect of superelastic springs: the amount of force they deliver can be changed by releasing a wire from a bracket on a tooth, and then tying or clipping it back into the bracket. The unloading curve changes with different degrees of initial activation (image 1). So there’s another magic trick with a superelastic wire. You can reactivate it just by releasing it and putting it back where it was. Clinically, that’s rarely important because the plateau is nearly flat anyway, but it’s an amazing property.

Still, even with an almost magical superelastic wire, you never escape some disadvantages. The formability of a superelastic wire is almost zero. The manufacturer can shape it by controlling temperatures as it is formed, but you can’t.

It’s interesting to contrast a superelastic force/deflection curve to the curve for a stainless steel wire, elastic nickel-titanium (Nitinol) and superelastic nickel-titanium (Chinese NiTi on this graph) (image 2). The superelastic wire has far greater range and much better springiness. For elastic wires, the activation and deactivation curves have the same slope; for superelastic wires, they’re different.

How much force would you want a superelastic spring to deliver? This is shown on the plot by the deactivation (unloading) segment of the curve. You have already learned the desired force magnitude for the various types of tooth movementt in your study of the biology of tooth movement.

Superelastic orthodontic wires are particularly useful in aligning crowded and irregular teeth, which usually requires tipping, rotation and extrusion (images 3, 4, 5). For all these movements, the optimal force is about 50 grams. So that’s what you’d like the superelastic wire to deliver during initial alignment.

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{{PAGE_102}} Bending moment (gm-mm) 1500 1000 500

Activation

Deactivation

Deflection (degrees) 0 20 40 60 80

Image 1: Activation (solid line) and deactivation (dashed line) curves for a superelastic wire.

Bending moment (gm-mm) 4000 2000 1000 500

Stainless steel Nitinol Chinese NiTi

Deflection (degrees) 0 20 40 60 80

Image 2: Force-deflection curves for steel, elastic NiTi and superelastic NiTi wires

Alignment was nearly completed without adjustment of the initial wire. Now a heavier wire can be employed as space closure begins.

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{{PAGE_103}} Design Factors in Orthodontic Appliances

Forces and Moments in Tooth Movement

Any force against an object, unless the force is directed through the object’s center of resistance, also creates a moment (image 1). A moment, by definition, is a force (grams for our purposes, newtons in scientific reports) that is delivered at a distance from the center of resistance. Its magnitude is force x distance (gm-mm). If the object is a tooth, the force has to be applied to its crown, but the center of resistance is at about the middle of the part of the root that is encased by bone. Thus we can say that not only is there a force (F) when an orthodontic spring contacts the crown of a tooth, there also is a moment of the force (MF). The effect would be to displace the crown, while tipping the tooth so that it rotates around its center of resistance (which also is the center of rotation when there’s only a single force). The periodontal ligament would be compressed maximally at the root apex on one side and at the height of the alveolar crest on the other side.

A couple, two equal forces in opposite directions, creates a pure moment (MC) that would rotate the object but not displace it. If you had equal forces in opposite directions on the corners of a box, it would just spin around (image 2).

Consider what would be required to move a tooth bodily, using a force of 100 gm (which you have already learned is about the right magnitude for bodily movement). What’s the magnitude of the moment of this force? That depends on the distance from the point where the force is applied to the center of resistance. For a maxillary incisor, it’s about 15 mm from the center of the crown (where a bracket typically would be placed) to the center of resistance. So now we have MF = 1500 gm-mm. What MC would be needed to move the tooth bodily? That’s right, you’d need 1500 gm-mm in the opposite direction to the moment created by the force against the crown (MF) so that MC cancels MF.

{{PAGE_104}} 50 50 Couple

Image 1: Diagram showing a tooth with labels for “X = Center of rotation”, “L = Moment arm”, and a force vector labeled “50 gm”.

Image 2: A couple is a pure moment—so this object would spin around but would not be moved to a new location

Effect of M

More generally, we can say that the type of tooth movement that is created will depend on the ratio of the moment of the couple (represented by a curved line in these diagrams) to the moment of the force. If MC/ MF is zero (i.e., there is no couple), the tooth tips (image 1), rotating around its center of resistance, and the root goes one way while the crown goes the other. As the MC/ MF ratio becomes positive, the tooth still tips, but now the center of rotation is displaced away from the center of resistance (image 2), so the amount that the root apex goes the other way from the crown decreases. When MC/ MF = 1, the tooth moves bodily, and now the center of rotation is displaced infinitely far away from the center of resistance (image 3). When MC/ MF becomes greater than 1, torque is created so that the root apex moves more than the crown (image 4) and the center of rotation is displaced beyond the crown.

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{{PAGE_106}} Image 3: When MC/ MF is 1, the center of rotation is displaced infinitely far away from the center of resistance, and the tooth moves bodily (translates).

Image 4: When MC/ MF is > 1, the root apex moves more than the crown, and the center of rotation is displaced in the other direction.

Creation of a Couple

Why are fixed appliances used for almost all orthodontic treatment, and why is a fixed appliance almost always required if you want to do anything but tip a tooth? Because it takes 2 points of contact against the crown to create a couple, and a finger spring from a removable appliance gives you only a single point of contact. This produces a force and its moment that causes the tooth to tip around its center of resistance.

With great difficulty, two finger springs contacting the tooth at different points can create a couple, but they have to push in opposite directions (image 1), and that’s so difficult to arrange that it’s rarely practical. As image 1 shows, in theory you could put one finger spring against the center of the crown of an incisor, pushing it back, and another one against the lingual surface near the incisal edge, pushing it forward. That would create a couple, and because the spring at the incisal edge is further from the center of resistance than the one pushing against the crown, its force could be less. But to get a net force of 50 grams to move the tooth, you’d have to activate the first spring to 200 gm and the second one to 150 gm. Forces of that magnitude almost surely would displace the removable

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{{PAGE_107}} appliance. The bottom line: to do anything but tip a tooth, you have to have a fixed attachment and 2 points of contact.

How much force would be placed against the corners of a bracket if you wanted to move a tooth bodily in a mesio-distal direction (to close a space between 2 teeth, for instance)? Now we have to know MF and also the distance between the equal and opposite forces. Suppose you’re using a 100 gm force, it’s 15 mm to the center of resistance, and the bracket is 4 mm wide (image 2). Then MF would be 1500 gm-mm, and you’d need a 1500 gm-mm MC to cancel it. The moment arm across the bracket would be 2 mm (the distance to the center of the bracket), and the force on each corner of the bracket to get 1500 gm-mm would be 375 gm x 2 mm + 375 gm x 2 mm. That doesn’t sound like light force, does it? If the bracket were narrower, the force on the corners of the bracket would be proportionately larger. But if this couple only cancels the moment of the force, the periodontal ligament doesn’t feel anything but the force itself.

Torque

How do you create a couple to move a tooth root in a facio-lingual direction? That type of movement, of course, is torque. The edgewise appliance was created by Edward Angle in the 1920s as a torquing device, and it’s still the basis of modern orthodontics because it provides torque control. Its basis is the placement of a rectangular arch wire that is twisted so that its orientation is different from the orientation of a rectangular bracket slot into which it fits tightly. Then the twist in the wire is translated into torque for the tooth.

To create a couple within the bracket slot, the forces against the outer and inner edges of the bracket slot would have to be quite large, because the moment arm of the couple is so small. Without that heavy force within the bracket (which the periodontal ligament does not feel), you can’t create the necessary MC for torque.

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{{PAGE_108}} Effect of Bracket Width We already have noted the effect of bracket width on the force with which an archwire contacts the corners of a bracket to create a couple. The wider the bracket, the lower the force, because the moment arm across the bracket is longer.

If the tooth is to slide along an archwire (to close a space between the teeth), it tips until the archwire contacts the corners of the bracket. Then, if the wire doesn’t bend, a couple is created that keeps the tooth from tipping any further. The magnitude of the force that generates the couple becomes a factor in resistance to sliding. There are two sources of resistance to sliding: friction as the wire slides along the sides and base of the bracket, and contact of the wire against the corners of the bracket that creates binding.

Brackets and wires are sized by thousandths of an inch: a typical round wire size might be .016” in diameter, while a rectangular one might be .017” in one dimension, .025” in another. It’s easier if we translate those numbers into mils (milli-inches), so that .016” becomes 16 and .017” by .018” becomes 17x25. We’ll refer to wires with those numbers. For rectangular brackets, only the smaller number usually is given—if the bracket dimensions are .022 by .028” or .022 by [something else], it’s just a 22 slot bracket because the larger dimension has little effect.

For sliding, the wire must be undersized relative to the bracket. Otherwise, friction makes movement all but impossible. With an undersized wire that provides .003” clearance, however, friction can be ignored as soon as the couple is generated, because the couple creates almost all the resistance to

{{PAGE_109}} sliding. Sliding often is done on a 19x25 steel wire in a 22 slot bracket—that’s the way you’d read about it, with the numbers in mils instead of thousandths of an inch.

The bottom line: if you want to slide a tooth along an archwire, a wider bracket is better, but friction is not an important factor when an undersized wire is used. The major source of resistance to sliding on an appropriately sized wire is the contact with the corners of the bracket, not friction (despite advertisements that say just the opposite).

Effect of Bracket Slot Size

As we have noted, the edgewise appliance was developed by Edward Angle as a torquing device. Sliding a tooth along the archwire was not part of Angle’s treatment plan. He engineered the appliance to use .0215 x .028” gold wires in a .022 x .028” slot, choosing the wire material because gold was best material available at that time. He chose to use narrow brackets to increase beam length (thereby making the wire more springy and increasing its range), and selected the wire and slot sizes to obtain the torsional properties he desired. Why torsional properties? Because the rectangular wire was twisted, not bent, when it was placed in the rectangular slot.

When stainless steel became available, the wire and slot sizes for torque changed. A 17 x 25 steel wire has torsional properties close to a 21.5 x 28 gold wire if the beam length is maximized by using narrow bracket, and to be effective it would have to be placed into a smaller slot. An 18 slot bracket typically is used for torque with a rectangular steel wire.

From this background, it’s easy to see that undersized steel wires with wider 22-slot edgewise brackets would facilitate sliding, but would make torque more difficult because full-dimension steel wires would be too stiff. With narrow 18-slot brackets and steel wires, torquing properties would be good but sliding would be more difficult. At present, about one-third of American orthodontists use 18 slot brackets and two-thirds use 22 slot. The choice is based on the trade-offs. Which do you want more: to slide on an under-sized steel wire, or torque with a full size one? It’s not that one slot size is better, it’s more that they emphasize different properties.

Contemporary Removable Appliances

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Functional Appliances

Three types of removable appliances are frequently used in modern orthodontics:

• functional appliances for growth modification
• active plates with finger springs or screws for minor tooth movement, and
• clear aligners for certain types of malocclusion in adults.

The goal of a functional appliance is not to move teeth, but to guide the growth of the jaws—usually to encourage forward growth of the mandible in children with a Class II problem due to mandibular deficiency. Why is it called a functional appliance? Because the originator of this type of treatment thought it worked by changing muscle function (image 1). He called the first device of this type an activator for the same reason—it was supposed to activate the jaw muscles (the spring displaced it so that the patient had to constantly bite down to hold it in place), and this increase use of the muscles would cause growth. That turned out to be incorrect. The effects on growth are the same whether or not the child has to bite forward repeatedly, so long as the condyles are out of the fossa, but the functional appliance name has persisted.

There are many different functional appliances, as images 2-5 show. They usually are removable but can be fixed in place (image 4)—but it is more convenient for teaching purposes to discuss both types at the same time. What they have in common (for Class II children) is a construction bite that brings the mandibular condyles down and forward, out of the glenoid fossa. For a Class III child, the condyles stay in the fossa while the mandible is rotated down and back. That’s only effective if the child has a short face, so functional appliance treatment is almost totally for Class II children.

Adding springs to a functional appliance to move teeth is possible, but may be counter-productive. If you’re trying to produce forward growth of the mandible, reducing overjet by tipping upper incisors back and lower incisors forward decreases the amount of overjet reduction from growth that might be achieved.

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{{PAGE_111}} Image 1, Original activator: The activator caused the child to bite forward, and incorporated a spring to keep it from fitting tightly so that the child would have to keep biting to hold it in place—thus “activating” the muscles.

Image 2, Twin-block appliance: The twin block has separate upper and lower components, with a ramp on the lower to bring the mandible forward. A screw for maxillary expansion usually is included in the upper component.

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{{PAGE_112}} Image 3, Frankel appliance: The Frankel appliance is constructed to hold the lips and cheeks away from the dentition, and incorporates acrylic behind the lower incisors to produce a forward bite. Image 4, MARA appliance: Fixed functional appliances can’t be removed by the patient. The MARA appliance (right) forces the patient to bite forward in order to bring the teeth together. It is less bulky than the older Herbst design (left) that holds the mandible forward all the time, but it may not be as effective. Image 5, Hybrid functional: For a patient with a jaw asymmetry, a hybrid appliance like this can be helpful. It has a bionator-type bite block on one side, Frankel-type shields on the other side, and is made using a mandibular position that improves or corrects the asymmetry.

Functional Appliance Effects You’ve already been taught that functional appliances often do affect mandibular growth, but the major effect is an acceleration of growth rather than development of a mandible that’s significantly larger than it would have been without treatment. Let’s review that important concept. When somebody talks to you about the wonders of treatment with a new functional appliance, keep the lesson of this graph in mind. In a child who wears the appliance faithfully (unfortunately, not all

{{PAGE_113}} of them do that, even those with a fixed functional), the effect usually is faster growth of the mandible for a while, then a gradual tapering off of growth even if the child keeps wearing the device. In the long term, the mandible ends up quite close to the size it would have been without treatment.

So how do functional appliances work? The reaction of the soft tissues to holding the mandible forward is a backward force against the maxilla that tends to restrict its forward growth (in other words, a headgear effect). Even without springs, there’s also a force to move the lower incisors forward and the upper incisors back (a Class II elastics effect). The acceleration in mandibular growth is convenient even if it eventually tends to wear off. All these things contribute to the eventual result.

Active Plates Active plates once were the major appliance in European orthodontics, but have never been used much in the US and Canada. Their use has declined everywhere at this point, simply because they are not efficient even though they can produce most types of tooth movement.

The “split plate” shown in image 1 shows a European approach to arch expansion, using a screw mechanism in a removable plate to produce the tooth movement. The spring properties of screws, of course, are almost nonexistent, so the screw has to be reactivated by the patient at least every few days. A one-quarter turn of the screw opens it 0.25 mm. With a split plate, the force is distributed over most of the teeth in the dental arch. Even so, a heavy force that decays rapidly is produced, just the opposite of the light continuous force that is ideal for tooth movement.

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{{PAGE_114}} A finger spring extending from an active plate (image 2) produces more physiologic forces over a much larger range than a screw. An active plate of this design can be effective if the desired movement is tipping and the distance the tooth is to be moved is not very far (image 3).

Both types of active plates, however, really can’t compete with fixed appliances if tooth movement other than tipping is desired. We’ve already pointed out that to do anything else, you have to have two-point contact on the teeth. That’s easy with a fixed appliance, very difficult with finger springs or screws.

Clear Aligners: Invisalign

Clear aligners are “suckdown appliances” produced by vacuum-forming clear thermoplastic sheets over a dental cast. They were introduced into orthodontics originally as retainers, and still are used frequently for that purpose. But if you had a way to repeatedly move teeth a little on a cast, and then made a series of aligners after each small movement that would not quite fit intra-orally and so would move the teeth a little each time…

This patented idea has been turned into the Invisalign appliance. There is no way to account for growth changes in the dental occlusion with a series of aligners, so Invisalign is just for adults or older adolescents, but the fact that a clear aligner is almost invisible when it’s in the mouth appeals to

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{{PAGE_115}} some adults. Almost all types of tooth movement can be produced with clear aligners, so long as you’re willing to bond attachments to some teeth to improve the grip of the aligners. The attachments can be clear plastic, so they too are almost invisible.

The Invisalign technique requires a number of steps in fabricating the series of aligners:

• accurate impressions of the teeth are sent to the Invisalign laboratory, and CT scans are used to develop a digital model (image 1)
• with input from the doctor, the company’s software is used to reposition the teeth in the digital model (image 2) in a series of steps, with a new digital model for each step
• the digital models for each step are used to create a series of stereolithographic casts (image 3)
• a suckdown aligner is made to fit each of the casts (image 4)

Image 1: Impressions being placed into the CT machine. Image 2: Digital models as they look on the computer screen as a maxillary central incisor is being repositioned (0.5 mm or less in a single step). Image 3: A sequence of stereolithographic casts ready for vacuum-forming the aligners. Image 4: A stereolithographic cast and the aligner made using it.

Invisalign (cont.) Invisalign isn’t an automatic process in which the company’s technicians make all the decisions about steps in treatment. The doctor has to decide whether crowding of the teeth requires extraction (if so,

{{PAGE_116}} Invisalign isn’t a good choice as the appliance) or can be addressed by removing small amounts of interproximal enamel (which is feasible with clear aligners) (image 1). Some types of tooth movement, especially rotation and extrusion, can be accomplished only if bonded attachments are used, and where these are placed and how many are used also is up to the doctor (image 2). The attachments can be small tooth-colored blocks of composite plastic, so they aren’t obvious. Do you want to use Invisalign in your future general practice? You can, just as you can use fixed appliances, if you select the right patients and carefully monitor their progress. Sometimes teeth don’t move exactly as planned, and it becomes necessary to take a new set of impressions and start over. Invisalign isn’t a hands-off procedure for the clinician.

Image 1: Invisalign reproximation form, showing the amount of enamel to be removed and the stage of treatment when the resulting space will be closed.

Image 2: Location of bonded attachments, as viewed on the computer screen during treatment planning.

Invisalign Treatment

This series of images shows Invisalign treatment of a girl with a mild anterior open bite and mild crowding of the incisors. For her, elongating the upper incisors to close the open bite was appropriate because she needed greater incisor display—which is somewhat unusual in open bite patients.

To accomplish the incisor extrusion, attachments on the incisors were necessary, and removal of interproximal enamel on multiple teeth also was required to obtain ideal alignment. She had 19 upper aligners and 10 lower aligners. Treatment required 9 ½ months. (courtesy Dr. William Gierie)

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Contemporary Fixed Appliances

Bands vs. Bonded Attachments

Angle’s edgewise appliance, which introduced the use of rectangular arch wires in a bracket with a rectangular slot, remains the basis for almost all current fixed appliances, but it has undergone a major evolution, and changes in the appliance are continuing to occur. In this section, we will briefly examine five important changes in recent years: bonded brackets rather than bands, prescription brackets (the straight-wire appliance), self-ligating brackets, computer-fabricated arch wires, and lingual appliances.

Since bonding is used widely in all types of dentistry now, its use in orthodontics requires very little discussion. The major difference between orthodontic bonding and bonding for restorative dentistry is that at some point the brackets have to be removed without damage to the enamel. Some modern restorative materials make it quite possible to bond brackets so well they can’t be removed without fracturing the enamel—so using them for orthodontic bonding would be a major mistake.

The major indication now for bands is a need for greater strength, particularly when heavy force will be encountered. A good example is when a patient will be inserting and removing a headgear (images 1, 2)- the patient probably would break a bonded attachment loose. Bands also are needed to support a lingual arch (image 3). More generally, bands often are used on first molars and sometimes on

{{PAGE_119}} second molars in a complete fixed appliance (images 4, 5), while all other teeth have bonded attachments—but bonded attachments for molars are quite feasible for many patients.

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{{PAGE_120}} Image 1, Cervical headgear: The force to molar teeth from a typical headgear arrangement like this one is not very heavy—but heavy force can be encountered when it is being placed or removed.

Image 2, Inner bow placement: Fitting the inner bow into the tube on a molar can be difficult for a child, who may generate heavy force in pushing it into the tube.

Image 3, Lingual and labial arch wires: Use of a lingual arch, especially in connection with a labial appliance, requires a band to support the buccal and lingual attachments.

Image 4, Complete fixed appliance: Note the use of bands on the molars, with bonded brackets on other teeth, in this patient for whom premolar extraction spaces are to be closed.

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{{PAGE_121}} Prescription Brackets: the Straight-Wire Appliance

The original edgewise appliance used the same bracket on every tooth. This required careful formation of the arch wires to compensate for the different thickness of adjacent teeth, different inclination of the crowns, and control of facio-lingual root position.

When bonded brackets came into common use, it was feasible to produce different brackets for each type of tooth and to vary the thickness of the bracket, the inclination of the bracket slot and the torque of the slot. This didn’t eliminate bending wires, but it greatly reduced the amount of wire bending that was necessary prior to the final finishing stage. That, of course, greatly increased efficiency. It was the final step in establishing the modern edgewise appliance as the one used by almost all orthodontists now.

The effect of varying the thickness of the bracket base is shown in image 1—it can eliminate the in-out bends that previously had to be placed in every archwire. Changing the orientation of the bracket slot for the maxillary incisors eliminated the need to place bends in the wire to obtain the proper inclination of the teeth (images 2 and 3). The torque orientation of the slot eliminated the need to have twists in every rectangular wire to prevent unwanted torque movements (image 4).

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*Image 1: With the original edgewise appliance, in-out bends were needed with each archwire in both the upper and lower arches to compensate for the varying thickness of the teeth. These bends can be eliminated if the thickness of the bracket base is varied.* </td> <td> *Image 2: Bends in the archwire like these, to obtain mesio-distal inclination of the roots of teeth, can be eliminated if the inclination of the bracket slot is varied. Note that different bracket inclinations are required on the left and right sides, so the left incisors would require different brackets from the right incisors.* </td>

</td> <td> *Image 4: Unless the facio-lingual orientation of the bracket slot matches the desired root orientation, twist bends are required in each rectangular archwire to prevent the teeth from becoming too upright.* </td>

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{{PAGE_123}} Self-Ligating Brackets

It’s obviously important to hold arch wires in the brackets of a fixed appliance. A pin across the front of the bracket was used in Angle’s predecessor to the edgewise appliance, and a similar system was used in the Begg appliance. Wire ligatures were used with the original edgewise appliance. In modern treatment, these have been largely superceded by elastomeric modules (Alastics).

Some sort of built-in cap was offered in a number of variations of the edgewise appliance throughout the 20th century, and this idea has been widely adapted recently. The current self-ligating brackets can be placed into three categories: active clip, active-passive clip, and passive (rigid) clip. All of them also have a prescription built into the bracket, so they are modifications of the straight-wire concept.

In an active-clip bracket (image 1), the springiness of the clip would add to the springiness of the wire, reducing the force against a tooth during initial alignment. That was more important before superelastic wires became available but still can be desirable.

It is possible to make a bracket with a clip that is active while teeth are being aligned initially, but no longer forces the wire into the bracket after alignment is complete (image 2). Almost all active clip brackets have been modified to operate in this way, and active-passive clip brackets are the most widely used type now. A variant of the active / passive design has spring clips that keep the wire within the bracket, which are passive after initial alignment is completed (image 3).

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{{PAGE_124}} Image 1, Speed bracket: Speed (active-clip) bracket, with the clip open. Image 2, In-Ovation bracket: InOvation bracket, with clip that catches on a ledge so that it is not active after initial alignment. Image 3, SmartClip bracket: These brackets use NiTi springs on both ends to retain the wire. Since these springs are active only at the very beginning of treatment (if then), this can be considered an active-passive clip that is close to being entirely passive.

Self-Ligating vs. Conventional Brackets

A passive-clip bracket, the third type of self-ligating bracket, has a rigid cap and depends entirely on the springiness of the wire at all stages of treatment (images 1 and 2).

In the first years of the 21st century, passive-clip self-ligating brackets were advertised heavily by their major producer with the claims that they produced faster tooth movement and different outcomes from conventionally-ligated brackets, primarily by reducing friction as the bracket slid along a wire (or a wire slid through a bracket, which would be the same). This assumed that

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{{PAGE_125}} resistance to sliding is largely due to friction—but in fact elastic binding created by contact of the wire against the corners of the bracket is much more important.

Laboratory studies of resistance to sliding when there was contact of the wire with the bracket corners, as there would be in the mouth, showed no significant differences between conventional and self-ligating brackets (image 3). Note that there is little resistance to sliding when small superelastic wires are used, greater resistance with larger rectangular wires, and no differences among the types of self-ligating brackets. By 2010 a series of randomized clinical trials with human subjects had discredited essentially all the advertising claims for superior performance of passive self-ligating brackets.

In summary, evidence now shows that self-ligating brackets of all types do not change orthodontic treatment outcomes significantly. The good news is that they all perform just as well as conventionally-ligated brackets, and therefore are quite acceptable in routine use. The decision as to the type of ligation, therefore, is entirely a practice management one, based on what the doctor and staff prefer to use.

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{{PAGE_126}} Image 1, Damon bracket: Damon bracket, with a rigid clip that cannot force the wire against the bottom of the bracket. Image 2, Passive self-ligating bracket closed: A passive self-ligating bracket is equivalent to a tube when closed. Image 3, Resistance to sliding: This laboratory data, obtained when brackets are allowed to tip so that the wire contacts the corners of the bracket, predicts that self-ligation would make no difference in resistance to sliding—which has been confirmed in studies of clinical outcomes.

Binding (=Resistance to Sliding) this is the lab experiment that predicts what happens clinically

Coefficient of Binding (cN/degree) 60 50 40 30 20 10 0

SmartClip | Std Bkt steel ligs | Std Bkt Alastics | Damon 2 | InOvation | Speed | Time little or no difference between standard or any self-ligating bkt

Courtesy Dr. Robert Kusy | 0.014” Nickel Titanium | 0.016 X 0.022” Nickel Titanium | 0.019 X 0.025 Stainless Steel |

Custom Prescription Brackets The prescription of a straight-wire bracket—the bracket base thickness, inclination of the slot, and torque orientation—carries with it assumptions about the morphology of the tooth and about where the bracket will be placed on the tooth. A number of prescriptions have been offered by different manufacturers, all of which are based on measurements from a large number of extracted teeth.

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{{PAGE_127}} The result is what you’d expect: the prescription works very well for patients whose teeth closely resemble what is sometimes called “California average”. The major manufacturers are in southern California, and that’s where they collected the teeth to generate their average measurements. The further your patient is from that average, the more wire bending you’d have to do using the standard prescription. There are now prescriptions based on samples of teeth from other areas (an Asian prescription, for instance), but it’s still the case that the prescription won’t work very well for individuals who are outliers in the normal bell-shaped curve.

In this era of computer-assisted fabrication of almost everything, it’s technically possible to produce “the ultimate straight wire appliance” by producing a special custom-prescription bracket for every tooth of a specific individual, and custom archwires to produce the desired arch form and tooth positioning.

This is done using a 3-D scan of accurate dental casts to produce a digital virtual model (image 1), and using special software to reset the teeth to ideal position (image 2). Then it is possible to contour the bracket base for a precise fit at a certain point on each tooth and to cut the bracket slot with the exact prescription to place that tooth in ideal position at the end of treatment (image 3). Bonding jigs so that the bracket is placed in exactly the planned position on the tooth are critically important (image 4). Bracket positions may appear to be somewhat different from where they are usually placed (image 5), but the custom archwires will bring them very close to the planned alignment and occlusion.

Is that the way of the future? Possibly, despite the obvious problems created by loss of a bracket (the records are on file at the manufacturer, so you can get a new one in a week or two) and by problems in getting the bracket in exactly the right place on the tooth (positioning jigs are not as precise as ideal). At the finishing stage some adjustment of the arch wire may be necessary, so the best description is that custom brackets minimize rather than eliminate wire bending.

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{{PAGE_128}} Image 1, Virtual model: For this patient with a wide maxillary central diastema and improper occlusal relationships, accurate dental casts were scanned into computer memory to create a virtual model. Image 2, Virtual set-up: Then the teeth were reset into the desired relationship using proprietary software. Image 3, Cutting custom bracket slots: The digital data are used to mill a custom prescription bracket for each tooth. Image 4, Bonding jigs: Bonding jigs are fabricated so that the bracket can be bonded precisely in the planned location.

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{{PAGE_129}} Image 5, Custom brackets in use: The appliance in the mouth. Note the vertical position of the maxillary central incisor brackets, altered from the usual location to obtain ideal display of these teeth.

Computer-Fabricated Arch Wires

Custom brackets for each tooth and arch wires that are formed only to the desired arch form are one way to reduce wire bending to a minimum if not totally eliminate it. An alternative approach, now commercially available, is to use “plain vanilla” brackets with no prescription (or any prescription, so long as it is known), and then fabricate the necessary rectangular arch wires with a computer robot.

The technique requires a digital model (image 1), and at present this is achieved via an intra-oral scan after the teeth have been brought into initial alignment with small round superelastic wires. If a CT scan is available for the patient, it can substitute for the intraoral scan. Small round superelastic wires that have standard arch forms can be used for the initial alignment. The 3-D information from the scan is sent to the company’s laboratory, and a digital model produced from the scan is used with input from the doctor to set the teeth in the desired final position. Then a wire-bending robot (image 2) forms a sequence of rectangular arch wires to accomplish the changes in the digital model (images 3 and 4).

It seems quite likely that either the custom brackets approach or the computer-formed arch wire approach will win this contest between alternate CAD-CAM methods. At this point it’s too early to know how it will turn out. Which would you bet on?

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{{PAGE_130}} Image 1: Intra-oral laser scan, to produce a digital model of the dental Image 2: Computer-controlled wire-bending robot Image 3: Computer-formed arch wire before insertion Image 4: Arch wire after insertion

Lingual Appliances Could you put an effective fixed appliance on the lingual rather than the facial surface of the teeth? Obviously you could—if the patient could tolerate it and if the doctor could get to it to adjust it. A successful lingual appliance is a 21st century phenomenon, although efforts to develop such an appliance started many years before the turn of the century. Three major problems had to be overcome:

• bonding a thin bracket to the variable lingual surfaces of the teeth
• inserting a rectangular wire into a rectangular slot without great difficulty (disturbing the tonsils in doing so is not good!)

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{{PAGE_131}} forming the arch wire, which requires major bends to compensate for tooth thickness and careful use of tilt and torque

Computer-assisted appliance fabrication provided solutions to all three problems. Using scans of dental casts, it is possible to generate a bonding pad for each individual tooth (image 1). That has the advantage that the pad fits onto the lingual surface in only one position, so you can put it back accurately if it comes off. Then low-profile brackets are attached to the bonding pads, and the appliance is ready for intra-oral bonding (image 2). The brackets, which have a rectangular slot that opens to the top rather than the side so that the wire can be dropped in from the top after slipping it into a tube on the last molar, are the same for each tooth.

Lingual Appliances (cont.)

Now, how do you form the lingual arch wire? The teeth are reset into ideal occlusion, and a wire- bending robot is used to provide the exact bends needed in the arch wires for that individual patient (images 1 and 2).

The resulting appliance is at least reasonably tolerable for the patient, and it allows the treatment of all types of malocclusion. It’s the invisible appliance for patients whose problems are too severe to treat with Invisalign, and it is more precise and predictable than what can be achieved with the series of aligners. Laboratory costs are about the same for Invisalign and this approach to lingual orthodontics (expensive for both).

Because both the bracket slots and wires are formed to tighter tolerances in the Incognito lingual system than any current labial appliances, greater precision of tooth positioning is theoretically possible. A careful superimposition of the set-up with the actual outcome in a sample of 54 consecutive patients showed that the differences between expected and actual outcomes really were quite small. Image 3 (courtesy of Dr. Dan Grauer) shows the distribution of changes in the torque orientation of all the teeth. Similar charts were made for all 3 planes of space and all 3 orientations, i.e., locating each tooth with 6 degrees of freedom.

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{{PAGE_132}} Studies of this type have not yet been accomplished for the custom-bracket and computer-formed facial-arch-wire methods, so how close the treatment outcomes with these techniques come to the virtual set-ups is not known. Note in image 3 that the precision of torque orientation for the 2nd molars was not as good as for other teeth, probably because the last segment of a continuous arch wire functions as a cantilever beam. Compensation for this can be built into the arch wire fabrication software. It is apparent that comparing actual to planned outcomes will be an important tool in improving the software algorithms for robotic forming of arch wires, whether the wires go on the facial or lingual surface of the teeth.

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{{PAGE_133}} Image 1: Image 2: Image 3:

Self-Test Referral The self-test section of this program is designed to help you be sure you have understood the material.

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{{PAGE_134}} Now that you have gone through the module, do the assigned reading in Contemporary Orthodontics (pages 347-382 in 5th ed.; pages 395-429, 4th ed.) Then take the self-test, and use it as a guide for further study and review.

Copyright 2013, UNC Dept. of Orthodontics

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{{PAGE_135}} 5. Orthodontic Anchorage and Controlled Tooth Movement

Orthodontic Anchorage

Learning Objectives An important aspect of carrying out orthodontic treatment is that the goal almost always is to move some of the teeth (perhaps a lot) while maintaining other teeth in their original position (or at least as close to it as possible). How do you do that? The answer is “With some difficulty, because this requires consideration of orthodontic anchorage”.

The objectives of this module are to help you understand anchorage, how it traditionally has been controlled, and how temporary skeletal anchorage is used in contemporary treatment.

In addition to going through the module, read pp. 328-346 and pp. 667-683 in the 5th edition of Contemporary Orthodontics, or pp. 377-393 and pp. 674-683 in the 4th edition**.

Orthodontic Anchorage: What Is It? Anchorage usually refers to how a ship is held in place by an anchor that grasps the bottom beneath it, and is generalized to to how other structures are held in place against forces that would displace them.

Orthodontic anchorage is a special use of the term. It is best defined as “resistance to unwanted tooth movement”, which contains a reference to what the dentist desires. That seems a strange way to put it, but it’s the clearest way to understand what happens in typical orthodontic treatment. The dentist or orthodontist activates an orthodontic appliance to produce certain desired tooth movements. For every action, there’s an equal and opposite reaction, and the anchorage is the resistance to reaction forces. Usually it is provided by other teeth, occasionally by the palate, head or neck, and increasingly now by anchors screwed into the bone.

Let’s begin with other teeth as anchor units. The essence of anchorage is its use to produce differential tooth movement, which is more movement of the teeth you’re trying to move than of the teeth that you don’t want to move. That’s done by taking advantage of the relationship of tooth movement to pressure in the periodontal ligament (PDL), as shown in this graph. What’s the optimal force for moving a tooth? It’s the lightest force (and resulting pressure in the PDL) that will produce tooth movement at a near-maximum rate. Why is that the optimal force? Because of anchorage considerations that we need to discuss.

{{PAGE_136}} Pressure in the PDL and Differential Tooth Movement How can you arrange things with dental anchorage so that differential tooth movement occurs? The slope of the curve gives the answer: that could occur only if the pressure in the PDL of the anchor teeth was less than the pressure in the PDL of the tooth you’re trying to move, and if you were on the vertical leg of the graph.

Consider the situation when the reaction force for movement of one tooth (M1 on the graph) is spread over two or more teeth that serve as anchors (A1). The reaction force is equal and opposite, but because it is distributed over a larger PDL volume, PDL pressure would be lower for the anchor teeth. They would move less than the tooth on which the force was concentrated. The larger the PDL volume over which the reaction force was distributed relative to the tooth we’re trying to move, the greater the relative movement would be.

As a general guideline, the ratio between PDL pressures for the movement tooth and anchor teeth should be at least 3:1. The larger the ratio, the better the anchorage, and vice versa.

{{PAGE_137}} Pressure in the PDL and Differential Tooth Movement (cont.)

What would happen if you increased the force against the tooth you’re trying to move, so that it was greater than the optimal force? Dentists often act as if they believe the old adage that “if a little bit is good, more must be better”. Increasing the force against the movement tooth would equally increase the reaction force. The effect would be to move M2 and A2 further along the response curve.

Note the unintended effect: now the movement tooth moves only a little more than it did with the optimal force, while the anchor teeth move a lot more. The heavier force was meant to increase the desired tooth movement. What it really did was increase the undesired tooth movement. This is referred to as “burning the anchorage”, or perhaps as “blowing or “slipping” it. Whatever you call it, using more than the optimal force destroys the anchorage value of the teeth that were meant to provide anchorage.

The important principle is that light forces allow control of anchorage and heavy forces make anchorage control impossible.

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{{PAGE_138}} Anchorage Value of Different Teeth

It should be apparent now that all teeth are not created equal from an anchorage perspective. The bigger the tooth and the larger its PDL volume, the greater its anchorage value should be.

It’s hard to calculate the PDL volume, but because only one part of the total volume is loaded at any one time, a two-dimensional representation of PDL area actually is more useful in determining the anchorage value of a given tooth. It’s certainly true that the anchorage value of a canine is greater than that of an incisor, and the anchorage value of a molar is greater than that of a canine. You can turn that around. If a canine is to be moved, more force will be needed to do so, and there would be more stress on the anchor teeth than if an incisor was being moved.

This image shows the calculated anchorage value of various teeth. The relative values, not the absolute numbers, are what we’re interested in.

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Types of Anchorage

Reciprocal Anchorage

The simplest type of anchorage is called reciprocal anchorage. It would occur when movement of one tooth (like a maxillary central incisor) was pitted against one exactly like it (the other central incisor). That could easily occur in the closure of a maxillary central diastema. Not surprisingly, the result would be that both teeth would have the same pressure in the PDL, and they would move toward each other equally.

The same thing would happen if one group of teeth were pitted against another group with equal anchorage value. Suppose the first molar and second premolar were the anchorage unit for retraction of the canine and two incisors after first premolar extraction. If you look back at the previous figure, you’ll see that the anchorage values of those teeth are close to the same. All other things being equal, the tooth movement would be reciprocal or close to it.

{{PAGE_140}} Reinforced Anchorage Suppose the goal was to close a first premolar extraction space by retracting the incisors and canine more than the premolar and molar moved forward. One strategy would be to add the second molar to the posterior anchorage unit. That would increase the anchorage value of the posterior unit. Now you would have to be sure that the force was optimal for moving the anterior teeth. If so, the force against the posterior anchor unit would produce a PDL pressure below the optimal level, and those teeth would not move forward as much (image 1). Adding the extra molar to the anchorage unit is described as “reinforcing the anchorage”. The principle of reinforcement of anchorage can be generalized to refer to anchorage outside the dental arch. One example is the use of extra-oral force (headgear) to reinforce anchorage. Could you control forward movement of posterior anchorage by having your patient wear headgear to the upper first molars (image 2)? The answer would have to be yes, because the headgear force would counteract some of the reaction force from the anterior teeth. Unfortunately neither the magnitude nor the duration of headgear force is conducive to reinforcing anchorage. Ideally, the amount of force would be light and its duration would be very long if not continuous. Headgear force tends to be heavy, and even the most conscientious patient doesn’t come close to wearing it all the time. Headgear for growth modification is more successful than headgear for anchorage reinforcement. The wrong force and duration for tooth movement explain why.

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{{PAGE_141}} Image 1: Graph showing the relationship between Pressure and Amount of tooth movement, with curves for Anterior Unit and Posterior Unit.

Image 2: Clinical photo of a girl wearing headgear. Headgear like this is ineffective as reinforcement of posterior anchorage.

Stationary Anchorage

Another strategy to increase the anchorage value is to take advantage of the different loading of the PDL for bodily movement vs. tipping. Since a force is distributed over twice as much PDL area for bodily movement as for tipping, you would move a tooth twice as much as its anchor if the anchor could only move bodily while the movement tooth was allowed to tip. Arranging the force and moments on the anchor unit so those teeth can only move bodily is called “stationary anchorage”.

This strategy applies very well to the situation when an anterior unit (canine plus incisors) is to be retracted more than its posterior anchor unit moves forward. If the incisors are allowed to tip while the posterior teeth have to move bodily, the optimum pressure for the anterior unit would be produced by about half as much force as if those teeth were to be moved bodily. The reaction force over the posterior unit would be only half the amount for optimal bodily movement, and then the anterior teeth would be retracted twice as much. This image shows the outcome.

That’s not a total net gain because the roots of the anterior teeth would have to be repositioned later, but two-stage movement of a unit to be moved often is used to reduce the strain on anchorage. Again, note that this works only if the force is kept below the optimum for movement of the anchor unit. Using too much force would totally destroy this method of anchorage control.

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{{PAGE_142}} Cortical Anchorage

It is obvious that anchorage could be both reinforced and stationary, and the combination of more teeth in the anchor unit and allowing the anchor teeth only to move bodily is frequently used clinically.

Another aspect of anchorage is seen in the different remodeling response of cortical and medullary bone. Teeth normally are in medullary bone, which remodels relatively quickly adjacent to a stressed PDL. Cortical bone can and does remodel, but it does so much more slowly. You can say that cortical bone has a higher anchorage value, but usually when it’s a factor in anchorage, it opposes the desired tooth movement, so it creates greater resistance to tooth movement rather than contributing to the anchorage.

A good example is seen in the area where a tooth was lost years previously (images 1 and 2). Almost always the alveolar process has become narrower and shorter. Closing a space like that requires remodeling of cortical bone, and that puts great stress on anchor teeth. An old extraction site, particularly in adults, can be so difficult to close that often it is not good judgment to try to do it.

{{PAGE_143}} Image 1: Clinical photo showing early loss of 2nd primary molars in a patient with congenital absence of 2nd premolars, resulting in narrowing of the alveolar ridge. The caption notes that closing this space will require remodeling of cortical bone.

Image 2: Dental cast model showing early loss of 1st molars in an adult, accompanied by significant loss of alveolar ridge width and height. This has created a space described as very difficult to close orthodontically.

Resistance to Sliding and Anchorage Control Strategies

In our previous discussion of modern fixed appliances, we have already touched on resistance to sliding, which cannot be avoided in patients (though it can in a laboratory where brackets are moved along a wire without the inconvenience of having roots on teeth). Laboratory data of that type can look good in an advertising brochure but are irrelevant to clinical treatment.

Let’s consider the impact of resistance to sliding (RS) on anchorage, using the familiar setting of closure of an extraction space when retraction of protruding incisor teeth is desired. A 60-40 ratio of anterior to posterior movement (image 1) often is all that is needed. Too much retraction of the anterior teeth can have an adverse effect on lip support and facial appearance.

If you wanted to retract the anterior segment further than that, what would you do? Reinforce the anchorage, of course, or make it stationary while allowing the anterior teeth to tip. It’s possible to use the teeth in the opposite arch for anchorage reinforcement (image 2), and there’s also an exciting new possibility that we’ll discuss at the end of this module.

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{{PAGE_144}} Image 1: After first premolar extraction, retracting the canine and 2 incisors creates about a 60:40 ratio—which often is as much incisor retraction as is desired.

Image 2: Anchorage can be reinforced is several ways, including elastics from the other arch.

Resistance to Sliding and Anchorage Control Strategies (cont.)

How much force does it take to move the canine and 2 incisors (on each side) posteriorly? The canine will need to move bodily (100 gm), the incisors can be allowed to tip back somewhat (~75 gm each), with a total of about 250 gm per side. But if an archwire has to slide through brackets on the posterior teeth, at least 100 gm has to be added to overcome the RS. The result will be force levels on the posterior teeth that pull them up toward if not onto the plateau of the force / response curve, decreasing their anchorage value.

The problem is aggravated because RS cannot be known precisely, and the clinician is tempted to be sure there’s enough force to close the space—so the amount of force is likely to be moved up. Even if the anchorage is reinforced by adding the second molars, the posterior teeth are likely to move forward as much as the anterior ones move back.

What to do? If space closure by sliding is planned, the space closure almost has to be done in two stages, first retracting the canine (100 gm + RS), then adding it to the posterior anchorage unit as the incisors are retracted (image 1). There still will be forward movement of the posterior anchor teeth, but this two-stage method allows more retraction of the incisors.

The alternative is to move segments of wire with the teeth attached, so that there is no sliding of brackets along a wire (image 2). This can be accomplished with loops that form the retraction spring. Then the typical incisor retraction could be achieved in one step, pitting the entire anterior segment against the posterior anchorage. The price of resistance to sliding then becomes clearer: the difficulty of controlling anchorage as teeth slide along an archwire increases treatment time to achieve the desired result.

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{{PAGE_145}} Two-stage space closure, retracting the canine by itself initially, then retracting the incisors, decreases the strain on posterior anchorage. Closing loops of this type eliminate strain on anchorage from resistance to sliding, and allow one-step space closure (which reduces treatment time).

Complex Force Systems

Determinate vs. Indeterminate Force Systems

By now you should be convinced that in order to control tooth movement, it’s necessary to be careful to use the right amount of force and use couples appropriately to obtain the desired MF / MC ratio. Otherwise, it’s easy to destroy the value of dental anchorage. It can be surprisingly difficult to be sure what forces and moments are produced by an orthodontic appliance because as the force system becomes more complex, it also is likely to become indeterminate.

A determinate force system means that you can measure, and therefore determine, the force and moments felt by all teeth. An indeterminate force system means that it is impossible to know exactly what forces and moments are produced.

What makes a force system indeterminate? The answer is that a one-couple system is determinate, while two-couple systems are indeterminate. What’s a one-couple system? Simply a system in which a couple is present at only one place, and a force or forces without a couple are felt elsewhere. Two-couple systems have forces and couples present in at least two places.

{{PAGE_146}} Complex Force Systems

One-Couple System —> Determinate Two-Couple System —> Indeterminate

Determinate vs. Indeterminate Force Systems (cont.)

Removable appliances rarely become two-couple systems—they have enough trouble producing one moment, much less two. So although they are not very efficient, you can at least know what forces and moments are being delivered.

Multi-bracketed fixed appliances with a continuous arch wire easily become two-couple systems. A two-couple system is guaranteed if you place a continuous rectangular wire in more than one rectangular bracket. The rectangular wire creates a couple within the bracket in the torque plane of space if there is any twist when it is inserted (image 1), and the wire also can create couples across the bracket in the mesio-distal inclination plane of space if it is not passive when it is inserted. A round wire also can produce couples across a bracket if the wire is flexed as it goes into the bracket (image 2). You can picture that with this wire, the root of the maxillary right central incisor would be moved mesially as the teeth are aligned because of the inclination of the wire across the bracket. The result is that with a typical fixed appliance, it’s impossible to know exactly what the forces and moments on individual teeth are.

Up to this point you’ve probably thought about orthodontic tooth movement being a smooth continuous action. With a fixed appliance it’s not, it’s more like a dance of the teeth. When the wire is inserted, a complex force system (almost surely a two-couple system) is established. As tooth movement begins, one tooth moves a little, which changes the force system and produces a response by another tooth, and so on. If you made a movie from images taken every 10 minutes over several

{{PAGE_147}} days, you’d have something like one you’ve probably seen that shows a flower unfolding as the blossom opens. The dance is tolerable because the form of the arch wire limits how far teeth can move, and so the outcome is predictable although the steps to get there are not.

Image 1: A rectangular wire in a rectangular bracket produces a torque couple unless its orientation precisely matches that of the bracket.

Image 2: A round wire like this produces couples across brackets, so the force system becomes indeterminate.

Intrusion: A Major Indication for a One-Couple System

You will remember that intrusion requires very light force, on the order of 10 gm / tooth. With even the springiest superelastic NiTi wire, the force between brackets on adjacent teeth will be about 50 gm, which is perfect for tipping teeth into alignment. But if the brackets are not vertically aligned, there also will be an extrusive force of 50 gm on one tooth and a 50 gm intrusive force on the somewhat elongated one next to it. That’s also perfect for extrusion, but the 50 gm intrusive force has no effect until undermining resorption can take place, and that’s particularly slow because of the dense bone beneath the teeth.

The result is that a continuous arch wire is extrusive. It aligns the brackets much more by extruding a relatively depressed tooth than by intruding an elongated one.

How do you set up an appliance for intrusion? This requires two things: a long span of wire that bypasses some teeth (so that it will deliver the necessary light force), and the absence of a couple where it attaches to the incisor teeth. A light round steel wire from bands on first molars to brackets only on the incisors (image 1) can meet both of these requirements. It’s often called a 2 x 4 appliance: 2 molar bands, 4 incisor brackets. It can generate a light vertical force at the incisors, and it produces a couple across the molar tube but doesn’t create a couple anteriorly. The problem is that the two molars aren’t much anchorage, and the reaction force from the incisors tends to tip the molars distally.

Better control is obtained by creating a multi-tooth anchorage segment, and using a one-couple intrusion arch formed from a rectangular wire. It inserts into an auxiliary tube on the molar but

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{{PAGE_148}} bypasses the brackets on other teeth, and is tied to a segment of wire in the teeth to be intruded (image 2). Tying it to the wire rather than placing it into the brackets keeps it a one-couple system, so that the intrusion force can be measured precisely. For two teeth to be intruded, it should deliver 20 gm.

Image 1: 2 x 4 appliance. The long span from molar to incisor allows the light force necessary for intrusion.

Image 2: Auxiliary intrusion arch to intrude two maxillary central incisors. The intrusion arch is formed from rectangular wire but is tied anteriorly so that there is no couple.

Intrusion: Where Force Magnitudes Are Really Important An easy way to see the effect of changing from a one-couple to a two-couple system is to tie a rectangular intrusion arch into the brackets on incisors. This creates one couple at the molar, another couple anteriorly. The clinical observation is that such an auxiliary intrusion arch is unpredictable in its effects. Sometimes excellent intrusion is obtained. Sometimes there is little intrusion, with reaction forces strongly expressed at the molars. Why? Because even though the wire is adjusted so that it would deliver a 30-40 gm intrusion force (appropriate for 4 lower incisors) when it’s deflected up to the level of the bracket, as soon as it’s tied into the anterior brackets, the actual intrusion force is unknown.

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{{PAGE_149}} The bottom line: you can’t intrude teeth with a continuous arch wire, and for all practical purposes you can’t do it with a two-couple system even if it bypasses brackets to reduce the force it delivers. You can do it predictably with a one-couple system.

Applications of Temporary Skeletal Anchorage

Ankylosed Teeth: Perfect Anchorage

Suppose you had to deal with an ankylosed tooth that needed to be repositioned. What would happen if you pulled against it? Once even a small part of the tooth root is fused to the bone (no PDL in that area), the tooth can’t move no matter how hard you pull on it. Any tooth movement would occur in what was meant to be the anchorage unit.

That unfortunate situation occurs sometimes during efforts to bring a badly impacted maxillary canine into the arch. If the canine ankyloses before it reaches its normal position in the arch and efforts to move it continue, there can be major displacements of the other maxillary teeth. The only way to move an ankylosed tooth is to create a bony segment that includes it, and move the segment by distraction osteogenesis. Although that’s possible, it’s rarely feasible. Usually it’s better to extract such a tooth—especially now that an implant to replace it is quite possible.

On the other hand, an ankylosed tooth would be perfect anchorage if it was in the right place. Since an implant is the equivalent of an ankylosed tooth, could it serve as an anchor for tooth movement? Yes, it can. Implants easily withstand the light forces needed to move other teeth. An implant that was placed to support a restoration can also serve as an orthodontic anchor, as these images show.

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{{PAGE_150}} Image 1: Clinical photo of lower anterior teeth showing an edentulous space behind the second premolar, with caption describing its intended restoration and use as an orthodontic anchor. Image 2: Radiograph showing an implant incorporated into an orthodontic appliance for anchorage during retraction of anterior teeth. Image 3: Close-up clinical photo of Class II elastics attached to a bone screw placed in the alveolar bone, serving as temporary skeletal anchorage during pre-prosthetic treatment.

Temporary Skeletal Anchorage Would it be possible to place a skeletal anchor that, unlike a typical implant for restorative purposes, would be easily removable after its orthodontic use? The answer to that also is yes. Temporary anchorage devices (TADs) now are coming into widespread use in orthodontics, especially in adults but also in adolescents. The lower age limit for TADs is about age 11. In children younger than that, the bone is not mature enough to maintain a bone anchor. TADs take two forms: bone screws placed in alveolar bone, and mini-plates that are attached to basal bone beneath the teeth by multiple bone screws. Bone screws work very well when they serve as the anchorage for repositioning specific teeth. Mini-plates are advantageous when more extensive tooth movement is needed. A typical application of a bone screw as an anchor for tooth movement is shown in the attached images. The goal for this patient, who had lost almost all her posterior teeth, was to align the remaining lower anterior teeth without protruding them. There weren’t any posterior teeth to serve

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{{PAGE_151}} as anchorage. A single bone screw was placed into the remaining alveolar bone on the right side. It provided the anchorage to retract the first premolar so that the canine could be rotated into proper position. Then comprehensive prosthodontic treatment could be done without concern for the occlusal interference posed by the displaced canine or any need to adapt the partial denture (whether fixed or removable) to the irregular anterior teeth.

Image 1: Canine crossbite and interference prior to treatment

Image 2: Pre-treatment occlusal view

Image 3: Retraction of the first premolar

Image 4: Rotation of the canine (with a superelastic wire segment)

Image 5: Alignment completed, ready for comprehensive prosthodontics

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Uses of Temporary Skeletal Anchorage

There are now four major applications of temporary skeletal anchorage in adults:

• positioning individual teeth when no other satisfactory anchorage is available (as shown on the previous screen)
• intrusion of posterior teeth to close an anterior open bite
• retraction and intrusion of protruding maxillary incisors
• distal movement of molars (and the entire maxillary or mandibular arch if needed)

It now is possible also to use skeletal anchorage in growth modification treatment for young adolescents.

The above list shows an increasing need for mini-plates as opposed to isolated alveolar bone screws.

Protruding Maxillary Incisors

Retraction of maxillary incisors with TADs can be done in three ways: using bone screws in the palate to stabilize a lingual arch that holds the posterior anchor teeth in position (image 1); using bone screws in the alveolar process posteriorly (image 2); or using mini-plates placed at the base of the zygomatic arch, with an arm projecting into the vestibule (images 3 and 4).

Bone screws provide excellent anchorage, but their position determines the vertical aspect of force against them. Often it is desirable to intrude severely protruding incisors as they are retracted, because tipping them to produce a more normal orientation brings the incisal edges down. Palatal anchorage makes this very difficult. The vertical position of bone screws can be adjusted to some extent, but screws need to be placed in attached gingiva rather than alveolar mucosa, which can make it difficult to obtain a vertical direction of force.

Mini-plates have the great advantage that a wire can be attached to the arm that extends into the oral cavity, so that the point of force application can be almost anywhere.

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{{PAGE_153}} Image 1: A palatal bone screw stabilizes a premolar-to-premolar lingual arch, to provide anchorage to retract the protruding and spaced upper incisors. Image 2: A bone screw in alveolar bone between the premolars provides anchorage to retract the incisors without forward movement of the posterior teeth. Image 3: Mini-plates usually are placed at the base of the zygomatic arch above the roots of the teeth and are held by multiple screws. Image 4: A wire segment can be extended from the tube at the end of the mini-plate. This allows the anchor point to be adapted as needed, so that a straight posterior pull as shown here (or any other force direction) can be obtained.

Protruding Maxillary Incisors (cont.)

The young adult seen in these images had protruding maxillary incisors, and was treatment-planned for retracting the incisors into a first premolar extraction site. For him, skeletal anchorage was particularly indicated because he needed space closure with maximum incisor retraction—and he had posterior bone loss from previous periodontal disease that decreased the anchorage value of the posterior teeth. The lower arch would be treated without extraction, bringing the lower incisors forward.

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{{PAGE_154}} Image 1: Clinical photo showing maxillary incisor protrusion and deep bite anteriorly, prior to treatment. Image 2: Clinical photo showing pre-treatment molar relationship described as nearly a full Class II; goal was to maintain the posterior segment in that relationship. Image 3: Lateral cephalometric radiograph showing extent of incisor protrusion in pre-treatment view.

Retraction of Maxillary Incisors

Miniplates were placed bilaterally at the base of the zygomatic arch, and a superelastic NiTi spring delivering 200 gm was attached to the mini-plate and used to begin retraction of the incisors with an upward and posterior direction of force (image 1). This is termed direct anchorage, i.e., the spring is attached directly to the TAD. Then, to obtain a more directly posterior force direction, an arm was fastened to the mini-plate’s intra-oral attachment to stabilize the maxillary molar. This use of TADs is termed indirect anchorage (image 2).

The dental occlusion at the completion of treatment (which required 18 months) is shown in images 3 and 4, and cephalometric superimposition from before to after treatment is shown in image 5. Note the lack of movement of the upper molar as the maxillary incisors were retracted.

{{PAGE_155}} Image 1: NiTi spring attached to end of arm from mini-plate. Image 2: An auxiliary arm from the miniplate attachment, stabilizing the molar to allow horizontal direction of retraction force. Image 3: Post-treatment occlusion, overjet and overbite corrected. Image 4: Posterior occlusion, Class II molar relationship with good interdigitation. Image 5: Superimposition tracings on cranial base, maxilla and mandible.

Distal Movement of Maxillary Arch

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{{PAGE_156}} Placing a bone screw between teeth makes it impossible to move all the teeth posteriorly. With mini-plate anchorage against the zygomatic buttress, above the roots of the teeth, it is possible to move the entire maxillary arch posteriorly—but there has to be space available, so extraction of third molars is necessary before this can be done. For some patients, second molar extraction may be needed so that the rest of the teeth can be moved back.

For this patient, orthognathic surgery to advance the mandible was one possible treatment; distalization of the maxillary arch was an alternative because it also would give acceptable facial appearance. Moving the upper teeth back, of course, is a form of camouflage—which is satisfactory only if the facial appearance is satisfactory after treatment.

As the facial images and ceph show, her maxillary incisors were already crowded and tipped lingually, so she already had a form of Class II camouflage. The upper incisors could end up almost in their initial position if they were retracted after being aligned.

{{PAGE_157}} Image 1: Pre-treatment frontal view Image 2: Pre-treatment profile Image 3: Pre-treatment right lateral view: note the Class II molar relationship Image 4: Pre-treatment maxillary occlusal view: note the incisor crowding

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Distal Movement of Maxillary Arch (cont.)

Initial alignment of the upper incisors created overjet (image 1). At that point 150 gm retraction force was employed (image 2), and after 3 months it was increased to 300 gm (image 3). At 14 months, the molar relationship was Class I (image 4), the maxillary incisors were in their planned position, and a lower fixed appliance was added.

In the cephalometric superimposition from pre-treatment to 14 months (image 5), note the distal movement of the molar and the position of the upper incisors (which were tipped forward into alignment, then retracted along with the rest of the maxillary arch).

{{PAGE_159}} Image 1: Note the incisor protrusion after initial alignment Image 2: NiTi coil spring to stabilize the canine position during alignment of the incisors. Image 3: Heavier coil spring force for distalization of entire arch Image 4: Progress, molar relationship now Class I Image 5: Superimposition tracings at completion of distalization

Distal Movement of Maxillary Arch (cont.)

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{{PAGE_160}} The post-treatment smile and profile (images 1 and 2) were quite satisfactory, as was the dental occlusion and alignment (images 3 and 4). Treatment time was 26 months. Note that the mini-plates are still present in the final photographs—they were removed soon thereafter.

The cephalometric superimposition for the total treatment is shown in image 5. Note that the mandible was rotated down and back somewhat in the final stage of treatment, giving her greater anterior face height at the cost of decreased chin prominence.

{{PAGE_161}} Image 1: Post-treatment smile Image 2: Post-treatment profile Image 3: Post-treatment occlusion: Class I molar relationship achieved Image 4: Post-treatment alignment of maxillary incisors

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{{PAGE_162}} Image 5: Post-treatment superimposition tracings showing the alignment of maxillary incisors without protrusion.

Posterior Intrusion to Correct Anterior Open Bite Mini-plates above the maxillary posterior teeth also allow a vertical force for intrusion. For many years there was a joke about intruding those teeth, which often are elongated in anterior open bite patients: “If only there was a sky hook over the patient’s head…”. Now mini-plates can provide the sky hook.

Not all open bite patients, of course, need intrusion of maxillary posterior teeth, but usually an anterior open bite is caused by posterior teeth that are elongated rather than incisors that haven’t erupted enough. Often the vertical position of the posterior teeth is influenced by downward rotation of the maxilla during growth, and the more severe the skeletal problem, the more likely that the patient would need orthognathic surgery. But for patients with a mild to moderate open bite, intrusion of maxillary posterior teeth now offers an alternative to the maxillary surgery that was the only treatment until quite recently.

The patient shown in these images had a 6 mm open bite and mild elongation of her lower face. Intruding the maxillary posterior teeth would allow the mandible to rotate upward and forward, closing the anterior open bite and improving her facial proportions.

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{{PAGE_163}} Image 1: Pre-treatment: note the display of the maxillary incisors, which is good—so elongation of these teeth to correct her anterior open bite would be unesthetic. Image 2: This degree of anterior open bite had developed as she experienced vertical growth after orthodontic treatment in early adolescence. Image 3: Anterior open bite from canine to canine. Image 4: Class II tendency in posterior occlusion.

{{PAGE_164}} Posterior Intrusion Treatment

The plan for this patient was intrusion of the maxillary posterior segments initially, and then a complete orthodontic appliance after the bite was closed.

Rather than a miniplate, a long bone screw was placed into the base of the zygomatic arch, after the roots of the 1st molars and 2nd premolars had been diverged to make space for it (image 1). A splint with lingual arches off the palate was bonded (image 2), and vertical force was applied against the maxillary segments via NiTi springs to the splint from the bone screw (image 3). Note that there is a spring anterior and posterior from the arm, giving a largely vertical pull.

After 6 months, the open bite was closed, the splint was removed, and a complete orthodontic appliance was placed. The archwire was tied to the bone screws to control re-elongation of the intruded teeth during the fixed appliance treatment (image 4).

Total treatment time was 15 months, with closure of the open bite (image 5) and successful maintenance of appropriate display of maxillary incisors (image 6). The result was stable at 2 years post-treatment, but 5 year follow-up is required to adequately evaluate long-term stability.

{{PAGE_165}} Image 1: Separation of the 2nd premolar and 1st molar roots was accomplished with bonded brackets and a segmental wire, so there would be space between them for placement of a long bone screw as anchorage.

Image 2: A bonded splint with trans-palatal connectors is an effective way of preventing the posterior teeth from being tipped facially by force applied on the facial side.

Image 3: The splint also helps to prevent the lower molars from erupting as the upper ones are intruded.

Image 4: During post-intrusion fixed appliance treatment, the molar tube is tied to the bone screw with a wire ligature, to maintain the intrusion.

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{{PAGE_166}} Posterior Intrusion Stability

A key question, of course, is whether posterior intrusion will be stable, or whether the intruded teeth will re-erupt so that the open bite recurs. Follow-up data are not yet extensive enough to be sure about long-term stability. It is known that moving the maxilla up is one of the most stable surgical movements. Perhaps this is an encouraging sign that intruding the teeth also will be stable—but 5-year follow-up on a reasonably large sample of patients is needed to know.

All that can be said now is that preliminary results are encouraging. For the patient whose treatment you just saw, cephalometric superimpositions at the end of intrusion (image 1) show the change in tooth positions during treatment. From that point to the end of treatment (image 2) the changes were maintained, and at 29 months post-treatment there still was no relapse (image 3). What it will look like at 5 years? We still do not have enough data (as of early 2013) to be sure about that. Results from a sequence of at least 25 consecutive cases with 5 year recall is needed.

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*Image 1* </td> <td> *Image 2* </td>

**At present, stability is encouraging, but data to document it do not yet exist** *Image 3* </td> <td></td>

TADs vs. Orthognathic Surgery

As we discussed in a previous module, Class III elastics to bone plates can modify the pattern of growth in children with maxillary deficiency and have some potential to alter the pattern of mandibular growth as well. In this module, we have discussed using TADs for intrusion of maxillary posterior teeth as a way to decrease anterior face height in patients with a long-face/open bite problem—which previously could be accomplished only with surgery to superiorly reposition the maxilla. Moving the entire maxillary or mandibular arch posteriorly now can be accomplished with skeletal anchorage, and this is a way to correct Class II or Class III malocclusion, respectively.

Does this mean that many fewer patients now will need orthognathic surgery? It is difficult to answer that question at this point, because using TADs to correct or camouflage skeletal problems is so new that there are no good data for long-term outcomes. It has been observed many times that jaw relationships apparently corrected by growth modification in children tend to recur during adolescence. Will that be the case with the new TAD-supported approach? We won’t know for sure until these patients have been followed all the way through their teen years.

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{{PAGE_168}} Intrusion of maxillary posterior teeth is done primarily for patients who have little or no growth left, so post-treatment growth would not be a major reason for relapse—but intruding posterior teeth beyond 4 mm is very difficult, and most patients experience 1 mm re-eruption of the intruded teeth despite efforts at retention. So a net intrusion of 3 mm is about as much as can be hoped for. Two mm closure of anterior open bite is achieved for each mm of posterior intrusion—which means that closing an open bite of more than 6 mm probably is not feasible with this method. The more severe long face patients will continue to need surgery.

Correcting Class II problems by moving the maxillary teeth posteriorly, or Class III by moving the mandibular teeth posteriorly, does not correct the jaw relationship. This succeeds only if protrusion of the upper or lower incisors was a significant part of the problem. Moving the anterior teeth back too much makes the facial appearance worse, and that is particularly a limit in Class III patients with a prominent chin because retracting the lower incisors makes the chin even more prominent.

The probable effect of TADs as a replacement for orthognathic surgery is summarized in image 4. The greatest effect is likely to be on treatment of long face patients; there will be some effect on treatment of skeletal Class III problems from a combination of growth modification and camouflage, with the magnitude of the effect still in doubt; there will be perhaps some, but not much, effect on treatment of skeletal Class II problems. With extraction of upper premolars, we already can camouflage Class II patients, and the limitations of doing that are not changed by extracting more posteriorly and retracting the whole dental arch.

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Impact of TADs on Patients and Treatment Difficulty

What’s it like to be the patient who receives TADs? Placing bone screws requires anesthesia (a potent topical usually is enough)—then the patient feels pressure as it is inserted, but minimal pain. There is some tissue irritation during the next few days. Mini-plates require more potent anesthesia, a flap to expose the bone, then sutures while it heals. Some swelling occurs that peaks at 2 days, and tissue irritation persists for another few days.

Despite the reaction to this amount of surgery, experience with both types of TADs shows that patients tolerate them quite well. The amount of pain and swelling reported in a group of 97 mini-plate patients is shown in image 1. This is acceptable to patients if they expect it.

The effect of mini-plates on the subsequent orthodontic treatment also is interesting. The 30 orthodontists who treated the same group of 97 patients expected the treatment to be difficult, usually very difficult (image 2, blue bars) because of the type of malocclusion and the treatment plan. They found it to be much easier than they expected. The majority of the cases were judged to be moderately easy (green bars), and none were judged to be very difficult.

The bottom line: TADs of both types are well tolerated by patients, and make tooth movement possible that couldn’t be done any other way. Skeletal anchorage won’t revolutionize orthodontics, but it certainly will be widely used to overcome anchorage problems.

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Image 1: Patient reports of pain and swelling after placement of mini-plates, in patients treated in Belgium (UCL) and NC. Note that pain during surgery to place the mini-plates and during orthodontic treatment was quite low for the great majority of patients, while post-surgical swelling was experienced by almost all.

Image 2: Orthodontist expectations of treatment difficulty in patients with mini-plates compared to their actual experience. The orthodontists judged that treatment of 75% of the patients would be very difficult without TADs; with TADs over 90% of the actual treatment was judged to be moderately easy or very easy.

Self-Test Referral

The self-test section of this program is designed to help you be sure you have understood the material.

Now that you have gone through the module, be sure you also have done the assigned reading in Contemporary Orthodontics(pages 383-388 in 5th ed.; pages 382-383, 4th ed.) Then take the self-test, and use it as a guide for further study and review.

Copyright 2013, UNC Dept. of Orthodontics

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