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{{PAGE_1}} LEVEL II · DIAGNOSIS

Unit A Malocclusion Definition and Prevalence · Known Causes of Malocclusion · Equilibrium Theory and the Etiology of Malocclusion

Proffit Instruction — generated for offline reference

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Contents

1. Malocclusion Definition and Prevalence
2. Known Causes of Malocclusion
3. Equilibrium Theory and the Etiology of Malocclusion

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1. Malocclusion Definition and Prevalence

Changing Goals of Orthodontic Treatment

Purpose of This Module

This module discusses the concept and characteristics of malocclusion, introduces the classification of malocclusion, and reviews the epidemiologic data for the prevalence of types of malocclusion. At its conclusion you should be able to discuss the:

• 21st century goals of orthodontic treatment
• Epidemiology of malocclusion
• Prevalence of the major characteristics of malocclusion
• Need and demand for orthodontic treatment

The Concept of Malocclusion

Crowded, irregular and protruding teeth have been a problem for some individuals for most of human history. The first efforts to correct this go back to at least 1000 BC, and as dentistry developed in the 18th and 19th century, a number of devices to “regulate” the teeth came into use. This treatment was focused entirely on aligning irregular and crowded teeth, and thereby improving the appearance of the teeth and face. In an era when an intact dentition was a rarity, little attention was paid to bite relationships, and the details of the way upper and lower teeth contacted each other was considered unimportant.

It was necessary to consider how the teeth should fit together when prosthetic replacement of missing teeth was planned. The term “occlusion of the teeth” was introduced to describe this, and concepts of prosthetic occlusion appeared in the late 1800s. Edward Angle, the “father of modern orthodontics”, is credited with most of the development of the concept of occlusion of the natural dentition. He provided the first clear and simple definition of normal occlusion, invented the term malocclusion to describe deviations from normal occlusion, and developed the first useful classification of malocclusion.

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Angle’s Characteristics of Normal Occlusion

Angle’s postulate was that the maxillary first molars were the “key to occlusion” because of their position at the base of the zygomatic arch, and that the mesiobuccal cusp of the upper first molar should occlude in the buccal groove of the lower first molar. If this molar relationship existed, and if the teeth were aligned along a smoothly curving “line of occlusion”, then normal occlusion would result. This statement, which is correct unless there are aberrations in the size of the teeth, brilliantly simplified normal occlusion.

Angle’s line of occlusion passes through the central fossae of the maxillary posterior teeth and across the cingulum of the upper canines and incisors. The same line runs along the buccal cusps of the mandibular posterior teeth and the incisal edges of the lower canines and incisors. The line thus specifies the occlusal and interarch relationships of the teeth, once the molar relationship has been established.

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Angle’s Classification of Malocclusion

Having described normal occlusion, Angle then was able to specify three classes of malocclusion based on the first molar relationship:

Class I: normal relationship of the first molars, but line of occlusion incorrect because of incorrectly aligned teeth

Class II: lower molar distally positioned relative to upper molar; line of occlusion not specified

Class III: lower molar mesially positioned relative to upper molar; line of occlusion not specified

Note that the Angle classification has four classes: normal occlusion and the classes of malocclusion. Normal occlusion and Class I malocclusion share the correct molar relationship but differ in the way teeth relate to the line of occlusion. The line of occlusion may or may not be correct in Class II and Class III, but the molar relationship is incorrect.

{{PAGE_6}} Image 1: Normal occlusion Image 2: Class I malocclusion Image 3: Class II malocclusion Image 4: Class III malocclusion

Goals of Treatment: from Angle to the Present

By the early 1900s, the goal of orthodontic treatment had evolved beyond the alignment of irregular teeth to the correction of malocclusion. This led to less emphasis on the appearance of the teeth and face—Angle’s view was that if the dental occlusion was correct, dental and facial esthetics also would be ideal. Unfortunately, as time passed it became clear that even an excellent occlusion was not an excellent orthodontic treatment result if it was achieved at the expense of proper tooth-jaw relationships and facial proportions.

The advent of cephalometric radiology, which came into routine clinical use in orthodontics in the mid-20th century, made it clear that many Class II and Class III malocclusions resulted from jaw relationships, not just from malposed teeth. A small mandible relative to the maxilla came to be called a skeletal Class II malocclusion because it almost required a Class II molar relationship, and a large mandible relative to the maxilla became skeletal Class III. Correction of jaw as well as tooth relationships (which might require jaw surgery as well as orthodontics) became a goal of treatment.

In the early 21st century, occlusion still is important, but achieving correct relationships of oral and facial soft tissues, rather than focusing just on teeth and bone, has become a primary goal of treatment. It’s not just teeth any more. This is a paradigm shift—a change in the conceptual basis for treatment.

{{PAGE_7}} Paradigm: A set of shared beliefs and assumptions that represent the conceptual foundation of an area of science of clinical practice.

Orthodontics

20th Century Angle Paradigm

21st Century Soft Tissue Paradigm

Goals of Treatment: from Angle to the Present (cont.) The images with this screen outline the difference between the goals of treatment in the 20th century, dominated by Angle, and the 21st century shift toward the “soft tissue paradigm”, which offers a broader view of treatment goals than just the correction of malocclusion.

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OCR Text

Treatment in the Soft Tissue Paradigm Era Because of Angle’s emphasis on the antero-posterior position of the teeth and jaws, profile photographs and lateral cephalometric radiographs received the greatest emphasis in orthodontic Level II Diagnosis — Unit A · 8 / 97

{{PAGE_9}} diagnosis until quite recently. But for this girl, treated by David Sarver and colleagues in the early 2000s, the frontal facial views show both the initial problem and the effects of treatment much more clearly.

When they sought treatment at age 11, this girl and her parents were concerned about her facial appearance and especially her “no teeth” appearance when she smiled. On smile, she exposed only 1 mm of her malformed maxillary incisors (image 1). To say that she had an Angle Class I malocclusion would be accurate but remarkably unhelpful, because the major problems were her short lower face and tooth-lip disproportions, neither of which are specified when a patient is classified Class I.

Her orthodontic treatment focused on increasing face height and positioning the malformed teeth for temporary restoration (image 2) and later permanent restoration. The favorable effect on her facial growth can be seen in the cephalometric superimposition (image 3)—another thing you will learn a lot more about in this course.

At age 18 (image 4), permanent laminates were placed on the anterior teeth, which would not have been possible without the increase in face height and balance that was created by modern orthodontics. Coordinated orthodontic / restorative or prosthetic treatment is done much more frequently now because it produces better results.

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

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{{PAGE_11}} Epidemiology of Malocclusion

Population Studies: Dental Crowding / Malignment

Several studies of the prevalence of malocclusion were undertaken between 1930 and 1965, using the Angle classification. The results varied tremendously: the prevalence of malocclusion in the United States was reported as anywhere between 35% and 95% of the population. The problem was a lack of consensus as to how much deviation from Angle’s normal (really ideal) occlusion could be tolerated before an individual was judged to have malocclusion. The conclusion was that the Angle classification just couldn’t be used for population studies.

More recently, studies in most developed countries have used specific characteristics of malocclusion to obtain information about their prevalence. As part of the NHANES-III study, a large-scale evaluation in the 1990s of the health of the white (European descent), black (African descent) and Mexican-American population groups in the United States, the alignment of the teeth was calculated by the irregularity index (image 1). This is the total of the distances between incisor contact points, so it allows a quantitative measurement of the extent of incisor irregularity.

The irregularity data, without differentiating the three population sub-groups, are shown in image 2. Note that one-third of the total population have at least moderately irregular (usually crowded)

{{PAGE_12}} incisors, and nearly 15% have severe or extreme irregularity.

Population Studies: Overjet

Overjet is defined as the horizontal overlap of the incisors (image 1). Normally the upper and lower incisors are in contact, with the upper incisors ahead of the lower only by the thickness of their incisal edges, i.e., normal overjet is 2-3 mm. Excessive overjet correlates with Class II molar and jaw relationships. If the lower incisors are in front of the upper incisors, this is reverse overjet, which correlates with Class III molar and jaw relationships.

Overjet is a better indicator of Class II and Class III problems than is the molar relationship that Angle described, and it now is used in population studies instead of molar relationship. Data for overjet in the American population are shown in image 2. Note that only one-third of the population have ideal overjet, and another one-third have only slightly increased overjet. Moderate overjet (5-6 mm) is found in 10%. Overjet of 7 mm or more can create severe problems. This affects about 5% of the American population.

Increased overjet associated with Class II is much more prevalent than reverse overjet associated with Class III. Reverse overjet of -3 mm or more represents a severe problem. There is a significant racial difference in the prevalence of reverse overjet: up to 5% of people of Asian origin have severe or extreme reverse overjet, versus 0.5% for African-Americans blacks and 0.3% for Americans of European descent.

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{{PAGE_13}} Image 1: Overjet is defined as the distance from the facial incisal edge of the lower central incisor to the lingual incisal edge of the upper central incisor. Ideally, it is 1-2 mm. Image 2: US population percentages for overjet, NHANES-III. Population Studies: Overbite / Open bite Overbite is defined as the amount of vertical overlap of the incisors (image 1). Normally the lower incisal edges contact the lingual surface of the upper incisors at or above the cingulum, i.e., normal overbite is 1-2 mm. In open bite, there is no vertical overlap, and the vertical separation (in negative numbers) is measured to quantify its severity.

Data for overbite and open bite are shown in image 2. Half the population have an ideal vertical relationship of the incisors. Note that moderate and severe deep bite are much more prevalent than moderate and severe open bite, but extreme deep bite is only slightly more prevalent than extreme open bite.

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Population Studies: Changes with Age

Changes in the prevalence of ideal alignment, severe crowding, excess overjet and open bite by age group are shown in this graph. As age increases, note the increase in crowding and the decrease in ideal alignment, and the decrease in severe overjet. As we will see later in this course, these changes are related to mandibular growth. As you can see, open bite prevalence is not affected and stays the same.

{{PAGE_15}} Population Studies: Racial / Ethnic Differences There are only modest differences in incisor irregularity between the three American racial groups studied in NHANES-III. Two differences in antero-posterior and vertical facial and dental proportions are worth noting:

1. Deep bite is much more prevalent than open bite in the population as a whole, but vertical relationships vary greatly between racial groups. Average open bite is significantly larger in African-Americans than the other groups (image 1). More clinically relevant, severe open bite is 5 times more prevalent in African-Americans. The average deep bite is significantly larger in those of European descent, and severe deep bite is nearly twice as prevalent.
2. In the three major groups in the NHANES-III study, severe reverse overjet is found in less than 1% of the population—but in people of Asian descent, up to 5% have severe reverse overjet (image 2).

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Epidemiologic Data For Malocclusion

From NHANES III – Key Points

Racial/ethnic differences

• Open bite much more prevalent in blacks
• Deep bite more prevalent in whites

Average Open Bite (mm) [Bar chart showing Whites, Blacks, Hispanics with p < .001]

Average Overbite (mm) [Bar chart showing Whites, Blacks, Hispanics with p > .001]

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Epidemiologic Data For Malocclusion

From NHANES III – Key Points

Racial/ethnic differences

• Open bite much more prevalent in blacks
• Deep bite more prevalent in whites
• Other differences are modest
• Whites/Black/Hispanic Class III = ~ 1%
• Asians, Class III = Up to 5%

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Population Studies: Summary

The key points to remember from modern population studies of malocclusion:

• About half the population have well-aligned teeth or slight irregularity, while about 15% have irregularity severe enough that either major expansion of the dental arches or extraction of a permanent tooth in each quadrant of the arch is likely to be needed if the teeth are to be aligned.
• With increasing age, incisor irregularity gets worse and the number of people with ideal alignment decreases.
• 15% of the US population are Class II, and about half of these individuals have overjet severe enough to predispose them to problems in social interactions.
• Open bite is much more prevalent and deep bite less prevalent in African-Americans.
• Class III and reverse overjet are much more prevalent in those of Asian descent.
• Other racial / ethnic differences are modest and unimportant.

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Problems due to Malocclusion

Malocclusion: Types of Problems

Protruding, irregular or maloccluded teeth can cause three types of problems for patients:

1. Psychosocial problems due to discrimination because of dental and/or facial appearance
2. Problems with oral function difficulties in jaw movement (muscles not well coordinated, pain) temporomandibular joint dysfunction problems with mastication, swallowing or speech
3. Greater susceptibility to trauma, periodontal disease or tooth decay

These are listed in the general order of their importance. Let’s consider them one at the time.

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{{PAGE_17}} Psychosocial Problems Studies have confirmed what you’d probably think anyway, that severe malocclusion is likely to be a social handicap and a threat to self-image and self-esteem. Every time you meet someone new, they size you up based largely on your facial appearance. The caricature of a stupid person always includes protruding upper incisors (image 1), so if your front teeth stick out, you’re not expected to be very smart. A girl or woman with a deficient midface and prominent lower jaw is a witch (image 2), expected to be unpleasant and perhaps even dangerous. A tall, skinny, long face man probably has no education or social skills, and is not to be taken seriously until he proves otherwise. And so on … there are a lot of caricatures along these lines.

Fortunately, as others get to know you better, the appearance of your face and teeth becomes less important, but it’s not a trivial handicap to have to overcome negative first impressions all the time—it affects your whole adaptation to life. This places the concept of handicapping malocclusion in a larger and more important context. “It’s just cosmetic, no big deal” misses the problem in a major way. If it affects your interaction with others, it is a big deal because it decreases your quality of life and your chance to succeed in life.

Why do parents seek treatment for their children? Why do adolescents and adults seek treatment for themselves? The major reason is to lessen or remove a psychosocial handicap that can have a major effect on their ability to get ahead in the world.

Image 1: Caricature of a stupid person - note the malocclusion. Image 2: Witches almost always have prominent chins.

Psychosocial Problems (cont.) McGregor’s studies of the effect of a disfiguring dental or facial condition offer an interesting insight into its psychosocial effect. An individual who is grossly disfigured can anticipate a consistently negative response. An individual with an apparently less severe problem, such as a deficient chin or protruding incisors, sometimes is treated differently because of this and sometimes not.

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{{PAGE_18}} It seems to be easier to cope with a defect if the response of other people is consistent than if it isn’t. Unpredictable responses create anxiety and can have a major impact on self-image and interaction with others.

Dentofacial Abnormality

Consistent Negative Response ↓ Adaptation

Consistent Negative Response (Sometimes negative sometimes not) ↓ Anxiety

Psychosocial Problems: the Effect of Self-Esteem

An individual’s self-esteem also makes a difference in how much a dentofacial abnormality affects him or her. As the chart shows, there are several possible responses to the same level of abnormality. You have met individuals who try so hard to overcome negative responses that they succeed despite what would seem a severe handicap. Some others with a similar condition adapt and accept a level of discrimination. Some begin to use the abnormality as a reason not to try, or literally fight back against what they see as persecution. It’s interesting that one way to improve the chance that a prisoner will not be back in prison after release is to treat a facial deformity while he or she is there.

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{{PAGE_19}} Dentofacial Abnormalality

Response

• Tries harder, succeeds anyway
• Accommodates, adapts
• Deformity as an excuse for failure
• Pathologic behavior

Psychosocial Problems: Who Seeks Treatment? The variety of responses to apparently similar levels of dentofacial abnormality mean that something that is not a problem to some individuals can be a major problem, or a problem but only a minor one, for others.

When the patient shown in these photos sought treatment as an adult (image 1), she said her major reason was that she was tired of people staring at her when she went out to dinner, and finally now she could afford treatment. But she held a good job, was married, had a positive attitude toward life, and expected that treatment would make an acceptable situation better—not that it would finally allow her to succeed. She had already made a great effort to succeed despite her handicap.

In fact, people who seek treatment for problems like this tend to score higher on psychologic scales of satisfaction and optimism than the general public. Those who see themselves as hopeless failures are less likely to seek treatment than those with a positive attitude who are sure they would benefit from it.

Both her dental and facial appearance were greatly improved by retraction of her protruding upper incisors and augmentation of her deficient chin (by a lower border osteotomy of the mandible to move her chin forward) (image 2). Five years after treatment (image 3), she held a much more responsible job—one that she was capable of handling before treatment, but almost surely would never have obtained because of her appearance.

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{{PAGE_20}} Image 1: Age 29, Pre-treatment Image 2: Age 31, Completion of treatment

{{PAGE_21}} Image 3: Age 36, 5-year recall

Problems With Oral Function

It seems obvious that poor dental occlusion would make it more difficult to chew your food normally. Some people with a severe malocclusion do report difficulty in eating, and say they can eat better after orthodontic treatment—but the great majority of those who seek orthodontic treatment report no problem with eating. When there is a problem due to malocclusion, it’s often more that it’s difficult to be socially acceptable while managing your food, and so you avoid eating in public and learn to avoid certain foods that are hard to manage. Pizza is an excellent example—can you bite cleanly through the crust and not get any of the filling on you? That’s really difficult if you have an open bite or excess overjet.

Would a severe malocclusion make you more likely to develop temporomandibular pain / dysfunction (TMD)? Usually, no. If there is a relationship, it’s a weak one, and there’s an important link to stress. It’s probably true that if you have a dental occlusion that requires you to shift your jaw when you bring your teeth together, and that you react to stress by clenching and grinding your teeth, you are at greater risk of TMD. There’s little or no evidence that’s also true for other types of malocclusion.

Can malocclusion lead to difficulties in swallowing and speech? Perhaps, but that’s rare. The ability of individuals with extreme malocclusion to speak normally is impressive, and almost always they also can swallow normally—although the adaptive tongue position needed to do this can look abnormal. We’ll review this later in some detail later in the course.

So is improving oral function a major reason for orthodontic treatment? Yes, for some patients, but not for most of them.

Problems With Injury and Dental Disease

Are you more likely to suffer injury to your dentition, or to develop dental disease, if you have malocclusion?

Protruding upper incisors and excess overjet produce a greater risk of injury to the teeth during childhood than normal overjet. There is about 1 chance in 3 that a child with untreated Class II malocclusion and excessive overjet will experience trauma to the upper incisors, but usually the result is only minor chips in the enamel. Most accidents that damage the teeth are associated with normal activity, not sports—but mouthguards for contact sports definitely can reduce damage to the teeth.

Extreme overbite can lead to damage to the tissues lingual to the upper incisors and/or labial to the lower incisors, and can even cause loss of the affected teeth. Soft tissue damage is an indication for orthodontic treatment.

Are you more likely to develop caries because your teeth are irregular and hard to clean? Although that sounds reasonable, there is no evidence that poor alignment of the teeth is a significant risk.

{{PAGE_22}} factor for caries.

Are you more liable to develop periodontal disease for the same reason if you have crooked teeth? Perhaps, but other factors, like how well you take care of your teeth and how good your immune system is, are much more important. The conclusion of an extensive study published in 2008 is that there is no evidence to support the claim of a periodontal health benefit from orthodontic treatment.

The bottom line: malocclusion can lead to increased risks to dental health, but this is the least likely of our three major reasons for orthodontic treatment.

Need and Demand for Orthodontics

Population Estimates of Need for Orthodontic Treatment

We now have good estimates of the prevalence of various characteristics of malocclusion. How does that relate to need for treatment?

If you consider only the severity of malocclusion as an indicator of need, there are two possible approaches. The first is to score each of the characteristics (irregularity, overjet, etc.) and add up the score. That has been tried but for all practical purposes just doesn’t work. If you have a little bit of several characteristics, you may get a high score when they’re added up, but that doesn’t relate well at all to how much you really need treatment.

The second approach is to accept the idea that the severity of the worst characteristic of a malocclusion is the major indicator of treatment need. For instance, if you have 10 mm overjet, that

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{{PAGE_23}} in itself is an indicator that you need treatment, and mild vs moderate irregularity of incisors is almost irrelevant. This approach now is used in many countries, in the form of the IOTN (Index of Treatment Need) system for grading treatment need. Patients can quickly be placed in one of 5 grades, from grade 1 “no need” to grade 5 “extreme need”. Children in grades 4 and 5 get priority for treatment, and perhaps financial help in obtaining it. The IOTN grades are shown in the attached charts.

Pay particular attention to the characteristics of grades 4 and 5, which are considered definite indications for treatment. Does that mean nobody in grade 3 needs treatment? No, because the psychosocial effect varies—and a child with real problems socially related to the appearance of his or her teeth needs treatment even if the malocclusion is not graded as severe.

{{PAGE_24}} Table: IOTN Grades 1-5 Characteristics

Grade 1 (No Need) 1. Extremely minor malocclusions including contact point displacements less than 1 mm.	Grade 3 (Moderate/Borderline Need) 3.a Increased overjet greater than 3.5 mm but less than or equal to 6 mm with incompetent lips. 3.b Reverse overjet greater than 1 mm but less than or equal to 3.5 mm. 3.c Anterior or posterior crossbites with greater than 1 mm but less than or equal to 2 mm discrepancy between retruded contact position and intercuspal position. 3.d Contact point displacements greater than 2 mm but less than or equal to 4 mm. 3.e Lateral or anterior open bite greater than 2 mm but less than or equal to 4 mm. 3.f Deep overbite complete on gingival or palatal tissues but no trauma.
Grade 2 (Mild/Little Need) 2.a Increased overjet greater than 3.5 mm but less than or equal to 6 mm with competent lips. 2.b Reverse overjet greater than 0 mm but less than or equal to 1 mm. 2.c Anterior or posterior crossbite with less than or equal to 1 mm discrepancy between retruded contact position and intercuspal position. 2.d Contact point displacements greater than 1 mm but less than or equal to 2 mm. 2.e Anterior or posterior openbite greater than 1 mm but less than or equal to 2 mm. 2.f Increased overbite greater than or equal to 3.5 mm without gingival contact. 2.g Pre-normal or post-normal occlusions with no other anomalies.
Image 1: Characteristics of IOTN grades 1 and 2.	Image 2: Characteristics of IOTN grade 3.
Grade 4 (Severe/Need Treatment) 4.h Less extensive hypodontia requiring pre-restorative orthodontics or orthodontic space closure (one tooth per quadrant). 4.a Increased overjet greater than 6 mm but less than or equal to 9 mm. 4.b Reverse overjet greater than 3.5 mm with no masticatory or speech difficulties. 4.m Reverse overjet greater than 1 mm but less than 3.5 mm with recorded masticatory or speech difficulties. 4.c Anterior or posterior crossbites with greater than 2 mm discrepancy between retruded contact position and intercuspal position. 4.l Posterior lingual crossbite with no functional occlusal contact in one or both buccal segments. 4.d Severe contact point displacements greater than 4 mm. 4.e Extreme lateral or anterior open bits greater than 4 mm. 4.f Increased and complete overbite with gingival or palatal trauma. 4.t Partially erupted teeth, tipped, and impacted against adjacent teeth. 4.x Presence of supernumerary teeth.	Grade 5 (Extreme/Need Treatment) 5.i Impeded eruption of teeth (except third molars) due to crowding, displacement, the presence of supernumerary teeth, retained deciduous teeth, and any pathological cause. 5.h Extensive hypodontia with restorative implications (more than one tooth per quadrant) requiring pre-prosthetic orthodontics. 5.a Increased overjet greater than 9 mm. 5.m Reverse overjet greater than 3.5 mm with reported masticatory and speech difficulties. 5.p Defects of cleft lip and palate and other craniofacial anomalies. 5.s Submerged deciduous teeth.
Image 3: Characteristics of IOTN grade 4	Image 4: Characteristics of IOTN grade 5.

Population Estimates of Need for Orthodontic Treatment (cont.) It is possible to take the data from NHANES-III for characteristics of malocclusion, and calculate the percentage of the major population groups in the US with IOTN scores that indicate mild, moderate and severe treatment need.

This graph shows the calculated IOTN grades for adolescents (age 12-17) in the major American population groups. Note that about 15% of the white and Hispanic (Mexican-American) groups are IOTN grades 4 or 5, indicating definite treatment need, while 20% of the black population are in this

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{{PAGE_25}} category. Moderate treatment need, grade 3, occurs in approximately another one-third of the white and black groups, and over 40% of the Hispanic group.

Severe (IOTN 4,5)	Severe (IOTN 4,5)
Moderate (IOTN 3)	Moderate (IOTN 3)
Mild (IOTN 2)	Mild (IOTN 2)

Population Estimates of Psychosocial Need for Treatment

How do the IOTN grades relate to the psychosocial aspect of need for treatment? You can’t assume a close coordination between severity of some aspect of malocclusion and its impact on appearance.

To deal with this, an “Aesthetic Component” of IOTN now has been added (note the British spelling, they developed it). It consists of a series of photos of the teeth. A dental appearance like photos 1-3 (image 1) is considered to indicate no need for treatment; photos 4-7 (image 2) are borderline; and photos 8-10 (image 3) indicate definite treatment need.

How the teeth are perceived is very much affected by the soft tissue frame in which they are presented to the world. So these pictures of teeth alone can be misleading, but at least adding the Aesthetic Component does acknowledge that how it looks and the effect of dental and facial appearance on social interactions is important in evaluating treatment need.

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{{PAGE_26}} Image 1: IOTN esthetic scale 1-3, no need for treatment Image 2: IOTN esthetic scale 4-7, borderline need

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{{PAGE_27}} Clinical photo: IOTN esthetic scale 8-10 showing severe dental malocclusion requiring treatment. Demand for Treatment in the USA By no means do all those who are classified as needing treatment actually seek it. Some do not feel they really need it or do not want to accept it. Others agree that they need treatment but can’t afford it or can’t obtain it.

More parents and children in urban/suburban areas than rural areas think that they need treatment. As this graph shows, family income is a major determinant of how many receive treatment. This reflects two things, one obvious and the other perhaps not so obvious. Obviously, high income families can more easily afford orthodontics. But it’s also true that a good dental / facial appearance is more important if you want to achieve more prestigious social positions and better-paying occupations. The higher the aspirations for the child, the more likely the parents are to seek orthodontic treatment, and upper income parents tend to have higher aspirations.

Although every state supports at least some orthodontic treatment for low-income children through its Medicaid program, this pays for very little of the total treatment that is done. Note in the graph that even in the lowest income group, more youths and adults receive treatment than Medicaid or other third-party sources cover. The amount of treatment that is done for the lower income segments of the population reflects the importance that many of these families place on it. It’s widely accepted that if you really want John or Susie to get ahead in the world, you need to get those teeth fixed—and

{{PAGE_28}} much of what little discretionary income exists for low-income families is used for orthodontics surprisingly often.

Demand for Treatment in the USA (cont.) The effect of financial constraints on orthodontic treatment is seen most clearly by the response when treatment is largely or completely covered by third-party plans, so that the cost of treatment no longer is a factor in deciding whether to seek it. In one such setting where comprehensive dental care including orthodontics was provided, the dentists recommended treatment for 55%, and 50% accepted it. It seems likely that as money is available to support it, demand for treatment will reach at least the 35% level at which parents readily recognize a need for treatment. The NHANES-III data show that even in the 1990s, 35-50% of the children in more affluent areas were receiving orthodontic treatment. It is interesting that demand for treatment among adults has greatly increased over the last 30 years. A survey of orthodontic practitioners in 2010 showed that, on average, nearly 30% of orthodontic patients were adults (age 19 or older). Wearing braces as an adult now is more socially acceptable than it was not so long ago, though no one really knows why. Older adults (over age 40) now have orthodontic treatment in conjunction with other types of dental treatment, because it now is widely accepted that better periodontal / restorative / prosthetic results can be achieved by improving the dental occlusion as part of the treatment. Level II Diagnosis — Unit A · 28 / 97

{{PAGE_29}} Special Considerations in U.S. Adult Orthodontics

Summary Important points from this teaching program: Angle classification:

• 4 classes, not 3 (normal occlusion, 3 malocclusion)
• molar relationship plus line of occlusion Extended Angle classification: Jaw relationship that predisposes to molar relationship Modern goals of treatment
• soft tissue proportions / tooth-lip relationships
• jaw relationship
• dental alignment / occlusion

Summary (cont.) Irregularity index (NHANES III data)

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{{PAGE_30}} 33% crowded incisors, 15% very crowded gets worse with increasing age

Overjet (NHANES III data) one-third severe or negative <1% Class III except Asian 5%

Overbite / open bite (NHANES III data) deep bite more frequent than open bite in total population severe deep bite more prevalent in those of European descent severe open bite much more prevalent in African-Americans

Reasons for orthodontic treatment psychosocial: discrimination, self-esteem oral function: mastication, swallowing, speech injury / disease

Summary (cont.)

Need for treatment determined by IOTN method: Most severe characteristic

IOTN grades for US population:

• 15% of whites and Mexican-Hispanic have severe malocclusion
• 20% of African-Americans have severe malocclusion

IOTN aesthetic component is based on view of teeth only, no facial component

Dentists vs parents: dentists: 55% need = all IOTN grade 3, some grade 2 parents: 35% need = all IOTN grades 4-5, some grade 3

Demand for treatment: affected by patient perception of problem, socio-economic status 50% want treatment if no financial constraint high demand from lower income groups: perception that orthodontics is important for future success of child

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 2-18, 99-103 and 133-145 in the 5th ed.; pages 2-22, 111-119, 145-158 in the 4th

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{{PAGE_31}} ed.) Then take the self-test, and use it as a guide for further study and review. Be sure you understand the correct answer to all questions that you didn’t get right on your first try.

Copyright 2013, UNC Dept. of Orthodontics

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2. Known Causes of Malocclusion

Introduction

Learning Objectives

Malocclusion is the result of a complex process of growth and development. For that reason, often it is difficult to precisely specify the cause. But it is important to understand the possible causes, and to know how specific events can influence the development of malocclusion.

The purpose of this program is to outline what is known about specific possible causes of malocclusion, and to put these specific causes in perspective. In a minority of the patients with severe malocclusion that you will meet in the future, you will be able to say that one of these causes was the etiology. In the majority, more than one of these causes may have played a role, so that the etiology would not be known clearly. Even so, you need to know what the possible causes might have been, and how important any of them might have been in your specific patient.

After viewing this program, you should be able to discuss the role of four major factors in the etiology of malocclusion and recognize the etiology of the clinical problem presented by patients affected by them:

1. Hereditary factors
2. Interference with normal development
3. Trauma
4. Disturbance in normal function

Effect of Malocclusion on Function

As we discussed in the first unit of this module, malocclusion is so prevalent in contemporary populations that only about 35% of the US population have occlusion that would be classified as normal. Another way to look at it is to consider the effect on function, both physiologic and social. In essence, this combines the functional and aesthetic IOTN grades that you learned about in a previous module.

The worst 5% of the population have a malocclusion so severe that they are handicapped in their ability to function both physiologically and socially. About 20% have a severe problem, another 20% are moderately affected, and another 20% are mildly affected.

The severe and handicapping problems usually have a skeletal component, that is, the position or size of the jaws makes it impossible for the teeth to fit together correctly even if the teeth are well aligned. Moderate and mild problems usually are just displacement of teeth relative to well-proportioned jaws.

{{PAGE_33}} Malocclusion

5% Handicapping 20% Severe 20% Moderate 20% Mild 35% Normal

Etiology: Overview Malocclusion almost always is the result of a distortion of normal development, and it can be very difficult to precisely specify the cause.

As a result, the etiology of most malocclusions is not well understood. In some situations, however, we can identify a specific cause of malocclusion, and when that is possible it is important to do so. At present only about 5% of patients fall into that category (image 1).

The 5% with known etiology overlap with the 5% with handicapping malocclusion, but they are not exactly the same group (image 2). A known etiology does not necessarily produce a handicapping problem.

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{{PAGE_34}} image 1: US population percentages for known / unknown etiology of malocclusion. image 2: For some patients with a handicapping malocclusion, the cause is known. For most, there is no single known cause. Does the 5% overlap between known cause and handicapping malocclusion mean it’s 2.5% each. No—it’s not that simple.

Interaction Between Genetic and Environmental Influences

For the patients whose malocclusion cannot be attributed to a specific cause, a complex interaction between genetic and environmental factors is involved.

As progress is made in future years, it should become possible to specify more clearly both the genetic or environmental factors and how they interact. In turn, that would greatly improve our ability to predict the outcome and stability of treatment.

Let’s examine some possible specific causes, beginning with hereditary factors.

Hereditary Factors

Evolution?

Certainly it is possible that inheritance of mismatched teeth and jaws could be a major cause of malocclusion, and at one time most malocclusion was thought to be related genetically determined.

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{{PAGE_35}} Under primitive conditions where populations were genetically quite homogenous and presumably everyone carried the same information for tooth size and jaw size, malocclusion occurred but was much less prevalent than it is now. In image 1, note the excellent alignment of teeth in this jaw fragment from 100,000 years ago, which is typical of skeletal remains from times up until quite recently. Crowding and malalignment did occur long ago, however (image 2). This individual had the largest teeth of any of the 80 specimens from the same source as image 1.

Fig. 1: Well-aligned teeth are observed in most jaws taken from ancient archeologic excavations. Fig. 2: Irregular teeth in an ancient jaw fragment—so this problem did occur at that time.

Evolution? (cont.) There are several possible causes for increased malocclusion that could relate to changes over a long period of time. An interesting one is the evolutionary trend toward a decrease in the size of the jaws and the individual teeth, as well as a decrease in the number of teeth. If the reduction in the size of the jaws did not match the reduction in tooth size and number, then malocclusion probably would be the result. Note the decrease in the size of human teeth from the Qafzeh anthropological site 100,000 years ago, to Neanderthal man 10,000 years ago, to a modern (English) population (image 1). In image 2, look at how the number of teeth decreases as we follow the primate evolutionary path from basic mammalian to man. Four premolars and three incisors per side has been reduced to two

{{PAGE_36}} premolars and two incisors. In man, third molars are so often missing now that the trend toward only two molars is apparent, and upper second (lateral) incisors and both upper and lower second premolars seem to be under some pressure. Over the same time span, jaws also have been reduced in size.

Genetic Drift However, this slow genetic drift over many generations and tens of thousands of years cannot explain the recent increase in malocclusion. It has been only in the last thousand years, perhaps even less, that significant human malocclusion has appeared on the scene, and there simply has not been enough time for evolutionary pressures to act. You could argue that malocclusion has appeared along with urbanization and population growth. We know of other “diseases of civilization” that have appeared on the same timetable, atherosclerotic heart disease being a prominent example. Is there something about the development of civilization that could contribute to the inherited aspects of malocclusion? Although recent research has shown that the expression of genes can change more quickly than was previously believed, this explains only a small amount, if any, of the recent increase in malocclusion. A recent sensationalist book called “Jaws” says the whole cause of malocclusion is decreased biting force that leads to small jaws, but that makes no sense in the context of major change in so few years—and in fact the jaws of 500 years ago are the same size as they are now.

Hereditary Factors: Outbreeding? Some day as you point out a child’s deficient chin and tell mother that’s why his upper teeth protrude, she’ll say “But he looks just like his father”. We have all observed familial tendencies in facial features, and there is no doubt that jaw proportions leading to malocclusion can be passed on from one generation to the next.

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{{PAGE_37}} Could the recent increase in malocclusion be the result of greater problems because mother’s small jaws matched up with dad’s large teeth, or the child inherited dad’s large upper jaw and mom’s small lower jaw? You could argue that malocclusion should be one result of marriage between different racial and ethnic groups.

Indeed, the US is proud to be a genetic melting pot (and the fact that people from different racial backgrounds come together is good), and we also have one of the highest rates of malocclusion in the world. Modern mobility has created a much more heterogenous gene pool than primitive populations had. Does that mean that the child of parents of different racial backgrounds (i.e. those who were previously geographically separated) is at greater risk of severe malocclusion? No, it’s not that simple.

Hereditary Factors: Outbreeding? (cont.)

Observation of the effects of outbreeding indicate that there are no independently inherited tooth and jaw characteristics that could produce malocclusion when mixed. It’s simply not the case that dental characteristics and jaw characteristics are inherited independently.

Although it’s attractive to think that kids can inherit mom’s jaws and dad’s teeth, it doesn’t work that way. Multiple gene effects and linkage between characteristics keep it from being so simple.

Let’s look at some of the evidence for that contention.

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{{PAGE_38}} Concept

Dental/facial characteristics do not follow simple inheritance rules.

(i.e., multi-gene effects)

Heriditary Factors: Animal Experiments

The temptation to blame malocclusion on mixing mom’s jaws and dad’s teeth, or mom’s upper jaw with dad’s lower jaw, was reinforced at first by the results of breeding experiments with dogs.

Crossing various breeds of dogs resulted in striking dentofacial malformations that seemed analogous to human malocclusions, and this illustration from an influential textbook published in 1941 makes the connection explicit. If you cross a small breed of dog with a large one, you can get jaw combinations that look a lot like human Class II or Class III jaw relationships.

{{PAGE_39}} Hereditary Factors: Animal Experiments (cont.)

What the early researchers didn’t realize, however, is that many of the involved breeds of dogs carry the gene for achondroplasia.

You will remember from Level 1 that this condition is characterized by deficient growth of primary cartilage, which causes midface deficiency and short limbs. It’s a dominant trait with variable penetrance in dogs. If you cross a Bassett hound like the one shown here, who is quite achondroplastic, with another breed of dog (a collie, for instance) who isn’t achondroplastic, you’re likely to get strangely-proportioned puppies with a malocclusion, but it turns out that the malocclusions are not a true reflection of the mixing of various dentofacial characteristics. Instead, this reflects the variable penetrance of achondroplasia.

{{PAGE_40}} Hereditary Factors: Human Data If breeding experiments in dogs are not a good model, why not look directly at humans?

Chung et. al. did just that by conducting a study of the prevalence of malocclusion in Hawaii, whose population probably shows the greatest degree of interracial mixing in the world.

What they found was that interracial mixes do not produce a major increase in malocclusion. Severe malocclusions are no more frequent in Hawaii than anywhere else, and no extreme combinations of parental characteristics are observed.

So, tempting as it might be, we really can’t blame the recent increase in malocclusion on interbreeding among different human groups.

Hereditary Factors: Familial Characteristics Nevertheless, children tend to look like their parents. Studying family members by observing similarities between parent and child and between siblings can provide a clue as to the amount of genetic influence on malocclusion.

Among family members, the correlation is higher for skeletal than for dental characteristics, which seems reasonable since tooth position should be more easily changed by environmental influences like habits.

{{PAGE_41}} Occasionally, inheritance of a specific pattern of malocclusion is observed. The most apparent is the familial tendency toward mandibular prognathism, well documented in many circumstances including the Hapsburgs, the European royal family. Note the very prominent chins of the members of the Spanish royal family over a period of more than 200 years. But most types of malocclusion aren’t inherited within families in that clear-cut way.

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{{PAGE_43}} Image 3: Charles IV, 1800. Note that his mother on the left has a strong chin, and so does his infant son. But his wife (right) has normal facial proportions.

Twin Studies

To separate genetic from environmental contributions to a malocclusion, researchers have looked at twins. Since identical twins share essentially the same DNA composition, differences between them are thought to be mostly of environmental origin, while differences in fraternal twins who share only part of their DNA can be of genetic or environmental origin. Comparing identical vs. fraternal twins gives a way to estimate how much variation is hereditary and how much is environmental.

At age 11, as you would expect, these identical twins have remarkably similar facial proportions (images 1 and 2). They’re distinguishing themselves by having different hair styles and clothes. They show a finding, however, that illustrates the extent to which genetic controls affect things that you might think were just random variation. Almost always during the transition from the mixed to the permanent dentition, there is a left-right asymmetry, with the teeth on one side erupting a little ahead of those on the other side. In identical twins, the dental arches will be a mirror image at that point—the teeth on one side ahead in one of the twins, the other side ahead in the other.

Note that for Tom, the maxillary right canine is erupting ahead of the maxillary left one, while for Chris the reverse is true (images 3 and 4). The photos of both boys were made on the same day.

{{PAGE_44}} Image 1: Identical twins, Tom (left) and Chris (right), age 11 Image 2: Identical twins, Tom (left) and Chris (right), age 11 Image 3: Note that on the right side Tom’s teeth are further along in eruption that Chris’ teeth. Image 4: On the left side it’s the reverse, Chris ahead of Tom.

Twin Studies (cont.)

The proportion of variation in a given characteristic that is attributed to genetic rather than environmental factors is called heritability.

Twin studies are, indeed, useful in estimating heritability of traits which is expressed as a quotient that varies from 1 to 0. If the heritability quotient is 1, then 100% of the variability in the trait is due to genetic influences. Conversely, if the heritability is zero, then variation in the trait is purely due to environmental influences.

One of the problems with twin studies is that they’re usually living in the same family at the same time, and therefore they have a very similar environment as well as similar genomes. It’s important in twin studies to try to control for similar environments because it places the subjects under similar non-genetic influences.

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Concept

Twin studies allow an estimate of heritability.

(Question: Did they control for similarity of environment?)

Contribution of Hereditability to Malocclusion

There are currently opposing views on the contribution of heritability to malocclusion. One end of the spectrum is occupied by Lundstrom, a well known European orthodontic professor who reviewed his years of research in this area in the 1980’s. The other end is represented well by Corrucini, a professor of physical anthropology who worked with the geneticist Rose Potter at the University of Indiana in the 1990s.

By comparing dentofacial variables in identical twins, especially skeletal and dental overjet, Lundstrom concluded that up to 50% of malocclusions have their origin in hereditary factors. Corrucini and Potter, using a different statistical approach to the same material, came up with essentially no genetic contribution at all. Each group questions the other’s methodology, especially how well they controlled for similarity of environment.

But the data indicate that 50% may be the maximum contribution to malocclusion by hereditary factors, and that the environmental contribution is likely to be significantly higher than the genetic one. The skeletal component of malocclusion seems more influenced by heredity than the dental component—you’re more likely to inherit mismatched jaws than crooked teeth.

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Concept

Twin studies allow an estimate of heritability.

(Question: Did they control for similarity of environment?)

Interferences with Pre-Natal Development

Interferences During Pre-Natal Development: FAS

What are the environmental influences that can cause malocclusion? In this sense, everything that isn’t clearly genetic is environmental, which means that there are many possible environmental factors.

One clear-cut example of facial distortion and malocclusion due to interference with normal development during embryonic life is the fetal alcohol syndrome (FAS). Exposure to high levels of alcohol during the first trimester of pregnancy (often, before the mother knows she’s pregnant) produces the characteristic facial features of FAS.

Compare the drawing of the FAS facial features (image 1) with the face of this 7-year-old boy (image 2). The child’s problems, of course, are much bigger than his facial appearance. Mental retardation and decreased neuromuscular coordination are common.

{{PAGE_47}} FACIAL FEATURES OF FETAL ALCOHOL SYNDROME Discriminating Features Short palpebral fissures Flat midface Short nose Indistinct philtrum Thin upper lip

Associated Features Low nasal bridge Epicanthal folds Minor ear anomalies Micrognathia

Image 1: FAS facial features Image 2: Fetal Alcohol Syndrome in a 7-year old boy

Craniofacial Microsomia Another example of interference with normal growth during embryologic life is craniofacial microsomia (formerly referred to as “hemifacial microsomia”), which is caused by early loss of neural crest cells at the stage of origin and migration.

The result is a characteristic underdevelopment of the affected side of the face, as seen in the facial views of this girl who is only mildly affected (images 1 and 2). In a more severely affected child, the ramus of the mandible and the TM joint area may be missing (image 3).

{{PAGE_48}} Image 1: 12-year-old girl with craniofacial microsomia. Note the underdevelopment of the left side of the face. Image 2: Moderate distortion of the left ear is present. Image 3: CT image of a patient with severe craniofacial microsomia shows the loss of hard and soft tissue in the area of the TM joint.

Intra-Uterine Molding

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{{PAGE_49}} Pressure against the developing face can significantly interfere with normal development. Image 1 shows the severe maxillary deficiency in a young girl that was produced by a forearm positioned across the mid-face during intra-uterine development.

Pressure against the lower jaw because the head is flexed tightly against the chest produces extreme mandibular deficiency at birth. This occurs when there is a deficiency in the volume of amniotic fluid, which can arise from multiple causes. The condition is called the Pierre Robin sequence or anomolad. When the pressure against the mandible is released after birth, some affected children have normal growth and a complete or nearly complete recovery, while in others the mandible never catches up (image 2).

Image 1: Mid-face deficiency from mechanical interference with intra-uterine development of the mid-face Image 2: Severe mandibular deficiency from the Pierre Robin sequence

Pre-Natal Interferences with the Dentition While disturbances of embryologic development are unlikely to produce skeletal malocclusions without any other associated defects, pre-natal disturbances of dental development frequently occur as isolated defects. These can contribute to Class I malocclusions where jaw size and proportions are normal but the teeth are malaligned and irregular.

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{{PAGE_50}} The best examples are supernumerary (extra) teeth, congenitally missing teeth and distortions of tooth form. An extra tooth occurs relatively frequently in modern populations. The most frequent location is in the maxillary midline, and a supernumerary tooth in that location is called a mesiodens (images 1, 2). Its presence either keeps the maxillary central incisors from erupting, or displaces them to produce an obvious and unsightly malocclusion. Two, or perhaps three, supernumeraries occasionally occur in otherwise-normal children. A supernumerary tooth bud sometimes fuses with the developing crown of a normal tooth, distorting it (images 3 and 4). Multiple supernumerary teeth are unlikely in children who are not affected by some congenital syndrome, but are a characteristic of cleidocranial dysplasia (image 5).

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{{PAGE_51}} Image 1: Clinical photo of a patient’s smile showing a midline supernumerary tooth (mesiodens). Image 2: Radiographic view of mesiodens. Image 3: Panoramic radiograph showing multiple supernumerary teeth in a child with cleidocranial dysplasia. Image 4: Dental cast model demonstrating the distortion of the lower right 2nd molar, which fused with a supernumerary tooth at an early stage of development.

{{PAGE_52}} Pre-Natal Interferences with the Dentition (cont.) A congenitally missing tooth or teeth also occurs relatively frequently, with the maxillary lateral incisors and/or mandibular 2nd premolars most likely to be affected (but any tooth can be missing in an otherwise-normal individual).

Multiple congenitally missing teeth, like multiple supernumeraries, usually occur only in children with a syndrome of some type. One frequent cause of multiple missing teeth is ectodermal dysplasia, which should be suspected when a patient with a dentition like this one is seen, even if the diagnosis has not previously been made.

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{{PAGE_53}} Radiograph showing dental anatomy and bone structure Birth Injuries (?)

An injury during birth that leads to interference with growth is possible, but this is the cause of a facial anomaly only rarely—though many parents are convinced that if something is wrong with their child’s face, a birth injury was the cause. As the family dentist, you may have to explain that no matter how convinced the parents are that a birth injury occurred, craniofacial microsomia, severe mandibular deficiency or a malformed incisor was present long before birth and was not caused by something that happened at that time.

Trauma as an Interference with Post-Natal Development

Soft Tissue vs Hard Tissue Injuries

The effect of trauma to the face and jaws as a cause of malocclusion is understood best from the perspective of the functional matrix theory of growth. In most instances, facial growth is affected more by the extent and healing of facial and soft tissue injuries than the amount of hard tissue injury. In particular, scarring of the soft tissues around the facial skeleton and dentition can have marked effects on bone growth and tooth positions.

The unfortunate boy seen in images 1 and 2 experienced trauma to his mid-face in an automobile accident in early childhood, and his severe maxillary deficiency in adolescence is a direct result of the scarring of soft tissues that prevented the normal downward-forward growth of the naso-maxillary area. The deformity became progressively worse as the affected areas did not grow and the adjacent areas did. Surgery to reposition his jaws improved his situation, but the soft tissue scarring could not be completely corrected (images 3 and 4). Fractures of the nose and maxilla usually do not produce

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{{PAGE_54}} this much effect on growth, and in fact may have very little effect—if extensive soft tissue scarring did not occur.

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{{PAGE_55}} Image 1: Frontal view, mid-face deficiency 8 years after an automobile accident with maxillary fractures and soft tissue injuries

Image 2: Profile view shows the marked mid-face deficiency, created by scarring that prevented the normal downward-forward growth of the maxilla that should have matched the mandibular growth.

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{{PAGE_56}} the bridge of the nose.

Soft Tissue vs Hard Tissue Injuries (cont.)

At age 4, this boy fell from a tractor that then ran over his head. Although the soft ground helped, he had almost too many fractures to count (image 1), but after a month in critical condition, he survived. At that point forward traction in the orthodontic clinic succeeded in moving his displaced maxilla forward 3-4 mm (image 2), and no further treatment was done.

When he returned to the orthodontic department at age 12, the healing of the fractures and the extent of normal growth was remarkable (images 3, 4, 5). In his case, despite the severity of the hard tissue injuries, extensive scarring of the soft tissues did not occur. If you didn’t know his history, you wouldn’t believe that he had experienced trauma of that extent.

He’s an excellent example of how facial growth can continue reasonably normally in the absence of soft tissue scarring.

{{PAGE_57}} Image 1: Facial x-ray post-injury, showing the extensive fracturing of facial bones

Image 2: After his survival was no longer in doubt 2 months later, the maxilla still was displaced posteriorly.

Image 3: At 2-4 months post-injury, a facemask (reverse pull headgear) to a splint over the maxillary arch was used to move the maxilla forward 3-4 mm. No other orthodontic treatment was done after that.

Image 4: Age 12. Note the normal facial proportions and jaw relationships.

{{PAGE_58}} Image 5: Age 12. The teeth were mildly crowded and irregular, but the malocclusion was only moderately severe.

Soft Tissue vs Hard Tissue Injuries (cont.)

You already know that after a fracture of the condylar process of the mandible, there is a 75% chance of normal growth, and a 25% chance that an asymmetry will develop due to an interference with growth. What’s the difference between the children who subsequently grow normally and those who do not? That’s right, it’s largely whether there was enough soft tissue injury to create scarring around the TM joint that inhibited translation of the mandible as facial growth continued.

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Effects of Trauma on the Dentition

If a young child crawling around on the floor bites down on an electrical cord, the result is likely to be a severe burn at the corner of the mouth, as in the girl shown in image 1. As it heals and scars down, pressure against the teeth creates a marked asymmetry in the mandibular dental arch, and makes normal arch form almost impossible to maintain.

The reverse effect is seen when cheek tissues are lost (image 2). Then unopposed pressure by the tongue tips the teeth facially.

In both cases, the soft tissue injury led directly to displacement of the teeth.

{{PAGE_60}} Effects of Trauma on the Dentition (cont.)

Direct trauma to the teeth also can lead to malocclusion. If a tooth is lost, the adjacent teeth will tend to drift into its space, and normal alignment and position of the teeth will be lost unless the space is maintained until the lost tooth is replaced. Premature loss of a primary tooth can lead to loss of space for its permanent successor. Loss of a permanent tooth has the same potential for the development of malocclusion.

If a tooth is displaced but not lost as a result of trauma (image 1), the extent of soft tissue injury again becomes a key factor in the long-term outcome. In this situation the soft tissue is the periodontal ligament. If it is severely damaged, the displaced tooth will become ankylosed as the cementum of the root fuses with the alveolar bone. Then neither further eruption of the tooth nor orthodontic tooth movement is possible.

If the PDL is not severely injured, abnormal development of the root (dilaceration) can occur (image 2), but a tooth with a dilacerated root can continue to erupt and may end up in normal position in the dental arch. A displaced tooth with an intact PDL can be moved orthodontically if necessary (image 3).

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{{PAGE_61}} Image 1: Displacement of a central incisor from trauma. Ankylosis is likely with this amount of displacement; if the tooth is to be repositioned orthodontically, this must be done soon after the injury.

Image 2: Distortion of the root of this lateral incisor resulted from trauma at an earlier age that displaced the crown of the tooth relative to the developing root.

Image 3: Bringing displaced teeth back into the dental arch can be done with orthodontic tooth movement (if the PDL is intact).

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Disturbance in Normal Function

Habits: Thumbsucking

The final specific potential cause for the development of malocclusion is a disturbance in function. The dentition is always subject to forces from its environment, such as masticatory forces or pressure from cheeks, lips and tongue just resting against the teeth. Yet tooth position in most people is stable, which indicates that these forces must be somehow in balance or equilibrium. How could normal function be disrupted to result in malocclusion?

One of the more obvious examples of interference with normal function is a thumb habit. Most likely, you have already seen a thumbsucking child who shows the classic side effects of this habit: protruding upper incisors, anterior open bite, and maxillary constriction causing the child to shift into a unilateral posterior crossbite. For the child in image 2 (not the same one as image 1), you can see just where he places the thumb between the teeth.

Habits: Thumbsucking (cont.)

Let’s look more carefully at what thumbsucking does, and how it does it. Why are protruding upper incisors often seen in a thumbsucker? The answer is that the child often puts upward-forward pressure on the upper teeth. If the sucking habit is maintained for enough hours each day, this can displace the teeth and even lead to more forward growth of the upper jaw.

Images 1 and 2 show identical twins, one of whom sucked his thumb but the other did not. In the dental casts you can see the increased overjet in the thumbsucker, with protrusion of the upper incisors. In the superimposed cephalometric tracings, note the remarkable similarity of the cranial and facial structures. This is characteristic for identical twins, and would not be seen in any other pair

{{PAGE_63}} of individuals. For this thumbsucking child, the effect was a little more forward growth of the maxilla and major protrusion of the upper incisors. Unlike what happened in this child, the position of both the upper and lower incisors often is affected by thumbsucking: the lower incisors are likely to be displaced lingually as the upper incisors are tipped facially.

Thumbsucking Effect on Arch Width

Probably it’s not obvious to you how thumbsucking could lead to a narrower, V-shaped maxillary arch. This is due to two related factors: increased pressure against the teeth as the cheeks are tightened during sucking, and a lowered tongue posture.

This diagrammatic representation shows how the pressure balance against the teeth is altered during sucking, as cheek pressure increases while the tongue is displaced downward. The effect is a lingual movement of the upper but not the lower teeth, which are stabilized by the tongue. The change toward a V-shaped maxillary arch form occurs because the cheek pressure is highest at the corners of the mouth, so the canines (and first premolars to a lesser extent) are displaced lingually more than the posterior teeth.

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{{PAGE_64}} Other Oral Habits? Tongue Thrusting? Mouth Breathing?

Although thumbsucking is a clear-cut cause of malocclusion, some other “oral habits” are not. It is easy to conclude that if pressure from a thumb can displace maxillary incisors upward and forward, pressure from the tongue could do the same thing. A “tongue thrust swallow habit” often is blamed for the maintenance or worsening of a malocclusion that relates back to thumbsucking. In these patients, the position of the tongue probably is more an adaptation to the position of the teeth than the cause of their displacement. As you are going to learn in more detail, teeth are displaced by light force of long duration, not by intermittent force like the brief contact of the tongue with the teeth during swallowing.

Mouth breathing because of nasal obstruction also often is considered the cause of open bite, long face malocclusions. There is evidence that in some cases, nasal obstruction is an etiologic factor—but in other patients with the same dental and skeletal morphology, it is not.

The question of the etiology of malocclusion in these situations is difficult, and the answer is complex. This can be understood best in the context of equilibrium between opposing soft tissue pressures as they affect jaw position and tooth position. We have already noted some equilibrium effects, and equilibrium theory will be the subject of the teaching module that follows this one.

Summary Summary, Inherited Tendencies Let’s summarize what we have learned about inherited tendencies toward malocclusion:

• Neither genetic drift during evolution nor increased outbreeding explains the recent increase in malocclusion.

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• Certain types of malocclusion, especially Class III mandibular prognathism, seem to be inherited —but most are not.
• The heritability of jaw relationships and malocclusion characteristics is not more than 50% and probably less. Heredity can explain some malocclusions, but it cannot explain all or even most of them.

Summary, Interferences with Pre-Natal Development

Interferences with pre-natal development as specific causes of malocclusion include:

Facial Development

• Fetal alcohol syndrome
• Hemifacial microsomia
• Intra-uterine molding

The first two interferences occur early in the first trimester of pregnancy. Alcohol is the most frequent, but not the only drug that can produce problems during the critical events of early stages of development.

Craniofacial microsomia is an example of an accident in development that is not due to an external agent, and is not the only mis-step in development that can affect the face.

Intra-uterine molding, in contrast, affects the growth that occurs in the third trimester. It has an environmental, not a heritable etiology—even though the problem arose before birth.

Dental Development

• Supernumerary teeth
• Congenitally missing teeth
• Fusion of tooth buds

These problems arise at the stage when the dental lamina is formed, late in the first trimester of pregnancy. Can they be due to external influences like drugs? Certainly some drugs, like tetracycline, can affect mineralization, but most problems of dental development do not seem to be due to the intra-uterine environment.

Summary, Interferences with Post-Natal Development

Facial Development

• Facial trauma
• Condylar fracture

The effects of trauma on growth are due much more to the extent of soft tissue injury than the direct injury to the jaws, because the formation of scar tissue that restricts growth of the bones is the key element in affecting post-injury growth.

Dental Development

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• Tooth loss
• Tooth displacement: ankylosis?
• Dilaceration

Tooth loss leads to failure of development of alveolar bone. Ankylosis and dilaceration can be considered a reflection of injury to the periodontal ligament, so soft tissue injury is also a factor in problems of dental development.

Disturbance in Function

• Sucking habits
• Other habits?

How much pressure does it take to affect jaw growth and tooth eruption, and how much of the time does it have to be applied to have an effect? We will consider those important questions in the next module.

Referral to Self-Test

Before you proceed to the self-test, be sure to do the assigned reading in Contemporary Orthodontics (5th ed., pages 122-133; 6th ed., pages ). Then use the self-test as a guide to further study. If you didn’t get all the questions right, be sure you understand why you were incorrect.

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{{PAGE_67}} 3. Equilibrium Theory and the Etiology of Malocclusion

Equilibrium Theory Learning Objectives Function influences form and vice versa, but the extent to which function contributes to the etiology of malocclusion remains controversial. Certainly it plays a role in many cases, and in at least some it is a major cause. Function influences the form of the dental arches by changing the pattern of soft tissue pressures, affecting the equilibrium of forces against the teeth. The focus of this program is on potential environmental influences in the development of malocclusion. The environment of the teeth, in this context, is the soft tissues that surround them. You need to understand the role of potential etiologic factors that affect soft tissue pressures on the dentition and the pattern of jaw growth, with a special emphasis on what we know about the magnitude and duration of pressures against the dentition that can affect the position of the teeth.

Evidence for Equilibrium Effects on the Teeth We already have reviewed the evidence that at least 50% of malocclusions are related to non-inherited influences, which means that the environment of the dentition is a factor in the etiology of the majority of malocclusions. Exactly what is meant by environmental influences? In this context, we can say that the environment affects the developing dentition by changing the balance of natural-occurring pressures against the teeth. These diagrams illustrates a simplified view of that balance of pressures or equilibrium, showing the incisors are in a position of balance between the front of the tongue and the lips (image 1), and the posterior teeth balanced between the side of the tongue and the cheeks (image 2). Equilibrium theory says that the teeth are located in a position of balance among the opposing forces that are brought to bear on them. From the fact that teeth remain in stable positions most of the time although they are subjected to forces, and yet can be moved when additional forces (as from orthodontic appliances) are applied, we know that there is indeed an equilibrium. The question is “What defines the equilibrium?” in the context of both the magnitude and the duration of forces against the teeth.

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Evidence for Equilibrium Effects on the Teeth (cont.)

Looking at a child like this one, you can see how malocclusion might arise if, for instance, a patient thrusts his tongue forward constantly, increasing the pressure by the tongue against the incisors and pushing those teeth forward.

Could it be possible that in fact pressure against the teeth, created by normal or abnormal function, is a major contributor to malocclusion?

{{PAGE_69}} Pressure Against the Teeth During Chewing Whatever your dental alignment and occlusion is at this moment, it’s quite stable—that is, nothing much is going to change in the near future unless you undergo orthodontic treatment. But the teeth are subject to heavy force during mastication. Biting down often puts tens of kilograms of force against the teeth. If a tooth is not quite in the right position, why doesn’t the force against it during chewing move it to the right place, or displace it even further? The force is more than enough to move a tooth. Why doesn’t it move?

The answer provides an important insight into the equilibrium conditions. Teeth do move during chewing, not because they are displaced within the periodontal ligament space, but because the alveolar bone bends. The fluid within the PDL acts like a shock absorber on an automobile. When you bite down, in the short term (seconds) the fluid is incompressible, the force is transmitted to the bone, and the bone bends. If the force is maintained more than a few seconds, the fluid begins to be squeezed out and the PDL begins to be compressed. That hurts, and you open up again to remove the force.

Why don’t you chew a tooth to a new place in the dental arch? Because the force isn’t maintained long enough to produce tooth movement.

Duration of Force / Pressure

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{{PAGE_70}} The current concept is that the duration of any force against a tooth or teeth (which produces pressure within the periodontal ligament) must be at least 4 hours per day and perhaps a bit more than that, if it is to have any impact on the position of the teeth within the dental arches and thereby change the dental occlusion so that it could produce malocclusion.

There are two lines of evidence to support this concept. Some of the best evidence is derived from experiments that orthodontists run all the time, perhaps experiments in which you yourself participated during your own orthodontic treatment.

Orthodontists often use removable appliances—headgears, removable plates with springs (like the one shown here being used to bring central incisors together), retainers—and of course any appliance that can be removed will be. Some patients wear removable appliances faithfully, and others wear them very little or not at all.

How much does a patient have to wear a removable appliance in order for it to produce tooth movement? The answer is “Not all the time, but at least 4 to 8 hours per day.”

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{{PAGE_71}} Image 1: Clinical photo showing closure of a midline diastema with a removable appliance featuring fingersprings. Image 2: Occlusal view clinical photo showing teeth alignment with springs activated to bring the teeth together. Image 3: Clinical photo showing result after wearing an appliance, with text questioning daily wear time and explaining that 12 hours is sufficient, minimum is 4-8 hours, and near-full-time wear accelerates movement.

Duration of Force / Pressure (cont.) A plot of tooth movement, as a function of the number of hours per day that a removable appliance is worn, looks something like this.

{{PAGE_72}} Wearing a removable appliance nearly all the time results in tooth movement that is very similar to full-time force from a fixed appliance. If the patient wears the appliance half the time, there’s less response but the appliance still works. Note that there’s a threshold, which occurs in humans somewhere between 4 and 8 hours. The concept that force must be applied for several hours per day in order to produce a change in tooth position has well been established.

Duration of Force / Pressure (cont.)

The second line of evidence for a duration threshold has come from animal experiments, largely carried out by Davidovitch and co-workers.

In the experimental preparation they used, it is possible to monitor cellular activity and to follow the cascade of chemical changes that lead to remodeling of the alveolar bone around a tooth and tooth movement.

In experiments of this type, it can be observed that after about 4 hours, the “second messengers” that trigger the cellular changes necessary for tooth movement begin to appear.

If it takes 4 hours or so to induce the formation of cyclic AMP and other second messengers, it seems reasonable to presume that this establishes the duration threshold, and of course this result coincides rather nicely with what has been observed in human patients wearing removable appliances.

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{{PAGE_73}} Habits? Now let’s consider habits or other behavior patterns as influences on the position of teeth. Can you affect the position of the teeth with a habit like thumb sucking? It’s apparent that you can—but it’s a common observation that some children who suck their thumb have much more displacement of their teeth than others who seem to suck in the same way. How could you explain that?

That’s right, it’s probably how long (hours/day) Susie sucks her thumb, not how hard she sucks when doing it. It would make a lot of difference whether or not the thumb was in the mouth all night.

The same thinking would apply to other habits or behaviors. They would have an effect on the dentition only if their duration exceeded the 4-6 hour minimum. Could the pressure on your teeth generated while playing your musical instrument (trumpet, saxophone, flute, violin, etc.) displace them? Yes—but you’d have to practice and perform a lot more than the average person to get that effect. It’s interesting that some professional musicians do show an effect on the shape of their dental arches from their long hours playing the instrument. Even for them the effects often are subtle.

Duration vs Force / Pressure in Orthodontic Tooth Movement It looks as if the duration of force against the teeth is a critical variable in whether it affects tooth position, and therefore could be an etiologic agent for malocclusion. How much force does it take to move a tooth, if it’s maintained long enough?

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{{PAGE_74}} The answer is “not very much”. Even a force of just a few grams can lead to tooth movement if it’s maintained long enough. A plot of tooth movement versus pressure in the periodontal ligament looks like this figure. If there is a threshold, it’s very low. As pressure increases, the rate of tooth movement also increases, up to a point. Above that, tooth movement occurs at about the same rate as pressure increases, up to and beyond the point of pain.

Duration vs Force / Pressure

That leads to an important concept: for orthodontic tooth movement or for environmental influences on the developing dentition, the duration of pressure is more important than its magnitude. Almost any pressure, maintained for enough hours per day, can affect the dentition, and above a certain level, it doesn’t matter how high the pressure is.

{{PAGE_75}} Concept

The duration of pressure against the teeth is more important than the magnitude.

Tongue vs Lip / Cheek Pressures: Tongue Thrust?

Measurement of Tongue and Lip Pressures We have seen that force against the teeth during chewing can be ignored in equilibrium calculations, because it’s too heavy to be tolerated for more than a few seconds. After that it exceeds the shock-absorbing capacity of the periodontal ligament. We also have seen that habits, if continued for a long enough time, could displace teeth. What about pressures against the teeth from tongue and lip / cheek function?

We can measure tongue and lip pressures using miniature pressure-measuring devices (called transducers) of the type shown in this image. The active elements in the device, used here to measure lip pressues against the lower teeth, are the strain gauges that you see in the incisor and canine regions. Thin lead wires extend out the corner of the mouth to the recording equipment. Devices of this type can be placed immediately in front of and behind the lower incisors so that tongue and lip pressures can be measured simultaneously.

The fact that the measuring devices extend labially or lingually from the teeth is a potential problem, but experiments suggest that if the measuring device can be kept within 2 mm of the tooth surface, major errors are avoided.

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{{PAGE_76}} Tongue vs Lip / Cheek Pressure: Equilibrium?

It is interesting that when the data from direct pressure measurements are examined, it is very difficult to demonstrate a balance of tongue and lip pressures.

In fact, almost never is there any indication that the tongue-lip/cheek pressures really are balanced as our original diagram suggested. For instance, if the pressures are measured during swallow, the tongue pushes forward to create a pressure of about 50 gm/cm2 against the upper and lower incisors, while the lip pushes back with a pressure of only 20 gm/cm2.

If pressure by the side of the tongue and the cheek against the molar teeth is examined, both tongue pressure and lip pressure are significantly greater, but the ratio is about the same, i.e. tongue pressure during swallowing is 2-3 times greater than the corresponding lip or cheek pressure.

Does that mean that there’s no equilibrium? No, because if any object (like a tooth) is subjected to pressures (which it is, as you chew, swallow, whatever) but does not move, there is an equilibrium.

So we have not been careful enough in considering what determines the equilibrium. It appears that simple measurements of pressures during one type of oral function (like swallowing) are not enough —there must be other components of the equilibrium.

{{PAGE_77}} Tongue vs Lip / Cheek Pressure: Equilibrium? (cont.) 50 20 Swallow

Tongue and lip pressures against the teeth also occur during speech and when a person is sitting quietly at rest. Pressure during speech lasts only a fraction of a second for most sounds. For instance, every time you say / th /, your tongue presses against your upper incisors with a force of about 40 grams, but for only 0.1 second.

In contrast, pressure created by the drape of the lips and cheeks against the teeth, or pressure created by the tongue resting against the teeth, is quite small, in the range of 5-10 grams. But these resting pressures last for hours.

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Soft Tissue Force Characteristics

Component	Magnitude	Duration
Speech	50-200 gms	1 second
Speech	15-40 gms	0.1 second
Rest	5-10 gms	hours

Equilibrium: Resting Pressures Only?

It is easy to see that the dentition is designed to withstand forces of short duration, such as the heavy forces encountered during chewing, and of course that would enable the teeth to withstand other short-duration forces like those from swallowing and speaking.

What would happen if we looked for balance just in the resting pressures against the teeth? That’s difficult because the magnitude of resting pressures is small and the patient must be still and relaxed in order to measure them, but it can be done.

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{{PAGE_79}} Equilibrium: Resting Pressures Only? (cont.)

Interestingly, the resting pressures do not balance either. At some places, as for example the lower incisors, resting tongue pressure tends to be greater than resting lip pressure, often by a ratio of 1.5:1. Resting tongue pressure also is greater against lower molars than is the balancing cheek pressure, by about the same ratio.

But in other locations, as for instance on the lingual of upper molars or canines, there is little or no resting tongue pressure, at least while the patient is awake, and resting cheek pressure is higher than tongue pressure.

Even if only the long duration pressures are considered, therefore, it is very difficult to demonstrate a balance between tongue and lip / cheek pressures.

But we know there is an equilibrium. The only conclusion would be that there is more to the balance of pressures than just tongue and lip / cheek pressure. What could the other elements be?

Equilibrium Components

Let’s examine again the possible contributors to equilibrium that we have considered up to this point. What could we add to this chart?

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{{PAGE_80}} Equilibrium Components

Component	Intensity	Duration
Occlusal Force	Very high	Very short
Lip or tongue pressure:
-Swallow	High	Short
-Speech	Low	Very Short
-Rest	Low	Long

Equilibrium Components (cont.)

Force that can move teeth also is generated within the periodontal ligament by the mechanism that produces eruption. Erupting teeth move, and the force that moves them is developed within the periodontal ligament. Furthermore, eruption can occur at any age, which means that force continues to be produced within the PDL after eruption apparently has stopped. For example, if a lower molar is extracted and the upper molar does not have an antagonist, the upper molar will begin to erupt again, even in patients in their 40’s and 50’s.

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Equilibrium Components

Component	Intensity	Duration
Occlusal Force	Very high	Very short
Lip or tongue pressure:
- Swallow	High	Short
- Speech	Low	Very Short
- Rest	Low	Long
Eruption Force	Low	Long

Equilibrium in the Eruption of Teeth

In Level 1, you already learned that as teeth erupt, light pressures of long duration opposing eruption (those from the soft tissues at rest) affect the amount of eruption, while heavy pressures (those from chewing or clenching the teeth together) do not.

The vertical equilibrium, in short, is like the equilibrium in other planes of space: the duration of pressure opposing the eruption of a tooth is more important than its magnitude.

Periodontal Ligament Contributions to Equilibrium

The present view is that force generated within the periodontal ligament, by the same mechanism that produces tooth eruption, also plays a role in stabilizing the teeth in other planes of space, and that this is the missing element in the equilibrium equation.

Dentists often observe that teeth in adults begin to drift to new locations when periodontal breakdown occurs. Perhaps that happens because stabilization by the periodontal ligament is lost. There has been no direct experimental demonstration of that, however, and so it must be labeled at this time as a plausible theory.

How much force can the eruption mechanism produce? That’s really difficult to measure, but it’s clear that it’s quite small, probably less than 5 grams. So how much of an imbalance in tongue vs lip / cheek pressures could PDL stabilization overcome? That’s right, you could think that if resting lip

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{{PAGE_82}} pressure was enough greater than 5 grams or so, the teeth would move toward the tongue until the opposing resting lingual pressure was close enough to balance for the PDL to be effective.

This concept of active stabilization by the periodontal ligament does explain why sometimes it appears that there is a threshhold for orthodontic force (5 grams or so) below which the tooth doesn’t respond, and why sometimes it appears there is a lower or no threshold. Do you understand that? That’s right, if the PDL’s stabilizing capacity was not being used, you’d have to add some light force to overcome it before tooth movement began. But if all the stabilizing capacity was being used, adding even a little more force in the direction that was being opposed by the PDL would result in tooth movement.

Habits? Tongue Thrust?

Tongue vs Lip / Cheek Pressure: Measurement

In our discussion of known causes of malocclusion, we have already discussed thumbsucking as an etiologic agent. Equilibrium considerations make it easier to understand why some children create a major effect on their teeth with a sucking habit while others do not, even though they seem to be doing the same thing.

It’s not how hard you suck, it’s how long—the same effect of duration being more important than magnitude.

Many children clench and grind their teeth. Whether this is a habit—an extraneous and learned activity—or a developmental stage is not understood as well as we’d like. A pertinent question in a discussion of equilibrium, however, would be: “Can you decrease the amount of tooth eruption with a clenching / grinding habit?”

The answer would be:

“Clenching, maybe, if you succeeded in having enough hours with pressure against the teeth”. More than four hours of clenching would be needed, and it would be hard to reach that level.

“Grinding, no. You’d never get hours of pressure during grinding the teeth.” Enamel would be worn away, but eruption wouldn’t be affected.

Tongue Thrust Swallow

One oral activity that has been labeled as a cause of malocclusion for a long time is “tongue thrust swallow”.

Tongue thrust swallow receives confusing labels in the dental literature, but usually it is defined as extension of the tip of the tongue between the incisor teeth during swallowing. When the lips are pulled apart as a child swallows, this can be seen—and almost always, in such a child, there is a space between the incisors, with excessive overjet and/or open bite. It’s easy to jump to the conclusion that the tongue thrust is the cause of this malocclusion.

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{{PAGE_83}} In young children (up to age 2-4), it’s normal to place the tongue tip against the lower lip during swallow. All children do it at those ages, and the percentage who are labeled as tongue thrusters declines with advancing age. About half have what could be called a tongue thrust swallow at the time they begin school.

The first misconception about tongue thrust swallow is that it’s a habit—an extraneous learned activity. It’s not. Instead, it’s a normal developmental stage, one that may be retained after it should have been replaced by the next stage. The concept that an activity is a habit leads immediately to the thought, “We need to break that habit”. That’s not the way to start thinking about tongue thrust swallow.

Prevalence of Malocclusion in Tongue Thrusters

Look again in the graph on this screen at the data for the prevalence of tongue thrust swallow in a reasonably typical American population, and note how the number of children with a tongue thrust compares to the number with anterior open bite. Tongue thrust swallow is about 10 times as prevalent as the malocclusion it is supposed to cause! If it’s an etiologic agent, it’s not a very potent one.

We would conclude that the epidemiologic data support what you would expect from consideration of equilibrium theory. It’s not the swallow—the duration isn’t right—and it doesn’t look like there’s much of a tongue posture effect either.

Note the prevalence of open bite and thumbsucking in this group of children. Open bite also is less prevalent than thumbsucking, which we know can cause it. How would you explain that?

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{{PAGE_84}} Thumbsucking wouldn’t make a difference either, unless you had a thumb in your mouth long enough each day, and it appears that a lot of children don’t.

Finally, note the difference in open bite between black and white children. That is a reflection of inherited skeletal proportions. Open bite is more prevalent in those of African descent; deep bite (not shown on this chart because nobody relates that to how you swallow) is more prevalent in those of European descent.

Tongue Thrust Habit?

Could you displace your incisors with a tongue thrust swallow? The duration of a single swallow is about 1 second. You swallow a few hundred times per day. Even if the shock absorber effect in the periodontal ligament didn’t totally accommodate this activity by the tongue, you’d be nowhere close to 4-6 hour duration needed for tooth movement.

So tongue thrust is not a habit, and it shouldn’t affect tooth position anyway. Does that mean that a child with a tongue thrust swallow could have no equilibrium effects on the teeth? Not necessarily. Although the swallow pattern is irrelevant, if a child like that had a different resting posture of the tongue, there could be a long enough duration of resting pressure to make a difference.

It does mean that trying to break a “tongue thrust habit” or teach a child to swallow differently would not be an effective way to remove a potential cause of malocclusion. In fact, it’s almost impossible to

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{{PAGE_85}} Effects from Speech?

Could you displace your incisors with tongue pressure during speech? Some children with a speech problem do show a lot of tongue during speech, and sometimes the speech problem is blamed for the development of a malocclusion. With a duration of tongue contacts during speech of only 0.1 second, nobody could talk that much.

Again, this doesn’t mean that an underlying adaptation in tongue posture would make no difference. It does mean that tongue posture, leading to resting pressure, would be the important thing in etiology. What you do with your tongue as you move it rapidly around makes no difference to the dentition. Where you position it at rest and while asleep could be a different matter.

The bottom line, however, is that lessons on how to swallow or speech therapy would be ineffective as a way to prevent or correct malocclusion. Some speech therapists do offer to help out the dentist and correct or prevent malocclusion, by correcting poor tongue position related to speech or swallowing. That doesn’t work clinically, and from what you now know about equilibrium, you wouldn’t expect it to.

Effects on Posture: Mouth Breathing?

Posture as a Determinant of Resting Pressure

So far in our examination of equilibrium we have established that only pressures of long duration should be important in affecting the equilibrium situation, and have shown that this theoretical prediction seems to be borne out rather well by clinical experience.

But resting pressures could be important, and they seem to be. What determines resting pressures? The answer to that would be, more than anything else, the posture of the head, jaw and tongue. Like everybody else, you have a characteristic posture of your head—and an equally characteristic, though harder to observe, posture of your mandible and tongue.

Moving the head, or the jaw, or the tongue to a different location and keeping it there, would change the pattern of resting pressures of both the tongue and the lips.

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{{PAGE_86}} Influences on Posture: Respiration We could assume, therefore, that influences on posture might well change a normal pattern of development to an abnormal one leading to malocclusion.

And we know that a major influence on posture of the head, jaws, and tongue is respiration. Respiration has a very high physiologic priority. If you can’t breathe, very quickly it does not matter what else you could have done if you had survived.

The influence of respiration on posture goes back literally to the first moments of life after birth. When you are delivered into this world, you have only a few minutes (not more than 4 or 5) to take the first breath of life. To do that, you have to open up an airway, which requires:

• lifting the head, so that it isn’t held down against the chest as it was throughout intra-uterine life
• bringing the mandible down and forward
• bringing the tongue forward

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{{PAGE_87}} Opening the Airway at Birth

• Elevate the head
• Bring the mandible down and forward
• Bring the tongue forward

The First Breath of Life In a series of experiments in the early 1960s that will never be repeated because of concern now about the effects of even small amounts of radiation, Bosma made x-ray movies of the first breath of life of a series of infants. That required taking the infant as it emerged from the birth canal and going quickly to the cine-radiographic machine that was in the delivery room. His x-ray movies clearly show the sequence of events that are necessary to open up the airway so that the newborn can breathe. In this view of a single image from the movie and a tracing from it of the airway, you can see the head elevation that is part of opening the airway so the new-born child can breathe.

{{PAGE_88}} Influence of Respiratory Mode on Posture

Once you have established an airway, of course you must maintain it for the rest of your life. It’s easy to demonstrate to yourself that posture of the head, jaw and tongue are adjusted to meet respiratory needs.

Try this experiment right now: pinch your nostrils shut and hold it so you can’t breathe through your nose. Keep holding it. After 30 seconds or so, you will have an irresistible urge to tilt your head up and move your mandible away from the maxilla, which also drops your tongue down. You’re opening up an oral airway so you can breathe through your mouth as long as your nose is blocked. The figure with this screen shows the results of an experiment with dental students, whose head posture was measured while their nose was blocked. Note that when the obstruction was removed, head posture immediately went back to what it was before the experiment.

Interestingly, newborn babies can’t breathe through their mouth. It takes a few weeks to develop that ability. So a totally blocked nose for a newborn is a medical emergency.

The bottom line, however, is that from the first minutes of life, respiratory needs are a major determinant of head, jaw and tongue posture.

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{{PAGE_89}} Does Respiratory Mode Affect Facial Growth? Could the way you breathe affect the way your jaws grow and the way your dental arches develop, and thereby be an etiologic agent for malocclusion? The answer is, “Yes, it could be”. When we speak of “mode of respiration” we are referring to the extent to which an individual breathes through the nose as opposed to the mouth. Nasal respiration is a mode of respiration, and so is mouth breathing. The current concept is that the mode of respiration can be an etiologic factor in malocclusion, because changing the respiration mode affects posture, and thereby changes resting pressure. Let’s examine the two parts of that concept individually, starting with whether the mode of respiration makes a difference. Then we’ll consider how it makes a difference.

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{{PAGE_90}} Concept

Mode of respiration can make a difference in the alignment and occlusion of the teeth because it alters postural relationships and that changes resting pressures.

Adenoid Facies

For over 100 years, there have been descriptions of the “adenoid facies” (image 1) in the dental literature. The concept was that if you have large adenoids, you can’t breathe well through your nose, therefore have to breathe through your mouth, and this will give you a long narrow face.

The strongest evidence to support the idea that nasal obstruction by enlarged adenoids can produce something like the adenoid facies (and an associated malocclusion) comes from observations in Sweden by Linder-Aronson. He studied children who required surgery for medical reasons to remove enlarged adenoids that presumably were at least partially blocking the airway and causing an increased proportion of mouthbreathing. Often tonsils were removed as well.

What Linder-Aronson demonstrated was that on the average, children who required adenoidectomy had longer faces than normal children, because the mandible was rotated downward and backward. In the computer-generated composite superimposition of the normal and T&A children (image 2), you can see the different facial proportions. The average difference was not large, but it was statistically significant, and it fits with the adenoid facies concept.

{{PAGE_91}} Image 1: Clinical photo of a patient with long, narrow face and anterior open bite. Image 2: Schematic diagram showing computer-generated composites of children with and without adenoidectomy; red lines indicate downward-backward rotation in the adenoidectomy group compared to normal black lines.

Growth After Adenoidectomy In this graph, also from Linder-Aronson, you can see that after the adenoids were removed, the mandibular plane angle, which decreases during normal growth, decreased more in the children who had this surgery. There was a tendency for these children to return toward the facial proportions of the normal group, although the recovery was not complete.

A weakness in the study is that nasal obstruction was presumed, not measured. One other group of researchers who repeated a similar study had the same findings. Another group were unable to replicate the result. But it does seem reasonable that some degree of nasal obstruction and increased mouth breathing was present in the children who needed T&A.

{{PAGE_92}} Nasal vs. Oral Respiration

One of the things that makes it difficult to evaluate the impact of respiratory mode on dentofacial development is that all humans breathe partially through the mouth.

You may think that you are a 100% nasal breather. If you do, run up four flights of steps, and recheck your mode of breathing. Even serious mental concentration, something we hope is occurring while you study this material, can increase oxygen consumption to the point that nasal breathing cannot totally supply it, and then a switch to partial mouth breathing occurs. At maximum effort in a person with no nasal obstruction, the air flow is about 50% through the nose, 50% through the mouth.

Probably what counts in the effect on growth of oral vs. nasal breathing is how many hours per day a high percentage of oral breathing is maintained. In the laboratory, it is possible to measure the percentage of oral breathing while physical activity is minimal—as it would be at rest and during sleep, when long-duration soft tissue pressures would be most likely to affect both growth and tooth positions.

This graph shows the percentage of a group of adolescent subjects with normal facial proportions and a long-face group, as a function of their nasal-oral ratio during a period of time in the laboratory when total air flow and nasal air flow were measured. The difference would be the amount of oral air flow. Note three things: (1) one-third of the long face group had low or very low nasal-oral ratios, i.e., were largely mouth breathers; (2) one third of the long face group had high ratios, i.e., were largely nasal breathers; and (3) the majority of the group with normal facial proportions, but by no means

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{{PAGE_93}} all, were largely nasal breathers. It looks as if impaired nasal respiration may be a risk factor for growth in the long face pattern, but does not seem to be its major determinant.

Severity of Nasal Obstruction

There is no doubt that total nasal obstruction, which is very rare in humans, can lead to severe malocclusion. The striking downward-backward rotation of the mandible shown in this cephalometric superimposition shows what happened in a patient with cleft palate and leakage of air into the nose during speech. A pharyngeal flap was placed surgically to reduce the nasal leakage, and it inadvertently completely blocked off the nose.

We have every reason to believe that if partial nasal obstruction is severe enough, there also can be an impact on growth. What we do not know is how severe the obstruction must be to significantly alter growth, and how much effect could be expected from varying degrees of partial obstruction.

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{{PAGE_94}} Detection of Mouth Breathing Often it is presumed that individuals who have their lips separated at rest are breathing through their mouth. If you look like a mouth breather, are you one? Not necessarily, because the rear of the oral cavity can be closed off with the tongue, and 100% nasal breathing is quite possible when the lips are separated. You can’t tell who’s breathing through their mouth just by looking at them.

The best way to characterize the mode of respiration is in terms of the nasal-oral ratio. This requires measuring the amount of air that passes through the nose as a percentage of the total air flow. Both of these things can be measured, using a body plethysmograph that fits around the chest for total air flow, and a flow meter over the nose for nasal air flow.

In a study at UNC, the percentage of nasal respiration in long-face vs. normal adolescents was measured using the nasal mask / body plethysmograph technique. In this graph of the results, note that one quarter of the long face group had less than 40% nasal respiration, while none of the normals had nasal respiration percentages that low. But you also can see that most of the long face group were predominantly nasal breathers. The data suggest that impaired nasal respiration may contribute to the development of long face / open bite problems, but this is not the sole or even the major cause.

{{PAGE_95}} Influence of Respiratory Mode

The second part of the concept that respiratory mode can be a factor in the etiology of malocclusion is that it acts largely by changing head, jaw and tongue posture, and this leads to changes in resting pressures against the dentition. Does that really happen?

Experiments with monkeys have shown that placing an obstruction in the roof of the mouth, so that tongue and jaw posture must be altered, has an effect on both how the jaw grows and on the form of the dental arches. Also in monkeys, it has been demonstrated that blocking the nose so that the monkey has to breathe through the mouth leads to a change in the jaw relationship and affects tooth positions.

In humans, such experiments are not possible, but if posture changes, resting pressures against the teeth would also change. We are left with the same question: how great a change in posture would be needed to generate a significant change in resting pressures? On the average, the postural change in Linder-Aronson’s adenoidectomy patients was quite small. All the data suggest that in the more severely affected individuals, changes would be expected, while there would be little or no effect in the less severely affected ones. How do you determine who’s severely affected enough? We don’t know.

Should a dentist suggest tonsillectomy and adenoidectomy for the snotty-nosed kid with a long face and anterior open bite? When the best data say that nasal obstruction is not the major cause of the facial growth pattern, that wouldn’t be a good idea.

Summary

Summary: Equilibrium Concepts

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{{PAGE_96}} When environmental influences on the developing dentition are considered, the key concept is that the duration of pressure against the teeth is much more important than its magnitude. Examining possible contributors to pressure from this perspective shows that resting pressure and a contribution from metabolic activity in the periodontal ligament are the key items.

Summary: Tongue Thrust? On clinical examination it can seem obvious that a tongue thrust swallow habit creates protrusion of incisors and/or anterior open bite. The problems with that:

• tongue thrust swallow isn’t a habit, it’s a normal developmental stage that may be maintained into the primary school years or later
• tongue pressure during swallow isn’t maintained long enough to affect the position of the teeth
• therapy (usually called myofunctional therapy) to break this “habit” or teach a child to swallow “correctly” doesn’t work

The resting posture of the tongue may be different in children with a tongue thrust swallow. If this tongue posture is necessary in order for the child to breathe, trying to change it isn’t going to work either.

Summary: Mouth Breathing? Total nasal obstruction that is maintained all the time (which is very rare in humans) can lead to marked changes in the pattern of growth and severe malocclusion. The long face, open bite (adenoid facies) type of malocclusion is:

• more prevalent in adolescents with a low percentage of nasal respiration
• found most often in adolescents with normal percentages of nasal respiration

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• best considered as possibly due to impaired nasal respiration but only in a minority of the long face children Recommending tonsillectomy and adenoidectomy primarily to improve the pattern of facial growth rarely is a good idea.

Referral to Self-Test Before you take the self-test, be sure to do the assigned reading for this module: pages 133-145 in the 5th ed. of Contemporary Orthodontics; pages 145-158 in the 4th ed. Then use the self-test to be sure you have understood this important topic, and be prepared to discuss it with your seminar leader.

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