Level I Growth and Development - Unit C_formatted_text

Level I · GROWTH AND DEVELOPMENT

Unit C

Eruption of Permanent Teeth · Process of Tooth Eruption · Physical Growth at Adolescence · Patterns of Facial Growth · Maturational Changes

Proffit Instruction — generated for offline reference

Level I Growth and Development — Unit C · 1 / 191

Contents

1. Eruption of Permanent Teeth
2. Process of Tooth Eruption
3. Physical Growth at Adolescence
4. Patterns of Facial Growth
5. Maturational Changes

Level I Growth and Development — Unit C · 2 / 191

1. Eruption of Permanent Teeth

Dental Age: Eruption Sequence and Timing

Introduction

In this program we will discuss the pattern and sequence of eruption of the permanent teeth. We also will observe the position of the permanent teeth as they erupt and some of the concurrent dento-alveolar changes that occur.

In addition to viewing the program, read pages 74-91 (5th ed) or 86-105 (4th ed) in Contemporary Orthodontics.

Learning Objectives

After viewing this program, you should be able to:

• define dental age and describe the normal eruption timing and sequence for the permanent teeth
• determine the dental age of a child age 6-16 from panoramic radiographs
• describe common variations in eruption and their clinical significance
• discuss the dimensional changes of the dental arches from the primary dentition to age 18
• indicate the sources of space to overcome incisor liability

Dental Age: Definition

Dental age is a developmental age scale, based on three things:

• The amount of crown or root development of the permanent teeth
• The degree of root resorption of the primary teeth
• Which teeth have erupted

The age at which various aspects of tooth development and eruption can be given in tables showing the age at which various things occur. Tables of this type are important background for what we really want you to know - how to estimate an individual child’s dental age from radiographs.

As a beginning, look carefully at these two charts - but you don’t have to memorize them, and you shouldn’t be surprised to see small differences in charts derived from different data sets. There is enough variation among normal children that the average age tends to vary a little among authors. It’s not enough to make a clinical difference, and not enough to worry about. For what it’s worth, the numbers here are pretty modern, and the numbers in some other texts are based on studies from many years ago.

<table border="1">
  <tr>
    <th colspan="8">Chronology of Tooth Development, Permanent Dentition</th>
  </tr>
  <tr>
    <td rowspan="2">Tooth</td>
    <td colspan="2">CALCIFICATION BEGINS</td>
    <td colspan="4">CROWN COMPLETED</td>
  </tr>
  <tr>
    <td>Max.</td>
    <td>Mand.</td>
    <td>Max.</td>
    <td></td>
    <td>Mand.</td>
    <td></td>
  </tr>
  <tr>
    <td>Central</td>
    <td>3 mo.</td>
    <td>3 mo.</td>
    <td>4½ yr.</td>
    <td></td>
    <td>3½ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>Lateral</td>
    <td>11 mo.</td>
    <td>3 mo.</td>
    <td>5½ yr.</td>
    <td></td>
    <td>4 yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>Canine</td>
    <td>4 mo.</td>
    <td>4 mo.</td>
    <td>6 yr.</td>
    <td></td>
    <td>5¾ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>1st Premolar</td>
    <td>20 mo.</td>
    <td>22 mo.</td>
    <td>7 yr.</td>
    <td></td>
    <td>6¼ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>2nd Premolar</td>
    <td>27 mo.</td>
    <td>28 mo.</td>
    <td>7¾ yr.</td>
    <td></td>
    <td>7¼ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>1st Molar</td>
    <td>32 wk. in utero</td>
    <td>32 wk. in utero</td>
    <td>4¼ yr.</td>
    <td></td>
    <td>3¾ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>2nd Molar</td>
    <td>27 mo.</td>
    <td>27 mo.</td>
    <td>7¾ yr.</td>
    <td></td>
    <td>7¼ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>3rd Molar</td>
    <td>8 yr.</td>
    <td>9 yr.</td>
    <td>14 yr.</td>
    <td></td>
    <td>14 yr.</td>
    <td></td>
  </tr>
  <tr>
    <td colspan="8"><b>Image 1, Permanent Dentition Calcification:</b><br>Knowing these average values will help you estimate dental age.</td>
  </tr>
</table>
 
<table border="1">
  <tr>
    <th colspan="8">Chronology of Tooth Development, Permanent Dentition</th>
  </tr>
  <tr>
    <td rowspan="2">Tooth</td>
    <td colspan="2">ERUPTION</td>
    <td colspan="4">ROOT COMPLETED</td>
  </tr>
  <tr>
    <td>Max.</td>
    <td>Mand.</td>
    <td>Max.</td>
    <td></td>
    <td>Mand.</td>
    <td></td>
  </tr>
  <tr>
    <td>Central</td>
    <td>7¼ yr.</td>
    <td>6¼ yr.</td>
    <td>10½ yr.</td>
    <td></td>
    <td>9½ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>Lateral</td>
    <td>8¼ yr.</td>
    <td>7½ yr.</td>
    <td>11 yr.</td>
    <td></td>
    <td>10 yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>Canine</td>
    <td>11½ yr.</td>
    <td>10½ yr.</td>
    <td>13½ yr.</td>
    <td></td>
    <td>12¼ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>1st Premolar</td>
    <td>10¼ yr.</td>
    <td>10½ yr.</td>
    <td>13½ yr.</td>
    <td></td>
    <td>13½ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>2nd Premolar</td>
    <td>11 yr.</td>
    <td>11¼ yr.</td>
    <td>14½ yr.</td>
    <td></td>
    <td>15 yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>1st Molar</td>
    <td>6¼ yr.</td>
    <td>6 yr.</td>
    <td>10½ yr.</td>
    <td></td>
    <td>10½ yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>2nd Molar</td>
    <td>12½ yr.</td>
    <td>12 yr.</td>
    <td>15¾ yr.</td>
    <td></td>
    <td>16 yr.</td>
    <td></td>
  </tr>
  <tr>
    <td>3rd Molar</td>
    <td>20 yr.</td>
    <td>20 yr.</td>
    <td>22 yr.</td>
    <td></td>
    <td>22 yr.</td>
    <td></td>
  </tr>
  <tr>
    <td colspan="8"><b>Image 2, Permanent Dentition Eruption:</b><br>Knowing these average values will help you estimate dental age.</td>
  </tr>
</table>
 
# Dental Age 6
 
Now let’s begin with a series of drawings that show the normal pattern of erupted and unerupted teeth at various dental ages, beginning with age 6 and the first appearance of permanent teeth. Remember that dental age correlates with chronologic age but often varies from it.
 
The first permanent tooth to erupt is, as a rule, is a mandibular central incisor, but it may be a mandibular first permanent molar or occasionally a maxillary first permanent molar. Usually the mandibular molar will precede the maxillary molar. These teeth erupt so near the same time that it is quite within normal variation for either of the first molars to slightly precede the mandibular central incisors or vice versa.
 

Dental Age 6: Radiograph Resorption of roots of primary teeth and the amount of root development of permanent teeth are the other characteristics that differentiate dental age.

At age 6, the erupting mandibular central incisors and the first molars in both arches have completed between 2/3rds and 3/4ths of their root development, which is typical of root development of all permanent teeth at the time they first appear in the mouth. As we will discuss in more detail in a later module, the amount of root formation when a tooth emerges is affected by the extent to which the permanent tooth was uncovered when its primary successor was lost. The maxillary central and lateral incisors have completed less than half their root development, and the canines and premolars are still in the crown stage of development.

Resorption of primary roots begins about the time permanent root development starts, so at age 6 you would see resorption of maxillary primary central and lateral incisor roots and mandibular primary lateral incisor roots.

Dental Age 6: Summary To summarize the characteristics of dental age 6: Erupting teeth: - mandibular central incisors - mandibular first molars - maxillary first molars Root development of permanent tooth / resorption of primary roots apparent - maxillary central incisors - maxillary and mandibular lateral incisors

Dental Age 7

Level I Growth and Development — Unit C · 6 / 191

In the second stage of eruption of permanent teeth, at dental age seven, the maxillary central incisors and the mandibular lateral incisors erupt. These teeth arrive about a year behind the mandibular central incisors. At dental age seven, the maxillary lateral incisor has advanced root formation but has not yet erupted. The canines and premolars are still in the stage of crown completion or just at the beginning of root formation.

Dental Age 7: Summary To summarize the characteristics of dental age 7: Erupted teeth: - mandibular central incisors - maxillary and mandibular first molars Erupting teeth: - mandibular lateral incisors - maxillary central incisors Root development / resorption of primary roots - maxillary lateral incisors

Dental Age 8 Dental age eight is characterized by the eruption of the maxillary lateral incisors, the beginning of root formation of canines and premolars, and the beginning of root resorption of primary canines and molars.

Image 1, Dental age 8: Maxillary lateral incisors erupting. Image 2, Dental age 8: Can you identify the key indicators of dental age 8 in this image?

Dental Age 9-10

After the maxillary laterals come in, no other permanent teeth erupt for the next two or even three years. Dental ages nine and ten are characterized by the presence of the permanent central and lateral incisors and permanent first molars in both arches, and by the presence of the primary canines, first molars, and second molars.

At dental age nine, root development on the premolars is obviously occurring. Approximately one-third of the root of the mandibular canine and the mandibular first premolar are completed. Root development is just beginning, if it has started at all, on the maxillary canines and maxillary and mandibular second premolars.

The difference in dental age nine and dental age ten (not shown in a separate drawing) would be the greater degree of root resorption of primary canines and molars, and greater root development of their permanent successors. Another indication of dental age 10 would be completion of the roots of the mandibular incisor teeth and near-completion of the roots of the maxillary lateral incisors.

Image 1, Dental age 9: Root development takes place at dental age 9, but eruption does not. Image 2, Dental age 9: Can you identify the hallmarks of dental age 9 in this image?

Dental Age 11 Dental age eleven is characterized by the beginning of eruption of canines and premolars. At this time, the mandibular canine, mandibular first premolar, and maxillary first premolar all erupt more or less simultaneously. In the mandibular arch, sometimes the first premolar erupts before the canine, but sometimes the canine is ahead. The odds slightly favor the prior emergence of the first premolar but not by much. In the maxillary arch the first premolar erupts well ahead of the canine or second premolar.

Dental Age 11: Radiograph

The important point to remember is that dental age 11 calls for the presence of the canine and first premolar in the mandibular arch but only the first premolar in the maxillary arch. The primary canine and second molar still are present in the maxillary arch. The primary second molar is the only remaining primary tooth in the mandibular arch.

At this time, root development is well advanced on the maxillary canine and second premolar and the mandibular second premolar. Root resorption is apparent on maxillary primary canines and second molars and on the mandibular primary second molars. Root formation beyond half to 2/3rds of the root is a signal of impending eruption, and an erupting tooth usually is in the mouth by the time 3/4ths of the root is completed.

The roots of the incisors were not complete when they first erupted. It takes about 2 years for root formation to be completed after a tooth erupts, so by dental age 11 the roots of the incisors in both arches should be completed.

Level I Growth and Development — Unit C · 10 / 191

Dental Age 12

At dental age twelve, the remaining succedaneous permanent teeth erupt into the mouth. Succedaneous refers to permanent teeth that have a primary predecessor. Thus, canines and premolars are succedaneous teeth, but permanent molars are not.

In addition, at dental age twelve the second permanent molars in both arches are nearing eruption. Typically the last succedaneous teeth erupt prior to the eruption of the second molars, but sometimes the second molars erupt before the maxillary canine and/or the mandibular 2nd premolar. That’s well within normal variation.

Dental Age 12: Radiograph

Although the third molar tooth buds are not shown in the dental age twelve drawing, often it is possible to see the early beginnings of these teeth at that stage. You cannot count on third molars being absent if their crowns have not begun to form at dental age twelve, but usually these teeth are obviously present by then.

Note that the lower second molars and second premolars should arrive in the mouth at the same time, with the upper second molars a little behind. That’s well within normal variation.

Dental Ages 13-15

It would be possible to calculate the dental age of a child as 13 or 14 based on an increasing but not quite complete degree of root formation in the maxillary and mandibular canines, second premolars, and second permanent molars.

By dental age 15, the roots of all permanent teeth except the third molars should be complete, and third molars should be apparent on the radiographs even though they will not have erupted.

Eruption Sequence / Timing: Summary

Clinically, it is more helpful to remember how teeth erupt in groups to indicate dental age than to try to memorize a table with precise ages for the eruption of individual teeth.

You have to know what teeth erupt when, and how much root formation and primary root resorption exist at each age, but it’s easier to take normal variation into account if you remember that the permanent teeth erupt in groups and when that happens. What’s the first or the last permanent tooth to erupt? Different sources are likely to give you different answers. It’s really a matter of normal variation. Within a group of teeth that erupt at about the same time, which one was first or last has little clinical significance.

Level I Growth and Development — Unit C · 14 / 191

<table>
    <tr>
        <th>Dental age</th>
        <th>Erupting teeth</th>
    </tr>
    <tr>
        <td>6</td>
        <td>Mandibular central incisors, maxillary / mandibular 1st molars</td>
    </tr>
    <tr>
        <td>7</td>
        <td>Maxillary central incisors, mandibular lateral incisors</td>
    </tr>
    <tr>
        <td>8</td>
        <td>Maxillary lateral incisors</td>
    </tr>
    <tr>
        <td>9-10</td>
        <td>None</td>
    </tr>
    <tr>
        <td>11</td>
        <td>Mandibular canines and 1st premolars, maxillary 1st premolars</td>
    </tr>
    <tr>
        <td>12</td>
        <td>Mandibular 2nd premolars, maxillary canines and 2nd premolars</td>
    </tr>
    <tr>
        <td>12+</td>
        <td>Maxillary and mandibular 2nd molars</td>
    </tr>
</table>
 
# Longitudinal Radiographs
 
## Longitudinal Sequence: Age 6, Dental Age 7+
 
Now let’s look at a series of radiographs of a child who participated in a longitudinal growth study at the University of Kentucky in the 1960s. Longitudinal radiographic data on normal untreated children now are almost impossible to obtain. Fortunately, the dentition still develops very much as it did 50 years ago.
 
This girl was rather advanced for her age in most characteristics, so you might expect her dental age also to be ahead of her chronologic age. In our first panoramic radiograph, our patient K.G. was chronologically six years three months of age.
 
You can note in the radiograph, however, that the maxillary and mandibular central and mandibular lateral incisors are in place, and that the maxillary lateral incisors are close to erupting. Thus, the dental age is seven and a somewhat advanced seven at that. Note the state of crown development of the canines and premolars--root development has not begun at dental age seven. The permanent second molars are also in the stage of crown formation only.
 
Level I Growth and Development — Unit C · 15 / 191
 

Longitudinal Sequence: Age 6 1/2, Dental Age 8 K.G. 4-67 age 6-3

Longitudinal Sequence: Age 6 1/2, Dental Age 8 Four months later, at chronological age six years and seven months, K.G. has reached a classic picture of dental age eight. Her central and lateral incisors in both arches are present and she is at the stage of completion of the crowns of the mandibular canines and first premolars.

Longitudinal Sequence: Age 6 1/2, Dental Age 8 K.G. 8-67 C.A. 6-7 D.A. 8

Level I Growth and Development — Unit C · 17 / 191

In the cephalometric radiograph taken at six years seven months, the movement of the lateral incisors to the same occlusal level as the central incisors, a characteristic of dental age 8, can be observed readily.

Longitudinal Sequence: Age 7, Dental Age 8

The lateral cephalometric radiograph for 6 months later, chronological age seven years one month, gives an excellent view of a patient at a slightly advanced dental age eight.

Note that root formation of the maxillary canine has begun since the previous radiograph was taken, and the canine has started to move down from the very high position where it was seen previously. It is interesting that teeth begin their eruptive movements when root formation starts. They don’t move within the bone until crown formation is completed.

Radiograph showing a lateral skull X-ray with handwritten annotations “K.G. 2-68” and “age 7-1”. Longitudinal Sequence: Age 7 1/2, Dental Age 9

At chronological age seven years, seven months, K.G. has acquired the dental characteristics of dental age nine. The mandibular primary canines have advanced root resorption and will soon be lost. Root formation is beginning on all premolars and the canines. The crowns of the second molars are nearly complete.

Note that in the mandibular arch the first premolar and canine are no longer running neck and neck. It is now apparent that the canine in the mandibular arch is going to erupt slightly ahead of the first premolar. That’s normal variation.

Level I Growth and Development — Unit C · 19 / 191

Longitudinal Sequence: Age 7 1/2, Dental Age 9 (cont.)

In the lateral cephalometric radiograph for the same age, note again the extent of the descent of the maxillary canine. This tooth is often called the “eye tooth” by lay people because it begins its development so far superiorly that it does in fact seem to be in the floor of the orbit in some instances.

Long before it erupts it is moving in a path downward and somewhat forward toward the position where it will eventually emerge.

Level I Growth and Development — Unit C · 20 / 191

Longitudinal Sequence: Age 8, Dental Age 10

At chronological age eight years two months, K.G. has the dental characteristics of age ten. Her mandibular permanent canines are now erupting. Her maxillary and mandibular first premolars are nearly ready to erupt. Note that root resorption is far advanced on the primary first molars and that about half the root is completed on the first premolars.

Level I Growth and Development — Unit C · 21 / 191

Longitudinal Sequence: Age 8, Dental Age 10 (cont.) In the lateral cephalometric radiograph for age eight years and two months, the characteristics of dental age ten also are apparent. Note the advancing root development on the premolars, particularly the maxillary second premolar, which is the best clue to the dental age in this particular radiograph. Continuing descent of the maxillary canine can be noted.

Longitudinal Sequence: Age 8 3/4, Dental Age 11 At age eight years nine months, K.G. has the dental characteristics of age eleven. Note that mandibular canine and first premolar have erupted. The mandibular canines and first premolars have erupted, and the maxillary first premolars are just about to erupt. The maxillary canines and the second premolars in both arches have less than half their roots completed and so would not be expected to erupt for another 6-12 months, but root resorption is now advanced on the maxillary and mandibular primary second molars. Root formation is now well advanced also on the permanent second molars in both arches.

Longitudinal Sequence: Age 9 1/4, Dental Age 11+

At age nine years three months, K.G. has the characteristics of dental age eleven or even eleven plus. Only her second premolars in both arches have not erupted. Her canines are in place in both arches. The degree of root formation of the mandibular second premolars is what you would expect to see just prior to the time of normal eruption, with about half the normal root length completed. There is very little root retaining the scond primary molars at this time, and that plus the amount of root formation of the second premolars pretty much guarantees that the primary molars will be lost soon and the second premolars will erupt with no delay. It looks as if the second permanent molars will be erupting before long, but the degree of root completion indicates that they will be a few months behind the second premolars.

Level I Growth and Development — Unit C · 23 / 191

Radiograph showing a lateral cephalometric view of a patient’s maxilla and mandible, with handwritten notes indicating ‘K.G. 3-70’, ‘C.A. 9-3’, ‘D.A.’, and ‘11’. Longitudinal Sequence: Age 9 1/4, Dental Age 11+ (cont.)

In the lateral cephalometric radiograph for age 9-3, compare the degree of root development on the maxillary first premolar, second premolar and canine.

The first premolar is in the arch and has been there for some months. The second premolar is about to erupt, yet the amount of root formation on the first premolar does not seem greatly in excess of that of the second.

Is this normal variation? Would it lead to change your mind about the dental age?

Level I Growth and Development — Unit C · 24 / 191

Radiograph showing a lateral skull X-ray of a child with dental age 11+ at chronological age 9 1/4, labeled “K.G. 3-70 age 9-2”, displaying mixed dentition and root development patterns consistent with normal variation for dental age 11.

Longitudinal Sequence: Age 9 1/2, Dental Age 11+

This cephalometric radiograph taken 5 months later gives an excellent view of the amount of root completion which is normally associated with eruption. Note that the mandibular second permanent molars are coming into occlusion with about half of their root completed. The mandibular second premolar is also up into occlusion with about half to two-thirds of its root completed.

For the first time, the developing third molars can be seen distal to the second molars. In the mandibular arch, the third molar is at the stage of beginning calcification of the cusp tips, while in the maxillary arch the third molar has progressed slightly further.

Level I Growth and Development — Unit C · 26 / 191

Longitudinal Sequence: Age 9 3/4, Dental Age 11+

At chronological age nine years, nine months, our patient K.G. has almost all the dental characteristics of age twelve. One second premolar has not erupted on the maxillary right side. All the other permanent teeth are in place except the maxillary second permanent molars. The roots of the canines and premolars are not yet completed.

Longitudinal Sequence: Age 10, Dental Age 12

At chronological age ten years, two months. K.G. has reached a classic picture of dental age twelve. All the succedaneous teeth are in place and root formation is well advanced, though not complete. The mandibular second molars have erupted into occlusion while the maxillary second molars have not yet moved down to the occlusal plane.

It is normal for the mandibular second molars to precede the maxillary second molars—but remember that the reverse also is normal, just not as likely. In general, mandibular teeth tend to erupt a little ahead of maxillary teeth that are in the same group.

Longitudinal Sequence: Age 10 1/2, Dental Age 12+

At chronological age ten years eight months, the dental characteristics are closer to age thirteen than age twelve. Although the maxillary second molars have not quite erupted into position their root development is far advanced.

Note that the roots of the succedaneous teeth in the mandibular arch are now nearly complete—an event that should occur about two years after a tooth comes into the mouth.

Longitudinal Sequence: Age 11, Dental Age 14

At chronological age eleven years two months, K.G. would grade as dental age fourteen based on the completion of the roots of the succedaneous teeth in both arches and the advanced root development o the second molars.

Slow development of the maxillary and mandibular third molars can be seen at this stage.

Longitudinal Sequence: Age 11, Dental Age 14 (cont.)

Level I Growth and Development — Unit C · 29 / 191

The lateral cephalometric radiograph taken at chronological age eleven years two months, dental age fourteen, was not well oriented - two shadows of the lower border of the mandible can be seen.

This radiograph does allow us to point out, however, that the eruption of any tooth (with the possible exception of the third molars) is not complete at the time it first makes occlusal contact.

Vertical growth is continuing rapidly at this time. The mandible is growing away from the maxilla, creating a space into which teeth have to erupt in order to maintain their contact. Thus, although incisor teeth erupted into occlusion at age six, seven and eight, these teeth have been continuing to erupt during the whole period of replacement in the rest of the primary dentition with the permanent teeth.

Longitudinal Sequence: Age 11, Dental Age 14 (cont.)

Take a look at the level of the root apex of the maxillary central incisor as related to the height of the palatal vault. The palatal vault shows up in the radiograph as a horizontal line just above the dentition. At one time the root of the maxillary central was at the height of the vault. Now it is well below it, even though the vault is remodeling downward.

That tooth has erupted 6 or 8 more millimeters during the five years since it first reached the occlusal plane. It had to, to stay in occlusion.

Level I Growth and Development — Unit C · 30 / 191

Longitudinal Sequence: Age 11 1/2, Dental Age 14+ The panoramic radiograph of K.G. taken at chronological age eleven years, seven months is a classic for dental age of nearly (but not quite) fifteen. Note that root completion of all teeth except the second molars has occurred.

Third molars are in the crown stage of development. These teeth would be advanced enough to erupt, based on the information in this radiograph, in approximately four years. Whether there will be enough room to accommodate them so that they can erupt will be determined by the extent of mandibular growth from this time forward. Unless the mandible grows longer, it is obvious that there will not be enough space to accommodate the developing mandibular third molars.

In the modern world, often there isn’t enough space, so impacted third molars are a common occurrence. Did you have enough room so yours could erupt, or were they impacted?

Level I Growth and Development — Unit C · 31 / 191

Longitudinal Sequence: Stages of Eruption

Our review of the eruption of the permanent teeth so far has emphasized the normal and usual pattern. In situations when two permanent teeth normally erupt about the same time, it really makes no sense to assign one tooth priority over the other, which is why you’re encouraged to think about stages of eruption rather then memorizing average eruption times for individual teeth.

But these stages are something you have to know!

Stage	Permanent Teeth Erupted
1	Mandibular Central Incisors Maxillary/Mandibular First Molars
2	Maxillary Central Incisors Mandibular Lateral Incisors
3	Maxillary Lateral Incisors
4	Mandibular Canines Maxillary/Mandibular First Premolars
5	Maxillary Canines Maxillary/Mandibular Second Premolars Maxillary/Mandibular Second Molars

Common Variations in Tooth Eruption

Eruption Variations

Level I Growth and Development — Unit C · 32 / 191

There are several reasonably normal variations in eruption sequence which are unusual enough to be outside the common path but which have clinical significance and which you should recognize. These are:

• eruption of second molars ahead of premolars in the mandibular arch
• eruption of canines ahead of premolars in the maxillary arch
• asymmetries in eruption between right and left sides.

Second Molars Before Premolars

Observe the eruption pattern of the premolars in this radiograph. Do you see anything abnormal? Look at the lower arch. The canine and the first premolar are ahead of the second premolar. That looks all right. Look at the maxillary arch. The first premolar is ahead of the canine and second premolar which seem to be developing at the same pace. That looks all right.

So where is the variation? Notice that the mandibular second molars have erupted and are in occlusion well ahead of the eruption of the mandibular second premolar.

Second Molars Before Premolars: Treatment

This is not an abnormal sequence, but it is an unfortunate sequence in a dental arch where the amount of room to accommodate the teeth is somewhat marginal. The eruption of the second molar ahead of the second premolar tends to decrease the space for the second premolar and may lead to its impaction.

In the case of this girl, a mandibular orthodontic appliance was used to hold the molars back and to open up some space into which the second premolars could erupt. Note that the one on her left has already erupted successfully. That would not have been possible for this girl without some dental intervention.

Level I Growth and Development — Unit C · 33 / 191

Second Molars Before Premolars: Consequences Compare also for this girl the potential amount of space for developing mandibular third molars with that which you saw in the previous page.

In the previous case, there just might be enough mandibular growth to allow the eruption of the third molars normally. It can already be said for this patient that there was barely enough room to accommodate 28 permanent teeth and there is no hope for 32.

Maxillary Canines with Premolars Another unusual but potentially significant normal variation is eruption of the maxillary canines essentially synchronously with the maxillary first premolars, like the normal situation in the mandibular arch. This can be seen happening for this patient in the radiograph.

Maxillary Canines with Premolars: Dental Age? Labial displacement of the maxillary canines often occurs when there is just not enough room to accommodate the permanent teeth, but labially displaced and prominent maxillary canines also can be an unfortunate consequence of an eruption pattern in which the canines simply arrive on the scene sooner than they should.

Based on what you know from what we have talked about up to this time, what’s the dental age of the patient?

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Maxillary Canines with Premolars: Treatment

Your answer should be 11 or 12 years. The upper arch is more like age 11, the lower arch more like age 12 (image 1). The atypical position of the maxillary canines would not be grounds for changing the dental age of the patient based on the other characteristics.

The intraoral photos show the alignment and occlusion of the same patient a few months later, when it was apparent that there was not enough space for the canine (image 2), and after orthodontic treatment to expand the arches and create space (image 3). Despite the fact that there was not a crowding problem, just an unfortunate eruption sequence, things would not have looked that good without some orthodontic treatment.

What would you say the dental age was at the end of treatment (image 4)? Do you see any abnormality with dental development?

Hopefully you would say dental age 15. Did you notice the congenitally missing third molar?

Asymmetry: Impaction

A third variation in eruption sequence is an asymmetry in position of teeth on two sides of the arches. Note that both maxillary canines have completed their root formation, with the right one in place in the dental arch and the left one impacted. (The panoramic radiograph is oriented as if you were looking at the patient, so the right side of the radiograph is the patient’s left side). It takes about two

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years after a tooth erupts for the root to be completed—so an asymmetry in canine eruption has existed for about that length of time. Now notice that the maxillary left primary canine is retained, and that the permanent canine is mesial to it. If an alert dentist had noticed 18 months ago that the crown of the permanent canine had missed its connection with the root of the primary canine and extracted the primary tooth at that time, there would have been a good chance that the permanent tooth would have moved toward the extraction space and erupted in normal position. Modest asymmetry between eruption on the right and left sides occurs in almost every child, but more than 6 months difference can be clinically significant and should be investigated. Impaction of maxillary canines often—of course not always—can be prevented by timely intervention. A few months of difference in eruption of the same teeth on the right and left sides probably is just normal variation. More than a few months is likely to be due to some sort of abnormality.

Asymmetry: Retained Primary Molars For this patient, notice also what is happening with the second premolars. What does the amount of root formation of the lower second premolars tell you? That’s right, with that amount of root formation these teeth should be close to erupting. The right one (on left of the x-ray) is close, the left one is a good way below where you’d expect it to be. What’s the problem? This patient is 14, and the amount of root formation makes her dental age 13 or 14. The primary second molars have not had the normal amount of root resorption and have not been lost on schedule. The lower right 2nd primary molar still has a lot of its distal root remaining and so is still retained, and much of the roots of the lower left 2nd primary molar are still there. The orientation of the upper second premolars makes their root formation harder to judge, but the maxillary second primary molars also are not resorbing normally. Why is the lower left 2nd primary molar below the adjacent teeth? It has become ankylosed (fused to the bone so that it can’t erupt any more, and as the other teeth erupt past it, it appears to be submerging.

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There is a moral to this story: the less the situation looks like the normal sequence of eruption, the more likely it is that something other than normal variation will be observed in the patient. What treatment do you think this patient needs now?

Asymmetry: Treatment

That’s right, retained primary teeth have been extracted, space for the maxillary canine has been opened, and the impacted maxillary left canine now has a bonded attachment so that it can be brought down into position on that side.

After the ankylosed mandibular second primary molar was removed, the second premolar erupted on its own, and brought alveolar bone with it. Then an attachment was bonded to it to bring it the rest of the way to its proper position. Whether it moves occlusally on its own or is moved orthodontically, a tooth brings bone with it. That’s the only way to get an increase in height of the alveolar ridges.

The “Ugly Duckling” Stage

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Ugly Duckling Stage: Diastema

Let’s now turn to another aspect of eruption—incisor positioning—and examine what is called the ugly duckling stage of development of the dentition. Dr. Broadbent, one of the early students of cephalometric x-rays, coined that term. The ugly duckling, of course, turns into the beautiful swan. The ugly duckling stage of normal development has come to refer to two different situations that often are self-correcting but may require treatment in their most severe form. First, as we have seen from the average graphs of space dimensions in the arches, there should be no excess space in the maxillary arch at the time the permanent central incisors erupt. Sometimes, however, in children who are developing normally, excess space will continue to be present at this stage, so that there is a space between the permanent maxillary central incisors. This girl has such a space, called a midline diastema, and also has severely distally-tipped maxillary lateral incisors.

Image 1, The “ugly duckling stage”: The space between the maxillary central incisors is called a diastema and characterizes this normal stage of development.

Image 2, The “ugly duckling stage”: The space between the maxillary central incisors is called a diastema and characterizes this normal stage of development.

Ugly Duckling Stage: Self-Correction

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A midline diastema like this one tends to close as the lateral incisors erupt. Notice in the panoramic radiograph that the permanent lateral incisors have nearly completed their eruption but their crowns are tipped distally with a good bit of space remaining in the incisor area. Further closure of the diastema is likely to occur when the maxillary canines erupt, and the lateral incisors tend to straighten up as the canines move toward the occlusal plane.

No treatment to align the incisors and close spaces is indicated (unless there are special circumstances) until the canines erupt. The guideline is that a 2 mm diastema almost always closes on its own, and a larger diastema is increasingly at risk of not closing completely without treatment. Eruption of the canines is the corrective factor.

We need to emphasize at this point that the ugly duckling stage is common enough to be noted as a variation of the usual pattern of eruption, but by no means should it be expected to occur for every child. Some children go through that stage, most don’t.

Image 1, The “ugly duckling stage”: The space between the maxillary central incisors - if it is less than 2mm - will self-correct with eruption of the lateral incisors and canines.

Image 2, Radiograph of the same patient: Note that the lateral incisors still have not completely erupted and that the canines still are high above the occlusal plane.

Ugly Duckling Stage: Patient Example

This young lady, age eleven at the time of the photographs were taken, demonstrates the facial appearance of the ugly duckling stage. She is a pretty girl until she smiles. The smile leaves a bit to be desired, both from her own point of view and the point of view of her parents, who would like the dentist to do something about it.

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Ugly Duckling Stage: Patient Example (cont.) A close-up view reveals more clearly the flaring (distal tipping of the crowns) and spacing of the four maxillary incisors. There is not enough room to accommodate the erupting permanent canines.

Measurements of the total amount of space show that there is enough room to accommodate all the teeth. If the canines could force their way into the arch, the position of the incisors would improve and the spaces between them would disappear.

Ugly Duckling Stage: Patient Radiographs Radiographs of this girl, as is usual in the ugly duckling situation, reveal canines in fairly good position to assume their place in the arch. But as we saw earlier, sometimes the crown of the

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permanent canine is not positioned so that it contacts the root of the primary canine, and that can become a problem.

Even though the permanent canines are in good position for this girl, notice the amount of root development that has occurred. Remember that a permanent tooth ought to erupt well before its root is completed. Certainly more than 3/4ths of the root of these maxillary canines has been completed without either being able to erupt. That reduces the chance of self-correction of the incisor position.

Image 1, Periapical radiograph of erupting canine: Note the root development of the permanent canine and the retained primary canine. Image 2, Periapical radiograph of erupting canine: Note the root development of the permanent canine and the retained primary canine.

Ugly Duckling Stage: Treatment

As part of treatment for this girl, it was decided to go ahead and extract both the primary canines and the primary second molars, which also were nearly ready for exfoliation.

At the time the primary canine was extracted, the permanent canine underlying it was exposed to make sure that there was no mechanical barrier to its eruption.

It looked clinically as though the canine had wedged itself between the lateral incisor and the first premolar.

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Ugly Duckling Stage: Resoloution

After extraction of the maxillary primary canines and primary second molars, which provided some space into which the first permanent premolar could shift distally, the permanent canines did nearly erupt into position. Only some limited and brief orthodontic treatment was needed to get all the teeth correctly aligned.

Notice how much the space between the lateral and the central has closed up, and how the inclination of the lateral incisors has changed. Most of that happened before the orthodontic treatment period.

In this case the ugly duckling stage was so extreme that it was not possible for the teeth to completely align themselves without some help. But the amount of improvement that occurred just with the continued eruption of the permanent canines does reveal how their eruption can close spaces between the maxillary incisors.

Spacing/Crowding in the Developing Dentition

Lingual Tooth Buds

Let’s now focus our attention on a view 90 degrees to the ones we have been using and look down on the dental arches from the occlusal. If we do this for a child who is still in the primary dentition period, and if we use x-ray vision to pick up the position of the permanent incisors, we will see that in both arches the permanent tooth buds lie lingual to the primary incisors as well as below them.

Lingual Tooth Buds: Displaced Lateral Incisors

This is particularly true of the lateral incisors, which are positioned more lingually than the centrals or the canines. The result is a tendency for the permanent lateral incisors to erupt somewhat lingual to their ideal position in the dental arch, even in children who have normally shaped dental arches and normal spacing within the arches.

Note the position of the lateral incisors in the mandibular arch in this girl. They are a perfect example of erupting lingually, in the same position as the tooth bud. Mild irregularity of the mandibular incisors, of the magnitude pictured here, is normal at age 7 to 8 when the permanent incisors and first molars have erupted but the primary canines and molars are retained.

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Lateral Incisor Displacement: Mandibular Arch

If there is a severe shortage of space to accommodate all of the permanent incisors, as in this patient, the tendency for the lateral incisors to be displaced lingually is accentuated and the central incisors may rotate or flare facially as the lateral incisors try to erupt.

Lateral Incisor Displacement: Maxillary Arch The same situation applies in the maxillary arch. The lateral incisors tend to erupt lingual to the central incisors and canines because of the lingual position of the tooth bud, and this is accentuated when there is not enough space for all the permanent teeth to be well aligned. In this case, it is also obvious that at least the left maxillary canine is going to be displaced from its normal position because there isn’t enough room for it.

The canine lies more directly above the primary canine than does the lateral or the central. If there isn’t enough room in the dental arch, it can go either lingually or labially, but most of the time it goes out to the labial.

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Image 1, Developing dentition: The maxillary incisors are likely be displaced lingually with crowding since the tooth bud lies lingual.

Image 2, Occlusal view of developing maxillary dentition: The maxillary incisors are likely be displaced lingually with crowding since the tooth bud lies lingual.

Normal Primary Spacing

Notice in this slide of a normal six year old the generalized spacing among the incisor teeth. This arrangement of the primary incisor teeth with the gaps between them may not be very pretty but it is the normal. You can see that where the mandibular primary central has exfoliated, there is enough room for the larger permanent central incisor.

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Image 1, Normal six-year old girl: Note the spacing of the primary incisors, which is normal and desirable. Image 2, Intraoral view: Note the spacing of the primary incisors, which is normal and desirable. Image 3, Maxillary arch: Note the spacing of the primary incisors, which is normal and desirable. Image 4, Mandibular arch: Less spacing is present, which is less desirable.

Absence of Normal Primary Spacing All dentists have the experience of dealing with a mother who is very concerned because now that her child’s permanent incisors have begun to erupt, they are crowded and irregular. (Even if you only treat adults, you’ll still get questions about that—you’ll just get them at the party or after the

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meeting). The mother says, “Jimmy had such beautiful baby teeth.” What mother means is that Jimmy’s primary incisors were in contact all the way across without the normal spacing. If you see this in a five year old you can be sure that you are looking at future crowding. Beautiful primary teeth with no spacing in the primary incisor region is an abnormal, not a normal finding. Normal looks like the child in the previous screen with the spacing of primary incisors.

Importance of Primary Incisor Spacing

The permanent incisor teeth are considerably larger than the primary incisors they replace. The mandibular permanent central incisor for instance, is about 5 ½ mm in width, while the primary central incisor is about 3 mm in width.

The extra space for the larger tooth has to come from somewhere. Since there is no growth of the bone that supports the teeth in the incisor area, the additional space has to come primarily from pre- existing spacing of the primary incisors.

Therefore, spacing between the primary incisors is not only normal, it is important so that there will be enough room for the permanent teeth to erupt.

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Primate Spaces

Usually the spacing in the primary incisor region is distributed among all the incisors, but the spaces distal to the maxillary lateral incisor and distal to the mandibular canine are particularly important.

These spaces, indicated by the arrows on the drawing, are called the “primate spaces” because permanent spaces appear in the dentition here in many mammals, including the higher apes whose dentition is similar to humans.

Average Amount of Primary Space in Each Arch

The spacing situation in the primary dentition can be diagrammed as shown in this picture.

On the average 28.8 mm of space is available in the maxillary incisor segment from lateral incisor across to lateral incisor. This is 2.6 mm more than the amount of space that would be necessary to accommodate the primary teeth. In the mandibular incisor segment, 22.3 mm of space is available on average, and that is 1.1 mm more than is required.

Incisor Liability With the eruption of the permanent central incisors in both arches, the situation changes markedly as far as spacing is concerned, since these permanent teeth are so much larger than the primary teeth they replace.

When the mandibular central incisors erupt, on average there is actually 0.1 mm less space available in the incisor segment than is needed to accommodate the teeth. Notice that the size of the mandibular primate space has decreased, presumably as the primary canines were tipped distally.

This lack of adequate space across the incisors is called “incisor liability” because the permanent incisors are likely to become at least slightly malaligned as they erupt.

+0.3mm +0.4mm +0.3mm

Overcoming Incisor Liability in the Mandibular Arch

In this slide of a child whose lower incisors have erupted, notice that the spacing in the anterior segment of the dental arch has disappeared. This is the normal situation at the time that the mandibular incisors erupt. The mandibular incisors take up pretty much all the excess space in the lower arch that was contributed by spacing of the primary dentition, and the primate space distal to the primary canines closes as the primary canines tip distally to make more room for the incisors.

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Maxillary Spacing

In the maxillary arch, the situation is somewhat more favorable. There is still, on the average, about 0.4 mm available after the permanent centrals erupt than is needed to align the teeth. For that reason, a little space between these permanent teeth when they first come into the arch is perfectly normal.

Maxillary Lateral Incisor Eruption In the absence of an ugly duckling pattern in the maxillary arch, the space situation becomes really tight when the lateral incisors erupt. On the average, there is just enough room to accommodate them. There isn’t any excess space nor should there be any particular crowding.

Mandibular Lateral Incisor Eruption In the mandibular arch, however, when the lateral incisors erupt the primate space decreases even more, and on the average there is 1.6 mm less space to accommodate the four mandibular incisors than would be required to place them in perfect alignment.

In other words, a period when the mandibular incisors are slightly crowded is a normal developmental stage, even if there will eventually be enough room to accommodate the incisors in good order. Normal variation says that in some children, there will be just enough room, and in others mild irregularity will be present.

Changes in Space for the Incisors Over Time Another way to show the changes in space in the incisor segment is to plot space vs time with a graph that has zero as the midpoint. The zero point is the point of balance between the size of the teeth and the amount of space available to accommodate them.

Excess space is indicated as plus and inadequate space as a minus. Notice that in the maxilla for males and females, spacing is the rule up until the time that the first permanent molar and the maxillary central incisor erupt.

This clearly shows incisor liability as the negative space that’s available as the incisors erupt: in both arches a period of temporary incisor irregularity is going to be present in many children who eventually will have enough - or almost enough - space for the incisors to be aligned.

Available space - incisor segment Maxilla Mandible

Changes in Space for the Incisors Over Time: Maxilla

In the maxillary arch there is a little excess space at the time the maxillary central incisor erupts. When the maxillary lateral incisor erupts, the space situation stabilizes at about the zero level.

There’s also a difference between boys and girls. Note that for boys (the chart on the left), on the average things go down right to the zero point and stay there. For girls there may be a stage of slight transitory crowding in the maxillary arch.

Available space - incisor segment Maxilla Mandible

Changes in Space for the Incisors Over Time: Mandible

In the mandibular arch, there is not as much spacing to start with as in the maxilla. When the central incisors erupt, the space situation goes down to zero or sometimes below. By the time the lateral incisors erupt, there is a definite shortage of space in both sexes.

With further development, the spacing situation improves, and on the average, by the time the canines erupt the graph is back to zero for boys and close to zero for girls.

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Moorrees Tables This concept of space changes is based on growth studies done in Boston some years ago, which were tabulated and published by Moorrees and other workers at Harvard. More details are given in Moorrees’ book, “The Dentition of the Growing Child”, which is available in the library.

Notice that on the average in girls, the space situation does not quite go back to zero. This simply means that mild crowding of lower incisors can be expected in some girls who otherwise have a perfect dentition. Remember that individuals will deviate from these averages.

Moorrees referred to this tendency toward incisor crowding, even in normal development, as “incisor liability”, and this term now is used to refer to that situation. Since it’s used a lot in what is written about the development of the dentition, you need to know what it means.

Overcoming Mandibular Incisor Liability The key question about developmental spacing / crowding that you must be able to answer is: “Where did the extra space come from to allow mandibular incisors that are crowded during the transitory period at age eight or nine to align themselves again as other teeth come in?”

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Most jaw growth is posterior - away from the dentition - so the easy answer “Jaw growth, of course” is not correct. The extra space comes from three different sources.

Overcoming Mandibular Incisor Liability: Distal Movement into Primate Space

One source of additional space in the lower arch is that the mandibular canine teeth not only widen out a little, but actually move distally in the arch into the primate space as the permanent incisors replace the primary ones.

There is very little, if any, growth in skeletal width of the mandible or maxilla. The distal movement of the canine into a wider part of the arch contributes to the slight inter-canine width increase that usually occurs.

Overcoming Mandibular Incisor Liability: Labial Eruption

A second source of space that is important in resolving the transitory crowding of the incisors is labial (forward) positioning of the permanent incisors relative to the primary incisors.

The primary incisors tend to stand quite upright. As the permanent incisors replace them, these teeth lean forward slightly relative to the primary teeth which they replace.

This of course puts them along the arc of a greater circle, and that typically contributes 1 or 2 mm toward the resolution of the crowding.

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Overcoming Mandibular Incisor Liability: Transverse Growth A third source of space is a slight increase in the width of the dental arch. The inter-canine width increases; not very much, but enough to contribute to the resolution of the crowding situation. On the average in the mandibular arch, about 2 mm of additional space is gained by expansion across the canines. More width is gained by boys than by girls, and so girls continue to have a greater liability to incisor crowding. Notice that arch width increases in all areas. The width increase is not very much, 2 or 3 mm at the most, but that amount of space certainly helps in the resolution of the crowding.

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Summary

In summary: Eruption of the permanent teeth is best described in stages:

• Stage 1, dental age 6: eruption of lower central incisors and all first molars
• Stage 2, dental age 7: eruption of lower lateral incisors and upper central incisors
• Stage 3, dental age 8: eruption of maxillary lateral incisors
• Stage 4, dental age 11: eruption of lower canines and 1st premolars, and upper 1st premolars
• Stage 5, dental age 12:: eruption of lower 2nd premolars, upper canines and 2nd premolars, and all second molars

Dental age is determined from three characteristics, in order of importance:

• which primary and permanent teeth have erupted and which primary teeth have been exfoliated
• the extent of root resorption of primary teeth
• the extent of root formation of permanent teeth

Summary (cont.)

Under normal circumstances, with spaces between the primary anterior teeth, there’s just enough room for the permanent teeth. Some additional space is needed in the mandibular arch, which is gained in three ways:

• intercanine width increase (~2 mm)
• distal movement of primary canines (~1 mm)
• labial movement of permanent incisors (~1 mm)

If there is little or no spacing between the primary teeth, crowding of the permanent teeth is inevitable in both arches.

A mid-line diastema between the upper central incisors tends to close without treatment when the canines erupt, but complete self-correction occurs only if the spaces are not too large (>2 mm may be too big, >4 mm almost surely is).

Self-Test Referral

Now take the self-test to consolidate what you have learned.

Before you do, be sure you have read pages 74-91 (5th ed) or 86-105 (4th ed) in Contemporary Orthodontics. Be sure you understand why your answers were correct or incorrect. This is critically important information that you will need to evaluate your child patients.

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2. Process of Tooth Eruption

Pre-Emergent Eruption

Introduction

Dentists, of course, deal with erupting human teeth all the time—because although tooth eruption declines to a very slow level in adults, it never really stops as long as the very slow jaw growth that is characteristic of human adults continues. We will be talking more about adult growth in a later module, but probably you already know that jaw growth usually continues in adults up to and often beyond age 50. In children and adolescents, you already have learned about the major changes in the developing dentition and when they occur.

But it has been very difficult to determine both the mechanism that produces eruption of the permanent teeth and how their eruption is controlled. This goal of this module is to bring you up to date on what we know about the process of tooth eruption.

In addition to viewing the module, be sure to read pages 74-80 in the 5th edition of Contemporary Orthodontics (87-93 in the 4th ed.)

Sequence of Steps in Tooth Eruption

Eruption of human teeth is most conveniently described in the context of two major phases and six total steps or stages.

The first major phase is pre-emergent eruption, defined as the events that occur while the tooth is moving within the bone of the jaws and penetrating the gingiva. It has two steps:

1. crown formation
2. movement toward emergence

The second major phase is post-emergent eruption, defined as the events that occur after the tooth has emerged into the oral cavity. It has four steps: 3. the post-emergent spurt, as the tooth moves up to the occlusal plane 4. juvenile occlusal equilibrium 5. pubertal eruption spurt 6. adult occlusal equilibrium

In the latter three steps, the rate of eruption coincides with the rate of vertical growth of the face.

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Sequence of Steps in Tooth Eruption

Pre-Emergent Eruption

1. Crown formation
2. Movement to emergence

Rate of Movement None Slow

Post-Emergent Eruption

3. Post-emergent spurt
4. Juvenile equilibrium
5. Pubertal eruption spurt
6. Adult occlusal equilibrium

Rate of Movement Fast Very slow Slow Very slow

Crown Formation

In a sense, crown formation is the first step in tooth eruption, but only in an honorary sense—because the tooth doesn’t move from the spot where crown formation began until formation of the root begins. It’s what doesn’t happen that’s important.

You have learned about cell types and their interaction during crown formation in histology, and we do not need to review the differentiation of cell types and formation of enamel and dentin at this point. What you need to know now is how these events relate to eruptive movement of the developing tooth. The point is, of course, that they don’t. In contrast, root formation and formation of cementum do relate to eruption. Eruptive movement of a tooth begins as soon as enamel formation ends and root formation begins.

You already have learned something about the use of implants in the jaws to create markers that don’t move as surface changes in the bone occur. How do we know that teeth don’t begin eruptive movements until crown formation starts? Because you can see this when implant superimpositions are done. The tooth stays in the same place relative to its surrounding bone until crown formation is completed. Knowing this now allows us to use teeth at this stage as points for superimposition of cephalometric radiographs, in the same way as metallic implants. Third molars are still in the crown formation stage at the ages when orthodontic treatment usually occurs, and are the teeth most likely to be used in this way.

Radiograph showing pre-emergent eruption of a tooth.

Pre-Emergent Eruptive Movements

In order for pre-emergent eruption to occur, two things have to happen: resorption of bone over the crown of the tooth, and development of a force to move the tooth. It seems reasonable that as root formation begins, the tooth is forced against the overlying bone and primary tooth roots (if it’s a succedaneous tooth), and this activates osteoclasts and causes the resorption.

Some years ago, an anatomist did an interesting experiment. He ligated tooth buds in young beagle dogs to the lower border of the mandible so that teeth couldn’t erupt. What happened? The teeth didn’t erupt, but bone resorption occurred anyway, opening a path along which the tooth was meant to erupt. The experiment showed that the controlling element in pre-emergent eruption wasn’t the development of an eruption force. Instead, resorption removed bone, and then the tooth moved along the path that had been cleared.

Odontoclasts begin to remove bone over the crown of a tooth just at the point that root formation begins. How do they know that root formation has started? As we outlined previously in Module 6, the inhibition of the osteoclasts over the crown that related to amelogenesis ends, and they are up-regulated and become active as cementogenesis begins.

Radiograph showing mandibular canines and molars with a visible fracture site.

Human Evidence Inadvertent experiments in humans have demonstrated that the same thing happens in humans when a forming tooth is ligated to the bone. This has happened several times when a jaw fracture was wired in place, and the wire passed through a tooth bud.

You have already seen the dramatic contrast in eruption of mandibular canines when one of them was ligated during treatment of a mandibular fracture (images 1 and 2). The eruption path was cleared on both sides, the unimpeded tooth followed the path and the ligated one didn’t. Image 3 shows the same thing, a cleared eruption path that the tooth couldn’t follow, with a second molar that was wired in place when a jaw fracture was treated in Raleigh, NC.

Normally, an erupting tooth follows closely along the path that has been cleared for it, as the right canine did in image 2. A cleared path with no eruption along it (described as an uncoupling of resorption from eruption) means that there’s a problem with the eruption mechanism—either mechanical interference, as in these patients, or because something is wrong with the mechanism itself.

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Image 1: Age 10, immediately after treatment for a mandibular fracture through the body of the mandible on the right side. Image 2: One year later (courtesy Dr. John Lin). Image 3: An inadvertently ligated second molar in another child treated for a mandibular fracture. Note that the eruption path has been cleared even though the tooth can’t move along it..

Altered Eruption Paths What would happen if the crown of an unerupted tooth was not oriented toward the occlusal plane? Would an eruption path aimed in the wrong direction be the result? The answer is yes—an eruption path in the wrong direction is entirely possible. For some reason this is particularly likely to occur with mandibular second premolars. It happens almost as often with maxillary canines, and can affect any tooth.

This series of images shows mandibular second premolars with various degrees of improper orientation and eruption in the wrong direction. In image 1, note the distally-oriented 2nd premolars bilaterally. On the right side, the 2nd premolar probably will be guided into position by the root of the 1st molar. On the left side the first molar has been lost, and the 2nd premolar has erupted toward the 2nd molar, which may guide it to erupt into the space where the first molar used to be. If the 2nd premolar is oriented almost horizontally, it can cause resorption of molar roots (images 2 and 3), and can go into the ramus and to the tip of the coronoid process if all the molars are lost (image 4). The osteoclasts just keep clearing a path, and the tooth moves along it!

When the osteoclasts run out of bone and into soft tissue that isn’t gingiva, they usually stop, but canines have been known to erupt into the nose.

Image 1: Note the distally-oriented mandibular 2nd premolars on both sides.

Image 2: Resorption over the crown of this horizontally-positioned 2nd premolar is likely to cause resorption of the roots of the 2nd molar.

Image 3: For this patient, both the 1st and 2nd premolars erupted beneath the 1st and 2nd molars, causing obvious root resorption.

Image 4: If all the molars are missing, a distally-oriented 2nd premolar can move into the mandibular ramus and erupt upward along the mandibular canal.

Eruptive Mechanism

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Radiograph showing a maxillary second premolar with less than normal root formation that has erupted.

module. It seems at present that the blood flow theory is likely to be correct, even if the exact mechanism has not yet been worked out.

3. Shortening of collagen fibers as they mature. There is no doubt that this happens, and as we will see in the next section of this module, the collagen mechanism is important in post-emergent eruption. But prior to emergence of the tooth, the collagen fibers are so randomly arranged that they couldn’t be the source of pre-emergent movements (images 1,2). For shortening of collagen fibers to be effective in producing eruptive movements, they have to be arranged as they are after a tooth comes into the mouth, hooked to the bone above the point where they connect to the tooth (images 3,4). This mechanism, therefore, can’t be the source of force to move a tooth along a path that has been cleared in the bone.

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Pre-Emergent Eruptive Movements: Summary The bottom line on normal pre-emergent eruption:

• resorption produced by osteoclasts over the crown of a tooth clears an eruptive path that the tooth follows
• an eruption mechanism that we still don’t fully understand, probably based on differential blood flow in the periodontal ligament, pushes the tooth along that path at a rate determined by the resorptive activity
• accelerated emergence of a succedaneous tooth occurs when inflammatory resorption exposes the crown, which is further evidence that clearance of the eruption path is the controlling element in pre-emergent eruption.

Post-Emergent Eruption Effect of Early Loss of Primary Tooth: Premature Eruption

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Effect of Early Loss of Primary Tooth: Delayed Eruption On the other hand, if a primary tooth is lost quite early and a considerable amount of bone remains over its permanent successor (which would be the case, for instance, when a 3-year-old knocks out a primary tooth), eruption of the permanent tooth often is delayed. This occurs because after the primary tooth is gone, bone that remains over the permanent tooth becomes less vascular and thereby more resistant to resorption—and if the rate at which the eruption path is cleared decreases, of course the tooth will take longer to emerge.

It would be nice if the permanent centrals erupted faster after the maxillary primary centrals were lost quite early to trauma, but it usually doesn’t work that way. Rather than appearing to be a

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Mechanism of Post-Emergent Eruption

Once a tooth comes into occlusal function, it’s exposed to the heavy forces that go with chewing and other forces that oppose eruption. At that point, as you have already seen, the periodontal ligament does organize itself so that the collagen fibers attach to the bone above the point where they attach to the root of the tooth. Then shortening of collagen fibers as the collagen matures and shrinks would move the tooth occlusally.

It was noted by sheep herders many years ago that if the animals ate a certain weedy plant, their teeth would become loose and might fall out. The problem turned out to be a plant constituent that inhibited cross-linking and maturation of collagen. Chemicals of this type are called lathyrogens.

That led to animal experiments on the effect of lathyrogens on tooth eruption. Sure enough, teeth that are already in the mouth stop erupting when lathyrogens are given. From this experimental evidence, it seems clear that shortening of collagen fibers is the mechanism of post-emergent eruption.

It’s important to keep in mind that collagen fibers in the periodontal ligament resorb and are replaced rather quickly. New ones are always forming, attaching and then shortening as they mature. So an eruption mechanism based on this would be active constantly.

Animal Experiments

It seems obvious that after a tooth emerges, forces against the tooth that oppose eruption must be a major part (if not all) of the mechanism that controls eruption. But exactly how does this work?

Experiments to determine the control of post-emergent eruption are difficult because an erupting tooth moves so slowly that extremely precise measurements are necessary to observe its movements. Three new measurement techniques were developed in the late 1980s and 1990s that could be used in tooth eruption experiments. The first two had a precision of 1-2 microns, the third could resolve a fraction of a micron (a micron is .001 millimeter).

The first technique used a variable capacitance displacement transducer (VCDT). If two plates are separated by a small distance, as they are in a capacitor, even a very small change in their separation produces a measurable change in capacitance. This allowed studies at UNC of eruption in rabbits, which were chosen because like all rodents, they have continuously erupting incisors that move much more rapidly than human teeth, and can be used in terminal experiments. The goal was to evaluate how much force was needed to affect eruption, and how long that force had to be maintained.

The instrumentation for the rabbit studies is shown in image 1. The rabbit has to lie very still—which means paralysis with curare. Then a respirator is needed, and body temperature must be controlled. The measuring device has one plate attached to the jaw, and another mounted on the tooth (image 2).

Animal Experiments: Force to Resist Eruption

In this close-up view, you can see that the two plates forming the VCDT that almost touch. A change in their position as small as 1-2 microns will cause a change in capacitance.

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You also can see a thin flat steel beam that contacts the incisal edge of the erupting tooth. For these experiments, the beam was mounted so that it could be moved into or out of contact with the tooth. A strain gauge on the beam allowed measurement of the amount of force it placed against the tooth. This allowed measurement of the force of eruption, by letting the tooth erupt into contact with the beam and observing how much force it took to stop the tooth. That turned out to be a very light force, only a gram or less. The same thing has been observed in other animal experiments—it takes only a very small amount of force to stop an erupting tooth if the force is continuous, as it would be if the beam were left in position. In the mouth, a tooth would almost never receive continuous force. What would happen if the force opposing eruption were intermittent?

Animal Experiments: Duration of Force to Resist Eruption These graphs show the effect of intermittent force, with a load of 1-2 grams against the erupting rabbit incisor. This force is more than enough to stop eruption with continuous application. The beam was brought into contact with the erupting tooth in an on-off fashion, with 3 cycles: on for 1 second, then off for 9 seconds (10% time) on for 1 second, then off for 3 seconds (25% time) on for 1 second, off for 1 second (50% time) After an hour of intermittent force application, the tooth was allowed to erupt without applied force, then another hour of force application followed, and then a final recovery hour. There were at least three animals in each group.

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Note that with 10% force application, there was little or no effect on the rate of eruption (image 1). The tooth continued to erupt at about the same rate when it was being loaded as when it was not. With 25% time, there was a marginal effect (image 2). Note that for all 3 animals, eruption stopped during one but not both of the hours that the tooth was loaded. With 50% time, eruption was stopped and intrusion began to occur (image 3). The tooth continued to erupt after the force was removed.

Image 1: Effect on eruption of 10% time force application (minimal effect) Image 2: Effect on eruption of 25% time force application (marginal effect) Image 3: Effect on eruption of 50% time force application (nearly total effect)

Animal Experiments: Eruptive Force and Orthodontic Tooth Movement

These results showed that in the rabbit model, if a force large enough to stop eruption is applied for 50% of the time, the effect is very similar to that of continuous force. The same force 10% of the time has little or no effect. Applying the force 25% of the time seems to be about the threshold for an effect on eruption.

This is remarkably similar to what has been learned about the amount and duration of force to move a tooth orthodontically. For orthodontic tooth movement, the amount of force can be quite small if it is maintained continuously. And if a removable appliance is used, unless it is worn for at least 4-8 hours/day (about 25% time), the teeth don’t respond. A removable appliance that is worn regularly

for 12 hours/day (50% time) is reasonably effective in moving teeth, although full time is at least somewhat better.

The bottom line: it looks as if heavy intermittent force opposing eruption, as from chewing, would not have enough duration to effectively control eruption. But light pressures from soft tissue contacts with the teeth, as from tongue or lip-cheek position during sleep, would have the right characteristics.

Duration of Light Force	Erupting Tooth	Orthodontic Movement
10% time	No effect	No effect
25% time	Threshold effect	Threshold effect
50% time	Eruption stops, intrusion?	Near-maximal movement
Continuous	Intrusion	Maximal movement

Human Experiments

Rodent incisors are rather different from human teeth. Similar experiments in humans would be possible only if non-invasive instrumentation could be used, so VCDT measurements would not be possible.

Robert Paterson, a senior engineer at IBM / Lexmark and long-time collaborator with dental research, suggested in the 1990s that a video microscope system might make it possible to follow the eruptive movements of a human second premolar in its post-emergent spurt (image 1). At this stage, the premolar erupts at an average rate of about 4 microns/hour. If an optical ruling was bonded to the premolar and viewed through a reference ruling bonded to a bar between the first premolar and first molar (image 2), it should be possible to observe 1-2 micron changes in position.

The major problem would be to get the two rulings in focus when a child with an erupting premolar was positioned for video microscope imaging. In image 3, the child’s cheek is retracted and the video microscope is being focused on the rulings. The video output is seen on the screen in front of the

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patient. By recording the video output and then capturing a video frame that did have the rulings in focus, the needed measurements were possible.

Video Microscope Studies The video microscope studies, with observations repeated each hour, produced an unexpected finding: during the post-emergent spurt, teeth only erupt in the evening, not during the day! Instead of erupting steadily at about 4 microns/hour (the average rate), they don’t erupt—and may and even intrude—during the day, then erupt surprisingly rapidly during the evening, at rates of 20 microns/hour or more.

The instrumentation was changed to allow a continuous view of eruption, using a flexible fiber optic cable to transmit an image of the rulings to the video microscope (image 1). Then children could spend the night on a cot in the laboratory, and eruption could be observed while they slept.

This showed that eruption started in the early evening and stopped around midnight (image 2). The tooth was likely to intrude somewhat after that until the next phase of eruption began in the early evening the next day.

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Moiré Magnification: Effect of Applied Force

Using the Moiré magnification device, a series of experiments with children with an erupting second premolar were carried out to evaluate the amount of force necessary to stop eruption and to judge the effect of intermittent force.

The results were very similar to those in the animal experiments: a light force maintained continuously against the tooth stopped its eruption and was likely to lead to intrusion. Intermittent force against the erupting tooth deflected it, but as you can see in this plot, eruption usually continued at the same rate.

It appears, therefore, that for control of eruption, as for orthodontic tooth movement, the duration of the force is much more important than its magnitude. Teeth are very well adapted to withstand quite heavy force of short duration during mastication. Heavy intermittent force from chewing does not affect the periodontal ligament because it is cushioned by fluid that fills the space between the tooth root and the bony walls of the socket.

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Subject with unimpeded eruption Tooth position in microns Time in seconds

Moiré Magnification: Effect of Decreasing Blood Flow

As we have discussed, blood flow in the periodontal ligament seems to be the major component of the mechanism of eruption prior to the time that a tooth erupts and the PDL becomes organized to support it. Would blood flow affect the eruption of a human premolar during the pre-occlusal spurt?

A series of experiments using Moiré magnification indicate that it would. Dental anesthetics often contain epinephrine to constrict blood vessels so the anesthetic agent is retained longer, which is particularly needed because most local anesthetics dilate vessels and increase blood flow.

As this image from a typical experiment with a child shows, injecting a local anesthetic with epinephrine over the apex of an erupting premolar stops eruption and may lead to intrusion of the tooth.

Blood flow experiments: eruption rate Erupting tooth: began to intrude after injection of vasoconstrictor

Moiré Magnification: Effect of Increasing Blood Flow In contrast, as this chart from another experiment shows, increasing blood flow causes eruption. Note that prior to the injection of a local anesthetic without epinephrine (a vasodilator), this premolar was intruding; it began to erupt after the injection.

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Blood flow experiments: eruption rate Intruding tooth: began to erupt after injection of vasodilator

Immediate Response

Short-Term Response

Injection, Vasodilator

What Causes the Daily Rhythm in Eruption? An increase in blood flow in the PDL appears to be the way that tooth eruption during the evening is created—but what leads to this cycle of increased blood flow? It is interesting that skeletal growth and tooth eruption follow the same pattern. A child is taller at midnight than at 6 PM, doesn’t grow any more for the rest of the night, actually loses a little height (but not all of the previous evening’s height gain) during the next day, then grows again the next evening. Tooth eruption mimics this pattern of growth. The growth in height, and probably also tooth eruption, is influenced by the cycle of release of growth hormone. A child has an increase in growth hormone levels beginning in the early evening that lasts until about midnight. That’s when he or she grows, and when a tooth erupts. Preliminary experiments with growth hormone-deficient children (who have delayed eruption of their teeth) suggest that an increase in the rate of eruption is one of the effects of injecting growth hormone. So it seems reasonable that the cause of the tooth eruption cycle is the same as the cause of the growth cycle: PDL blood flow probably changes because of growth hormone effects.

The Bottom Line It’s important to keep in mind that both the mechanism and control of pre- and post-emergent eruption are different.

For pre-emergent eruption:

• mechanism is pressure generated by differential blood flow in the PDL (exactly how remains unknown)
• control is by the rate of resorption over the crown of the erupting tooth

For post-emergent eruption:

• mechanism (after a tooth comes into function) is collagen fiber maturation and shortening. It appears still to be the blood flow mechanism during the pre-occlusal spurt.
• control is by force opposing eruption, but the force that counts is light force that’s prolonged for hours, not heavy intermittent force from chewing

Diagnosis and Treatment of Eruption Problems

Impacted Teeth: Third Molars

An impacted tooth, by definition, is one that has failed to emerge into the oral cavity. This can happen with any tooth from a variety of causes, but the usual cause is a displaced path of eruption because of lack of space in the dental arch. The most frequently impacted teeth are 3rd molars, especially mandibular 3rd molars, and maxillary canines.

In modern humans, it is unusual for the jaw to grow long enough to accommodate the 3rd molars. Remember that space for all the molars is created by resorption along the front of the mandibular ramus and addition of new bone at the maxillary tuberosity. If there isn’t enough room for it, the third molar may break through the bone but still be covered by soft tissue distal to the second molar, even if it’s upright (image 1). In this situation oral pathogens have access to the 3rd molars via a communication behind the 2nd molars.

Upper third molars are less likely to become impacted, in part because they are more likely to be small, and often erupt when lower third molars have problems (image 2). Even if lower third molars become horizontally impacted (image 3), there’s still likely to be a communication between the oral cavity and the area around the crown of the tooth. Because the oral flora have access to a 3rd molar that has penetrated through the bone, this type of impaction is more likely to be a clinical problem than a bony impaction.

Can third molars like these push the rest of the lower teeth forward and cause incisor crowding? That’s an important developmental question that we’ll come back to at the end of this module.

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Impacted Maxillary Canines

In a previous module, we have already noted that maxillary canines sometimes miss connecting with the root of the primary canine, and can become impacted if this happens.

Like any other tooth, the maxillary canines follow the path that has been cleared for them. It is interesting, and clinically significant, that if the primary canine is extracted when the path of the permanent canine is being deflected away from the primary root, the permanent tooth often changes its direction toward erupting in the right place.

This is an excellent example of a clinical intervention based on what we know about the control of pre-emergent eruption. If you clear the path for a tooth to erupt, it tends to follow the path. For this patient, one permanent canine was in poor position, mesial to the root of the primary canine (image 1). After extraction of the primary canine (image 2), the permanent canine erupted downward through the extraction site into its proper position.

Cleidocranial Dysplasia An unusual syndrome, cleidocranial dysplasia, is characterized by two things that would seem to have nothing in common: absence of clavicles (collar bones) and multiple unerupted permanent teeth. In a child with this condition, the primary teeth erupt normally, and so do the permanent molars. You remember, of course, that the permanent molars are derived from the primary dental lamina, and developmentally can be considered primary teeth that last indefinitely—so if the primary teeth can erupt, it makes sense that the permanent molars also would erupt.

As this image shows, the succedaneous teeth do not erupt, and the reason is obstruction of their eruption path. Three things work together to make it almost impossible for these teeth to make it into the oral cavity: multiple supernumerary teeth usually are present, the alveolar bone is resistant to resorption, and if a tooth does get past the first two obstacles, the heavy and fibrous gingiva still impedes eruption.

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Cleidocranial Dysplasia: Treatment

If the problem is mechanical obstruction, clearing the eruption path should lead to eruption of the impeded teeth, and it should be possible to move teeth orthodontically if they didn’t erupt on their own.

For this patient, supernumerary teeth and bone over the unerupted incisors were removed at age 10, and by age 11 the permanent incisors had erupted a considerable distance (image 1). At that point orthodontic attachments were placed on the incisors to bring them into normal position (image 2).

In addition to the eruption problem there was severe crowding in both arches. Note the position of the mandibular canines in image 2. These teeth were removed along with additional supernumerary teeth in the canine-premolar area, and later the maxillary first premolars were removed so there would be room to bring the maxillary canines and 2nd premolars into the arch.

Eventually, the remaining succedaneous teeth were brought into the dental arch except the maxillary right 2nd premolar (image 3), which was extracted after it proved impossible to move, probably because an area of its periodontal ligament was damaged during the surgery to remove other teeth and this led to ankylosis (fusion of cementum to bone).

The concept is important: if a tooth doesn’t erupt because it’s mechanically obstructed, it may do so when the obstruction is removed and can be moved orthodontically if necessary.

Primary Molar Ankylosis / Obstructed Permanent Successor

Image 1: Cleidocranial dysplasia, age 11, after surgical exposure of incisors. Image 2: Cleidocranial dysplasia, age 12, after alignment of incisors and removal of obstructions in the canine-premolar area. Image 3: Cleidocranial dysplasia, age 16, near end of orthodontic treatment.

Although most ankylosed primary teeth eventually are exfoliated and the underlying permanent tooth then can erupt, an ankylosed primary tooth can be a formidable mechanical obstruction. If a primary molar becomes ankylosed at an early age, it can disappear as other teeth erupt past it and gingiva covers it over. Then the crown of the primary tooth is retained even if the root resorbs, and resorption of enamel is very slow if it happens at all (image 1). For this patient, note that the roots of the obstructed 2nd premolars have continued to develop. The root of the premolar in the lower right has stopped at the border of the mandible.

The maxillary second premolars and the ankylosed primary molars were extracted, and space was opened between the 1st molar and 1st premolar in the lower left. The 2nd premolar erupted on its own and brought alveolar bone with it (image 2). With the premolar moved orthodontically the rest of the way to its normal position (image 3), alveolar bone height in the premolar area also was normal. Note that the root of the premolar has remained incomplete. A tooth with an incomplete root can still have eruptive potential.

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Image 1, Ankylosed primary molars: Primary second molars in all four quadrants were ankylosed and prevented normal eruption of the underlying premolars. Image 2, Ankylosed primary molars: All four ankylosed primary molars as well as the maxillary second premolars were extracted, and the lower premolars erupted. Image 3, Ankylosed primary molars: Despite its short root, the lower left premolar erupted into occlusion.

Molar Ankylosis vs Primary Failure of Eruption

The usual cause of a posterior open bite is failure of premolars or permanent molars to erupt normally. There are two possibilities when this occurs: isolated ankylosis of one or more first molars (image 1), or primary failure of eruption affecting multiple posterior teeth (images 2 and 3).

Ankylosis can be thought of as the ultimate form of mechanical obstruction—once a tooth fuses to bone, it can’t erupt and can’t be moved unless the ankylosis can be released, which is impossible except for brief periods and then only if the area of ankylosis is small. Primary failure of eruption (PFE) carries that name because the teeth do not erupt even though they are not ankylosed—which indicates a failure of the eruption mechanism.

In both conditions, the problem is first noted when some or all the first permanent molars stop erupting, and in both conditions the prognosis for the affected molar is very poor. If it’s ankylosed but the area of ankylosis is small, sometimes it is possible to gently luxate the tooth to break the ankylosis, and then move it orthodontically—but eventually it becomes ankylosed again.

In PFE the periodontal ligament is abnormal, and the affected teeth do not respond to orthodontic force. They can’t be moved even though they aren’t ankylosed. About the only effect of trying to move these teeth is that they do become ankylosed.

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Image 1: This mandibular first molar became ankylosed soon after it erupted, after an occlusal restoration had been placed. Note the amount that it submerged as the 2nd molar and premolars grew past it.

Image 2: Cephalometric radiograph showing posterior open bite. With so many teeth affected, primary failure of eruption is the likely diagnosis.

Image 3: Panoramic radiograph, primary failure of eruption affecting molars in all four quadrants..

Primary Failure of Eruption

The distinction between ankylosis of first permanent molars and primary failure of eruption is important, because it establishes the prognosis for the second molars. In isolated ankylosis of first molars, the second and third molars are likely to respond perfectly normally to orthodontic force—so the second molar can be moved into a first molar extraction site, creating room for the third molar to erupt.

In contrast, in PFE all teeth distal to the most mesial affected tooth are affected (images 1-3). First premolars sometimes are affected but usually are not; 2nd premolars may or may not be affected, as in this patient. Usually the 1st molar is affected but it may be just the 2nd and 3rd molars.

Because the periodontal ligament is abnormal and does not respond to orthodontic force, an attempt to move any of these teeth is guaranteed to fail. You’ll learn more about managing eruption problems

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later—but at this point you need to understand the significance of failure of a tooth to follow along as its eruption path is cleared. It is difficult to have to say to parents that nothing can be done to bring the affected teeth into the mouth—but at least ineffective treatment can be avoided if the correct diagnosis is made.

If a permanent first molar doesn’t erupt, it can be difficult to tell at age 7-8 whether it’s ankylosed or whether it is affected by PFE (image 1). If the second molar erupts normally, it’s an isolated ankylosis of the first molar. If the second molar also doesn’t erupt although its eruption path has been cleared, it’s primary failure, and the second premolar may be affected as well. This means that a definite

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diagnosis of ankylosis or primary failure cannot be made until it is obvious that the second molar is erupting normally (image 2) or is failing to erupt and also is affected.

When posterior teeth are not erupting normally, caution in proceeding with orthodontic treatment is advised until a definite diagnosis can be made, but early extraction of an abnormal permanent first molar often is better than waiting. For the patient in image 2, the first molar should have been extracted long before this image was taken. If it’s PFE, the early extraction does no harm. If it’s just an ankylosed first molar, the normal second molar can begin drifting mesially as it erupts, and then it brings alveolar bone with it.

You’ll learn more about managing eruption problems later—but at this point you need to understand the significance of failure of a tooth to follow along as its eruption path is cleared.

Image 1: An 8-year-old with ankylosed primary molars and failure of the maxillary right 1st molar to erupt. This might be PFE, but the 2nd molars are symmetric bilaterally, and a definitive diagnosis cannot be made yet.

Image 2: Obvious ankylosis of a maxillary 1st molar. The 2nd molar and 2nd premolar have erupted past it and the 2nd molar moved mesially over it.

Hereditary Pattern of PFE Primary failure of eruption runs in families and is the result, at least in part, of a specific genetic mutation that has been identified.

The PTH1R gene encondes information for a parathyroid hormone receptor protein. A novel mutation in this gene is associated with individuals who have PFE. Studies indicate that it is inherited in an autosomal dominant fashion.

These radiographs of mother (images 1 and 2) and daughter (images 3, 4, 5) show all quadrants affected in both. Sometimes PFE is present in one or two quadrants of the dental arch, while the other quadrants have only an ankylosed first molar, so there is some overlap between isolated ankylosis and PFE.

Eruption disorders like PFE have a complicated etiology, but it is likely that the specific causes will be worked out before too long as genetic studies of these individuals continue.

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Image 1: Mother, all 4 quadrants affected, after extraction of most of the affected teeth. Image 2: Mother, all 4 quadrants affected. Image 3: Daughter, age 7. Note ankylosed primary teeth and unerupted permanent molars. Image 4: Daughter, age 14. All four quadrants are affected but right side is more severe. Image 5: Daughter, age 14, bilateral posterior open bite.

Summary

Important points to remember: Pre-emergent eruption

• no eruptive movement until root formation begins
• mechanism: differential blood flow in PDL (?)
• control: resorption over crown

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• accelerated if crown is uncovered by loss of primary tooth, delayed if dense bone forms over the crown after very early loss

Post-emergent eruption

• fastest movement during pre-occlusal spurt
• continued slow eruption in concert with vertical jaw growth
• mechanism: blood flow prior to occlusion collagen maturation / shortening afterward
• control: soft tissue pressures, light force, long duration

Clinical application impaction:

• clearing eruption path often but not always can prevent it
• more likely when teeth are crowded by lack of space

ankylosis:

• created by fusion of an area of cementum to bone
• frequent in primary teeth, rare in permanent teeth

primary failure of eruption:

• abnormal periodontal ligament
• all teeth distal to most mesial affected one are affected
• teeth do not erupt although they are not ankylosed
• teeth do not respond to orthodontic force, cannot be moved

Self-Test Referral

Before you take the self-test, read pages 74-80 in the 5th edition of Contemporary Orthodontics (87-93 in the 4th ed.). Then use the self-test to be sure you have understand the material you have reviewed. It’s important background for treatment of children with eruption problems.

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3. Physical Growth at Adolescence

Definition/Significance

Purpose of this Program

The purpose of this instructional unit is to describe the physical events that occur as a child grows through adolescence. In this program, we are going to describe features associated with sexual maturation and the developmental events that relate to cranial and facial growth. Then, we will discuss the importance of normal variations in the pattern, timing, and rates of growth. Finally, we will address why it is important to be able to assess individual levels of maturity.

In addition to viewing the module, read Contemporary Orthodontics pages 92-96 (5th ed), 107-111 (4th ed). Be sure you are able to:

• describe the endocrinologic changes that lead to the onset of puberty
• describe the difference of timing of puberty in boys and girls
• describe the stages of sexual maturation in boys and girls
• discuss the impact of puberty on the growth of the major types of tissues (neural, general body, lymphoid, sexual)
• describe the effect of earlier vs. later maturation on ultimate body size
• discuss the relationship of jaw growth to the efficiency of orthodontic treatment during the adolescent growth spurt.

What is Adolescence?

Adolescence is a sexual phenomenon. It is defined as the stage of growth and development when sexual maturity is attained, and this stage of sexual development is referred to as puberty. It can be defined more specifically as the transitional period between the juvenile stage and adulthood, during which:

• Secondary sex characteristics appear
• Fertility is attained
• The adolescent growth spurt takes place
• Profound psychological changes take place

All these developments are associated with the maturation of the sex organs and the concomitant surge in secretion of sex hormones.

Growth Events: Clinical Significance

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Changes at adolescence significantly affect the face and dentition. Three major growth events affecting the dentition take place along with the onset of adolescence. These are:

1. Exchange of the dentition from mixed to permanent
2. An acceleration in the overall rate of facial growth
3. Differential growth of the jaws, i.e. more growth in some areas than others

Why do you need to understand these events? Because any dentist must understand the relationship between growth events that occur during adolescence so that he or she can:

• recognize and assess stages of development in patients
• help solve developing problems of dentofacial disharmony

Growth Spurt Characteristics

Adolescent Growth Spurt

Examine Scammon’s growth curves from the point of view of sexual maturation and the adolescent growth spurt. Note that upon the onset of puberty:

• There is an almost explosively rapid growth and development of the sex organs
• Lymphoid tissue decreases in size—tonsils, adenoids and the like shrink
• Neural growth is unaffected by sexual maturation—it has been largely completed by then
• General body growth shows changes in response to sexual growth

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Hormones: the Basis of Adolescence

Since puberty is a sexual phenomenon, and sex hormones are the mediators of the changes we are most interested in, we will start with an overview of the hormonal events. The genital growth curve is also representative of the change in blood level of the sex hormones. Significant amounts of the male and female sex hormones first appear at puberty. Hormones are released into the blood stream in a process called endocrine secretion—thus we can say that we are talking about the endocrinology of adolescence.

Remember that we want you to understand the big picture. Concentrate on that, not the fine details.

Hormones of the Growth Spurt Three types of hormones are sequentially involved in the growth spurt:

The hypothalamic releasing factors are the hormones that initiate the growth spurt. They are produced in the brain region called the hypothalamus. Their target is the anterior pituitary gland.

The pituitary gonadotrophins are the second type of hormones. They are produced by the pituitary gland, in response to the hypothalamic releasing factors. Their target is the testis in the male and the ovary in the female, with some effect on the adrenal cortex in both sexes.

The sex hormones themselves are the third type of hormones produced by the ovary, testis and adrenal cortex. They have varied effects on tissues throughout the body. Chemically, the sex hormones are all steroids—but not all steroids are sex hormones, only a few with special properties.

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Endocrinology-Step 1

The first events of puberty occur in the brain, and the stimulus for their unfolding remains unknown. For whatever reason, perhaps in response to some kind of internal clock, perhaps in response to a pattern of outside stimuli which we have not recognized, cells in the hypothalamus begin to secrete substances which are called releasing factors. Both the cells and their method of action are somewhat unusual. These are neuro-endocrine cells. They look like typical neurons but they secrete materials in the cell body that are carried by cytoplasmic transport down the axon toward a richly vascular area at the base of the hypothalamus near the pituitary gland.

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Labelled diagram showing the pituitary portal system. The image depicts the Hypothalamus, Releasing factors, and Anterior pituitary within a red background representing brain tissue.

The Pituitary Portal System The substances secreted by the nerve cells pass into capillaries in this vascular region, and are carried by blood flow the short distance to the pituitary gland. It is unusual in the body for capillaries to pass from one region to another, but here the special arrangement of the vessels seems made for the transport of materials from the hypothalamus to the pituitary. Accordingly, the special network of blood vessels is called the pituitary portal system.

The first step in the endocrinology of adolescence would be the arrival of our unknown stimuli in the hypothalmus, which cause the neuro-endocrine cells in that region to begin secreting their releasing factors.

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Hypothalamus Releasing factors Anterior pituitary

Endocrinology-Step 2 The second step occurs at the pituitary gland, where the releasing factors in turn stimulate pituitary cells to produce hormones called pituitary gonadotrophins. There are at least two of these hormones, with perhaps others involved. Their function is to stimulate endocrine cells in the developing sex organs to produce sex hormones. Their target is the testis in the male and the ovary in the female, with some effect on the adrenal cortex in both sexes.

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Male Endocrinology-Step 3

In the male, cells in the testis produce the male sex hormones, including testosterone, but there are also other cell types in the testis (especially the Leydig cells) that produce female sex hormones. Different gonadotrophins stimulates these cell types.

As you already know, every individual has a mixture of male and female sex hormones. There are a number of hormones, testosterone the primary one, that, in addition to their other functions, promote growth leading to a more masculine body form. These collectively are termed androgens. Conversely, there are a number of hormones, estrogen the primary one, that promote growth leading to a more feminine body form. These collectively are termed estrogens.

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Female Endocrinology-Step 3

In the female, the pituitary gonadotrophins stimulate secretion of estrogen by the ovaries, and later progesterone by the same organ. In the female, male sex hormones are produced in the adrenal cortex, and possibly some female hormones are produced in the male adrenal cortex.

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Endocrinology-Step 3

Under the stimulus of the pituitary gonadotrophins, sex hormones from the testis, ovary, and adrenal cortex are released into the blood stream in quantities sufficient to cause accelerated growth of the genitals and the development of secondary sex characteristics.

The increasing level of the sex steroids in the blood also causes other physiologic changes, including the acceleration in general body growth and shrinkage of lymphoid tissues.

Different levels of androgens and estrogens in males and females cause differences in the adolescent growth spurt.

Summary of Endocrinology

Perhaps you are wondering about the reason for such a complicated control system. The purpose of having a three-stage hormonal process is to amplify the small signal that begins in the brain.

In the hypothalamus, only a few nerve cells are involved, and they produce a few molecules of releasing factor—a very weak signal.

Those few molecules serve to stimulate a number of cells in the pituitary gland, and these cells in turn release a quantity of gonadotrophic hormones which is considerably larger than the original amount of releasing factor.

This amount of gondotrophic hormone in turn serves to stimulate a larger number of cells in the gonads, so that the amount of sex hormones ultimately produced is many thousands of times the amount of releasing factor back at the hypothalamus. Thus, a small signal from the brain has a large and continuing effect on the body.

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Male/Female Differences

Adolescent Growth

Puberty and the adolescent growth spurt tend to occur considerably earlier in girls than in boys. From cross-sectional incremental growth curves for boys and girls, you can see that on the average, puberty occurs one and a half to two years earlier in girls than in boys.

Why this occurs is not known—it is just one of the biological facts of life. Perhaps you remember those embarrassing junior high school days when most of the girls were very eager to attend a dance but had great difficulty in finding a boy who was willing to go. Two years later, all that had changed remarkably as sex hormone levels in the boys caught up.

Of course you also realize that there is a great deal of individual variation in the timing of adolescence in both boys and girls. The early maturing boys will reach puberty well ahead of the slow maturing girls, and chronological age has very little to do with where an individual stands developmentally.

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Physiologic Maturation It will be important to you to be able to judge the degree of physiologic maturation which your patients have achieved, so that you will have some ideas about their growth status. If you had growth records on every patient, that would be easy, but of course you won’t have that. How could you do it?

Obviously, you need to be able to assess some markers of physiologic maturation. The degree of development of the secondary sexual characteristics provides such a marker. While it is true you will not have the opportunity to evaluate all of these characteristics, many will be visible under normal dental office conditions. Fortunately, there is a good correlation between development of secondary sexual characteristics and the individual’s position on the growth curve.

Stages in Female Sexual Development

Let us begin by looking at the relationship between height gain in girls and secondary sexual characteristics. For convenience, we can identify three stages in development of the secondary sexual characteristics, labeled I, II and III on this graph. Stage I is at the beginning of the growth spurt; stage II is at the peak; and stage III is on the downslope toward the end of the growth spurt.

Female Stage I The first stage in secondary sexual characteristics for girls occurs very near the beginning of the growth spurt. The initial sign of sexual maturation is the appearance of breast buds and early stages of pubic hair.

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Stage 1:

• Breast buds
• Early pubic hair
• Benign height spurt

Female Stage I (cont.)

The breast and papilla elevate to a small mound and the diameter of the areola increases. Pubic hair is slightly pigmented and is sparsely distributed along the labia. The illustrations shown here may be slightly more advanced already than early Stage I.

Female Stage 2

The peak velocity for growth in girls occurs about one year after stage one, and coincides with stage two of development of secondary sex characteristics. At this time, there is noticeable breast

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development. Pubic hair is darker and more widespread. Hair appears in the armpits (axillary hair).

Stage 2:

• Noticeable breast development
• Dark pubic hair
• Axillary hair
• Peak of height spurt

Image 1, Height gain per year for girls: Stage 2 includes noticeable breast development as well as pubic and axillary hair.

Female Stage 2 (cont.)

Breast development shows growth and elevation of both breast and areola with contours being smooth. Pubic hair is darker, coarser, and curled.

Female Stage 3

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The third stage in girls occurs about one to one and one-half years after stage two. By this time, the growth spurt is all but complete, and this stage is marked by the onset of menstruation. By now, there is noticeable broadening of the hips with a more adult fat distribution, and development (though not necessarily growth) of the breasts is complete.

Stage 3, 12-18 months later:

• Broadening of hips
• Female fat distribution
• Menarche
• Complete breast development
• End of height spurt

Image 1, Height gain per year for girls: Stage 3 is marked by the beginning of menstruation.

Female Stage 3 (cont.)

The breast shows more adult contours with the areola and papilla forming a secondary mound. Pubic hair has a more adult distribution with some spread to medial surfaces of the thighs.

Male Sexual Characteristics

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The stages of sexual development in boys are more difficult to specifically define than in girls. The process of puberty begins later in boys and extends over a longer period of time.

Note in the graph that for boys, the process extends over about a five year period, while in girls puberty extended over a 3 ½ year period. We will define four stages in development of secondary sexual characteristics in boys, again indicating the position on height curve which roughly corresponds with the sexual characteristic stage.

Stage I is at the beginning of puberty, with a slight increase in the growth rate; stage II is at the beginning of the growth spurt; stage III is at the peak; and stage IV is toward the end of the growth spurt.

Male Stage 1 The first stage of sexual development for males is the “fat spurt”, as adipose tissue increases throughout the body. The initial sign of male puberty, interestingly, is the development of a more feminine body form. It appears that the estrogen-producing cells in the testis are stimulated by the pituitary gonadotrophins before the androgen-producing cells.

At this time also, the scrotum begins to increase in size and may show some increase or change in pigmentation, while the penis remains juvenile.

Male Stage 1 (cont.)

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During this stage, boys may appear to be obese and somewhat awkward physically. At this time also, the scrotum begins to increase in size and may show some increase or change in pigmentation, while the penis remains juvenile (image 2).

Male Stage 2 At stage two, about one year after stage one, the spurt in height is just beginning (image 2). At this stage, pubic hair appears above the penis, and there is growth of the penis, increasing size of the testes, and increasing size and pigmentation of the scrotum (image 3). There also is a redistribution and relative decrease in subcutaneous fat, so that the chubby appearance decreases and body form becomes more like that of an adult male.

Stage 2, one year later:

• Dark pubic hair
• Begin height spurt
• Growth of penis
• Loss of fat
• “Peach fuzz” on upper lip

Image 1

Image 2, Height gain per year for boys: Stage two sees the end of the fat spurt and a sharper increase in height gain.

Image 3

Male Stage 2 (cont.) As stage 2 develops, “peach fuzz” (fine unpigmented hair) appears on the upper lip, and as stage 3 and the height of the growth spurt approaches, pigmented hair starts to appear on the upper lip.

At age 10 yrs 2 months (image 1), this boy shows none of the characteristics of early adolescence. The fat spurt is more obvious in some individuals than others, but he has not reached that stage, and has no sign of any hair on his upper lip.

At age 11 yrs 4 months (image 2), his parents report that he has recently started to grow. It’s hard to see in the photo, but he has the beginnings of unpigmented hair on his upper lip. This is another sign that he’s entering his growth spurt. If you’re trying to take advantage of growth during orthodontic

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treatment, this would be a good time to start (in fact, if you look closely, you can see that he’s just had braces put on his teeth). There’s nothing to be gained by starting earlier. Image 1, Boy aged 10 years, 2 months: No evidence of early adolescence present. Image 2, The same boy fourteen months later: Hair on the upper lip is one good indicator that the growth spurt has begun. Male Stage 3 The third stage occurs 8-12 months following stage two and coincides with the peak velocity in gain in height. At this time, axillary hair appears. For most boys the first facial hair is “peach fuzz” (fine unpigmented hair) on the upper lip (as shown in the previous screen), which tends to appear before the peak of the growth spurt. The appearance of pigmented facial hair on the upper lip (no hair on the chin yet) is an indicator that a boy is about at the peak of growth. A spurt of muscle growth also occurs, along with continued decrease in subcutaneous fat and an obviously harder and more angular body contour. In this boy (image 3) at age 14-8 (when his orthodontic treatment ended), the pigmented hair on the upper lip is apparent. It first appeared several months previously.

Stage 3, 8-12 months later:

• Pigmented facial hair, upper lip only
• Axillary hair
• Gain in muscle mass, loss of fat

Image 1: Stage 3 occurs 8-12 months following stage 2. The greatest increase in height takes place during this phase.

Image 3: Dark pigmented hair on the upper lip is a good indicator of stage 3 in boys.

Male Stage 3 (cont.)

In stage 3, pubic hair distribution appears more adult but no spread to the medial of the thighs has occurred. Penis and scrotum are near adult size.

Male Stage 4

Stage four for boys is difficult to pinpoint. This occurs anywhere from 15 to 24 months after stage 3. At this time, the spurt in growth in height ends. By then there is the beginning of facial hair on the chin as well as the upper lip, further increase in muscle mass and redistribution of fat toward the body image of an adult male, and a spurt in muscular strength.

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Stage 4, 15-24 months later:

• Facial hair on chin
• Adult/pubis axillary hair
• Height spurt ending
• Weight gain
• Spurt in muscular strength

Image 2

Male Stage 4 (cont.) Pubic hair distribution is nearly adult with some spread to medial thigh and some spread beginning in a sagittal direction toward the umbilicus. The scrotum and penis have reached full adult size.

Differential Growth

Correlation

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If information about physical growth status is to be applied to dentofacial development, it is important to know how growth in the face and jaws correlates with growth elsewhere. Our information so far has correlated sexual development with growth in height. Does growth in height correlate with jaw growth? Fortunately, it does. The acceleration in height occurs at the same time as an acceleration in growth at the mandibular condyle and a slight acceleration at the sutures of the maxilla. So careful observation of the stage of secondary sexual characteristics would give a dentist insight into the stage of growth of the jaws as well as skeletal growth more generally.

Correlation: Differential Growth Note that these curves for increments of growth show that during the adolescent growth spurt, there is more growth at the condyles (mandibular growth) than at the sutures (maxillary growth). You have seen the same thing previously on distance curves for maxilla and mandible. The maturing face becomes less convex as the mandible becomes more prominent because of differential jaw growth.

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Growth timing boys

cm/mm Per year

10 Height 5 Condyles Sutures 0 8 10 12 14 16 18 20 22 Age in years

Treatment Timing: Headgear

Orthodontic treatment for the most common growth problem in children of European descent, protruding maxillary incisors because of deficient mandibular growth (i.e., skeletal Class II in the terminology with which you will soon become very familiar), is timed to coincide with the period of rapid growth at adolescence when possible because it is more effective at this time.

Suppose the lower jaw is somewhat behind the upper jaw—a common problem—but is growing forward relative to the upper jaw as it normally does during the adolescent growth spurt. We can apply force to the sutures of the maxilla with a “headgear” appliance (figures 1 and 2). Sutures react, and the external force can alter the direction of maxillary growth, holding the maxilla while the mandible grows forward.

Appliances like this can produce a considerable change in jaw relationship, but only if differential growth is occurring. Headgear to the maxilla only works if the mandible is growing. That’s why it’s so important to know where things are on the development scale in order to do orthodontic treatment at the most advantageous time.

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Treatment Timing: Functional Appliances

It isn’t necessary to use extra-oral appliance like headgear to take advantage of differential jaw growth. This device, one of a group of similar intra-oral devices collectively called functional appliances, is a bionator. By holding the mandible forward, it can influence the growth of both the mandible and maxilla, and it also guides the posterior teeth as they erupt.

This takes advantage of our final correlation: the transition from the mixed to the permanent dentition also tends to occur along with the other growth changes at adolescence. With all those changes going on, it’s a great time to be correcting any problems in the way teeth and jaws are arranged.

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OCR Text

Image 1, A Bionator appliance: By functioning, or biting together, with the appliance in place, the saggital relationship of the teeth and jaws is improved – a functional appliance, if jaw growth is occurring.

Image 2, A Bionator appliance: By functioning, or biting together, with the appliance in place, the saggital relationship of the teeth and jaws is improved – a functional appliance, if jaw growth is occurring.

Normal Variations

Maturation

So far, we have discussed the “why” of adolescence, in terms of its endocrinology, and the “what” in terms of physical growth and its relation to the appearance of primary and secondary sexual characteristics.

Can you describe the pattern of changes and discuss the timing of adolescence? By now, you should be able to draw and label the incremental growth curves from memory.

Now let’s talk about variability.

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Concepts in Physical Growth

Pattern Timing Variability

Maturation Variability

These 13-year-old boys, who participated in a growth study, are in the same class at school and only a few months apart in age. It’s obvious that chronological age is far from reflecting their growth status. The bigger boy is much more mature sexually. A close look at his face that showed you hair on his upper lip but not on his chin could tell you a lot about where he is on the growth curve. The timing of onset of puberty makes a big difference in size in the seventh grade.

What do you think the effect of early puberty is on final height as an adult? Would you be taller if you matured early? Shorter? The same size?

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Maturation: Effects on Bone Growth

You’ll remember that growth in height depends on endochondral growth at the epiphyseal plates of long bones. The answer to the effect of maturation timing on adult height derives from the fact that the impact of the sex hormone on endochondral bone growth is two-fold. First, the sex hormones stimulate the cartilage to grow faster, and this produces the adolescent spurt in height. Secondly, the sex hormones also cause more rapid maturation or transformation of cartilage into bone, and the speed-up in maturation is even greater than the acceleration in growth. Thus, during the adolescent growth spurt, the cartilage is used up faster than it is replaced. Toward the end of sexual maturation, the last of the cartilage is transformed into bone and the epiphyseal plates close. At that point, of course, growth potential is lost, and growth stops. Now, knowing that, what’s the answer? Would you be taller if you mature early? Shorter? The same size?

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Maturation: Timing and Ultimate Height All other things being equal, the earlier you mature sexually, the shorter you will be as an adult. Perhaps you remember seeing that happen among your contemporaries. Remember the boy who was the biggest guy in the 6th grade? But he is nearly the same size as an adult as he was then, and now is relatively short, while one of the runts then is one of the biggest men now. An early-maturing girl who was the tallest in her middle school class will watch many others who hit their growth spurt later grow taller. This phenomenon is responsible for much of the difference in adult size between men and women. Girls mature earlier on the average, and finish their growth much sooner. Boys aren’t bigger than girls until they grow for a longer time at adolescence. The difference arises because growth continues slowly prior to the growth spurt, and so for those who mature late, when growth spurt occurs it takes off from a higher plateau.

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Timing of Puberty Now let’s tie this relationship of timing of puberty and variation in final height together, to be sure you understand.

Look again at the growth velocity curves for girls who mature at various times. The early maturing girls tend to have a more intense but shorter growth spurt, and are likely to be all through growing by age 13 or so. They’d better have their orthodontic treatment early, too. The late maturing girls have a later and less intense growth spurt—but they are likely to be taller adults than the early maturers. The reason is that they grow at the pre-pubertal plateau level for a longer time, before the growth spurt and growth fade-out.

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Timing of Puberty: Other Factors

Timing of puberty is a major contributor to variability, but this timing in turn can be related to other factors. There is a genetic influence on timing of puberty—of course your inheritance has strong influences on your growth potential. Somatotype, or body-build characteristics, affects timing of the growth spurt. A boy classified as an ectomorph—a slender one, like all 3 of these boys—will have a later growth spurt than his mesomorph, muscular friends—and after being shorter for years, he may end up taller. Seasonal and climatic variations have an interesting impact. Children mature faster in warmer climates, slower in colder ones, and growth in height is faster in the spring than fall.

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Assessment of Maturity By now you should be thoroughly convinced that developmental age is important. The stage of sexual development correlates strongly with physical development, so assessment of sexual maturity by observing the secondary sexual characteristics is important when you are dealing with adolescents. Don’t forget that there are other ways of obtaining information about physiological maturity. Sequential height-weight charts and bone development as seen on radiographs are two that are important. You already know a lot about growth curves on height-weight charts and will be learning more about how to judge skeletal maturation from radiographs.

Summary The adolescent period of life is characterized by attainment of sexual maturation. This is mediated by sex hormones and controlled through the hypothalamic area of the brain and the pituitary gland. The adolescent growth spurt is characterized by sexual differences in timing, rate, and duration. Girls mature earlier—which is one important reason why they are smaller adults. Assessment of physiological maturity is necessary in treatment planning, especially when orthodontic treatment is to take advantage of differential jaw growth. Now that you have completed this module, be sure you have carefully read pages92-96 (5th ed), 107-111 (4th ed) in Contemporary Orthocontics. Then take the self-test in the next section to be sure you understand what you have learned.

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4. Patterns of Facial Growth

Facial Growth Patterns Purpose of this Program In this program we discuss facial growth in the context of different growth patterns, their clinical presentations, and their treatment considerations. Read pages 96-103 in the 5th edition of Contemporary Orthodontics (or pages 111-119 in the 4th ed.) in addition to viewing the teaching program. Be sure you are able to: describe the normal pattern of facial growth and development, specifying the changes in the cranium, cranial base, maxilla, and mandible differentiate skeletal Class I, Class II, and Class III jaw relationships and growth patterns describe the interaction between changes in the position of the dentition during growth and the facial growth pattern.

Facial Growth: Patterns and Principles Before we discuss facial growth patterns, it is important to quickly review the concepts of pattern and growth pattern that you learned in the first part of this course:

1. A pattern is defined as the proportional relationships of parts.
2. A growth pattern refers to how proportional relationships change, or fail to change over time. In the context of facial growth, pattern describes how various facial structures relate to each other. Growth patterns refer to how these facial proportional relationships change over time as we grow and mature.

Timing of Facial Growth with General Body Growth Remember, a major influence on the pattern of facial skeletal growth is the different timing of growth of different body tissues. By now you’re very familiar with Scammon’s curves, but it’s important to keep in mind that not all structures in the body grow at the same rate at the same time.

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Craniofacial Skeleton: Functional Units

You will recall that it is easiest to understand the facial skeleton as a set of major functional units. The major functional units of the craniofacial complex are:

1. Cranium and cranial base
2. Maxilla
3. Maxillary dentoalveolar process
4. Mandibular dentoalveolar process
5. Mandible

Let’s review the effect of different growth timing for different types of tissues on the facial growth pattern.

Craniofacial Growth: Cranium and Cranial Base The cranium and cranial base follow Scammon’s neural curve and complete most of their growth by age 7.

Craniofacial Growth: Maxilla and Mandible The growth pattern of the face is influenced by an interaction between neural and general body growth. The result is that the facial skeleton grows according to a cephalocaudal gradient—the structures closest to the brain, which follows the neural growth curve, grow faster initially and mature earlier than structures closer to the bottom of the head. At birth, the mandible is relatively underdeveloped relative to the maxilla, and it gradually catches up but really doesn’t do so until the adolescent growth spurt is completed.

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Craniofacial Growth: Facial Soft Tissue It is also important to consider the growth of the facial soft tissues. Of course the soft tissues that cover the cranium have to follow the neural curve. You will not be surprised to hear that the cephalocaudal gradient also can be seen in the nose. The nasal bone completes its growth around age 10, but the cartilage and soft tissues of the nose (which are further from the brain) grow and become more prominent during adolescence. The mandibular musculature grows and becomes stronger during adolescence, as do the lips. The soft tissue chin is something of an exception-it becomes more prominent as the mandible grows forward but the soft tissue itself does not thicken appreciably. Can you see these changes in the boy pictured in images 1 and 2?

Constancy of the Pattern Although all children more or less follow Scammon’s curves, it is obvious that facial proportions are different in different individuals. For example, some children have a relatively small mandible, some a relatively large nose, etc. These differences tend to stay relatively constant during growth, which is another way of saying that the facial pattern in childhood tends to stay the same as a person grows and matures. For example, a child with mandibular deficiency is likely to remain mandibular deficient during adolescence and adulthood, while a child with true mandibular excess is likely to exhibit mandibular excess during adolescence and adulthood. Although constancy of the facial pattern is the rule, there are exceptions. Trauma, hormonal disturbances, and a number of other growth problems can affect and change facial symmetry and proportions.

Assessing Skeletal Growth (cephalometrics) Cephalometrics

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The structural components of the cranium and face can be distinguished well in a lateral projection (image 1). A lateral cephalometric radiograph (image 2) is an excellent way to document and understand the relationships between facial skeletal structures, or facial pattern. Note that you could outline the structural units on the radiograph (which we will call a ceph from now on) just as you could on the anatomic drawing. These radiographs are particularly valuable in evaluating how the five functional units relate to each other.

Cephalometric Tracings

A single cephalometric radiograph provides a snapshot of what the facial pattern and relationships are at any single point of time. In order to analyze facial pattern, it helps to make a tracing of the radiograph that outlines the functional units and other important structures of the facial skeleton. The tracing reduces the amount of information contained on a radiograph to the essential structures, and makes more apparent how these structures relate to each other.

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Cephalometric Superimpositions

It is only by carefully comparing radiographs taken at different times that we can truly discover and understand the changes in facial proportions over time, the facial growth pattern.

When two cephalometric tracings from radiographs of the same individual are superimposed on stable structure, areas that do not overlap are areas where changes, typically growth, have occurred. The tracings shown here are superimposed on the cranial base, which completes its growth around age 7, and therefore is stable after that time. This makes the cranial base superimposition ideal for showing the growth of the face after the time that permanent teeth are beginning to erupt.

Cephalometrics: Stable Landmarks

This comparison of radiographs shows how facial structures grow—the absolute amount for individual structures, and the relative amount compared to adjacent structures.

The superimposition shows that relative to the cranial base, the facial structures grew downward and forward. Note the somewhat greater growth of the mandible than the maxilla, which you would expect from the cephalocaudal gradient of growth that we discussed previously.

Variations in Skeletal Growth

Maxilla and Mandible Variation in facial growth affects the relationship between the functional units of the craniofacial complex, and you are used to seeing differing facial patterns all around you.

Some degree of variation is normal, but extreme variation in the growth of one or more of the functional units can cause facial disharmony that creates problems for the affected individual and could be considered abnormal. These growth variations occur in varying degrees and directions. For our purposes, fairly extreme examples help to illustrate this important concept. It is obvious that the girl in image 1 has had less mandibular growth than normal, and that the boy in image 2 has had too much.

Differences in the facial pattern resulting from variations in facial growth are most easily understood when analyzing the functional units in separate planes of space: antero-posterior, vertical, or transverse.

AP Variations in Skeletal Growth: Class II

Antero-posterior growth variations are typically expressed in an altered A-P position of either the maxilla, mandible, or both jaws.

A skeletal Class II jaw relationship occurs when the maxilla is positioned a significant amount anteriorly relative to the mandible or the mandible is significantly posterior to the maxilla. This can have several causes that are illustrated below in block diagrams using the structural components: mandibular deficiency (image 1, image 4); maxillary excess, which also is called maxillary prognathism (image 2); or a combination of both (image 3, image 5). A Class II relationship typically results in increased overjet that is due to the displacement of the jaws, not to displacement of the teeth relative to their own jaw.

By definition, a Class II growth pattern is one that leads to a skeletal Class II jaw relationship.

Image 5: Severe Class II, deficient mandible

A-P Variations in Skeletal Growth: Class III

A skeletal Class III jaw relationship occurs when the maxilla is positioned behind the mandible, and of course a Class III growth pattern leads to this condition.

This A-P relationship in the position of the jaws can have several causes: mandibular excess, which is also called mandibular prognathism (image 1); maxillary deficiency (image 2); or a combination of both (images 3,4,5). These jaw relationships usually result in no overjet (end-to-end incisors) or reverse overjet.

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A-P Variations in Skeletal Growth: Class I (Bimaxillary Protrusion)

Sometimes an individual can have normal skeletal relationship between the maxilla and mandible (image 1), but have both the upper and lower teeth and dentoalveolar processes displaced anteriorly relative to their skeletal bases (images 2,3,4).

In patients like this, the growth pattern of the jaws is normal (which is called a Class I growth pattern). The patient’s condition, strictly speaking, is bimaxillary dentoalveolar protrusion, but it is called bimaxillary protrusion for short (except by anthropologists, who use bimaxillary protrusion to describe protrusion of both jaws relative to the cranium). The dentists’ bimaxillary protrusion is characterized by proclined maxillary and mandibular incisors on normally positioned jaws (see image 2).

Image 1: Normal skeletal and dental relationships Image 2: Bimaxillary dentoalveolar protrusion, normal jaw relationship Image 3: Bimaxillary dentoalveolar protrusion, normal jaw relationship Image 4: Bimaxillary dentoalveolar protrusion, normal jaw relationship

Vertical Variations in Skeletal Growth

For normal jaw relationships, both the a-p and vertical positions of the jaws must be normal (image 1). Variations in vertical growth result in an altered vertical position of either the maxilla, mandible, or both jaws.

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The two extremes are skeletal open bite (long face) and skeletal deep bite (short face) patterns (images 2 and 3, respectively). Naturally, the growth patterns that produce these conditions are called the long face (or open bite) and short face (or deep bite) growth patterns.

Vertical Variations in Skeletal Growth: Open Bite / Long Face

Skeletal open bite, characterized by anterior open bite and a long face (increased vertical face height, see image 1), often is caused by a maxilla that has grown more in the posterior than anterior. This results in a palatal plane that appears tipped down posteriorly and downward-backward rotation of the mandible (image 2).

It can also be caused by a short mandibular ramus, which also leads to downward-backward rotation of the mandible, often with an increased gonial angle and steep mandibular plane (images 3 and 4).

Image 1: Long face pattern Image 2: Skeletal open bite, downward-back rotation of mandible Image 3: Long face pattern, open bite Image 4: Long face pattern; note increased lower face height

Vertical Variations in Skeletal Growth: Deep Bite / Short Face

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At the other extreme is the skeletal deep bite pattern, characterized by a short face that almost always is accompanied by a deep bite (image 1).

This is often caused by a growth pattern that leads to a long mandibular ramus, decreased gonial angle and flat mandibular plane (images 2,3,4). This boy has maxillary incisors that are tipped forward and are protrusive. That’s not uncommon in patients with a deep bite, but it’s not part of the vertical problem—you can have a short face and deep bite without protrusion of upper incisors.

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Image 1: Short face, deep bite anteriorly (excessive overbite) Image 2: Short face pattern Image 3: Short face pattern (same pt.) Image 4: Short face pattern (same pt.)

Transverse Variations in Skeletal Growth

Transverse growth variations are typically expressed in an altered transverse position of the mandible. True maxillary asymmetry is rare, but mandibular asymmetry occurs frequently.

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A mandibular asymmetry with the chin to the right side can result from excess growth on the left, decreased growth or degeneration on the right, or a combination of both. Conversely, a mandibular asymmetry to the left (as shown here) can result from excess growth on the right, decreased growth or degeneration on the left side, or a combination of both.

Transverse Variations in Skeletal Growth: Patient Example

For this patient (image 1), the deviation of the mandible to her left can be seen even more clearly in a P-A cephalometric radiograph (image 2). She has a corresponding mandibular midline deviation to the left that can be seen in her intraoral photograph (image 3). Examination of her panoramic radiograph reveals a left mandibular condyle that is smaller than the right mandibular condyle (image 4).

Detecting the asymmetry may be easy in a case like this, but understanding the cause of the asymmetry can be much more difficult. Because her face height is abnormally long on the right side, the asymmetric mandibular growth was elongation of the neck of the ramus on the right, which pushed the chin to the left.

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Image 1: Frontal photograph, note chin asymmetry Image 2: P-A cephalometric radiograph, same pt. Image 3: Frontal intraoral photo, note midline deviation Image 4: Panoramic radiograph, note long condylar neck on right

Complex Variations in Skeletal Growth

Disturbances in facial growth that affect multiple planes of space can create complex deformities. Analyzing facial and facial growth patterns using three distinct planes of space helps you fully analyze and understand the variation.

This patient (image 1), more obviously than the previous one, has hemimandibular hypertrophy (excessive growth of the mandible on one side). Notice how the excess growth of the right condyle and ramus (image 5) has caused problems in all three planes of space, including mandibular asymmetry, a dental midline discrepancy, and lateral open bite (images 2,3). In the lateral

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cephalometric radiograph (image 4), you can see two distinct mandibular planes because the ramus is longer on the right side. In addition, there is a Class III molar relationship on the right side.

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Image 1: Frontal view, chin off to left Image 2: Note unilateral posterior open bite Image 3: P-A cephalometric radiograph (same pt) Image 4: Lateral cephalometric radiograph (same pt)

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Dental Compensations for Skeletal Discrepancies Now let’s look at the effect these skeletal variations have on the teeth and dentoalveolar processes. The dentition exhibits a remarkable ability to compensate for an unfortunate growth pattern of the jaws. These compensations allow individuals with different facial types to have teeth that fit together reasonably well despite the jaw relationships. Even if a severe malocclusion results from the growth pattern, often the dental compensations mask the severity of the skeletal problem, i.e., the jaw deviations are worse than the dental deviations. Again, it is easiest to understand these dental compensations by analyzing in three planes of space the dental compensations that accompany different facial patterns.

Dental Compensations for Class II Growth Patterns Class II patients often have upper incisors that are upright or more lingual relative to the maxilla, and lower incisors that are flared facially relative to the mandible (image 1). Notice how this patient has a deficient mandible (image 2) but a relatively minimal increase in overjet because of forward movement of the lower teeth relative to the mandible and lingual tipping of upper incisors (image 3). The lower incisors are not as far forward as they would have been if the growth pattern were normal and the mandible were not deficient.

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Image 1: Dental compensation for Class II growth </td> <td> Image 2: Skeletal Class II, mandibular deficiency </td>

Image 3: Forward position of mandibular teeth, decreasing overjet </td> <td></td>

AP Dental Compensations: Skeletal Class III

Class III patients often have upper incisors that are flared facially relative to the maxilla and lower incisors that are upright relative to the mandible (image 1).

Notice how this patient (image 2) has a Class III profile with the typical associated dental compensations. Her Class III skeletal problem (image 3) was so severe that even with the dental

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compensations she could not maintain a positive overjet. She was treated with surgical repositioning of the jaws.

AP Dental Compensations

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Many patients with Class II and Class III jaw relationships can have fairly normal overjet, despite the fact that there is a significant discrepancy between the AP position of the maxilla and mandible.

These two patients (images 1,2 and images 3,4) have relatively small differences in overjet and similar dental occlusion, but widely different jaw structures and relationships.

Vertical Dental Compensations: Skeletal Open Bite

Vertical dental compensations are expressed in the vertical position of incisors relative to their skeletal bases (maxilla or mandible). Remember that a skeletal open bite pattern is often associated

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with a short mandibular ramus and steep mandibular plane.

In order to compensate for this skeletal pattern, more eruption of incisors than normal would decrease the open bite (image 1). Patients with these compensations (images 2,3,4) can have relatively normal overbite relationships, especially when compared to their underlying skeletal pattern. For this girl, incisor eruption almost totally compensated for the downward rotation of the mandible—but she would benefit from correction of her Class III and long face characteristics. This would require surgical repositioning of both jaws.

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Vertical Dental Compensations: Skeletal Deep Bite

Remember that patients with a skeletal deep bite pattern typically have a decreased gonial angle and a flat mandibular plane. In order to compensate for this relationship, less than normal eruption of incisors would be required (image 1).

This short-face girl has reasonably normal overbite despite her short face (images 2,3,4,5). Note the facial tipping of the incisors in both arches.

Image 1: short face height predisposes a patient to anterior deep bite Image 2: short face patient Image 3: short face patient Image 4: minimal overbite despite short face pattern

Image 5: minimal overbite, but incisors are severely proclined

Transverse Dental Compensations: Skeletal Asymmetry

As with the other planes of space, transverse dental compensations usually accompany asymmetric jaw growth. If the jaw deviates to one side, the teeth shift back toward the midline so that the dental midline discrepancy often is less than the jaw discrepancy.

For this patient the dental compensation is apparent but not nearly enough to create normal dental occlusion, and treatment would have to be surgery to reposition the jaws. In order to get good jaw relationships, however, the dental compensation would have to be removed by presurgical orthodontic treatment.

PA cephalometric film: Note the mandibular asymmetry with the chin to the patient’s left. Same patient, intraoral view: The position of the mandibular teeth are compensated to mask the skeletal asymmetry of the mandible.

Orthodontic Treatment in the Context of Growth

Treatment for Growing Patients

For growing patients, it is important to recognize skeletal malocclusions so they can be treated at the appropriate time. Often this treatment will involve growth modification—trying to alter the position of the skeletal bases and/or dentoalveolar segments while an individual is growing. Of course, you can’t modify growth that isn’t happening, either because the child is in a period of slow growth prior to the adolescent growth spurt, or because an adolescent already is past the growth spurt.

For example, consider the problem presented by failure of the maxilla to grow forward as it should (image 1). This 7 year-old boy presented with the a convex profile and a facial appearance of maxillary deficiency and an anterior crossbite (images 2,3). Cephalometric analysis revealed a component of maxillary deficiency contributing to his Class III skeletal pattern and reverse overjet (image 4).

Depending on their physiologic maturity rather than chronologic age). If it is delayed beyond about age 10, the skeletal deficiency no longer can be treated without surgery.

Image 2: age 7, before growth modification treatment Image 3: age 8, after treatment

Treatment for Adults / Non-Growing Patients

In older patients who have completed the vast majority of their growth, there are fewer options for treating a skeletal malocclusion. Sometimes a jaw discrepancy is so great (image 1) that repositioning the jaws through orthognathic surgery is the only way to obtain an acceptable facial appearance (image 2) and get the teeth to fit together normally.

Early diagnosis of the facial pattern and growth pattern, and appropriate treatment at the ideal time are essential for effectively treating skeletal malocclusions. Unfortunately, it is almost impossible to control excessive mandibular growth, and an individual with this growth pattern is quite likely to end up needing surgery, despite efforts to treat it during growth.

Fortunately, as these pre- (image 1) and post-surgery (image 2) images show, it is possible now to correct jaw deformities with surgical repositioning of either or both jaws. Class III problems are more likely to require surgery because of the tendency of the mandible to grow after maxillary growth is nearly completed.

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Image 1, A girl with a Class III skeletal relationship: Her malocclusion was treated with surgical-orthodontics since she was no longer growing. Image 2, Same girl post-surgery: Her maxilla was moved forward surgically to correct her skeletal and dental Class III relationships.

Summary

In summary:

• The pattern of facial growth reflects an interaction between the neural and general body growth curves, so that growth of the face trails behind growth of the cranium and cranial base, and growth of the mandible trails behind growth of the maxilla
• Some individuals have a pattern of growth with excessive or deficient growth of the maxilla and/or mandible, which leads to what is best described as a skeletal malocclusion (by definition, a skeletal malocclusion means that the jaw position is abnormal and tends to create a dental malocclusion).
• For mild skeletal discrepancies, dental compensation during growth may create normal dental occlusion, and if not dental compensation created by orthodontic treatment can be satisfactory.
• For moderately severe discrepancies, growth modification can be successful treatment; for the most severe ones, surgery is the only option.
• Mandibular growth that is more downward and less forward leads to a Class II skeletal and dental relationship.
• Mandibular growth that is more forward and less downward leads to a Class III skeletal and dental relationship.

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The pattern of facial growth, whether normal or leading to skeletal malocclusion, is established early and rarely changes spontaneously. In children with skeletal malocclusion, dental compensation for the skeletal discrepancy usually occurs so that the dental occlusion is less abnormal than the jaw relationship.

Now take the self-test in the following section. Before you do, be sure you have read pages 96-103 in the 5th edition of Contemporary Orthodontics (or pages 111-119 in the 4th ed.). Then use the self-test to help you understand these concepts of evaluating patients and using that information to plan appropriate treatment.

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5. Maturational Changes

Introduction

Purpose of this Program

In this program we discuss changes that occur in adult life that are a continuation of the normal developmental pattern. Some of them represent further physical growth at a time when it was once thought that no further increase in skeletal dimensions occurred. Others are further steps along the path of development that we have followed in childhood and adolescence which lead to the changes we refer to (not very positively) as aging. But you’ll experience aging, and you need to know how it affects the patients you will be treating.

In addition to viewing this program, read pages 104-113 (5th ed) or 119-128 (4th ed) in Contemporary Orthodontics, and take the self-test at the end of this module.

Learning Objectives

After viewing this program, you should be able to:

• Section 1 – describe the timing and pattern of skeletal growth in adults
• Section 2 – describe the facial soft tissue changes that accompany aging and discuss their clinical significance
• Section 3 – describe maturational and aging changes in the dentition, including normal patterns of attrition and abrasion, as well as alterations in pulp chambers and vascularity
• Section 4 – discuss the possibilities for crowding of lower incisors in late adolescence and early adult life, and indicate which possibility is the most likely cause

Skeletal Growth in Adults

Weight Change in Adult Life

Obesity, of course, is now a public health concern of considerable magnitude. On the other hand, just about everyone gains weight between ages 20 and 50. A recent report showed that on the average, a college graduate can be expected to gain 40 pounds before the 25th anniversary of graduation. Some, of course, will gain a lot more than that, but it simply isn’t true that growth in the context of weight gain ends as one enters adult life.

Height Changes in Adult Life

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Does an increase in height occur in adult life? Interestingly, the answer is no, and you already know why—don’t you? That’s right, once the epiphyseal plates of the long bones have been bridged by bone, which normally occurs at the end of adolescence, any further increase in the length of these bones would have to be due to surface changes in the joints. In fact, beyond age 50 most individuals experience a decrease in height of an inch or two, as intervertebral discs become compressed. It’s probably true that the more weight you gain, the more height you’re likely to lose for this reason. That doesn’t mean, however, that all the epiphyseal plates close at the same time. The small bones of the hands and feet, especially the phalanges and tarsals, still can have endochondral growth into the early 20s, and growth of the cartilaginous nasal septum is possible for a long time because this cartilage persists indefinitely. Also, bones that do not depend on growth of cartilage would be capable of growth, no matter what happened to growth cartilages. The mandible is the best example of a bone like that. It’s not surprising, therefore, that hands, feet and lower jaws are likely to grow at least a little after growth in height is completed. The nose, as we will see, can grow a lot.

Men’s Height Percentiles with Age

Changes in Facial Dimensions in Adults The amount of facial growth that occurs beyond the early 20s was not appreciated until Behrents in the 1980s succeeded in recalling a number of individuals whose facial growth had been studied with

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serial cephalometric radiographs in the Bolton Growth Study in the 1930s, 1940s and early 1950s. Behrents was able to get follow-up cephalograms forty to fifty years after the last recall for this growth study, when the subjects were in their 30s. Superimposition on the cranial base showed that the same pattern of downward-forward growth of the jaws that occurred in adolescence continued, albeit very slowly, until quite late in life. Although the amount of growth in each year was small, over decades it added up to noticeable amounts. In the woman shown here, the black tracing is from age 34, and the red one is from age 83. Her facial growth changes over a this period of nearly 50 years were reasonably typical of what was observed: both the maxilla and mandible grew forward, in her case a little over 2 mm the ramus lengthened a couple of millimeters, reflecting further growth of the muscles of the pterygoid sling (masseter and internal pterygoid) the teeth in both jaws were carried forward by the late jaw growth the nose grew nearly twice as much as the jaws.

Changes in Facial Dimensions in Adults: Pattern Constancy Note the changes in this man from age 37 to 77. He had been treated orthodontically for a Class II malocclusion, the orthodontic description of the condition in which the upper teeth protrude. This usually is due to deficient growth of the mandible. In his case, the same pattern of growth that led to Level I Growth and Development — Unit C · 172 / 191

his Class II problem as a youth continued late into adult life. His maxilla grew forward more than his mandible, and the maxillary incisors were carried forward more than his mandibular incisors. The result, of course, was a partial return of the condition for which he had been treated successfully many years previously. Is this relapse? In one sense it is—the teeth returned toward their pretreatment relationship. In another sense it isn’t, because what really happened was growth long after treatment that tended to recreate the original dental relationship. He didn’t simply slip back toward the original dental problem. If it’s not relapse what is it? Lapse? That’s not a commonly used term, so we can laugh when it’s used this way—but a return toward a previous dental and jaw relationship because of late disproportionate growth isn’t like relapse in the usual sense of going back to the original situation.

Changes in Facial Dimensions in Adults: Dento-alveolar Changes

It is possible to superimpose on the bony structures of the maxilla and mandible, in order to get a closer look at the changes in the shape and size of the bones, and in the changes of the dentition relative to its bony base. These superimpositions, showing the mean changes in Behrents’ sample of individuals who were recalled after >30 years, make it clear that the pattern of change in adult life is very similar to what occurred during growth in adolescence. From one year to the next there is very little change, but over decades changes can and do occur.

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We really can’t say that growth stops in late adolescence. Instead, it declines to the slow rate that is characteristic of adults, and continues at that slow rate beyond age 50.

Facial Soft Tissue Changes with Aging

Characteristics of Facial Soft Tissue Changes With increasing age, we have seen that surprisingly large changes occur in the hard tissues of the face —but changes in the facial soft tissues are greater than the hard tissue change, and a dentist must understand them to offer the best care to older patients.

The soft tissue changes can be put into 3 categories:

1. decrease in lip fullness
2. downward movement of the lips relative to the teeth, leading to less display of the maxillary incisors and greater display of the mandibular incisors
3. deepening of skin folds and loss of skin tone

Let’s look at these in turn.

Decrease in Lip Fullness

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As males or females get older, not only does lip fullness decrease, the amount of lip vermilion also decreases (image 1). One of the ways you have learned to judge the age of the individual you just met is to assess the lip contours. You can imagine that dentists would have to be careful with treatment that decreased the amount of tooth support for the lips and accentuated the lip changes.

Fortunately, for most people the lip changes are not a problem and are accepted as just part of life. If it is a problem for someone who is fighting aging, plastic surgeons can thread denatured collagen matrix (Alloderm) into the lips to provide a more subtle and longer lasting increase in fullness (images 2 and 3) than is achieved with collagen implants. This material is soft, so after treatment it wouldn’t be like kissing a rope.

Decrease in Lip Fullness (cont.)

There is an adolescent spurt of growth in the lips, especially in girls, and lip fullness increases up to about age 16—and then begins to decline, to the point that some young women are troubled by this in their early 20s and want to do something about it. This advertisement, which appeared in a magazine (and probably elsewhere) not long ago, states it quite clearly: “full lips … are associated with beauty and youth.”

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Downward Movement of the Lips After adolescence, the lips tend to move downward relative to the teeth so that maxillary incisor exposure at rest and on smile decreases, while the lower incisors increasingly are exposed. The change can be seen between adolescence and the mid-20s (image 1) and between that time and middle age (image 2). It can make quite a difference in facial appearance. In fact, incisor exposure is one of the things that people judge when evaluating how old someone is.

Incisor Display as a Function of Age

This graph shows the amount of upper and lower incisor display at rest as a function of age, from a cross-sectional study published in the prosthodontic literature.

Note that below age 30, there is almost no display of the lower incisor at rest but >3 mm display of the upper incisor. By age 50, there is considerably more display of the lower than the upper incisor, and beyond age 60 the average individual has no display of the upper incisor and >3 mm display of the lower incisor.

Keep in mind that the chart does not show the considerable individual variation that exists. Nevertheless, it is obvious that more display of your lower than your upper incisors, at rest or when you smile, makes you look older. In orthodontic or prosthodontic treatment, positioning the incisors relative to the lips is a critical part of obtaining an esthetic result. You’ll rarely be thanked for making your patients look older.

Skin Folds and Tone

Typical soft tissue changes with increasing age include

• slight jowling (droop of the cheeks) as skin tone declines (skin becomes less elastic)
• submental fat deposition (under the chin)
• deepening of the paranasal skin folds, and
• droop of the commisures (outer ends) of the lips.

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The left image shows this woman at age 25, the right one at age 40. The changes are subtle but noticeable, and will become more apparent as she gets older.

Skin Folds and Tone (cont.) In plastic surgery the classic “face lift” approach was to pull the skin tighter to decrease the folds and wrinkles. In recent years it has been acknowledged that a more natural appearance is obtained by “filling up the bag”, increasing the amount of hard tissue support for the overlying soft tissues. This can be done to some extent by orthodontic or prosthodontic treatment (images 1 and 2), and to a greater extent by surgical repositioning of the jaws (image 3).

For the patient in late middle age who is seen in images 1 and 2, fracture of the crown of a lower incisor was a problem for two reasons. Not only was the rough edge of the tooth irritating to the tongue and lip, the broken tooth was clearly apparent when she smiled. When lower incisors are crowded in adult life and something happens to one of them, extracting the damaged tooth and closing the space orthodontically can be a good plan. For this patient, however, that would have decreased lip support, so a better plan was to align both the upper and lower incisors, moving them forward and increasing lip support, and then restore the fractured incisor. She appreciated looking a bit younger rather than older after treatment.

For the patient in image 3, who had always had less support for her facial soft tissues than ideal, a dramatic improvement in facial soft tissue appearance was created by surgery to move her maxilla forward. For her, the problem was lack of bony support for the soft tissues, and tightening up the skin would not have produced nearly as favorable a change.

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Image 1, Before orthodontic treatment: The fractured lower incisor was a problem for two reasons: it irritated the lip and was apparent on smile. Image 2, After orthodontic treatment: Aligning the incisors by tipping them facially has increased lip support and facilitated restoration of the fractured tooth. Image 3, Before and after orthognathic surgery to advance the maxilla: Note that advancing the maxilla tightened her skin and decreased the paranasal folds, giving her a more youthful appearance as well as better dental occlusion.

Changes in the Teeth and Supporting Structures

Aging Changes in the Teeth: Wear and Attrition

Aging changes in the teeth happen in two major ways: wearing away of the enamel surfaces and decrease in the size of the pulp chamber.

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Extensive wear of tooth surfaces was common among humans until quite recently, because a primitive diet included harder and more abrasive materials than the modern soft diet. It is instructive to look at Australian aboriginal skull specimens from the 19th century to see the progressive and extensive wearing away of the teeth that occurred.

Image 1 shows the mandible of a child of approximately age 8. Note that wear of the occlusal surfaces of the first molars already exists, although these teeth have only been in the mouth for a couple of years. By dental age 14 (image 2), wear has penetrated through the enamel on the occlusal surface of the first molars and extensive wear exists on the incisors. In an adult of indeterminate age, enamel is gone and dentin is being worn away on the surface of all teeth except the 3rd molars, and enamel has been penetrated on these teeth.

Image 1: Australian aboriginal child approximately age 8 Image 2: Australian aboriginal youth approximately age 14 Image 3: Australian aboriginal adult, indeterminate age

Aging Changes in the Teeth: Wear and Attrition (cont.) A lateral view of maxillary and mandibular teeth in occlusion, from the collection of aboriginal skulls, shows how the cusps of the teeth have been worn away so that a flat plane occlusion exists.

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It also shows that there is wear of the interproximal surfaces as well as the occlusal surfaces of the teeth. This occurs as the teeth rock back and forth relative to each other under the stress of heavy occlusal forces, which were necessary when none of the food had been cooked. With increasing age, the tooth loses both crown height and width.

Aging Changes in the Dental Pulp

Given the amount of wear on the teeth that you just observed, you could ask at what point pulp exposure occurs and the tooth is lost. The answer is that this is possible but rarely occurs. Why not? Because with increasing age the size of the pulp chamber decreases as new dentin is formed, and this process accelerates in teeth that are being worn away.

Note the reduction in the size of the pulp chambers, especially of the molars, in this typical modern individual who does not have extensive wear of the teeth. The upper radiograph was taken at age 16, the lower one at age 26. This process of gradually filling in the pulp chamber continues throughout adult life, but proceeds less rapidly as you get older—unless there is severe wear of the teeth.

Now that the teeth don’t wear down as they once did, this protective mechanism is less important, but it still occurs—and makes it easier to restore teeth in older individuals. The dentist doesn’t have to worry so much about a pulp exposures when the tooth is being prepared.

Exposure of the Crowns of the Teeth: Gingival Recession?

As teeth erupt, it is apparent that more of the crown is exposed beyond the gingiva. This continues after teeth have come into occlusion, as in the girl shown here. At age 10, only a part of the crowns of her maxillary incisors were exposed. At age 16, almost all the crown of each incisor is exposed, and the gingiva appears to have retracted.

At one time it was thought that this occurred primarily by “passive eruption”, which was not eruption at all but recession of the gingiva. There is no doubt that the gingiva retracts relative to the crown during adolescent growth. There is considerable doubt, however, that this really is gingival retraction. Instead, the tooth continues to erupt, and the gingiva remains about where it was. Note the increase in face height that occurred for this patient during the time that the incisors were increasingly exposed. The teeth had to erupt to stay in occlusion as the lower jaw grew downward away from the upper jaw. All that is necessary to create the appearance of gingival retraction is for the supporting structures of the teeth not to grow quite as much as the teeth erupt. Passive eruption, in short, is not nearly as important as active eruption in increasing the amount of the crown that is exposed.

Image 1: Exposure of the clinical crowns of maxillary incisors at age 10 and 16 Image 2: Smile photos at age 10 and 16. Note the increase in face height that required further eruption of the teeth.

Exposure of the Crowns of the Teeth: Gingival Recession?

The concept that retraction of the gingiva is part of normal development is important when aging is considered, because at one time it also was thought that gingival recession normally continued as patients got older.

In fact, in adults gingival recession (image 1) is an indication of pathology, not an inevitable part of getting older. If you take care of your teeth and the periodontal tissues remain healthy (image 2), gingival recession doesn’t occur during aging. If you develop periodontal disease, it does. Gingival recession isn’t a part of the normal aging process.

Knowing that retraction of the gingiva is not part of normal development is important when aging is considered, because at one time dentists thought that gingival recession normally continued as patients got older, and considered it just a part of the normal aging process. Now we know that it’s a sign of disease or trauma.

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Image 1, Gingival recession in an adult: Recession is the result of poor periodontal health, not a normal consequence of growth and aging. Image 2, A healthy adult dentition: Greater exposure of the crowns, up to but not beyond the crown-root interface, is largely due to eruption in response to vertical jaw growth that leaves the gingiva behind.

Late Lower Incisor Crowding: Why?

Prevalence of Lower Incisor Crowding by Age

It is a frequent observation that lower incisors become more crowded after adolescence. Quite typically, lower incisors that were nicely aligned at age 15 or 16 become slightly (sometimes more than slightly) crowded and irregular by the early 20s. Those that were mildly or moderately irregular get worse, as this graph from American epidemiologic data shows. Did that happen to you?

This is frustrating for orthodontists, of course, because it is so difficult to keep lower incisors perfectly aligned after braces are removed during adolescence. But late crowding of these teeth occurs whether or not they had been aligned orthodontically. Why does this happen?

Late Incisor Crowding: Third Molars?

Lots of people in the general population are sure that third molars cause the late incisor crowding. It makes sense because of the timing: the crowding develops about the time the third molars are erupting, or should erupt but can’t because there’s no room for them. It also makes sense because it seems reasonable that the third molars could exert pressure against the other teeth as they try to wedge their way into the dental arch. When you see an impacted third molar on a panoramic radiograph, you can picture how it would be pressing against the other teeth.

For many years, dentists encouraged patients to believe that the third molars caused crowded incisors. So, when Susie’s incisors begin to crowd while she’s off in college, her grandmother is very likely to tell her that her third molars are the problem. The orthodontist often listens to former patients say “I know I should have gotten my third molars out, but I didn’t get around to it—and now look at my teeth”. It’s easy for the doctor to agree with that even if he or she is wondering if it’s correct.

Problems with the 3rd Molar Theory The theory that third molars are the cause of the late incisor crowding has two big problems:

1. Lower incisor crowding develops in people whose third molars are congenitally missing, at about the same level of prevalence as in those with third molars. For the patient shown in this radiograph, the lower incisors became quite crowded during his late teens, but third molars couldn’t have been the cause because he didn’t have any.

2. Numerous studies of the relationship between third molar presence, position, or other characteristics have reported little or no correlation with incisor crowding. If the third molars have a role in the development of incisor crowding, it’s clear that it isn’t a major one.

When you think about it, it’s a bit hard to see how the third molar could win a contest against all the rest of the teeth. One little tooth at the back of the dental arch pushes all the rest of them around? Powerful tooth indeed if it can do that!

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Late Incisor Crowding: Interproximal Wear of Teeth?

A second theory, which offers an explanation for crowding more generally, but also relates to late incisor crowding, was based on the aboriginal skeletal material that we’ve already looked at. Raymond Begg, an influential Australian orthodontist of the mid-20th century who collected aboriginal skeletal remains, noted that the Australian aboriginals had (and have) remarkably well-aligned teeth. They do not develop late incisor crowding, while their fellow citizens of European descent often have both incisor crowding and impaction of third molars.

Begg came to believe that interproximal wear of the teeth was necessary to provide enough space in the dental arch for all the permanent teeth, and that this was the key to the excellent alignment of teeth in the aboriginal population. He suggested that extraction of first premolars, to provide the space that wasn’t being produced by attrition of the other posterior teeth, was necessary in most people to obtain good dental alignment now that almost everyone is on a relatively soft diet.

Would interproximal wear in the aboriginal group but not in the European group really explain the difference in dental crowding?

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Problems with Begg’s Theory

The interproximal wear theory also has a major problem: modern Australian aboriginals have a diet very different from that of their ancestors, and quite similar to the European soft diet. Yet these modern aboriginals who did not experience interproximal wear as they were growing up don’t develop incisor crowding.

This dissected skull, showing the dentition in a young adult who has some wear on the teeth but not a lot, is from an unknown source and probably is not a modern Australian aboriginal. But both the minimal wear on the teeth (note how little loss of the width of teeth has occurred) and the excellent alignment are typical of Australian aboriginals now.

In the aboriginal population, the primary effect of the dietary change has been to greatly increase the prevalence of periodontal disease, not to produce crowding of the teeth. The significant difference between the European-derived and aboriginal populations turns out to be a shorter neck and higher tongue position in the aboriginals, and a wider face. As you will learn in the teaching module on equilibrium effects, resting pressures by the tongue and lips/cheeks against the teeth are a major determinant of dental arch dimensions and the position of the teeth.

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Late Incisor Crowding: Late Mandibular Growth? We have noted already that a small increment of mandibular growth often occurs after growth of the other facial structures has declined to the very slow adult growth rate. Could this be related to late incisor crowding? The answer is yes. How it happens can be understood best by looking at what happens to lower incisors in patients with excessive mandibular growth. In a typical patient with this growth pattern (images 1 and 2), superimposition of serial cephalometric radiographs shows that as the mandible grows forward, the incisors are tipped lingually and forced into a more upright position as lip pressure against them increases. The effect is to shorten arch length, and this leads to crowding of the incisors. In almost everyone, the mandible grows forward a little relative to the maxilla in the late teens or early 20s. As this happens, the lower incisors tip lingually and crowding develops. Both x-rays and clinical examination show that backward movement of the incisors relative to the chin, not forward movement of the posterior teeth, almost always is what happens as late crowding develops. To put it in terms we’ve used before: the determinant of late lower incisor crowding is the small amount of mandibular growth that usually occurs at that time; the mechanism is lip pressure against the incisors.

Image 1: Cephalometric superimpositions, excessive mandibular growth. Note the lingual tipping of the mandibular incisors as the mandible grew forward.

Image 2: By the end of adolescence, the upright position of the incisors was apparent, and incisor crowding was developing.

Late Incisor Crowding: Summary

For late lower incisor crowding, late mandibular growth explains almost all of what happens. It seems clear that lack of wear on the teeth has little or nothing to do with it.

Is it possible that pressure from third molars that are trying to erupt contributes to crowding? Perhaps the third molars sometimes are “the straw that broke the camel’s back”, but their influence is minimal compared to mandibular growth.

Maturational Changes: Summary

Important points to remember about maturational changes:

Facial growth continues, very slowly, at least into the 50s and perhaps beyond:

• the nose grows more than anything else
• for most people, there’s a little more mandibular than maxillary growth
• the original growth pattern is followed, so a partial re-creation of a jaw relationship corrected by orthodontic treatment may occur many years afterward

Facial soft tissue changes:

• decrease in skin tone (less elasticity), which causes wrinkles
• deepening of facial folds, especially the nasolabial fold
• downward movement of the lips → more display of lower incisors, less display of upper incisors

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Dental changes:

• enamel wear (but much less with modern soft diet than previously)
• decrease in size of pulp chambers

Crowding of lower incisors in late teens / early 20s:

• mostly due to differential forward growth of mandible at that time
• as this growth occurs, lower incisors tip lingually in response to lip pressure against them
• third molars play a minor, if any, role in producing incisor crowding

Self-Test Referral

Now, before you take the self-test in the following section, be sure you have read pages 104-113 (5th ed) or 119-128 (4th ed) in Contemporary Orthodontics. Then use the self-test to be sure you understand the important points you learned in this module.

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