01 - 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.

Image 1, Permanent Dentition Calcification: Knowing these average values will help you estimate dental age.	Image 2, Permanent Dentition Eruption: Knowing these average values will help you estimate dental age.

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

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.

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.

Image 1, Dental age 15: Root formation should be complete for all teeth but the third molars.	Image 2, Dental age 15: Can you identify the key indicators of dental age 15 in this image?

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.

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.

Longitudinal Sequence: Age 6, Dental Age 7+ (cont.)

A lateral cephalometric radiograph taken on the same day as the panoramic radiograph that you just observed provides different view of the same child. This is helpful in observing the eruption position of the teeth. Focus your attention for a moment on the maxillary arch.

Note that at dental age seven the maxillary central incisor has erupted into occlusion but the maxillary lateral incisor is still above the occlusal plane. Look how high the maxillary canine is relative to the first premolar and also to the second premolar. The first premolar will erupt ahead of the canine but the canine will catch up with the second premolar and usually will erupt about the same time it does. In the mandibular arch, the canine is slightly below the first premolar but not very much. In that arch the two teeth will erupt about the same time.

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

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.

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.

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.

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.

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.

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?

Longitudinal Sequence: Age 9 1/4, Dental Age 11+ (cont.)

No, this radiograph shows all the characteristics of dental age 11, and some variation in the amount of root development is entirely within the range of normal. Individual patients usually have some small deviations from the usual pattern. As they say in Australia, “Not to worry”.

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.

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.)

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.

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?

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!

Common Variations in Tooth Eruption

Eruption Variations

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.

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.

With this particular sequence of eruption, it is quite likely that the maxillary canine will be forced labially. When that happens, parents notice. The result may be a phone call to the dentist with a comment like “I’m afraid our child has begun to develop fangs”. One day in the future you’ll have to reassure somebody’s mother that the child isn’t about to become a vampire.

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?

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?

Image 1: Panoramic radiograph, dental age 11	Image 2: Intraoral view a few months later
Image 3: Intraoral view after orthodontic treatment to obtain enough space	Image 4: Panoramic radiograph after treatment

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 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.

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

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 cephlometric 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

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.

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 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.

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.

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.

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.

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 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.

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.

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.

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.

Image 1, Space used as the maxillary laterals erupt: Often the eruption of the lateral incisors uses the last available space across the incisor region.	Image 2, Radiograph of erupting maxillary laterals: Often the eruption of the lateral incisors uses the last available space across the incisor region.

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.

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.

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.

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?”

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.

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.

Summary

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 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.

Self-Test

Question 1

Key differences between dental age 9 and dental age 10 are:

(1) root completion of lower incisors

(2) degree of resorption of primary canines

(3) degree of root formation of canines and premolars

1. 1 and 2
2. 2 and 3 ✓
3. 1 and 3
4. 1, 2 and 3

Correct

That’s right. At dental age 10, the primary canines show more resorption and the canines and premolars have more root formation than at dental age 9, but the appearance of the lower incisors is very similar at ages 9 and 10. It takes 2 years to complete the roots after an incisor erupts, so the lower incisors are likely to already be complete at age 9.

Incorrect

No, you’re wrong. At dental age 10, the primary canines show more resorption and the canines and premolars have more root formation than at dental age 9, but the appearance of the lower incisors is very similar at ages 9 and 10. It takes 2 years to complete the roots after an incisor erupts, so the lower incisors are likely to already be complete at age 9.

Question 2

What is the dental age when the roots of all the permanent teeth (except third molars) are completed?

1. 12
2. 13
3. 14
4. 15 ✓
5. 16

Correct

That’s right, the roots of all permanent teeth except third molars are completed at dental age 15. The guideline is that it takes two years after eruption for the roots to be completed, and the second molars often haven’t completed their eruption at dental age 12, so root completion wouldn’t be expected at age 14 or younger.

Incorrect

No, that’s wrong. The roots of all permanent teeth except third molars are completed at dental age 15. Remember the guideline that it takes two years after eruption for the roots to be completed. The second molars often haven’t completed their eruption at dental age 12, so root completion wouldn’t be expected at age 14 or younger.

Question 3

Which of the following is not characteristic of the first stage of eruption, dental age 6?

1. eruption of the first permanent molars
2. eruption of the mandibular central incisors
3. eruption of the maxillary central incisors ✓
4. formation of half the root of the mandibular lateral incisors
5. formation of less than half the root of the maxillary lateral incisors

Correct

That is correct. The maxillary central incisors erupt a year later than the permanent first molars and the mandibular central incisors, so their eruption isn’t part of dental age 6.

Incorrect

No, that’s incorrect. The maxillary central incisors erupt a year later than the permanent first molars and the mandibular central incisors, so their eruption isn’t part of dental age 6. At age 6, the first molars and mandibular central incisors erupt, the mandibular lateral has half its root formed and the maxillary lateral somewhat less than that.

Question 4

With regard to space for the permanent teeth as they erupt, what does the term “incisor liability” mean?

1. space for incisors normally is less than required ✓
2. space for incisors normally is only equal to required
3. if space is a problem, it’s almost always space for incisors that’s the problem.
4. you’re more likely to get sued if the incisors are involved

Correct

That’s right, incisor liability means that under normal conditions there isn’t quite enough space for the permanent incisors when they erupt, which can lead to temporary crowding even when there’s ultimately enough space.

Incorrect

No, that’s not the best answer even if it’s not totally incorrect. By “incisor liability” we mean that under normal conditions there isn’t quite enough space for the permanent incisors when they erupt, which can lead to temporary crowding even when there’s ultimately enough space, so the correct answer is the first choice.

Question 5

During normal develoment, which of the following is not a source of additional space to overcome incisor liability?

1. increase in intercanine width
2. distal movement of the maxillary canine into primate space ✓
3. distal movement of the mandibular canine into primate space
4. labial positioning of the permanent incisors
5. they’re all sources of additional space

Correct

That’s correct, the maxillary canine can’t move distally into a primate space that’s mesial to it, but the other possibilities all help overcome incisor liability.

Incorrect

No, that’s wrong. Remember that the primate space in the maxillary arch is mesial to the canine, not distal to it as it is in the mandibular arch. The maxillary canine can’t move distally into a space that’s mesial to it, but an increase in intercanine width, distal movement of mandibular canine and labial positioning of permanent incisors all help overcome incisor liability.

Question 6

To a dentist, a beautiful smile in the primary dentition reveals large spaces between the primary incisors. Which of the following is not a reason for that attitude?

1. permanent incisors are likely to erupt in crossbite unless spaces are present. ✓
2. spacing is required to provide enough space for the permanent teeth
3. large spaces are better than no spaces
4. crowding of the permanent teeth is likely even if small spaces are present.
5. in the absence of spacing, the permanent teeth are more likely to be displaced as they erupt.

Correct

That’s correct. Large spaces between the primary incisors are necessary if there’s to be enough room for the permanent incisors, but the amount of spacing has little or nothing to do with whether the teeth erupt in crossbite.

Incorrect

No, that’s wrong. Large spaces between the primary incisors are necessary if there’s to be enough room for the permanent incisors, but the amount of spacing has little or nothing to do with whether the teeth erupt in crossbite.

Question 7

At which dental age is eruption of the first permanent molars completed?

1. 6
2. 12
3. 15

4. 15 ✓

Correct

That’s right. All the teeth have to continue to erupt after they reach the occlusal plane, to stay in occlusion while the face is growing taller. Eruption doesn’t stop until growth is completed—and even late in life, a tooth is likely to start erupting if its antagonist is lost. So the first molars have to erupt beyond age 15, as long as vertical growth continues.

Incorrect

That’s wrong. All the teeth have to continue to erupt after they reach the occlusal plane, to stay in occlusion while the face is growing taller. Eruption doesn’t stop until growth is completed—and even late in life, a tooth is likely to start erupting if its antagonist is lost. So the first molars have to erupt beyond age 15, as long as vertical growth continues.

Question 8

(A) Eruption of the mandibular second molars ahead of the second premolars can be a problem because (B) this decreases the space for the second premolar and may lead to its impaction.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s correct. The statements are true and related. Even though eruption of the second molars ahead of the second premolars is a “normal variation”, it’s unfortunate when space in the dental arches is marginal.

Incorrect

No, that’s wrong. The statements are true and related. Even though eruption of the second molars ahead of the second premolars is a “normal variation”, it’s unfortunate when space in the dental arches is marginal.

Question 9

At dental age 7, where is the maxillary canine located?

1. not visible, crown formation hasn’t started yet
2. crown formation beginning just distal to the lateral incisors
3. crown partially complete, slightly above and distal to the lateral incisors
4. crown partially complete, very high above and slightly distal to the lateral incisors ✓

Correct

That’s right, the maxillary canines initially are very high above and only slightly distal to the lateral incisor. Their location almost in the floor of the orbit is why they’re sometimes called “eye teeth”.

Incorrect

No, that’s wrong. The maxillary canines initially are very high above and only slightly distal to the lateral incisors. Their location almost in the floor of the orbit is why they’re sometimes called “eye teeth”.

Question 10

What is the effect of a primary dentition with little or no space between the primary incisors on incisor liability?

1. decreases it
2. little or no effect
3. increases it ✓
4. unpredictable, primary dentition spacing is unrelated

Correct

That’s right, if there’s little or no spacing in the primary incisor region, the amount of crowding of the permanent incisors when they erupt will increase.

Incorrect

No, that’s incorrect. If there’s little or no spacing in the primary incisor region, the amount of crowding of the permanent incisors when they erupt will increase. When incisor liability is calculated, normal spacing between the primary incisors is taken for granted.

Question 11

At dental age 12, which teeth are expected to be erupting?

1. maxillary canines and mandibular second premolars
2. maxillary canines and maxillary second premolars
3. all four second premolars
4. maxillary canines and all four second premolars
5. maxillary canines, all four second premolars and all four second molars ✓

Correct

That’s correct. At dental age 12, the maxillary canines, all four second premolars erupt, and all four second molars erupt. The second molars usually are a bit behind the other teeth, but erupt so close to the same time that they’re part of the age 12 year development.

Incorrect

No, that’s wrong. At dental age 12, the maxillary canines, all four second premolars erupt, and all four second molars erupt. The second molars usually are a bit behind the other teeth, but erupt so close to the same time that they’re part of the age 12 year development.

Question 12

Which of the following are not succedaneous teeth?

1. central incisors
2. canines
3. first premolars
4. second premolars
5. first molars ✓

Correct

That’s correct, the first molar is not a succedaneous tooth because it has no primary tooth as a predecessor. Developmentally, the permanent molars can be considered primary teeth that erupt late and last a lifetime.

Incorrect

That’s wrong. All the permanent teeth except the molars are succedaneous teeth because they have a primary tooth as a predecessor. The first molars are not succedaneous teeth because they aren’t preceded by a primary tooth in their location. Developmentally, the permanent molars can be considered primary teeth that erupt late and last a lifetime.

Question 13

The ugly duckling stage of dental development is characterized by:

1. duck-bill protrusion of the incisors
2. upper incisors ducked back toward the lower incisors
3. spacing of upper and lower permanent incisors
4. spacing of upper but not lower permanent incisors ✓

Correct

That’s correct, the ugly duckling stage has spaces between the upper incisors, with the upper laterals inclined distally into the canine space. As the canines erupt, these spaces tend to close—but an extreme ugly duckling stage can indicate potential impaction of the canines.

Incorrect

No, that’s wrong. The ugly duckling stage has spaces between the upper incisors but not the lower incisors, with the upper lateral incisors inclined distally into the canine space. As the canines erupt, these spaces tend to close—but an extreme ugly duckling stage can indicate potential impaction of the canines.

Question 14

(A) When the arches are crowded, lateral incisors usually erupt facially and maxillary canines erupt lingually because (B) teeth tend to erupt on the side where the tooth bud was lying initially.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true ✓
5. A and B false

Correct

That’s correct. The first statement is false—lateral incisors usually go lingually and canines facially—but the second is true, teeth tend to erupt on the side the tooth buds were lying initially.

Incorrect

No, that’s wrong. The first statement is false—lateral incisors usually go lingually and canines facially—but the second is true, teeth tend to erupt on the side the tooth buds where lying initially. The lateral incisors start out lingual, the canines nearer the middle of the arch but often slightly facial.

Question 15

(A) In a patient with a potential problem in eruption of the maxillary canines, early extraction of the maxillary primary canines should be considered because (B) this increases the chance of the permanent tooth erupting reasonably normally instead of becoming impacted.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s correct, the statements are true and related. Early extraction of maxillary primary canines should be considered when the position of the permanent canines suggests potential impaction.

Incorrect

No, that’s wrong. The statements are true and related. Early extraction of maxillary primary canines should be considered when the position of the permanent canines suggests potential impaction.

Question 16

(A) Eruption of the maxillary canine coincident with the maxillary first premolar rarely is a problem because (B) the erupting canine tends to close spaces between the incisors and creates space in that way.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true ✓
5. A and B false

Correct

That’s correct, the first statement is false and the second one true. Whether or not there’s incisor spacing, if the canine erupts along with the first premolar, before the second primary molar is lost, it’s likely to be displaced facially, and that’s a problem, because orthodontic treatment will be needed to get it back.

Incorrect

No, that’s wrong. The first statement is false and the second one true. Early eruption of the canine is likely to create a problem, even though its eruption tends to close spaces between the incisors and some space is gained in that way. Whether or not there’s incisor spacing, if the canine erupts along with the first premolar, before the second primary molar is lost and some additional space becomes available in that area, it’s likely to be displaced facially, and orthodontic treatment will be needed to get it back.

Question 17

Which of the following is not characteristic of dental age 7?

1. eruption of the maxillary central incisors
2. eruption of the mandibular lateral incisors
3. formation of more than half the root of the maxillary lateral incisors
4. crown formation of maxillary and mandibular canines
5. approximately half the root formed on maxillary and mandibular first premolars. ✓

Correct

That’s right. At age 7, root formation of first premolars is just beginning, if they are beyond the crown stage at all.

Incorrect

No, that’s wrong. At age 7, root formation of first premolars is just beginning, if they are beyond the crown stage at all. Canines are at the crown stage, the maxillary centrals and mandibular laterals erupt, and the maxillary laterals are still about a year away but have more than half their root formed.

Question 18

How much additional space for the incisors is gained by the normal mechanisms to overcome incisor liability?

1. the width of one lower incisor
2. ½ the width of a lower incisor ✓
3. ¼ the width of a lower incisor
4. unpredictable, depends on how much was needed and how the child grows.

Correct

That’s correct, the normal mechanisms contribute about 3 mm, or half the width of a lower incisor. If more space than that would be needed to obtain incisor alignment, it probably won’t be forthcoming from normal development alone.

Incorrect

No, that’s wrong. The normal mechanisms contribute about 3 mm, or half the width of a lower incisor. If more space than that would be needed to obtain incisor alignment, it probably won’t be forthcoming from normal development alone.

Question 19

Dental age 11 is characterized by eruption of:

1. mandibular canine and all first premolars ✓
2. maxillary canine and all first premolars
3. mandibular canine and mandibular first premolars
4. mandibular canine and maxillary first premolars
5. all canines and mandibular first premolars

Correct

That’s right. At dental age 11, the mandibular canines and all four first premolars erupt at about the same time. The maxillary canines erupt a year later, at dental age 12.

Incorrect

No, that’s wrong. At dental age 11, the mandibular canines and all four first premolars erupt at about the same time. The maxillary canines erupt a year later, at dental age 12.

Question 20

In a child whose dental age is 8 years at chronologic age 6 years, what would you expect her dental age to be at chronologic age 12?

1. 10
2. 12
3. 14 ✓

4. 14

5. no way to predict

Correct

That’s right, a child whose dental development is two years ahead of her chronologic age at 6 is likely to still be two years ahead when she’s 12. Children tend to stay in the same developmental percentile channel.

Incorrect

That’s wrong, you’d expect dental age 14. A child whose dental development is two years ahead of her chronologic age at 6 is likely to still be two years ahead when she’s 12. Children tend to stay in the same developmental percentile channel.