03 - Development of Occlusion in the Primary and Transitional Dentition

Primary Dentition Occlusion

Purpose of This Module

The purpose of this module is to review the development of occlusion in the primary and transitional dentition. In addition to pages 86-91 (5th Ed) or 97-103 (4th Ed) in *Contemporary Orthodontics,*carefully review Table 3-1 (5th ed)/Table 3-2(4th ed), Figure 3-2 (5th ed)/Fig 3-13 (4th ed), and Fig 3-35 (5th ed)/Fig 3-43 (4th ed). After this preparation and viewing the teaching program, be sure you are able to:

• describe the primary to permanent dentition pathways, especially the early and late mesial shifts
• describe the location and significance of dental spacing in the primary dentition
• discuss the pre-eruptive position of the permanent teeth and its significance
• identify the canine and molar relationships that can occur in the primary dentition and discuss the implication of each for development of the permanent dentition occlusion

Pre-Eruptive Development of the Dental Arches

All infants have a small, relatively under-developed mandible that gives them a very convex profile (image 1). As growth progresses, the mandible will grow forward more than the maxilla. This leads to a straighter facial profile and increased mandibular projection compared to the maxilla.

Intra-orally the maxillary and mandibular arch forms have different shapes in the edentulous infant. The maxillary arch is ovoid in shape (image 2), while the mandibular arch is V-shaped (image 3). The alveolar ridges contain the developing primary and permanent tooth buds.

Initially the upper and lower gums pads touch together with no vertical space. Vertical growth of the maxilla and mandible produces an increase in face height which accommodates the eruption of the primary teeth. The erupting teeth don’t push the jaws apart; the jaws grow apart which creates space for the eruption of the teeth.

Image 1, Infant Profile: The facial profile in an infant is very convex with the mandible being much less developed than the maxilla.	Image 2, Infant maxillary arch form: The maxilla in an infant is ovoid in shape.
Image 3, Infant mandibular arch form: The mandible of an infant is V-shaped in comparison to the maxilla arch.

Dental Spacing

The eruption of the primary dentition is usually complete by 24-30 months of age. Spacing is common and desirable in the primary dentition and is usually noticeable in two locations called primate spaces (image 1). Why that name for these spaces? Because they are also found in non-human primates.

In the maxillary arch the primate spaces are found between the lateral incisors and the canines. In the mandibular arch they are found between the canine and first molar (image 2).

Image 1: The crowns of the developing permanent incisors lie lingual to the crowns of the primary incisors. The arrows indicate the location of the primate spaces.	Image 2: Right buccal occlusion of a child with arrows indicating the location of the primate spaces.

Dental Spacing (cont.)

Spacing between the primary incisors is the normal condition in young children. As they grow and the alveolar processes develop, the spaces may become larger (image 1).

Lack of spacing in the primary incisor region is abnormal (image 2) and indicates future crowding problems in the transitional and permanent dentitions as the larger permanent incisors erupt.

Image 1: Incisor spacing in the maxillary and mandibular arches is common and helps accommodate the larger permanent incisors.	Image 2: This child has no interdental spacing in the primary dentition, which indicates there will be future crowding problems when the larger permanent teeth erupt.

Incisor Liability

The combined mesiodistal width of the permanent incisors (centrals and laterals) is larger than the width of the primary incisors. This size differential is called the incisor liability. In the maxilla the four permanent incisors are on average 7.6 mm wider than the primary incisors; in the mandible the permanent incisors are 6.0 mm wider.

Image 1: In the maxillary arch the sum of the widths of the permanent incisors is approximately 7.5 mm greater than the sum of the widths of the primary incisors.	Image 2: In the mandibular arch the sum of the widths of the permanent incisors is approximately 6 mm greater than the sum of the widths of the primary incisors.

Overcoming Incisor Liability

If the permanent incisors are larger than the primary incisors, where does the space come from to allow their eruption? This lack of space appears also to be compounded by the fact that most jaw growth occurs posteriorly in the ramus of the mandible and the tuberosity region of the maxilla, and not anteriorly in the dental arches.

In the maxillary arch the larger permanent incisors are accommodated by:

1. Utilization of any interdental spaces between the laterals and centrals. (image 1)
2. Utilization of the maxillary primate spaces between the laterals and canines. (image 2)
3. Labial eruption of the incisors increasing arch length and circumference. (image 3)
4. Increase in width of the dental arch across the canines (intercanine width)

In the mandibular arch the larger permanent incisors are accommodated by:

1. Utilization of any developmental spaces between incisors.
2. Increase in intercanine width.

Image 1, Interdental spaces: Interdental spaces between the primary incisors help to accommodate eruption of the larger permanent incisors.	Image 2, Primate spaces: In the maxillary arch primate spaces between the primary lateral incisors and canines help accommodate the eruption of the larger permanent incisors.
Image 3, Labial eruption of the incisors: In the maxillary arch the larger permanent incisors also erupt labially into a wider arc which increases arch circumference.

Classification of Molar Occlusion in the Primary Dentition

After eruption of all 20 primary teeth, occlusion of the primary dentition is established. The relationship of the distal terminal planes of the upper and lower second primary molars are used to classify posterior occlusal relationships. Three categories exist:

1. Flush terminal plane (image 1)

The distal surfaces of opposing primary second molars are in the same vertical plane. 2. Mesial step terminal plane (image 2)

The mandibular primary second molar terminal plane is located mesial to the terminal plane of the maxillary primary second molar. This corresponds to Angle Class I, the normal relationship in the permanent dentition. 3. Distal step terminal plane (Image 3)

The mandibular primary second molar terminal plane is located distal to the terminal plane of the maxillary primary second molar terminal plane. This corresponds to Angle Class II, a major type of malocclusion characterized by protruding upper incisors, in the permanent dentition.

In the primary dentition the distribution of the categories is as follows:

Flush terminal plane – 37%

Mesial terminal plane – 49%

Distal terminal plane – 14%

Image 1, Flush Terminal Plane: Flush terminal plane relationship with the distal surfaces of opposing primary second molars are in the same vertical plane.	Image 2, Mesial Step: In a mesial step relationship, the mandibular primary second molar terminal plane is located mesial to the terminal plane of the maxillary primary second molar.
Image 3, Distal Step: In a distal step relationship, the mandibular primary second molar terminal plane is located distal to the terminal plane of the maxillary primary second molar terminal plane.

Canine Relationships in the Primary Dentition

The antero-posterior relationships of the upper and lower primary canines are also used to classify occlusion in the primary dentition, just as they are in the permanent dentition. There are four categories. The first two are:

1. End-to-end (image 1) The long axes of the upper and lower canines are in the same anteroposterior plane.
2. Class I (image 2) The cusp tip of the upper canine occludes in the embrasure between the lower canine and lower first molar.

Image 1: In an end to end primary canine relationship, the long axes of the upper and lower canines are in the same anteroposterior plane.	Image 2: In a Class I primary canine relationship, the cusp tip of the upper canine occludes in the embrasure between the lower canine and lower first molar.

Canine Relationships in the Primary Dentition (cont.)

The third and fourth categories are:

3. Class II (image 1) The cusp tip of the upper canine occludes mesial to the lower canine in the embrasure between the lower canine and lateral incisor.

4. Class III (image 2) The cusp tip of the upper canine occludes distal to the embrasure between the mandibular first primary molar and canine.

Different relationships can exist on the right and left sides of the mouth. As well, different combinations of molar and canine relationships are possible. These asymmetric differences may be due to differences in skeletal growth, tooth size differences, and different spacing on the left and right sides of the dental arches.

Image 3: In a Class II primary canine relationship, the cusp tip of the upper canine occludes mesial to the lower canine in the embrasure between the lower canine and lateral incisor.	Image 4: In a Class III primary canine relationship, the cusp tip of the upper canine occludes distal to the embrasure between the mandibular first primary molar and canine.

Primary to Permanent Dentition

Transitions from Primary to Permanent Dentition

The relationship of the primary second molars and canines, and the position of the primate spaces, greatly influence the eruption of the first permanent molars.

One would expect that:

Mesial terminal plane → Class I molar permanent molars (image 1A)

Flush terminal plane → End-to-end permanent molars (halfway between Class I and II, image 1B)

Distal terminal plane → Class II molar permanent molars (image 1C)

However this is too simplistic. Sometimes it turns out that way, but often it doesn’t.

Leeway Space

The difference in the combined mesial-distal width of the primary canine, first molar, and second molar and the succedaneous permanent canine, first premolar, and second premolar is called the leeway space. Surprisingly, the width of these three primary teeth is larger than the three permanent teeth. In the mandibular arch the leeway space is on average approximately 2.5 mm per side (5.0 mm per arch) and 1.5 mm per side (3.0 mm per arch) in the maxillary arch. Most of the leeway space is due to the size differential between the mandibular primary second molar and the succedaneous mandibular second premolar.

The variability in leeway space between individuals is quite large due to variability in tooth size.

Early Mesial Shift

Patients with a flush terminal plane molar relationship in the primary dentition can, and often do develop into a Class I permanent molar relationship when the first permanent molars erupt.

Recall that primate spaces exist distal to the mandibular primary canines. At 6 years of age, when the mandibular first permanent molars erupt, they may shift forward, closing the mandibular primate space. This results in a mesial shifting of the lower molar relative to the upper molar and early establishment of a Class I molar relationship. This is called the early mesial shift.

Late Mesial Shift

With the exfoliation of all the primary second molars (age 11-12 years of age), the permanent maxillary and mandibular molars shift mesially into the leeway space. Because the leeway space is larger in the mandibular arch, the permanent mandibular molars shift more mesially than the permanent maxillary molars. This is called the late mesial shift. This allows a flush terminal plane in the mixed dentition to transition to a Class I molar relationship in the permanent dentition.

Influence of Jaw Growth

The early mesial shift and late mesial shift do not solely determine the occlusal outcomes during the transition to the permanent dentition. Differential growth of the mandible relative to the maxilla is also an important contributor to molar position. Growth of the jaws carries the dentition along with it, and if one jaw grows forward more than the other, this affects how the upper and lower teeth will be related in space.

In general, the mandible grows forward relative to the maxilla during the transition from the mixed dentition to the early permanent dentition, carrying the lower teeth with it, and this aids in the development of normal (Class I) occlusion of the permanent teeth.

Pathways of Development

This figure summarizes the possible pathways during the transition of occlusal relationships between the primary and permanent dentition.

The green arrow shows what will happen if there is more forward growth of the mandible than the maxilla and there is a mesial shift, either early or late.

The dashed green arrow shows what will happen if there is a mesial shift but little or no differential growth.

The blue arrow shows what will happen if there is neither differential growth nor a mesial shift.

Flush Terminal Plane Transition

Let’s start with a child with a flush terminal plane, the most frequent relationship in the primary dentition.

For a child with a flush terminal plane, which is the norm in the primary and mixed dentition, the transition to the normal Class I relationship in the permanent dentition requires a one-half cusp (3-4 mm) relative forward movement of the lower molar. This is accomplished most readily by a combination of more mandibular than maxillary jaw growth and a mesial shift of the mandibular molars (the solid green arrow).

If there is a mesial shift (early or late) (dashed green arrow) but no more mandibular than maxillary growth, there will be about half as much movement in the direction of Class I. If neither growth nor a mesial shift occurs, an end-to-end relationship of the permanent molars would persist.

About 75% of children with a flush terminal plane in the primary dentition will develop into a Class I molar relationship in the permanent dentition. Because of unfavorable jaw growth and/or minimal mesial shift, the other 25% will not complete the transition to a complete Class I relationship.

Distal Step Transition

With differential growth of the mandible and mesial shift of the molars, a distal step terminal plane in the primary dentition may develop into an end-to-end relationship in the permanent dentition (which is usually described as a ½ cusp Class II), but that is about as much improvement as one could expect. If minimal differential jaw growth occurs, a full cusp Class II relationship in the permanent dentition will result. It is highly unlikely that a distal step terminal plane will develop into a Class I relationship in the permanent dentition.

This means that children with a distal step relationship in the primary dentition are quite likely to require appropriately timed orthodontic intervention in the future to have a Class I molar relationship in the permanent dentition.

Mesial Step Transition

For most children, a mesial step relationship in the primary dentition usually was due to an early mesial shift, and it develops into a Class I molar relationship in the permanent dentition—unless there is significantly more mandibular than maxillary growth during the transition period.

However, if excessive mandibular growth occurs, a mesial step may develop into a Class III molar relationship and Class III malocclusion (lower teeth in front of the upper teeth) in the permanent dentition.

In the primary and early mixed dentitions (before the period of transition from the mixed to the permanent dentition), mesial step relationships are quite uncommon and usually are a sign of excessive mandibular growth or deficient maxillary growth. These growth patterns usually persist during the transition period into the permanent dentition years, often changing the mesial step into a Class III occlusal relationship that becomes more severe with increasing age.

Summary

The purpose of this module was to review the development of occlusion in the primary and transitional dentition. You should now understand:

a) Dental spacing in the primary dentition b) Classification of occlusion in the primary dentition c) Implications of occlusion in the primary dentition d) Possible pathways from the primary to the permanent dentition

In addition to pages 86-91 (5th Ed) or 97-103 (4th Ed) in *Contemporary Orthodontics,*carefully review Table 3-1 (5th ed)/Table 3-2(4th ed), Figure 3-2 (5th ed)/Fig 3-13 (4th ed), and Fig 3-35 (5th ed)/Fig 3-43 (4th ed) before you take the self-test. It’s important to be sure you understand these important concepts.

Self-Test

Question 1

Primate spaces in the primary dentition are usually located:

1. In the maxillary arch between the canine and first molar
2. In the maxillary arch between the lateral incisor and canine ✓
3. In the mandibular arch between the canine and lateral incisor
4. In the mandibular arch between the first and second molar

Correct

That’s right; in the maxillary arch the primate spaces are between the lateral incisors and canines while in the mandibular arch they are between the canines and first molars.

Incorrect

No, that’s incorrect. In the maxillary arch the primate spaces are between the lateral incisors and canines while in the mandibular arch they are between the canines and first molars.

Question 2

In the maxillary arch, which of following facilitates the eruption of the larger incisors to replace the primary incisors?

1. Labial eruption path of the permanent incisors ✓
2. Primate spacing between the primary canine and first molar
3. Expansion of the midpalatal suture
4. Increase in arch length due to growth in the tuberosity region

Correct

That’s right; in the maxillary arch, gaining space for eruption of the larger permanent incisors is facilitated by their facially directed eruption, which increases arch length and circumference. The maxillary primate space also helps their eruption, but it is located between the canine and lateral incisor, not the canine and first molar. Midpalatal suture expansion and growth in the tuberosity are important aspects of maxillary growth but they play a minimal role in the eruption of the permanent incisors.

Incorrect

No, that’s incorrect. In the maxillary arch, gaining space for eruption of the larger permanent incisors is facilitated by their facially directed eruption, which increases arch length and circumference. The maxillary primate space also helps their eruption, but it is located between the canine and lateral incisor not the canine and first molar. Midpalatal suture expansion and growth in the tuberosity are important aspects of maxillary growth but they play a minimal role in the eruption of the permanent incisors.

Question 3

In the primary dentition, the terminal plane of the mandibular second primary molar is located behind the terminal plane of the maxillary second primary molar. What is the molar classification for this patient?

1. Mesial step terminal plane
2. Flush terminal plane
3. Distal step terminal plane ✓
4. Class III terminal plane

Correct

That’s right; the classification would be distal step, because the lower molar is distal relative to the upper molar. The preferred relationship is a mesial step relationship where the terminal plane of the lower molar is mesial to the terminal plane of the upper molar.

Incorrect

No, that’s incorrect. The classification would be distal step, because the lower molar is distal relative to the upper molar. The preferred relationship is a mesial step relationship where the terminal plane of the lower molar is mesial to the terminal plane of the upper molar.

Question 4

In the primary dentition which of the following relationships is most common?

1. Mesial step terminal plane ✓
2. Flush terminal plane
3. Distal step terminal plane
4. Class III terminal plane

Correct

That’s right; mesial step terminal plane occurs in 49% of children while flush terminal plane occurs in 37% and distal step terminal plane occurs in 14%.

Incorrect

No, that’s incorrect. Mesial step terminal plane occurs in 49% of children while flush terminal plane occurs in 37% and distal step terminal plane occurs in 14%.

Question 5

The early mesial shift occurs at the time of the:

1. Exfoliation of the primary canines
2. Eruption of the first permanent molars ✓
3. Exfoliation of the primary buccal segments
4. Eruption of the second permanent molars

Correct

That’s right; the early mesial shift occurs at the time of the eruption of the first permanent molars or approximately 6 years of age. As the permanent molars erupt they drift forward closing the primate spaces, especially in the lower arch. The late mesial shift occurs with the exfoliation of the primary buccal segments in the late mixed dentition.

Incorrect

No, that’s incorrect. The early mesial shift occurs at the time of the eruption of the first permanent molars or approximately 6 years of age. As the permanent molars erupt they drift forward closing the primate spaces, especially in the lower arch. The late mesial shift occurs with the exfoliation of the primary buccal segments in the late mixed dentition.

Question 6

1. Too small to help patients attain a Class I molar relations
2. A crucial part of the early mesial shift
3. Larger in the maxillary arch than the mandibular arch
4. Due to differences in the size of posterior primary and permanent teeth ✓

Correct

That’s right; leeway space is primarily due to the difference in the combined mesial distal widths of the primary canine, first molar, second molar and the permanent canine, first premolar and second premolar. The three primary teeth are wider than the permanent teeth. Leeway space is a crucial part of the late mesial shift which helps patients attain a class I molar relationship. The leeway space is larger in the mandibular arch (5mm) than the maxillary arch (3mm).

Incorrect

No, that’s incorrect. Leeway space is primarily due to the difference in the combined mesial distal widths of the primary canine, first molar, second molar and the permanent canine, first premolar and second premolar. The three primary teeth are wider than the permanent teeth. Leeway space is a crucial part of the late mesial shift which helps patients attain a class I molar relationship. The leeway space is larger in the mandibular arch (5mm) than the maxillary arch (3mm).

Question 7

What percentage of patients with a flush terminal plane relationship in the mixed dentition will develop into a Class I molar relationship in the permanent dentition?

1. 25
2. 40
3. 60
4. 75 ✓

Correct

That’s right; 75% of patients with a flush terminal plane relationship in the mixed dentition will develop into a class I molar relationship in the permanent dentition. This occurs due to the late mesial shift and differential mandibular growth.

Incorrect

No, that’s incorrect. 75% of patients with a flush terminal plane relationship in the mixed dentition will develop into a class I molar relationship in the permanent dentition. This occurs due to the late mesial shift and differential mandibular growth.

Question 8

In the maxillary arch the incisor liability is approximately:

1. 3 mm
2. 4 mm
3. 5 mm
4. 6 mm
5. 7 mm ✓

Correct

That’s right; because the maxillary permanent incisors are bigger than their primary predecessors by slightly more than 7mm, so that amount of additional space must be gained in some way. The additional space needed to align the permanent incisors is called the incisor liability.

Incorrect

No, that’s incorrect. The maxillary permanent incisors are bigger than their primary predecessors by about 7mm, so that amount of additional space must be gained in some way. The additional space needed to align the permanent incisors is called the incisor liability.

Question 9

The profile of a newborn infant can best be described as:

1. concave
2. straight
3. convex ✓
4. any of the above due to birth molding

Correct

That’s right; the profile of a newborn infant is quite convex because of the relative underdevelopment of the mandible at the time of birth.

Incorrect

No, that’s incorrect. The profile of a newborn infant is quite convex because of the relative underdevelopment of the mandible at the time of birth.

Question 10

What is the chance that a child with a distal step relationship of the primary molars will self-correct to Class I permanent molars?

1. Extremely small ✓
2. 10%
3. 75%
4. very likely

Correct

That’s right; the chance of spontaneous correction of a distal step relationship is extremely small, much less than 10%.

Incorrect

No, that’s incorrect. The chance of spontaneous correction of a distal step relationship is extremely small, much less than 10%. The positive changes associated with the late mesial shift and differential growth of the jaws usually are not large enough to overcome the distal positioning of the lower molars in a patient with a distal step relationship.

Question 11

The mandibular arch form of a newborn infant is:

1. ovoid
2. V-shaped ✓
3. trapezoidal
4. square

Correct

That’s right; the mandibular arch form is rather V-shaped, which is noticeably different than the ovoid maxillary arch.

Incorrect

No, that’s incorrect. The mandibular arch form is rather V-shaped, which is noticeably different than the ovoid maxillary arch.

Question 12

In a child, a lack of primate spaces in the dentition should be interpreted as:

1. supernumerary teeth are likely to be present
2. spacing in the arch is likely to occur long term
3. crowding of the permanent teeth is likely to occur ✓
4. child is evolutionarily advanced

Correct

That’s right; a lack of primate spaces strongly suggests that crowding of the permanent teeth will occur.

Incorrect

No, that’s incorrect. A lack of primate spaces strongly suggests that crowding of the permanent teeth will occur. Even though the spaces are found in less evolved primates, a lack of the spaces doesn’t indicate a higher level of evolutionary status.