Occlusion (Dental Anatomy, Physiology and Occlusion) Part 2

Mixed (Transitional) Dentition

The transition from primary to permanent dentition begins at about 6 years of age with the emergence of the mandibular permanent central incisors (Figures 16-10 and 16-11) or the first permanent molars at 6 to 7 years of age. The timing of the shedding of the primary teeth has an effect on the emergence of the permanent teeth (i.e., early shedding of primary teeth advances the emergence of the permanent teeth).19

Arch Dimensions and Tooth Size

The growth of the jaw provides space for the teeth to erupt and emerge into the mouth. Forward rotation of the maxil-lomandibular complex, which is the more usual pattern, influences the magnitude of tooth eruption as well as the ultimate anteroposterior position of the incisors. Lingual positioning of the mandibular incisors relative to the mandible often accompanies forward rotation during growth.20,21 Changes in the anteroposterior position of the incisors with rotation reflect a major influence on changes in arch length. It has been suggested that changes in arch length, which occur in both arches with rotation during growth associated with movement of the incisors, may be of relatively greater importance than forward movement of the molars.15

An important portion of the dental arch in the development of the occlusion of the permanent dentition is the premolar segment. In this section, the erupting premolars are significantly smaller in mesiodistal dimension than the primary molars, which they replace. The dynamics of this change in arch dimensions, particularly in the mandibular arch, is important to a proper understanding of the development of occlusion and malocclusion.


 Occlusal contact relations of primary dentition. A, Contacts on maxillary teeth (top) and contacts on mandibular teeth (bottom). B, Casts of normally developed teeth of a child 6 years of age. Top, Frontal view. Bottom, Sagittal view.

Figure 16-8 Occlusal contact relations of primary dentition. A, Contacts on maxillary teeth (top) and contacts on mandibular teeth (bottom). B, Casts of normally developed teeth of a child 6 years of age. Top, Frontal view. Bottom, Sagittal view.

Drawings of a sagittal section through the permanent and primary incisors. The labial surface at the cervical margin is oriented in the same plane. Note that the midalveolar point of the permanent incisors (B) is more lingual than the same point of the primary incisor (A) but that the incisal edge of the permanent incisor is more labial than that of the primary incisor.

Figure 16-9 Drawings of a sagittal section through the permanent and primary incisors. The labial surface at the cervical margin is oriented in the same plane. Note that the midalveolar point of the permanent incisors (B) is more lingual than the same point of the primary incisor (A) but that the incisal edge of the permanent incisor is more labial than that of the primary incisor.

Observe available space and size of emerging permanent incisors and incisal edge mamelons. Child is 6.5 years of age.

Figure 16-10 Observe available space and size of emerging permanent incisors and incisal edge mamelons. Child is 6.5 years of age.

An often-confusing point in the analysis of the mixed dentition is the actual decrease in arch perimeter during the growth of the mandible. The arch perimeter of the permanent dentition, as measured from the mesials of the mandibular first molars, decreases an average of about 4 mm.22 This occurs at the same time that the mandible and basal bone are experiencing significant growth posteriorly. Because this change is found to a greater extent in the mandible than in the maxilla and because of the pronounced tendency for the lower molars to drift mesially, occlusal relationships are in a flux during the later stages of the mixed dentition.

When a discrepancy is evident between the aggregate mesiodistal dimension of the teeth and the size of the bony supporting arches, crowding or protrusion can occur. The discrepancy may relate to small or large teeth, small or large bony bases, or some combination of these dimensions. It was shown by Howe et al23 that arch width and perimeter dimensions can relate to differences between crowded and uncrowded dentitions.

ARCH WIDTH

Arch width is measured between corresponding lingual points (e.g., intersection of the lingual groove and gingival margin) on the teeth in the same arch (e.g., transpalatal width between second molars).

Early mixed dentition in a child with a full complement of primary teeth and first permanent molars. A, Maxilla. B, Mandible.

Figure 16-11 Early mixed dentition in a child with a full complement of primary teeth and first permanent molars. A, Maxilla. B, Mandible.

Maxillary intermolar width, for example, can be as much as 6 mm less in crowded dentitions than in uncrowded dentitions. A transpalatal width increase of 2.5 to 3.0 mm occurs in untreated persons from 7 to 15 years of age. According to a study by Spillane and McNamara,24 if an inadequate transpalatal width is evident in a narrow dental arch (less than 31 mm) in the early mixed dentition, it is unlikely that the child will reach adequate arch dimensions through normal growth mechanisms. The distance (perimeter) around the dental arch as measured from the mesial surfaces of right and left maxillary second molars is significantly greater in uncrowded dentitions than in crowded dentitions. Other researchers have obtained similar findings. Orthodontists undertake the prevention or treatment of tooth and arch size discrepancies. However, it is in the best interests of their patients for practitioners to know something of the significance of the differences in size between the primary/deciduous teeth and the permanent teeth and between tooth size and arch size.

Differences in Tooth Size

The relationship of the size of the dental arches to differences between the size of the primary/deciduous teeth and the size of the permanent teeth is important relative to the development of the permanent dentition. For example, the aggregate size of the posterior permanent teeth is generally smaller in the mesiodistal dimension than that of the primary teeth, and the posterior succedaneous teeth (permanent teeth) generally have a smaller total mesiodistal dimension than the primary teeth. The difference is related to the leeway space, or the amount of space gained by the difference in the mesiodistal dimensions of the premolars and the primary molars.14,15 The average mesiodistal size of the primary dentition in that area is 47 mm, which, when compared with the succeeding aggregate tooth dimension of 42.2 mm, indicates an average gain of 4.8 mm in available space. In effect, the difference in size of the posterior suc-cedaneous/permanent teeth and the primary teeth provides for the mesial movement of the permanent molars (Figure 16-12). Thus a simple comparison of tooth size of the two dentitions can indicate a need for additional space in both dental arches as soon as the primary incisors are lost, except when sufficient interdental spacing is evident in the primary dentition.25

Leeway space. Difference in mesiodistal dimensions between primary teeth (A) and permanent teeth (B). Arrows indicate the mesial movement of the permanent molars after loss of primary molars and eruption of the second permanent premolar.

Figure 16-12 Leeway space. Difference in mesiodistal dimensions between primary teeth (A) and permanent teeth (B). Arrows indicate the mesial movement of the permanent molars after loss of primary molars and eruption of the second permanent premolar.

Some of the space made available by the leeway space (the difference in sizes between the premolars and primary molars) must be used for alignment of the lower incisors, because these teeth erupt with an average of 1.6 mm of crowding.26 The mandibular molar will use the remainder of the space. This movement of the mandibular molar may correct an end-to-end molar relationship (normal for the mixed dentition) into a normal molar relationship in the permanent dentition (i.e., the mesial lingual cusp of the maxillary first molar occludes in the central fossa of the mandibular first molar and the mesial buccal cusp of the maxillary first molar occludes between the mesial and distal buccal cusps of the mandibular first molar) (see Figure 16-27).

INCISAL LIABILITY

If the second molars erupt before the premolars erupt fully, a significant shortening of the arch perimeter occurs, and malocclusion may be more likely to occur.27 Because of the discrepancy in mesiodistal crown dimensions between the primary and permanent incisors, some degree of transient crowding may occur (incisal liability) at about the age of 8 to 9 and persist until the emergence of the canines, when the space for the teeth may again be adequate.

Permanent Dentition

The sequence of eruption of the permanent dentition is more variable than that of the primary dentition and does not follow the same anteroposterior pattern. In addition, significant differences in the eruption sequences between the maxillary arch and the mandibular arch do not appear in the eruption of the primary dentition (Table 16-3).

The most common sequences of eruption in the maxilla are 6-1-2-4-3-5-7-8 and 6-1-2-4-5-3-7-8. The most common sequences for the mandibular arch are (6-1)-2-3-4-5-7-8 and (6-1)-2-4-3-5-7-8.28 These are also the most favorable sequences for the prevention of malocclusion (Figure 16-13). As noted earlier, should the second molars erupt before the premolars are fully erupted, significant shortening of the arch perimeter resulting in malocclusion is likely to occur, even if the alveolar bone arch dimensions are adequate for the size of the permanent dentition.27

The eruption of permanent teeth also follows the tendency for the mandibular tooth of one type to erupt before the maxillary tooth erupts. This tendency is reversed in the premolar eruption sequence. This is the result of the difference in eruption timing of the canines in the two arches. In the mandibular arch, the canine erupts before the premolar, whereas in the maxillary arch the canine generally erupts after the premolar.

The timing of eruption of the permanent dentition is not critical as long as the eruption times are not too far from the normal values. The sequence of eruption varies somewhat, with the dentition in girls erupting an average of 5 months earlier than that in boys. However, sexual differences are less significant than the tendency exhibited by the individual in the eruption times of previously erupted teeth.

Table 16-3 Chronology of Permanent Teeth*

tmp57-159

Tooth

Eruption (Years)

CI

8, 9

7-8

LI

7, 10

8-9

C

6, 11

11-12

P1

5, 12

10-11

P2

4, 13

10-12

m1

3, 14

9-10

m2

2, 15

12-13

m3

1, 16

17-21

Maxillary Teeth

tmp57-160
tmp57-161

Mandibular Teeth

CI

24, 25

6-7

LI

23, 26

7-8

C

22, 27

9-10

P1

21, 28

10-12

P2

20, 29

11-12

m1

19, 30

6-7

m2

18, 31

11-13

m3

17, 32

17-21

CI, Central incisor; LI, lateral incisor; C, canine; ml, first molar; m2, second molar; m3, third molar.

^Universal 1 numbering system for the permanent dentition.

Favorable emergence sequence (numerical) of permanent teeth.

Figure 16-13 Favorable emergence sequence (numerical) of permanent teeth.

If any tooth has erupted early or late, succeeding teeth will also be early or late in their eruption.

Next post:

Previous post: