The morphogenesis of the dental arches occurs discontinuously, alternating between active and stable phases, and proceeds for about 20 years.
Eruption schedule of the primary dentition
The timing of eruption is highly variable, so these values are only approximate:
1. At 6 months: primary mandibular incisor
2. Between 6 and 13 months: the other primary incisors
3. Between 12 and 18 months: primary first molars
4. Between 18 and 24 months: primary canines
5. Between 24 and 36 months: primary second molars
After the emergence of the 20 primary teeth, the primary dentition remains relatively stable for about 4 years.
Functions of the primary dentition
One of the chief functions of the primary dentition is to provide a mechanism for chewing. Occlusion in the primary dentition is rudimentary, and the dental morphology is rather flattened. The morphology is not as complex as that of the permanent dentition, which will function at a time when more sophisticated mastication will be required.
The primary dentition also participates in the development of facial height. As the mandible and maxilla grow downward and forward, corresponding points in the two jaws move away from each other, following divergent lines. To compensate for these emerging gaps, maxillary structures, especially the alveolar process, grow vertically, and the child passes from the edentulous state of the newborn into the successive stages of the primary, mixed, and permanent dentitions.
In addition, the primary dentition sets the stage for the emergence of the permanent teeth by guiding them as they erupt and by establishing and preserving the space needed for the permanent dentition.
Definition of the terminal plane
The permanent first molars emerge along the distal surfaces of the maxillary and mandibular primary second molars, which have been described as the terminal plane (Fig 2-1).
The terminal planes predetermine the position the permanent first molars will occupy in the dental occlusion (see “Establishment of the mixed dentition”). However, other factors, such as maxillary and mandibular growth rates, the leeway space, the size and the shape of the teeth, and environmental factors such as caries and premature loss of primary teeth, can play a role. In addition, so-called noxious habits and the functional matrix can exert considerable influence.
Skeletal and neuromuscular characteristics of the primary dentition
In newborn babies, the glenoid fossae of the temporomandibular joint are flat; in the absence of teeth and the masticatory movements that would accompany them, the mandible moves only horizontally, back and forth, sucking on the mother’s breast or on the bottle. When the primary teeth begin to erupt, the articular discs organize for more sophisticated action, the glenoid fossae deepen, and the condylar slope augments progressively. As teeth continue to erupt, the muscles of mastication learn to accomplish all the movements required for functional activity.
Because it is transitory and operates on evolving basal bone, the occlusion of the primary teeth can be classified into three categories (Tollaro 1990): the normal primary dentition; the at-risk primary dentition; and the pathologic primary dentition.
Primary dentitions that are in normal occlusion exhibit the following traits (see the photograph at the beginning of the chapter):
1. Anterior diastemata
2. Primate spaces
3. Little or no overbite
4. Virtually perpendicular relationship between most teeth and basal bone, with interincisal angles of 150 degrees
5. Class I molar relationship: flush terminal plane or terminal plane with a mesial step
6. Articulation of the maxillary canine with the mandibular canine and the primary mandibular first molar
7. Flat occlusal plane
8. Ovoid shape and transverse harmony of the dental arches
9. Alignment of the maxillary and mandibular frena
Although the primary dentition may be normal, this is no guarantee that the permanent dentition will be free of malocclusion. In general, indications of malocclusion can make their first appearance as the permanent teeth erupt, but their primary etiologic factors and especially the influence of the functional matrix manifest themselves during periods of more rapid growth (see chapter 3).
Characteristics of at-risk and pathologic primary dentitions
When any of these conditions is present, a primary dentition can be considered to be at risk of developing a malocclusion:
1. Absence of diastemata