Preeruptive tooth movement. Made by the deciduous and permanent tooth germs within tissues of the jaw before they begin to erupt.

Eruptive tooth movement. Made by a tooth to move from its position within the bone of the jaw to its functional position in occlusion. This phase sometimes is subdivided into intraosseous and extraosseous components.

Posteruptive tooth movement. Maintaining the position of the erupted tooth in occlusion while the jaws continue to grow and compensate for occlusal and proximal tooth wear.

Preeruptive Tooth Movement

When the deciduous tooth germs first differentiate, they are extremely small, and a good deal of space is available for them in the developing jaw. Because they grow rapidly, however, they become crowded. A lengthening of the jaws, which permits the deciduous second molar tooth germs to move backward and the anterior germs to move forward gradually, alleviates this crowding. At the same time the tooth germs are moving bodily outward and upward (or downward, as the case may be) with the increasing length and width and height of the jaws.

The origin of the successional permanent teeth was described in Chapter 5. Those tooth germs develop on the lingual aspect of their deciduous predecessors, in the same bony crypt. From this position they shift considerably as the jaws develop. For example, the incisors and canines eventually occupy a position, in their own bony crypts, on the lingual side of the roots of their deciduous predecessors, and the premolar tooth germs, also in their own crypts, finally are positioned between the divergent roots of the deciduous molars (Figures 10-4 and 10-5).

The permanent molar tooth germs, which have no predecessors, develop from the backward extension of the dental lamina. At first, little room is available in the jaws to accommodate these tooth germs. In the upper jaw the molar tooth germs develop first, with their occlusal surfaces facing distally, and then swing into position only when the maxilla has grown sufficiently to provide room for such movement (Figure 10-6). In the mandible the permanent molars develop with their axes showing a mesial inclination, which becomes vertical only when sufficient jaw growth has occurred.

These preeruptive movements of deciduous and permanent tooth germs place the teeth in a position within the jaw for eruptive movement. These preeruptive movements of teeth are a combination of two factors: (1) total bodily movement of the tooth germ and (2) growth in which one part of the tooth germ remains fixed while the rest continues to grow, leading to a change in the center of the tooth germ. This growth explains, for example, how the deciduous incisors maintain their position relative to the oral mucosa as the jaws increase in height.

Preeruptive movements occur in an intraosseous location and are reflected in the patterns of bony remodeling within the crypt wall. For example, during bodily movement in a mesial direction, bone resorption occurs on the mesial surface of the crypt wall, and bone deposition occurs on the distal wall as a filling-in process. During eccentric growth, only bony resorption occurs, thus altering the shape of the crypt to accommodate the altering shape of the tooth germ. Little is known about the mechanisms that determine preeruptive tooth movements, including whether remodeling of bone to position the bony crypt is important as a mechanism or merely represents an adaptive response.

Eruptive Tooth Movement

The mechanisms of eruption for deciduous and permanent teeth are similar, resulting in the axial or occlusal movement of the tooth from its developmental position within the jaw to its final functional position in the occlusal plane. The actual eruption of the tooth, when it breaks through the gum, is only one phase of eruption.

Histologic Features

Histologically, many changes occur in association with and for the accommodation of tooth eruption. The periodontal ligament (PDL) develops only after root formation has been initiated; when established, the PDL must be remodeled to accommodate continued eruptive tooth movement. The remodeling of PDL fiber bundles is achieved by the fibroblasts, which simultaneously synthesize and degrade the collagen fibrils as required across the entire extent of the ligament. Recall also that the fibroblast has a cytoskeleton, which enables it to contract. This contractility is a property of all fibroblasts but is especially well developed in PDL fibroblasts, which have been demonstrated to exert stronger contractile forces than, for example, gingival or skin fibroblasts. Ligament fibroblasts exhibit numerous contacts with one another of the adherens type and exhibit a close relationship to PDL collagen fiber bundles.

The architecture of the tissues in advance of erupting successional teeth differs from that found in advance of deciduous teeth. The fibrocellular follicle surrounding a successional tooth retains its connection with the lamina propria of the oral mucous membrane by means of a strand of fibrous tissue containing remnants of the dental lamina, known as the gubernacular cord. In a dried skull, holes can be identified in the jaws on the lingual aspects of the deciduous teeth. These holes, which once contained the gubernacular cords, are termed gubernacular canals (Figures 10-7 and 10-8). As the successional tooth erupts, its gubernacular canal is widened rapidly by local osteoclastic activity, delineating the eruptive pathway for the tooth. The rate of eruption depends on the phase of movement. During the intraosseous phase, the rate can attain 10 mm per day; it increases to about 75 mm per day once the tooth escapes from its bony crypt. This rate persists until the tooth reaches the occlusal plane, indicating that soft connective tissue provides little resistance to tooth movement.

When the erupting tooth appears in the oral cavity, it is subjected to environmental factors that help determine its final position in the dental arch. Muscle forces from the tongue, cheeks, and lips play on the tooth, as do the forces of contact of the erupting tooth with other erupted teeth. The childhood habit of thumb-sucking is an obvious example of environmental influence of tooth position.