30 Theories and techniques of oral implantology

Fig. 2-7. By the time maturity is reached, the mandibular canal is typically located nearer to the inferior border of the mandible than to the alveolar crest.

Progressive increase in depth and elongation, especially behind the mental foramen, provides room for the permanent molars. The mental foramen assumes its adult position, and the ramus is almost at a right angle to the body of the mandible (Fig. 2-7) .

The mandible remains in this state as long as the teeth and jaws are healthy. However, disturbances in the peridontal tissues that support and invest the teeth occur in from 75% to 80% of the adult population. The incidence of these disturbances rises with age and accounts for the loss of more teeth than do caries. The disturbances may result from local conditions such as calculus, filth, and irritation (simple gingivitis). If these conditions are not corrected, loss of the teeth results. Consequently, alveolar bone resorption occurs, beginning at the crest and gradually removing the bone down to the dense underlying basal bone. Eventually only a broad, flat ridge is left.

The resorption of alveolar bone naturally influences the position of the mandibular canal and the general appearance of the mandible. Where a great deal of resorption has occurred, the canal may be separated from the surface of the mandible by only a thin plate of bone, and the mental foramen will lie close to the upper border of the mandible (Fig. 2-8).

The fate of alveolar bone and the position of the mandibular canal are of prime importance in considering whether or not to perform an implant intervention (Fig. 2-9). An endosseous implant must be set deeply enough in the bone so that bone can grow around it and hold it securely. Also, the implant cannot impinge upon the mandibular canal; the nerve may be damaged and facial paralysis result.

Accordingly, a table has been drawn up indicating those cases in which endosseous implants could be conternplated. Not too surprisingly, the divisions of favorable, possible, and impossible are directly related to the amount of alveolar bone remaining and the position of the mandibular canal. Where an endosseous implant cannot be performed, the subperiosteal implant is suggested (Fig. 2-10) .

Fig. 2-8. Immediately after extraction, a good deal of alveolar bone usually remains for a time between the crest of the ridge and the mandibular canal and the mental fora-men. However, with time the bone resorbs to a broad, flat ridge and the alveolar crest, canal, and foramen are close together.