42 Theories and techniques of oral implantology

direct forces applied by the muscles of mastication. One is found in the region of the mandibular angle; another starts at the tip of the coronoid process, fanning out into the mandibular body. The region of the mandible above and in front of the angle is almost free of stress. In this region the trabeculae of the spongy bone are thin and the marrow spaces wide, and as a result the site should be avoided as an implant site.

The region of the chin where the two sides of the mandible join must bear a great deal of force from both the bending forces of the masticatory muscles

and those of the neck that attach to it. The mental region is strengthened by rather massive compact bone that forms the mental protuberance and by trajectories of the spongy bone, which cross each other at right angles (Fig. 2-20) .

The ways in which the maxillae are structured to bear stress are strikingly different from those of the mandible (Figs. 2-21 and 2-22). In the first place, the maxillae are firmly anchored to the upper face and base of the skull. The maxillae are therefore an integral part of a unit, with stress patterns extending back into the skull. The presence of the maxillary

Fig. 2-21. The kinds of bone found in various areas of the maxilla are shown in these horizontal cross sections of the upper jaw. Note that around the teeth and near their roots the most concentrated amounts of alveolar bone are found. (From Updegrave, W. J.: New horizons in periapical and interproximal radiography, Elgin, Illinois, 1966, Rinn Corporation.)

Fig. 2-22. In these vertical cross sections through the maxillary jaw, the relationship between the teeth and the sinus is demonstrated. Alveolar bone can clearly be seen flanking the sinus. However, it is usually very thin, therefore contraindicating, in most instances, implants placed buccal and palatal to the sinus. Careful radiography can sometimes reveal a site that might otherwise be overlooked, and in the region of the sinus the areas flanking it should be carefully examined as possible sites when not enough bone is available between the floor and the crest.