The tissues involved in implant procedures 41

(Fig..2-19). Here another kind of pressure--masticatory pressure exerted upon the teeth—must be considered. This pressure is passed as traction to the lamina dura, or alveolar bone proper, via the principal fibers of the periodontal membrane. If this pressure were transmitted directly down into compact bone, a great deal of force would be exerted upon bone not structurally adapted to withstand it. How-ever, this tendency is counteracted by the spongy nature of the bone arising on the outer surface of the lamina dura. The trabeculae of this bone unite to form a series of trajectories for the distribution of stress.

In the mandible, the trabeculae arise on the outer surfaces of the lamina dura and end, in part, on the compact alveolar plates or on the laminae of adjacent teeth. Some of the spongy trabeculae surrounding the apical parts of the sockets unite as a trajectory, which runs backward below the sockets and then diagonally upward and backward through the ramus to end in the condyle. In this way, masticatory pressure is finally transmitted to the base of the skull. Naturally the greater the distance a force passes, the more resistance it meets, which dissipates its strength.

Other trajectories are formed in response to the

Fig. 2-19. Vertical cross sections through the mandible show the dense cortical bone shell enclosing more loosely arranged cancellous alveolar bone. (From Updegrave, W. J.: New horizons in periapical and interproximal radiography, Elgin, Illinois, 1966, Rinn Corporation.)

Area of weakness

Fig. 2-20. The trajectories of the lower jaw correspond to the kind of bone found in the mandible. The zone of weakness consists of very loosely arranged alveolar bone consisting mostly of hemopoietic marrow. This area is to be avoided in endosseous interventions. (Redrawn from Weinmann, J. P., and Sicher, H.: Bone and bones: fundamentals of bone biology, ed. 2, St. Louis, 1955, The C. V. Mosby Co.)