The tissues involved in implant procedures 43

sinuses, orbits, and nasal passages also influences its stress patterns. The maxillary sinuses are pneumatic cavities. Together with the other sinuses in the skull, they lessen its overall mass. This may not seem too dramatic in the human, but the absence of sinuses in an elephant, for example, would make the skull so heavy that the animal could not lift his head.

In addition to lightening the skull, a maxillary sinus also performs an interesting architectural function. It acts in the upper jaw as an arch does in a wall   it helps shape and direct stress. From a mechanical point of view, the sinus is analogous to the large marrow spaces in long bones. The resulting tubular structure of the long bone is better able to withstand stress by deflecting it around the bone rather than directly into it.

In addition to its sinus, each maxilla has three vertical pillars that conduct forces from the basal part of the alveolar process toward the base of the skull. These move upward around the nasal cavity and orbit and are connected horizontally by bony formations that act as buttresses. The three pillars are the canine, zygomatic, and pterygoid pillars. Their pathways, and those of their connective processes, are marked by strengthened compact bone (Fig. 2-23) .

As for the teeth themselves, they and the alveolar bone in which they are set beautifully illustrate

Wolff's law—functional stresses affect the bone (Fig. 2-24).

The teeth are connected to the alveolar bone by a modified periosteum known as the periodontal membrane. This membrane provides the tooth cementum and bone with nutrition, maintains and re-places bone and cementum, provides proprioception and pain reception for adjustive mechanisms for occlusal functions, and supports a tooth by organizing and distributing the stress on it caused by occlusion. In other words, the fibers of the periodontal membrane transmit the pressure on a tooth as tension to the bone. The bone that directly receives this pressure is alveolar bone (Fig. 2-25) .

Alveolar bone is continually being resorbed and replaced. Because the stimulus for osteogenesis is pressure, the loss of tension on the bone causes loss of alveolar bone. Knowing what happens to alveolar bone when teeth are lost is important to the implantologist because it helps him evaluate the condition of the bone.

The resorption of alveolar bone caused by tooth loss is called disuse atrophy. Disuse atrophy may be minor or severe, localized or total. The simplest kind of localized atrophy results when a tooth loses its opposing tooth. Although much of the pressure usually brought to bear upon this tooth is lost, enough remains from neighboring teeth functioning

Pterygoid pillar

Fig. 2-23. Because the maxillae are strongly fused to the skull, the stress lines in them pass upward via other bones in the skull. This is not the case in the mandible. (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.)