different ways. This is reflected in both the outer shape and the inner structure of the bones involved. One type of adaptation is the internal arrangement of spongy bone along lines of stress. In response to vertical force, the trabeculae thicken and form orderly tracts, or trajectories, bridged by shorter, thinner trabeculae. This type of internal arrangement is exemplified by the mandible. The maxillae illustrate another type of adaptation — the reduction of force and its consequent dissipation by deflecting it over curved, reinforced pathways.

The maxillae are solidly fused to other bones of the upper face and skull with which they share nondental functions. Because of this attachment, these bones share mechanical stress. Normally, the major source of mechanical force on the upper face is mastication, initiated — of course — on the alveolar processes of the

maxillae. Up to 200 pounds per square inch of force can be exerted on the molar area by healthy teeth in jaws with strong muscle attachments.

Masticatory forces are diverted over, across, and around the bones of the upper face and eventually toward the base of the skull in somewhat the same way as an architectural arch makes the weight of a wall tolerable. The pathways for stress distribution in the maxilla are determined, in large part, by structures performing nondental functions. The nasal and paranasal structures, orbits, and the foramina supplying them interrupt an otherwise solid upper face with rounded cavities. The bone forming the borders of these cavities is adapted as the so-called stress pillars and their buttresses.

The maxillary stress pillars are familiarly illustrated as two-dimensional pathways drawn on the surface of an intact skull. However, these depictions actually represent a three-dimensional system that directs stress or force away from the dental arch in any possible direction consistent with other functions of the upper face.

The internal structure of the stress pillars is particularly note-worthy because it provides the bulk that limits the expansion of the maxillary sinus and the extent of the nasal cavity. Arising from the dental arch is spongy bone continuous with the alveolar bone of the dental arch. This spongy bone forms the core of the pillar and is surrounded by denser, more compact bone. The cancellous core, like spongy bone elsewhere in the body — including the alveolar crest — is far more responsive to the presence or absence of mechanical stimulation than is dense, compact bone. When healthy teeth are in good functional occlusion, the core is sizeable with

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