(12) that helps divert occlusal forces upward, away from the dental arch. The cuspid's labial plate (13) is unusually thick, and heavier than that of its mandibular counterpart. The cuspid also flares out-ward less than do the maxillary incisors, additional security against dislodgement.

Posteriorly, the dental arch is threatened by loss of bone height from two directions: the crest of the ridge, and the floor of the sinus.

As in the anterior regions in a mature adult with healthy teeth and periodontia, the posterior ridge is usually at least as tall (14) as the roots of the teeth (15) and often taller (16). Bone is lost from the crest through periodontal disorders, functional trauma, and so on. Also, as in the more anterior regions of the maxillae, that bone on the outer (buccal) aspect of the teeth is more prone to resorption due to mechanical distress because of the inclination of the teeth and the fact that they lie buccal to the mandibular teeth. Thus commonly the outer portion of each side of the arch tends to "migrate" toward the palate, often faster than the ridge loses height. Again, this resorption picture contrasts to that classically presented, in which the arch simply and gradually flattens after tooth loss. Again, the persistence of this incorrect image seems inconsistent with the commonly observed knife-edge ridge.

A young adult whose permanent teeth have erupted, but whose facial structure has not yet reached its full maturity, usually has a very generous amount of bone above the molars. As the sinus expands — and it is sinus expansion that gives the upper face its mature height — the bone height above the roots may lessen. The amount of bone above the teeth depends on several factors, many of them nondental in origin.

Heredity, of course, plays a role. Some individuals have taller upper faces than others. The individual's sex also plays a role in facial height. Men, for example, in accordance with their overall larger skeletal proportions, have larger and broader faces and

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