mountain mellows to a hill and eventually flattens to a plain. My own clinical observations of hundreds of patients' exposed ridges and current investigations with laboratory animals, in which good teeth are experimentally extracted and the ridge periodically ex-posed, indicate another trend with important implications for the conventional restorationist, as well as the implantologist. These implications can be logically predicted by observing the pre-edentulous arch.

The alveolar ridge of the maxillary arch with a full complement of healthy teeth flares pronouncedly outward in the anterior region. Less bone flanks the teeth labially/bucally (1) than it does palatally (2). Over the incisors, the bone forms only a very thin plate. When the teeth become less stable, their outward inclination has already predisposed the principal direction in which they will be unseated. This tendency, combined with the thinner labial bony support, causes greater resorption on the labial/buccal face of the ridge (3). The maxillary ridge thus tends to resorb inwardly (4) and obliquely off the horizontal plane faster than the height of the crest reduces (5). Even if a healthy tooth is sacrificed from a healthy socket (!) , its outer plate of bone is thinner and prone to more rapid resorption because less scaffolding remains for rebuilding the socket. The palatal side of the residual ridge, usually less adversely affected than the labial/buccal, is thus taller. It is these resorptive tendencies that create the knife-edge (6) common in the edentulous maxilla, and familiar to most periodontists and prosthodontists who have had occasion to expose the ridge. This is an accurate description of what happens in most maxillary arches, and