will have lost their teeth principally because they have lost bone — the old picture of alveolar crest resorption found in every standard dental or oral anatomy book is misleading. The classic arch pro-files are surface observations, and do not accurately portray tissue arrangement and proportions within the ridge. In most cases, it is the mucoperiosteal tissues (7) padding the narrowing ridge that form its gently diverging walls.

Misconception of the true tissue profile has had serious implications in implantology. It has lead to repeatedly trying to design an implant shaped like the root of a tooth. Such designs will be of only limited application because the bone narrows more rapidly than it loses height. Studies on experimental animals in which a healthy tooth is extracted and shortly thereafter a root-shaped implant inserted are unrealistic. We will not be extracting from our human patients a healthy, strongly supported tooth. Our patients in a good many cases, lost many of their teeth several years ago, and the ridge will reflect the consequences of this loss.

In addition to the height of the bone investing the roots of the various teeth, a variable amount of cancellous bone may lie above the apices of the roots, further separating the dental sites from other nondental structures in the maxillary complex. This bone may be used as a continuation of the ridge when contemplating an endosteal implant, as long as the implant does not invade or interfere with the nondental structures. For example, anteriorly some people have several millimeters of bone (8) between the apices of the tooth roots and the floor of the nasal cavity. Others have very little (9) , sometimes only a thin plate of bone separating the two. This characteristic is genetically determined, and has little influence on dental health. The individual with several millimeters of bone above the roots naturally has more bone remaining after tooth loss (10) than does the person with little bone (11) between the two features. The former also has the advantage over the latter after tooth loss from an implantologist's viewpoint, and cosmetically as residual support for the orbicularis oris muscle.

Naturally, there is a wide range of individual variation between these two extremes. This one example illustrates the tip of the iceberg of maxillary variability that will confront the implantologist. Anteriorly and posteriorly, the maxillae vary more than does the mandible, not only from one person to another, but from one maxilla to its bilateral counterpart in the same individual.

The cuspid is better supported than its fellow maxillary teeth because the bone in which it is located forms a strengthened column

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