tize alveolar bone and cause its destruction, or strengthen it by encouraging the formation of more numerous and thicker trabeculae.

Alveolar bone is important both as the investing support for natural teeth and as the base for some form of substitute dentition. Thus, the features contributing to its health — how it is lost, re-forms, or reshapes — and the consequences to other structures in the maxillary complex of its degradation or loss must be explored in detail.

Initially in a young adult, a healthy dental arch is a U-shaped ridge investing the natural teeth. In the maxilla its height is deter-mined by two factors: the height of the root of the tooth and the amount of bone between the apex of the root and the closest non-dental feature. Strictly speaking, only that bone investing the teeth is the dental arch proper, even though contiguous bone is similar in shape and construction.

The maxillary, arch is, in a typical mouth, normally deeper anteriorly than posteriorly, simply because the incisors' roots are longer than those of the molars'.

The healthy dental arch begins to lose bone principally due to periodontal disease associated with systemic disorders. Less common causes, at least initially, are trauma from occlusal disharmonies or inadequate prosthetic devices, and disuse atrophy.

First the bone's internal architecture alters with the trabeculae thinning and becoming fragile, having smooth surfaces devoid of osteoblasts. Up to 75% of the bone's mass may be lost before the external shape of the ridge alters.'` Thus judging a site's health by radiographs alone can be misleading. Tall bone does not mean strong bone. An operator clinically observing a tooth with no antagonists, a tooth in poor occlusion, or a tooth with periodontal disease, and so on, should anticipate a loss of mass even though radiographs indicate very little loss of bone height.

The loss of bony support, rather than disease of the tooth itself, is the prime cause of tooth loss. The empty socket then re-builds, forming much more fragile alveolar bone because the mechanical stimulus for solid bone formation is absent. This bone unstimulated, or even traumatized by undue pressure from other teeth or a poorly fitting conventional appliance, loses even more substance. This is the classic picture of bone loss in the maxillary arch. However, it is erroneous to assume that the dental arch shrinks in height as alveolar bone is lost in the same manner as a

* Silverman, Sidney I., Oral Physiology. St. Louis: C. V. Mosby, 1961. (p. 292)

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