sturdy trabeculae. When the functional stimulation necessary for osteogenic activity is lacking, the spongy bone undergoes a type of disuse atrophy very similar to that occurring in the alveolar crest. The spongy bone loses mass and eventually the core diminishes in size, losing the bulk that maintains significant barriers between the cavities of the upper face. The loss is most profound near the dental arch, for here the bulkier portion of the core initially existed.

The origin of all maxillary pillars is in the alveolar bone of the dental arch. Thus tooth loss leads to cancellous bone atrophy and resorption in both the ridge and its associated pillar. Conversely, the functional stimulation that maintains the alveolar bone of the dental arch also maintains the bulk of the pillar.

The maxillary pillars are particularly important to the implantologist as deterrents to the expansion of the maxillary sinus. As the core of a pillar narrows, the pillar retreats and the sinus expands. This lateral expansion is almost always accompanied by a dropping of the sinus toward the crest of the ridge.

The base of the canine pillar (1) is the alveolar bone in-vesting the canine tooth (2), primarily, and the first bicuspid and lateral incisor, secondarily. The pillar ascends toward the lateral border of the piriform aperture (3), continues as the frontal process of the maxilla, and terminates on the medial edge of the supra-orbital rim (4) .

The canine pillar intercedes between the anterior extents of the maxillary sinus (5) and the nasal cavity (6). Seen in cross section, the base of the pillar forms a triangular-shaped barrier (7) with a compact outer layer and a dense central spongiosa. Be-cause the maxillary sinus enlarges passively as bone retreats (8), the loss of stimulation in the canine pillar area directly leads to sinus enlargement anteriorly (9). The sinus advancement threatens the security of other anterior teeth by weakening their support. They become more prone to tipping and drifting, complicating

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