sinuses than do women. This observation does not imply, how-ever, that the male's larger sinuses are gained at the expense of bone in the dental arch. It merely points out another variation that may affect ridge height.

Another nondental factor affecting sinus size is disease involving the sinuses. A chronically inflammed, irritated, or infected sinus membrane may destroy bone. When the floor of the sinus above the dental process is eroded (17), the sinus expands at the expense of the bone between the sinus' floor and the teeth.

It is not uncommon to see healthy, apparently firmly sup-ported molars with the sinus "draped" (18) over the apices of their roots. The tooth is firm, not because its entire root system is en-cased in bone, but because the bone holds the tooth close to the crown (19) — keeping the fulcrum advantageously positioned against the lateral forces causing dislodgement. A molar with some bone around its neck (20) is far more secure than if the same amount of bone encased the apices of its roots (21). However, in this apparently secure situation, the fact remains that the total ridge height has been reduced. If a molar secured in such a fashion were then afflicted with periodontal problems that reduced bone at the crest, a minor loss of crestal bone (22) would be sufficient to cause rapid loss of total height and perhaps loss of the tooth.

Another pertinent ridge change is the actual dropping of the dental arch (23) as a tooth overerupts when it has no functional antagonist (24). In a posterior maxillary case, the crest not only follows the tooth, but the sinus (25) usually follows the over-eruption as well. In standard therapy to fit a conventional removable maxillary appliance, the tooth might be extracted and the ridge made level by an alveolectomy (26), thus bringing the crest of the ridge even closer to the sinus (27). An implantologist would probably approach the situation differently. A relatively stable tooth might be retained as a natural abutment (28) for a fixed prosthesis in which the interocclusal spatial irregularity would be compensated. This would retain the tooth and maintain bone height. The opposing site, of course, must also be restored to pre-vent further eruption of the natural tooth.

Without osteogenic stimulation or traumatization by surface pressure, the maxillary ridge (29) will eventually resorb until it becomes a flattened surface (30) continuous with the palatal bone. If the mucoperiosteal tissues follow the bone resorption and do not thicken, the former horseshoe-shaped upper arch will become in-distinguishable. However, this is a rare phenomenon in the maxilla.

9