of the inferior and anterior walls of the pyramid, lies approximately 1 cm. below the level of the floor of the nose (12).

As the individual ages and the sinus expands, the pyramid shape — never more than a handy descriptive similarity — be-comes more obscure. That part angled toward the teeth and forming the lowest point may expand anteriorly toward the canine eminence (13) or posteriorly toward the tuberosity (14) or in both directions (15, 16). The apex of the former pyramid may invaginate the zygomatic arch, and those sections near the ethmoid and frontal sinuses may balloon into them. Inferiorly the sinus may also expand into the hard palate if its cancellous core resorbs and retreats toward the midline. Thus, the implantologist examining radiographs of an older patient with moderate or extensive tooth loss cannot rely upon the classic textbook reference shapes in tracing the sinus' morphology.

When the sinus is small, its walls are thick, and a great deal of bone separates the sinus from the dental arch. As the individual grows and his teeth erupt, the sinus becomes larger as the walls become thinner. (It is interesting to note that the increase in facial height typical of maturation is related to the expansion of the sinus, while the reduction in facial height as a consequence of aging results from tooth loss and subsequent alveolar bone loss. These are important observations in restoring facial dimension with a dental prosthesis.)

Enlargement of the sinus — in no matter which direction — occurs at the expense of bone. The fate of the bone between the sinus floor and the dental arch concerns the implantologist. If