in any direction over the sinus, including upward onto the zygomatic arch, requires lodging them against thin bone. Also, above the mucogingival junction and over the lateral wall of the sinus, the soft tissues are loosely arranged, loosely attached to bone, and very mobile — unsuitable for binding an implant strut to bone.

The cancellous core of the hard palate can also resorb, permitting sinus expansion between the plates (27). Deep expansion toward the midline is rare, and sinus expansion in any direction usually occurs in the posterior portions of the arch. Thus sinus expansion into the hard palate may not contraindicate using a subperiosteal-type implant on the anterior portion of the palate.

The paranasal sinuses aid respiration by warming the air. Their lining also moisturizes the air and removes particulate matter, thus helping to reduce bacterial invasion of the deeper respiratory pas-sages. Because these passages connect, what affects one might well affect all. Infection in a maxillary sinus can spread to the other sinuses, the nasal cavity, the throat, and deeper into the lungs. Thus, the integrity of the soft tissues lining the passages and separating them in structure and function from other parts of the body should be maintained.

The bony walls of the maxillary sinus are covered by a 1 to 2 mm thick membrane (28). This tissue is a mucoperiosteum consisting of three layers. The most superficial layer is a pseudostratified, ciliated columnar epithelium. The lower two layers, which are fused together and form the lamina propria, are a granular layer with mucous and serous glands and a modified periosteum.

The maxillary sinus mucoperiosteum is continuous with that lining the other paranasal sinuses and the nasal cavity. It differs from the latter in being thinner and having fewer goblet cells and fewer blood vessels, making it paler in color. The sinus' mucous and serous glands are concentrated mostly around the opening of the sinus into the middle meatus of the nasal cavity, the osteum maxillare.

The character of the lamina propria is of particular interest. It contains relatively fewer elastic fibers than that of the nasal cavity, which make it less firmly attached to the bone. This is an advantage when inserting a bladevent in shallow bone below the sinus. If the implant accidentally intrudes into the sinus (29), the membrane tends to easily separate from the bone if gently pushed up. Because the membrane detaches easily, there is less danger of breaking through the tissue sealing the sinus from the oral cavity. This leaves the bladevent in somewhat the same situa-

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