the ridge, closely followed by a loss of height. However, whereas the anterior ridge loses height only from the crest, posteriorly the ridge becomes shallower as bone at both the crest (4) and from the floor of the sinus (5) resorbs.

The bone's condition, as well as the height of the ridge, helps determine the implant type. The totally edentulous posterior ridge is very likely to have fragile bone with widely spaced, narrow trabeculae. This characteristic may suggest using a pterygoid extension implant rather than the specially-designed shallow bladevent, even if the ridge is tall and wide enough for the bladevent.

A bladevent's innate stability depends upon broad surface con-tact with bone. The shallow body of the implant is a design disadvantage, and widely spaced, fragile trabeculae even further minimize the essential bone-implant contact.

The shallow maxillary bladevent works most successfully in dense bone, and is therefore more appropriate under the sinus between stable teeth than in a totally edentulous site. It is not recommended as the posterior abutment for a unilateral bridge, although in some situations it can be used in conjunction with a full arch, fixed prosthesis.

When using a shallow bladevent design seems suitable in a posterior situation, the ridge must be at least 3.5 mm wide at its narrowest point (6). This is wider than for a similar anterior situation because of the greater lateral stress on posterior implants during chewing and grinding. When undercuts are present, the ridge usually must be remodeled (7). However, very careful estimates must be made when considering reducing a shallow posterior site. Too much reduction, and even the shallowest bladevent design will not fit. Bone below the sinus is too precious for experimental reduction.

The special, shallow double-posted sinus bladevent requires at least 7 mm of bone for properly deep insertion. Its shoulders must be buried in bone — with the bottoms of the posts touching the crest (8) for good retention.

In a shallow implant site there is always the danger of penetrating the sinus. Thus the 700 XL bur is used very cautiously. Typically, the bone is fragile and offers little resistance. A heavy

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