closely resembles in shape and character the bone around a healthy natural tooth, except that its overall dimensions are diminished and the bone trabeculae are thinner and less numerous.

Such a ridge is rare in a totally edentulous arch. Generally, the causes of tooth loss and the effects of disuse and misuse have had time to take their toll. However, it would be a mistake to assume that the entire ridge of a totally edentulous arch exhibits the same bone all around and that implantation in a totally edentulous ridge is always difficult or precarious. Even assuming that all teeth are lost simultaneously — an unusual occurrence unless they are deliberately extracted — both natural and artificially-induced wear patterns would probably occur in different ways at different times in different portions of the arch. Also, events in the opposing jaw would affect changes in the maxillae. Thus a totally edentulous maxillary ridge generally has a varying range of implant potential, sometimes with a good bone in the anterior portion of one or both maxillae.

A good ridge is not unusual in the anterior portion of a partially edentulous maxilla with limited tooth loss, as for example in a young adult whose remaining teeth are well attended. The edentulous area may be the site of a recent tooth loss, with the healed socket receiving some osteogenic stimulation from neigh-boring teeth in good occlusion.

The problems of a totally edentulous arch differ slightly from those of a partially edentulous arch, even if the actual implant site is the same position in both. Because the purpose of this text is to explain specific concepts in detail, totally edentulous arch situations are presented first to illustrate generalizations about a specific area. Partially edentulous situations tend to be modifications or elaborations of the basic surgical concepts and, therefore, are covered as addenda.

The Single-Posted Bladevent

Balanced support for a fixed prosthesis is the goal of any implant procedure. In the totally edentulous arch, balanced support may be achieved by placing six abutment posts strategically around the arch. These posts should protrude at fairly regular intervals,

ideally into the lateral incisor or cuspid area, and the second bicuspid and the second molar area. These six posts may arise from any combination of implants. The best situation, when bone morphology permits, is four bladevents: one single-posted implant

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