fragile for bars or struts to sit on top of it and press on it. This contra-indicates the maxillary crest for an implant incorporating these features. However, the bone in the anterior maxilla from canine to canine is as dense as most areas of the mandible, except for the symphyseal region, and thus can support cross struts. Posteriorly, the tuberosity is frequently found to have a layer of compact bone over its crest, and thus can bear weight. If the bars or struts can be extended over that portion of the crest below and distal to the sinus from weight-bearing surfaces grounded on more solid bone elsewhere in the maxillary complex, some form of subperiosteal implant may be considered.

The Mucoperiosteal Tissues

The mucoperiosteal tissues covering the dental arch are concerned in several phases of implant procedure and prosthesis design. Because these tissues conceal any endosteal or subperiosteal implant site, they are always surgically involved. They must be cleanly retracted from the intended site to examine the bone and, in subperiosteal cases, to take an accurate impression of the bone for fabricating the implant itself. When reseated, they must form a tight elastic cuff around the posts of all types of implants, and securely bind the buried portions of a subperiosteal-type implant to bone.

Although an implant-supported restoration is not borne by the soft tissues, as is a conventional denture, it does contact them. Therefore, potentially irritating situations, such as ulcerations, tender spots, and other conditions that would retard healing, should be treated. In some cases surgery, followed by a suitable healing period, must be done prior to implantation.

The operator must be familiar with the differences characteristic of specific areas of the mouth so he can anticipate both prosthodontic and implantation problems.

Primarily two types of mucosa are concerned in implantations: masticatory mucosa and lining mucosa. Masticatory mucosa is the mucosa of the gingiva and hard palate. Its epithelium is thick, hornified, and heavily keratinized, with a dense, firm lamina propria. Both gingival and hard palatal mucosae are firmly attached to the periosteum of the underlying bone by dense bands and trabeculae of connective tissue fibers that extend from the lamina propria to the periosteum. However, whereas the submucosal layer is not well defined in the gingiva (1) , in the palate it is a distinct layer (2) containing adipose tissue anteriorly and glands pos-

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