CHAPTER 12 Subperiosteal implants

1—Although this volume is primarily devoted to endosseous implants, there are certain situations in which a subperiosteal implant is preferable. The two groups of implants are not competitive; one type of implant cannot possibly be utilized successfully where the other one is indicated.

There are two schools of thought about the appropriateness of a subperiosteal implant. Both agree that the subperiosteal implant should be used only when the alveolar bone has almost completely re-sorbed. However, the American school, which includes such men as Gershkoff, Goldberg, Lew, Jermyn, Bodine, Cranin, Weber, Linkow, and members of the American Academy of Implant Dentistry, advise that a subperiosteal implant is successful over a prolonged period of time only when set over basal bone. Adequate amounts of this type of bone exist only in the mandibular arch, not in the maxillary arch. Maxillary bone is primarily of a cancellous, more porous structure. Therefore, with few exceptions, the edentulous maxilla is inappropriate for subperiosteal restoration

A subperiosteal implant should be utilized for the atrophied mandible only. It should not be at-tempted on a mandible where much alveolar bone exists, since over a relatively short period of time—within 2 to 3 years   the alveolar bone will resorb. Subsequently the implant will probably have to be removed. In situations where teeth still exist, there is usually too much alveolar bone.

Certain Europeans, including the Spaniards Salagaray and Sol and the Frenchman Audoire, do use subperiosteal implants in the maxilla. However, these are uniquely designed to include a good portion of the hard palate, zygomatic arch, canine eminence, nasal spine, and even the pterygoid process. These men report gratifying results with their strongly braced implants. American operators, however, prefer to limit subperiosteal restorations to the mandible,

where the anatomy is more favorable for prolonged success.

The subperiosteal implant is usually cast in Vitalliumu (a chrome-cobalt alloy) and consists of two major parts: a substructure and a superstructure (Fig. 12-1) . The substructure, the implanted part, rests on the bone directly underneath the periosteum. It is held in position by its accurate fit and the fibromucosal tissue that tenaciously binds to it. The superstructure snaps over the protruding posts of the substructure and fits over, but does not touch, the healed soft tissues. The superstructure is incorporated into the prosthesis.

The substructure of the subperiosteal implant consists of several parts (Fig. 12-2) :

1. The struts, of which there are three types:

1. Primary struts, which bear the abutment posts. These struts should be the only ones traversing the crest of the ridge to minimize long-range problems associated with bone resorption.

2. Secondary struts, which connect the labial and buccal peripheral strut with the lingual strut.

3. Peripheral struts, which outline the peripheral shape of the implant.

2. The abutment posts. There should be four abutment posts, evenly spaced around the arch, for a full arch restoration. These have narrow necks around which the tissues will be sutured. Their heads will fit into the superstructure.

The substructure framework must extend to the external oblique ridges, circumvent the bundle of nerves exiting the mental foramina, extend down deep to the symphysis on the labial surface, extend deep to the genial tubercles lingually, and not go be-low the mylohyoid ridges posteriorly on the lingual side.

The superstructure of the implant is horseshoe-shaped and consists of four atypically clasped copings

*Howmedica, Inc., New York, N. Y.

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