The soft tissues are separated from the bone to expose the anterior portion of the hard palate, specifically the premaxillary area; the palatal surface of the residual ridge mesial to the molar area; and the labial face of the ridge from the first bicuspid area to the first bicuspid area. An impression material is molded over these sites, paying special attention to pack it against surface variations in the bone. A bite registration is also taken.

In designing the implant, the struts should be placed as bilaterally symmetrically as bone variations allow. Positioning the struts in bony valleys (7) or against prominences (8) gives greater security against dislodgement than does standing them on flat planes (9).

The thickness of the struts is important because as little non-living material as is possible should be interposed between living tissues. Ideally, the struts should be thin and narrow. However, a narrow strut (10) does not provide the bracing action of a wider strut and can be more easily turned within the tissue. Therefore, narrow segments are recommended only where other segments pro-vide structural counterbalance or where several struts are joined together in a framework. In some cases, a broad flat strut can be fenestrated. Thinness is a fabrication problem and its importance should be stressed to the laboratory. The struts should be no more than 0.5 mm thick.

That portion of the toroplant to be seated on the anterior premaxillary region should cover a wide enough area to prevent lateral rocking. The implant can cross the midline on the palate but should avoid it on the ridge. In the premaxillary region, the struts can be relatively narrow because the entire framework of the implant — not the individual struts — provides the bracing action. The narrowness and thinness of the segments allow the soft tissues to regrow through the implant and bind it securely to bone, a characteristic essential to subperiosteal implant stability.

Formerly to prevent torque, the antero-posterior dimension (12) of the palatal framework extended almost to the posterior nasal spine (13), under the glandular zone of the soft palatal tissues. Because the palatine torus (14) was an important land-mark in its design, the implant was named toroplant. However, ex-tending the implant beyond the rugae into the glandular region provoked adverse effects in some cases, principally poor resealing of the soft tissues to bone and consequently infection. Also, the more posterior portion of the palate is composed of thinner bone. Current designs limited to the more anterior, thicker portion of

96