should include a precautionary note about the careful separation of the impression from the plaster so as not to distort the bite impression. Most recently the author started taking impressions using a palateless Omnivac plastic tray taken from the stone model, duplicating the soft tissue arch. Rubber base impression material is used inside the tray and when it hardens it is picked up with a plaster impression.

The implantologist designs each pterygoid extension implant individually on the master stone model. All stress-bearing portions must rest on compact, not cancellous, bone. The proposed sites should be compared with preoperative radiographs to detect weak or flawed areas to avoid.

Anteriorly, the implant covers a wide enough area of the pre-maxilla (46) to discourage tipping. Bone variations are used to lodge or brace the struts. The premaxillary portion bifurcates, avoiding the midline of the ridge, and extends over the ridge in the cuspid region (47). Each labial segment continues upward and divides high on the ridge into side-arms or "wings" (48) as further buttressing and to prevent labial tipping. Currently these struts are always joined together beneath the anterior nasal spine.

Extending posteriorly and bilaterally from the premaxillary portion of the implant is a palatal strut (49). This is a wide, flat piece that is closely fitted to the palatal surface of the residual ridge at its highest peak just before it blends with the hard palate. As it approaches the greater palatine vessels, it may narrow (50) to avoid lying directly under them. The palatal strut stabilizes the implant and prevents lateral movement.

The palatal strut passes into the hamular notch where it should be wedged against the pterygoid process and emerges buccally to wrap a short distance forward, remaining on the thicker
bone of the tuberosity (51). It does not proceed anteriorly to lie over the eggshell-thin bone over the sinus. When the buccal tuberosity strut is flat and wide, it should be fenestrated for greater soft tissue reattachment-to-bone potential. Fenestrations also lighten
the implant, a particularly important consideration in the maxilla. In the tuberosity region, a strut crosses the crest (52) and links the buccal and palatal peripheral struts. This cross-strut gives further strength to the posterior portion of the implant, and serves as the posterior terminus for the modified Andrews or Dolder bar. The weight brought to bear upon the cross-strut during occlusion is deflected toward the peripheral and pterygoid struts. If positioned correctly, the cross-strut also lies behind the most posterior

120