muscles, and the material tends to slip and distort as it is worked. The prototype is a thin, perforated piece of highly malleable metal (24) with a tab (25) for inclusion in the impression material. The prototype is first shaped roughly (26) with the fingers outside the mouth, then lodged behind the tuberosity in the hamular notch with its arms extended forward to embrace the tuberosity palatally (27) and buccally (28).

To prevent the prototype from slipping, an assistant braces it in the hamular notch with a thin, rigid implement (29) and secures it against the unworked side (30) with a finger as the operator molds the prototype against the bone. Using the rounded side of a thin periosteal elevator (31) , he presses the prototype into bony variations, working from the notch anteriorly.

After the prototype has been shaped to the bone, the impression material is used. With the prototype still braced in place by one assistant, another assistant begins supplying the operator with impression material. With the patient's mouth wide open, the opera-tor begins by packing the impression material over the palatal arm (32) of the prototype, starting in the notch area. He gradually works anteriorly, making sure to include the entire palatal surface of the residual ridge and the crest.

Caution: The impression material should be kept away from the foramina and not be allowed to creep around the vessels. All of it must be removed to avoid irritation, and possibly infection and bone damage.

The operator progresses anteriorly until he has crossed the midline (33) and included the entire premaxillary area. He has now attended about two-thirds of the palatal surface of the arch. Even though a one-piece full arch implant is intended, he stops. He doesn't continue around the entire arch. Experience has shown that in this situation, two simultaneous "bilateral" impressions are far more accurate than a full arch impression of the maxillae.

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