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The long-term dimensional stability of three non-aqueous elastomeric impression materials, a silicone, a polyether and a polysiloxane, were investigated. The testing apparatus was an A.D.A. specification No. 19 test block and mould. The impressions were measured by the use of a travelling stage reflecting microscope and a Quantimet 720 Image Analyzing computer at time zero and again at 4, 24 and 48 h, and at 1, 2, 3 and 4 weeks. Impressions poured immediately had the greatest accuracy for all the materials. The polysiloxane (0.025 mm) and polyether (0.033 mm) showed very little change in dimension after 4 weeks when compared to the test block. However, the silicone showed significant change at 4 h (0.14 mm) and 0.2 mm after 4 weeks. After 4 weeks the polysiloxane maintained the best surface detail, the polyether was adequate but the silicone lost much of its detail in as little as 24 h. The reasons for these findings are discussed.

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The posterior palatal seal area is described and its anatomical borders are defined. The methods used to achieve a seal are (1) scraping the cast, (2) a selective loading impression technique, and (3) a physiological impression technique. Each method is described and its problems are discussed. It is believed by the authors that the physiological impression technique using wax requires the least amount of skill and experience to master competently.

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