EFFECT OF IMPRESSION TRAY DESIGN AND IMPRESSION TECHNIQUE UPON THE ACCURACY OF STONE CASTS PRODUCED FROM A PUTTY-WASH POLYVINYL SILOXANE IMPRESSION MATERIAL

This study examined the accuracy of stone casts produced from impressions taken in stock polycarbonate trays, some of which had been strengthened with autopolymerizing polymethyl methacrylate resin. Three techniques were used to make the impression of an acrylic master model of the mandibular arch on which two extracoronal preparations for bridgework and one intracoronal inlay preparation had been carried out. Each preparation had been indented with a reference point for later measurement. The impression material was a putty-wash polyvinyl siloxane material. Five impressions were taken for each type of tray for each impression technique and these were cast in die-stone after 24 h. The distances between the points were measured with a reflex microscope and the means determined for each design of tray. The mean difference between casts produced from the various tray designs and the acrylic master model were determined for each of the distances between the three measuring points for the various impression techniques. Statistical analysis showed that, with the polycarbonate stock trays, there were significant differences between some of the modifications and between them and the acrylic model. for the three distances (P < 0.05). These differences were limited to one measurement for one design of tray for each of the two-stage impression methods. With the one-stage technique the unreinforced tray and those reinforced with acrylic, over the heels and anteriorly, and the barred design were statistically significantly different from the acrylic model for measurement A-B. It would appear from this study that the design of tray, or the impression technique employed, has little effect on the accuracy of impressions made with the polyvinyl siloxane material tested when used as a putty-wash and a two-stage technique. but that the accuracy of the impression material within the bulk of the material (A-B) was affected adversely using a one-stage technique.