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A dental die is intended to be an exact replica of a tooth so that the restorations made on it are as accurately fitting as possible. A working cast-and-die system must provide surface detail, dimensional accuracy, strength and resistance to abrasion. With the present laboratory practice of placing a die spacer on the replicated, prepared tooth surfaces of stone dies that allow space for the luting agent, the necessity of accurate surface reproduction may not appear to be as important.1 Type IV dental stone, are composed of cuboidal shaped particles of á hemihydrate with reduced surface area, which provide the required properties of strength, hardness and minimal setting expansion. Therefore, the dimensional stability of the dies is a prime requisite to a serviceable, well-adapted cast restoration. The first step in the search for an accurate die is a perfect impression material. There are numerous impression materials available ranging from alginate to the elastomers. The elastomers now have overshadowed every other impression materials with their superior properties. Among the elastomers, the addition silicone and polyether exhibited superior properties over others. Owing the credit to various researchers, the addition silicone and polyether with their excellent surface quality reproduction, best dimensional stability and low Aim: The aim of the study was to compare the dimensional stability of dies obtained from addition silicone and polyether impressions that were reheated or subjected to vacuum treatment. Materials and methods: A master die was constructed of a rectangular stainless steel block on to which a photosensitive nylon printing plate attached which contains engraved test grooves. A custom-tray was fabricated and impressions were made using addition silicone and polyether before and after the two treatments. A total of 90 impressions and dies were made from both the elastomers and divided as control group, group I (reheated impressions) and group II (vacuum-treated impressions). A comparative analysis was done to determine which dies yielded the best dimensional accuracy compared to that of the master die. Results: The minimum percentage dimensional change was observed in test dies made from addition silicone impressions of group I (reheated impressions), followed by the test dies made from polyether impressions in control group. The maximum percentage dimensional change was observed in test dies made from polyether impressions in group I (reheated impressions), followed by the test dies made from addition silicone impressions in group II (vacuum-treated impressions). Conclusion: The dies obtained from reheated addition silicone impressions yielded the best dimensional stability overall when compared with that of the master die. The dies made of polyether impressions after reheating were not clinically acceptable, because of its hydrophilic nature swelled when they were reheated in a water bath.

In vitro Comparison of Dimensional Stability of Stone Dies Obtained from Two Elastomers after Two Treatments