Dimensional changes and pressure of dental stones set in silicone rubber impressions.

Objectives: A high expansion stone has been used for improving the adaptation of dentures. Although the method is based on the idea to compensate for the processing shrinkage of the denture base resin with the high expansion stone, there are few studies on the expansion behavior of stones in an impression. The purpose of this study is to clarify the dimensional changes of stones set in silicone rubber impressions using two types of impression tray.

Methods: Impressions of a master model that represents the alveolar ridge were taken using a silicone impression material and the dimensional changes in the stone casts were measured in different directions (x, y and z-axes). Two types of impression tray were used, one with an open surface and the other with a closed surface. In order to evaluate the setting expansion of the stones and the pressure against the impression, stones were poured in other impressions that were taken from a cubic master model. The thickness of the impression was set at 2 mm.

Results: A statistical analysis showed that there were significant differences in the dimensional change in the vertical direction (z-axis) and horizontal directions (x- and y-axes) of any stone casts set in the open tray. However, there were no significant differences in the dimensional changes in any direction of each stone cast set in the closed tray.

Significance: The impression tray influences the setting expansion of stones. Stones do not uniformly expand in the open tray and the dimensional changes in the stone casts in the three-dimensions increases, when used with a high expansion stone.