Accuracy of impression materials measured with a vertical height gauge.

The objectives of this study were to introduce a different method for evaluating the accuracy of impression materials using a vertical height gauge, and to determine the vertical (axial) and horizontal (transversal) changes of four impression materials. Comparison of means demonstrated the changes for addition and condensation type silicones, and larger changes for the agar/alginate combination and the visible light cured (polyetherurethanedimethacrylate) impression material. The results were in agreement with an earlier study of the agar/alginate combination, and the values for the condensation and addition silicones were about half those reported in previous studies. These small changes might indicate the use of a mandibular stock tray for the upper jaw when a putty/wash technique is employed. The findings suggest that, when the horizontal changes (delta X) of impressions taken with a mandibular stock tray have a negative sign (contraction), then the vertical changes (delta L) will have a positive value (expansion), and vice versa. Thus the pattern of distortion can be formulated as delta X/delta L less than 0.