The ratio between vertical and horizontal changes of impressions.

The purpose of this study was to analyze theoretically and experimentally the relations between the horizontal and the vertical changes of impressions taken with a mandibular stock tray. According to the theoretical analysis the ratio between the horizontal (delta X) and the vertical changes (delta Z) can be expressed as: [formula: see text] where mu is the Poisson's ratio and Lx and Lz are the horizontal and vertical dimensions of the die, respectively. With regard to the experimental findings the Poisson's ratio calculated by this expression was mu = 0.44 which fits the range of Poisson's ratio for rubber materials. In addition it was shown that the ratio between the vertical and horizontal strains is constant for a particular impression technique.