Evaluation of factors affecting the accuracy of impressions using quantitative surface analysis.

Impression material goes from a plastic to an elastic state during setting. Movement of the impression and excessive seating pressure during this transition can cause distortion in the impressions. The purpose of this study is to determine if the impression distortion is related to movement during setting or to distortion of the putty phase in the two-step impressioning technique. A master model of a maxillary quadrant of teeth was impressed using four different procedures: 1) one-step technique without movement (1S-NM); 2) one-step technique with movement (1S-M); 3) two-step technique without movement (2S-NM); and 4) two-step technique with movement (2S-M). An artificial oral environment and surface analysis technique of the Minnesota Dental Research Center for Biomaterials and Biomechanics were used to produce the impressions and measure their accuracy. A digitized image of the first premolar of the master model was aligned with a digitized image of the first premolar of each epoxy model using AnSur. The root mean squared difference (RMS) between the aligned images is a measure of the distortion. The corresponding RMS values for the different methods were: 1S-NM = 23.7 +/- 9.21; 1S-M = 20.4 +/- 3.9; 2S-NM = 20.5 +/- 7.7; 2S-M = 21.3 +/- 4.4. Statistical analysis using a two-way analysis of variance showed no difference at the 0.05 level of significance. Pairwise comparison using the Tukey method showed that neither technique (one-step vs two-step) nor movement is a significant factor. These results showed that low seating pressure will not cause any greater distortions in the two-step impression technique than in the one-step technique, and minor movement during the setting of the impression material will no cause distortion.