THE EFFECT OF MARGINAL THICKNESS ON THE DISTORTION OF DIFFERENT IMPRESSION MATERIALS

An impression of the margins of a prepared tooth and adjacent gingival sulcus must be of sufficient thickness to withstand distortion and te aring when the impression is removed from the mouth. The purpose of th is study was to compare the dimensional accuracy of Elite, Examix, and Express polyvinyl siloxanes; Permadyne polyether; and Permlastic poly sulfide elastomeric impression materials. These materials were used to make impressions of a metal model that simulated prepared abutments w ith gingival sulci of various widths. A traveling microscope was used to measure the abutments and impression widths, and the number of defe cts in each impression was recorded. Between 70% and 100% of the abutm ent impressions with sulcular widths of 0.05 mm exhibited defects, whi ch prevented accurate measurements in this group. Express material dem onstrated a high number of defects in the 0.10 and 0.16 mm sulcular wi dth groups. No great difference in average maximum distortion values o r coefficients of variation (CV) were detected among the materials use d to make impressions of abutments with sulcular width groups of 0.2 t o 0.4 mm. For the sulci of 0.16 mm and less, Examix and Permlastic mat erials exhibited distortion and a CV comparable to the impressions of the wider sulcular groups, whereas Elite and Permadyne showed greater distortions and CVs. The differences were not statistically significan t (analysis of variance) because of the larger CV among the groups. Th e larger coefficient of variation in the 0.1 and 0.16 mm sulcular widt h groups demonstrated inconsistencies in obtaining good impressions of abutments with such narrow sulcular widths.