STATEMENT OF PROBLEM: In dental implant restorations, a lack of passivity may be associated with mechanical failure. Passivity can be compromised during impression making. PURPOSE: The purpose of this study was to compare the distortion of mechanically mixed polyether and hydrophilic addition silicone impression materials, and to evaluate the effect of intercoping distance on distortion. MATERIAL AND METHODS: Twenty impressions (10 polyether and 10 silicone) were made from a single mandibular definitive cast with 5 abutment analogs using standardized laboratory and technique protocols. The direct impression technique and mechanical mixing were used. A precision measuring machine established spatial coordinates of the impression copings in 3 dimensions, with the operator blinded to materials. Linear distances (concentricity) and angular inclinations (perpendicularity, parallelism) were calculated to measure impression distortion relative to the positions/angulations of the implants in the definitive cast. Distortion differences between materials and implant intercoping distances were tested using 2-factor ANOVA with an interaction term. A Bonferroni 2-sided test was used (alpha=.05). RESULTS: No significant difference was found between the impression materials for parallelism (P=.91) and concentricity (P=.85). For perpendicularity, the silicone material had an average of 0.643 degrees less distortion (P=.004). With respect to intercoping distances, no significant differences were found for perpendicularity (P=.234), parallelism (P=.114), or concentricity (P=.346). An interaction term for material and coping distance was not significant. CONCLUSIONS: Hydrophilic addition silicone and polyether impression materials have similar distortion effects for transfer procedures when using the direct impression technique and machine mixing. Silicone demonstrated superiority for perpendicularity distortion, though of a magnitude unlikely to have clinical significance.
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