Purpose: The aim of this study was to investigate the deviations of the implant positions of both impressions and casts using different impression materials and techniques. Furthermore, the existence of a correlation between the deviations of the impression and those of the cast was investigated.
Materials and methods: A reference model was fabricated with 5 Frialit-2 implants parallel to each other. In a standardized experimental setting, 5 stone casts were produced with 5 different techniques using polyether (A) or polyvinyl siloxane (B through E). In 3 groups, a direct technique was used with a medium-viscosity material or a putty-tray material in combination with a light-viscosity syringe material (A to C). In 2 groups, an indirect technique (either 1-step [group D] or 2-step [group E] was used with a putty-tray material in combination with a light-viscosity syringe material. The center-to-center distances were measured for impressions and casts in the horizontal plane using a computer-aided microscope, and the relative and absolute deviations compared to the reference model were calculated. Analysis of variance followed by the post-hoc Scheffé test (parametric data) or the Kruskal-Wallis test followed by pair-wise Mann-Whitney tests (nonparametric data) were used for statistical analyses. Deviations of impressions were compared with their respective casts using paired t tests and the Pearson correlation coefficient.
Results: No significant differences for the relative deviations were found for impressions (-5 to -8 microm) or casts (+7 to +16 microm). Group E produced significantly higher absolute deviations for impressions (38 microm) and casts (39 microm) compared to the other groups (11 to 18 microm and 17 to 23 microm, respectively). A significant correlation between deviation of the impression and its respective cast was found for every group (r = 0.40 to 0.80) except group D.
Conclusions: The distortions in the horizontal plane of the casts obtained from the impression techniques of groups A to D would probably not affect the clinical fit of implant-retained superstructures. Because of the high variation of deviations (-113 to +124 microm), the 2-step technique cannot be recommended. The method to measure both impression and cast provided a better understanding of how inaccuracies are caused.