Evaluation of the accuracy of four different impression techniques for implant prostheses

The success of implant-supported prostheses directly depends on the success of passive fit between the implant itself and the connector of implant-prostheses. Therefore, impression and transfer of the exact positions of the implants on the working cast should be accurate. The most important steps for a passive fit is an accurate impression. From a practical standpoint, passive fit is impossible to achieve; instead, minimal misfit may be the clinical goal.This study consisted of two parts. Edentulous upper jaw was obtained using epoxy resin and four dental implants were placed in parallel to support overdenture prosthesis. This model was used as the main model for both parts of the study. Impressions were taken from the main model. A total of 100 gypsum models were prepared for the first (n=60) and second (n=40) part of the study. Type IV dental gypsum was used for preparing the study models. In the first part, main models and study models were examined under the profile projector and implant positions were recorded linearly. The main model was used as a reference and study models of linear dimension variations were statistically compared with it. In the second part, main model (n=10) and the working models (n=40) were used to obtain a total of fifty Ni-Cr alloy superstructures. In the maxilla total of a six strain gauges were cemented on distal and mesial servical regions of the implant, in which strain gauges were distributed equally on both sides of the midline. The total amount of strain which occurred during fixation of the superstructures were determined and analyzed statistically. For the first stage, models of group A (indirect technique) and B (direct technique) were found statistically significant difference was noticed on the main model, while in group C (splinting with acrylic resin + direct technique) and D (splinting with metal connector + direct technique) were not. According to the measurements of the second stage, all groups were found to be significantly different from the main model.The new method with metal connector + direct technique used in this study was identified as the most reliable impression technique. As a result, this technique can be an alternative to the classical impression techniques to achieve the minimal misfit which is necessary for the implant superstructure.