Influence of inlay preparation design on the trueness of different impression techniques

Statement of problem. Limited data is available on the effect of inlay preparation design on the trueness of digital impressions obtained by intra-oral scanners and conventional impressions.
Purpose. The purpose of this study was to evaluate the influence of inlay preparation design on the trueness of different impression techniques
Material and methods. Two typodont resin teeth representing maxillary first premolars were prepared for disto-occlusal inlay preparations with two different divergent angels (6° and 12°). The prepared teeth were mounted in two typodont dental models and were used as reference models. Each reference model was scanned using an extra-oral scanner. A total of 40 digital impressions of the reference models were obtained using two different intra-oral scanners (3Shape Trios and Cerec Omnicam). For the conventional impressions, each reference model was duplicated 10 times for each preparation design (n=10) using a stock metal-tray and a vinyl polysiloxane impression material to obtain a total of 20 conventional impressions. Then, all impressions were scanned using the previously used extra-oral scanner. Subsequently, all impressions were poured with modified dental stone, then each cast was scanned 10 times for each preparation design (n=10) using the same extra-oral scanner to obtain a total of 20 working casts. Both reference models and all (STL) files (n=80) were loaded into a 3D reverse engineering software. For the trueness measurement, color difference maps and reports (n=80) were obtained by superimposing the digital data sets of the scans from the two intra-oral scanners onto the reference model, then (3D) deviation analysis was performed with the best-fit algorithm method. By using this method, for each superimposition, the root mean square (RMS) of the amount of deviation at each measurement point was recorded.
Results. Two-way ANOVA revealed statistically significant differences between the taper, impression technique, and significant interaction. 12° taper showed statistically significant higher trueness values when compared to 6° taper. As for impression technique, the impression scan and the cast scan were statistically significantly better than the intra-oral scan. However, there was no statistically significant difference either between the impression scan and the cast scan or between the Omnicam and the 3Shape Trios scan.
Conclusion. Inlay preparation with 12 degrees axial wall divergence is significantly better than the 6 degrees axial wall divergence, in terms of trueness.
Clinical implications. Special care should be considered during capturing the image while using intra-oral scanners for inlay restorations.