Methods: 182 STL-data sets of zirconia frameworks (provided by BEGO Medical GmbH), obtained by the CAD-CAM process, were analyzed regarding convergence angle, abutment height and marginal preparation design using a 3D inspection and mesh processing software (GOM Inspect). The Mann-Whitney and Wilcoxon rank-sum tests were applied to detect significant differences at p<0.05.
Results: Mean convergence angle was 23.2±10.1° for molars, which showed statistically significant difference (p<0.001) when compared with angles for premolars (14.2±8.2°) and incisors (16.4±8.1°). Between crown and FPD preparations, no significant difference was observed (p=0.154). The abutment height showed a mean value of 4.8±1.0mm for molar preparations, 5.2±0.9mm for premolars and 6.9±1.2mm for incisors (p<0.010). In 92.46% of teeth the marginal preparation design was not ideal for ceramic restorations along the entire circumference of the abutment tooth, and in only 13 teeth (4.26%), the design met the clinical requirements for adequate all-ceramic preparations.
Conclusions: The results of this study showed that variance from ideal clinical preparation recommendations are primarily related to the convergence angle and marginal preparation. Apparently, the preparations of general dental practitioners differ from the ideal clinical preparation recommendations for zirconia-based restorations.