Tooth Preparation Analysis Obtained by CAD-CAM

Objectives: The aim of this study was to evaluate dental tooth preparations for zirconia-based (BeCe^® CAD Zirkon+) and cobalt-chrome metal-based (Wirobond^® C+) crowns and fixed partial dentures (FPDs) compared to ideal clinical preparation recommendations, performed by general dental practitioners in Germany.
Methods: 182 STL-data sets of zirconia frameworks and 186 STL-data sets of cobalt-chrome metal frameworks, 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). Kruskal–Wallis one-way analysis of variance and Mann-Whitney U test were applied to detect significant differences at p<0.05.
Results: Regarding the tooth preparations intended for all-ceramic restorations, mean convergence angle was 23.2±10.1° for molars, 14.2±8.2° for premolars and 16.4±8.1° for incisors. Compared with the results for metal-based restorations, no significant differences were found for molars (22.6±9.2°; p=.710) and premolars (12.6±7.5°; p=.121), but for anterior teeth (13.9±8.7°; p=.031). The mean abutment height was 4.8±1.0mm for molars, 5.2±0.9mm for premolars and 6.9±1.2mm for anterior teeth in the zirconia group, respectively 4.9±1.0mm, 5.1±1.0mm and 6.5±1.3mm in the metal group. Again, significant differences between both restoration materials occurred exclusively for front teeth (p=.025). In 92.46% of the 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 clinical requirements for adequate all-ceramic preparations were achieved. The cobalt-chrome group showed 30.6% knife-edge, 28.2% slight chamfer, 25.2% deep chamfer, 5.6% rounded shoulder, 5.6% indefinable, 3.5% beveled and 1.4% sharp shoulder preparations.
Conclusions: The preparations of general dental practitioners differ from the ideal clinical preparation recommendations, but were hardly influenced by the restoration material (except for front teeth).