Analysis of tooth preparations for zirconia-based crowns and fixed dental prostheses using stereolithography data sets

Statement of problem. Under clinical circumstances, it can be challenging for a dentist to achieve an adequate tooth preparation, which is essential for the long-term success of fixed dental restorations. This is particularly true for zirconia restorations fabricated by computer-aided design and computer-aided manufacturing (CAD-CAM) because of the difficulties involved in the scanning and milling process. Purpose. The purpose of this retrospective study was to use a digital approach to evaluate dental tooth preparations for zirconia-based crowns and fixed dental prostheses (FDP) performed by general dental practitioners in Germany and to find out whether the type of prepared tooth or the type of restoration influenced the outcome. Material and methods. Stereolithography data sets of 182 zirconia frameworks with a total of 305 abutment teeth obtained by the CAD-CAM process were analyzed regarding total occlusal convergence, abutment height, and finish line design using a 3-dimensional (3D) inspection and mesh processing software. The criteria for adequate preparations were defined based on current literature and compared with the measurement results. The Kruskal-Wallis 1-way analysis of variance and the Mann-Whitney U test were applied to detect significant differences (alpha=.05). Results. Only 13 teeth (4.3%) met the clinical requirements for adequate zirconia-based tooth preparations. The mean total occlusal convergence was 17.9 +/- 9.7 degrees. Molars (23.2 +/- 10.1 degrees) showed statistically significant difference (P<.001) when compared with premolars (14.2 +/- 8.2 degrees) and anterior teeth (16.4 +/- 8.1 degrees). No significant difference was observed between the crown and FDP preparations. The mean abutment height was 5.6 +/- 1.3 mm. Again, significant differences occurred when tooth types (4.8 +/- 1.0 mm for molars, 5.2 +/- 0.9 mm for premolars and 6.9 +/- 1.2 mm for anterior teeth; P <=.01) were compared. In 92.5% of teeth, the finish line design was not ideal for zirconia-based restorations. Conclusion. The tooth preparations of general dental practitioners differ from the ideal clinical preparation recommended for zirconia-based restorations. Difficulties are primarily related to the total occlusal convergence and finish line design. Although the outcome is strongly influenced by the type of tooth, the type of restoration seems to have only a minor effect on the tooth preparation quality.