Marginal and Internal Fit of CAD/CAM Crowns Fabricated Over Reverse Tapered Preparations

Purpose Intraoral digital scanning can accurately record single abutment tooth preparations despite their geometry, and the algorithms of the CAD software can be set to manage different abutment forms. The purpose of this in vitro study was to evaluate the marginal and internal fit of CAD/CAM zirconia crowns fabricated over conventional and reverse-tapered preparations. Materials and Methods Crown preparations with known total occlusal convergence (TOC) angles (-8 degrees, -4 degrees, 0 degrees, 8 degrees, 12 degrees, 16 degrees, and 22 degrees) were digitally created from a maxillary left central incisor and printed in acrylic resin. Next, casts were scanned with a TRIOS intraoral scanner, and crowns were designed with KaVo multiCAD software using default parameters (50 mu m cement space) on abutments with positive TOC angles, whereas reverse-tapered abutments (negative TOC angles) were digitally blocked out at 0 degrees and had an extra mesiodistal gap set to 50 mu m. Then, zirconia crowns were fabricated, and their marginal and internal discrepancies were recorded with the silicone replica technique. All replicas were examined under a stereomicroscope at 50x magnification. Collected data were analyzed with one-way ANOVA and post hoc Tukey test for marginal fit. For the axial and incisal fit, measured values did not follow a normal distribution; therefore, the Kruskal-Wallis and the Dunn/Bonferroni multiple comparison tests were applied (p = 0.05). Results The mean marginal fit of -8 degrees crowns (58.2 +/- 6.0 mu m) was statistically different (p < 0.0001) from all the remaining crowns (range 42.1-47.3 mu m). Also, the internal fit was statistically significant when comparing crowns fabricated over abutments with positive and negative TOC angles (p < 0.0001). The largest median axial discrepancies were found in the -8 degrees (165.5 mu m) and -4 degrees (130.8 mu m) groups; however, when evaluating the incisal fit, they showed the smallest discrepancies (67.3 and 81.8 mu m, respectively). Conclusions Under the conditions of this study, the marginal and internal fit of zirconia crowns fabricated over inverse-tapered preparations is within clinically accepted values.