Effect of Total Occlusal Convergence Angle and Rounded Preparations on the Marginal and Internal Fit of Incisor Monolithic Zirconia Crowns

Purpose: To evaluate the marginal and internal fit of anterior monolithic zirconia crowns fabricated over preparations with different total occlusal convergence (TOC) angles with and without a rounded incisal design. Materials and Methods: Crown preparations of 14 maxillary central incisors were digitally designed and fabricated using photosensitive resin. The crowns were designed with axial wall inclinations of-8,-4, 0, 8, 12, 16, and 22 degrees with and without rounded incisal corners and a 1-mm-wide chamfer-type finish line. Monolithic zirconia crowns (n = 10) were milled and assessed for fit using the dual-scan technique using an intraoral scanner. The STL files were analyzed in Geomagic Control X, measuring fit at 100, 150, and 175 points on the incisal, axial, and marginal areas, respectively. Data normality was tested (Shapiro-Wilk), and TOC angle groups were compared using one-way ANOVA, Levene test, Tukey HSD, and independent samples t tests (P < .05). Results: TOC angle groups showed statistically significantly differences (P < .001). The 22-degree preparation had the lowest marginal discrepancies (36 +/- 5 pm rounded, 61 +/- 5 pm nonrounded), while -8 degrees had the highest (117 +/- 9 pm rounded, 147 +/- 30 pm nonrounded). Axial discrepancies were lowest at 22 degrees (77 +/- 14 pm) and highest at -8 degrees (208 +/- 52 pm) in nonrounded samples (P< .001). The smallest incisal discrepancy was found in 22 degrees (95 +/- 13 pm), while the largest (163 +/- 31 pm) was at -4 degrees in the nonrounded groups (P < .001). Conclusions: TOC angle and rounded preparations have a significant impact on the fit of CAD/CAM-milled monolithic zirconia crowns. Rounded incisal corners and preparations with higher TOC angles increased marginal and internal adaptation of the crowns.