Effect of Total Occlusal Convergence Angle and Rounded Preparations on the Marginal and Internal Fit of Incisor...

To evaluate the marginal and internal fit of anterior monolithic zirconia crowns fabricated over preparations with different TOC angles with and without a rounded incisal design.

Crown preparations of fourteen maxillary central incisors were digitally designed and fabricated using photosensitive resin. The axial wall inclinations were -8°, -4°, 0°, 8°, 12°, 16°, 22° 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's test, Tukey HSD, and Independent Samples T-tests (p<0.05).

TOC angle groups showed statistically significantly differences (p<0.001). The 22° preparation had the lowest marginal discrepancies (36±5 μm rounded, 61±5 μm non-rounded), while -8° had the highest (117±9 μm rounded, 147±30 μm non-rounded). Axial discrepancies were lowest at 22° (77±14 μm) and highest at -8° (208±52 μm) in non-rounded samples (p<0.001). The smallest incisal discrepancy was found in 22° (95 ± 13 μm); while the largest (163 ± 31μm) was at -4° in the non-rounded groups (p<0.001).

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.