Impact of convergence angle of tooth preparation on the fracture resistance of incisor monolithic zirconia crowns.

Purpose: To evaluate the effect of convergence angles on the fracture resistance of monolithic zirconia crowns in maxillary central incisors.

Materials and methods: A maxillary central incisor preparation was digitally designed with four different angles (0°, 8°, 16°, and 22°). For each group, 10 abutment teeth were manufactured using a 3D printer with photosensitive resin and positioned in a maxillary typodont model for scanning procedures. Anatomically contoured monolithic zirconia crowns were fabricated for each group (n = 10), 40 crowns in total, using the subtractive manufacturing technique. The crowns were cemented onto their respective abutments using a self-adhesive resin cement. Fracture resistance testing was conducted using a universal testing machine and the fracture load was recorded in Newtons (N). The obtained data were statistically analyzed using one-way ANOVA for intergroup comparisons, and Tukey HSD post-hoc test was applied to identify significant differences among groups (p < 0.05).

Results: A statistically significant difference was found among the groups, with fracture resistance varying according to the convergence angle (p < 0.05). Crowns with 16° angle exhibited the highest values (2897.57 N), followed by 22° (2425.58 N) and 8° (2095.76 N) groups. The lowest value was found at 0° crowns (1830.3 N).

Conclusions: The results indicated that the convergence angle had a significant effect on the fracture resistance of monolithic zirconia crowns. Instead of preparing parallel axial walls (0°) in central incisors, creating a tapered preparation design with different levels (8°, 16°, or 22°) increased the fracture resistance. Specifically, 16° was related to the greatest fracture resistance. However, the absence of thermocycling and fatigue testing limits the simulation of long-term clinical performance.