Effects of Yttria Content and Margin Design on the Fracture Resistance of Monolithic Zirconia Crowns

Background: Zirconia ceramics are generally used in monolithic restorations, and their microstructural, mechanical, and optical properties continue to improve. Several factors affect the mechanical properties of these restorations; however, the combined effects of yttria content and margin design on the fracture resistance remain unclear. Methods: Sixty monolithic zirconia crowns were fabricated and assigned to six groups (n = 10) based on three different yttria contents (strength-gradient multilayer zirconia containing 3 mol% yttria tetragonal zirconia polycrystals in the dentin region and 5 mol% yttria-partially stabilized zirconia in the occlusal region: 3Y-TZP/5Y-PSZ [ZP], 3 mol% yttria tetragonal zirconia polycrystals: 3Y-TZP [HTML], and 4 mol% yttria-partially stabilized zirconia: 4Y-PSZ [STML]), and two different margin designs (chamfer and rounded shoulder). Crowns were adhesively bonded to standardized 3-dimensional-printed resin dies and subjected to thermal and mechanical aging (10,000 thermocycles at 5–55 °C, and 1.2 million mechanical cycles at 50 N, 1.6 Hz). Fracture resistance values were recorded in Newtons, and fracture types were evaluated. Data were analyzed using a two-way analysis of variance (ANOVA), and Bonferroni adjustment was used for multiple comparisons (α = 0.05). Results: A significant interaction between yttria content and margin design was found (p = 0.005). In the chamfer margin design groups, ZP (2208.5 ± 501.9 N) and HTML (2069.6 ± 463.3 N) showed significantly higher fracture resistance than STML (1444 ± 303.2 N) (p < 0.05). In the rounded shoulder margin design groups, no significant differences were observed among ZP (1662.8 ± 293.8 N), HTML (1940.9 ± 341.6 N), and STML (1795.6 ± 529.6 N) (p > 0.05). ZP and HTML showed higher fracture resistance values with the chamfer margin design, while STML showed higher fracture resistance with the rounded shoulder margin design. Conclusions: The fracture resistance of zirconia restorations is influenced by both the margin design and the yttria content. Designing the margin geometry based on the type of zirconia to be used can enhance the mechanical properties of the restorations and support clinical decision-making.