Evaluation of the Fracture Load of Multilayer Zirconia Onlay Restorations with Different Yttria Contents After Thermomechanical Aging: An In Vitro Study

This in vitro study compared the fracture load of multilayer zirconia onlay restorations with different yttria contents after thermomechanical aging. Standardized CAD/CAM-fabricated onlay restorations were produced from three multilayer zirconia materials: one color-gradient material (KATANA Zirconia UTML; 5Y-TZP) and two strength-gradient materials (KATANA Zirconia YML, 3–5Y-TZP; IPS e.max ZirCAD MT Multi, 4–5Y-TZP) (n = 10/group). The restorations were cemented onto three-dimensionally printed resin dies and subjected to 240,000 loading cycles and 5000 thermal cycles. Fracture load was measured using a universal testing machine, and the data were analyzed by one-way analysis of variance, Tukey’s post hoc test, and Weibull analysis (α = 0.05). Fracture load differed significantly among the materials (p = 0.017). The UTML group (395.73 N) exhibited a significantly lower fracture load than the YML group (618.19 N), whereas the ZirCAD MT Multi group (565.24 N) did not differ significantly from either of the other two groups. Weibull analysis revealed no significant differences in modulus among the groups, whereas the characteristic load of the UTML group was significantly lower than that of the other two materials (p = 0.002). Within the conditions of this in vitro study, the strength-gradient materials, which contain lower yttria concentrations in their underlying layers, showed higher fracture loads after thermomechanical aging than the color-gradient material. These findings are limited to the tested laboratory conditions and require clinical confirmation.