Mechanical Behavior of Multi-Layered Translucent Zirconia and Lithium Disilicate Crowns Following In Vitro Aging

Objective: The mechanical behavior of multilayered translucent zirconia (MLTZ) at varying occlusal thickness has remained questionable. This study assessed the fatigue and fracture resistance of two MLTZ crown materials at different occlusal thicknesses and compared their performance with lithium disilicate. Methods and Materials: Two typodont teeth were prepared with either 1 mm or 2 mm occlusal reduction and duplicated to produce 48 resin replicas. Standardized crowns were milled from three materials (n = 8): two MLTZ materials, 5 mol% and 6 mol% yttria-partially stabilized zirconia (StarCeram Z-Smile and Katana UTML, respectively), and lithium disilicate (IPS e.max CAD). Specimens underwent thermomechanical aging (500,000 cycles under a 50-Newton [N] load and 5,000 thermal cycles between 5°C and 55°C). The surviving specimens were loaded to failure to measure fracture resistance (N). Failure modes were examined using a stereomicroscope. Fracture strength data were analyzed using two-way ANOVA and Bonferroni post hoc tests, while failure mode data were analyzed using the chi-square test followed by Fisher's exact test. Results: Fatigue testing showed no statistically significant differences among groups ( p = 0.112), with 97.9% (47/48) of specimens surviving fatigue testing (one 1-mm lithium disilicate specimen failed). Both material type and thickness significantly influenced fracture resistance and failure mode ( p < 0.001). StarCeram Z-Smile demonstrated the highest fracture resistance at both 1-mm (1549.1 N) and 2-mm (3273.2 N) thicknesses, significantly exceeding Katana UTML (1183.8 and 1622.0 N, respectively; p < 0.05) and 1-mm lithium disilicate (1549.1 N; p < 0.001); 2-mm lithium disilicate (1459.5 N) was comparable to 2-mm Katana UTML and 1-mm StarCeram Z-Smile ( p = 1.0). Conclusion: Following in vitro thermomechanical fatigue, fracture resistance was material- and thickness-dependent. Both tested MLTZ exceeded the reported maximum clinical biting forces at 1.0 mm, although the material with higher yttrium content showed significantly lower strength, while lithium disilicate required a minimum thickness of 2.0 mm to ensure adequate mechanical performance.