Influence of CAD/CAM Milling Protocol and Mechanical Fatigue on Load-to-Failure and Failure Modes of Ultra-Translucent Monolithic Zirconia versus Lithium Disilicate Laminate Veneers An In Vitro Study

Objective: To systematically evaluate how different CAD/CAM milling protocols influence the post-fatigue mechanical performance and failure modes of ultra-translucent monolithic zirconia compared to lithium disilicate laminate veneers.Methods: This in vitro comparative experimental study employed a randomized design with four groups (n = 11 per group): ultra-translucent zirconia with standard milling (ZR-ST), ultra-translucent zirconia with fast milling (ZR-F), lithium disilicate with standard milling (LD-ST), and lithium disilicate with fast milling (LD-F). Specimens underwent mechanical fatigue testing (step-stress protocol starting at 20 N with 10 N increments every 1,000 cycles at 1.6 Hz) followed by failure mode analysis.Results: Fast milling protocol significantly improved fatigue resistance compared to standard milling (50.31 ± 9.3 N vs. 42.63 ± 10.45 N, p = 0.012). In the fast milling subgroup, lithium disilicate demonstrated significantly higher fatigue resistance than zirconia (54.28 ± 11.16 N vs. 46.33 ± 4.71 N, p = 0.048). Failure modes differed significantly between materials and protocols, with zirconia showing more debonding failures and lithium disilicate exhibiting more cohesive fractures.Conclusions: Fast milling protocols improve fatigue resistance for both materials, with lithium disilicate demonstrating superior performance under fast milling conditions. These findings provide evidence-based recommendations for CAD/CAM protocol optimization in ultra-thin veneer fabrication.