Fatigue performance and failure load of minimally-invasive occlusal veneers made of lithium disilicate and composition-gradient multilayered zirconia: An in vitro study

Purpose: To investigate the effect of ceramic material (lithium disilicate, LDS vs. composition-gradient multilayered zirco-nia [4Y-PSZ and 5-PSZ], Z) and ceramic layer thickness (0.5 mm, 1.0 mm, and 1.5 mm) on fatigue performance and failureload of occlusal veneers on molars.Methods: Seventy-two CAD-CAM-fabricated occlusal veneer restorations (IPS e.max CAD; IPS e.max ZirCAD Prime Es-thetic, Ivoclar Vivadent) were divided into six groups (n=12, LDS-1.5, LDS-1.0, LDS-0.5; Z-1.5, Z-1.0, Z-0.5). Restorations wereadhesively cemented (Variolink Esthetic DC, Ivoclar Vivadent) to dentin-analogue composite dies (Z100, 3M ESPE) andexposed to thermomechanical fatigue (1.2 million cycles, 49 N, 1.6 Hz, 5-55° C). Single-load-to-failure was tested with auniversal testing machine. Data were analyzed using ANOVA with Tukey post-hoc tests and t-tests (P < 0.05).Results: The overall success rate across all materials and layer thicknesses was 91.7%. Half of the specimens of group Z-0.5revealed cracks after chewing simulation. Occlusal veneers fabricated from LDS withstood significantly higher failure loadsthan gradient multilayered zirconia veneers in all tested thicknesses. The mean failure load values led to the followingranking: 3194 N (LDS-0.5)>2683 N (LDS-1.0)>2338 N (LDS-1.5)>1744 N (Z-1.5)>1310 N (Z-0.5)>1198 N (Z-1.0).Conclusions: Ultrathin LDS occlusal veneers outperformed thin and standard thick counterparts, as well as gradientmultilayered zirconia veneers at all thickness levels. Ultrathin gradient multilayered zirconia occlusal veneers were proneto cracks during thermomechanical fatigue. Individual mechanical properties need to be considered when aligning therestoration within the multilayered zirconia blank.