Fracture resistance and color stability of monolithic CAD/CAM crowns: effect of occlusal thickness and thermocycling

PURPOSE: This in vitro study evaluated the effects of occlusal thickness and thermocycling on the fracture resistance and color stability of monolithic CAD/CAM crown materials. MATERIALS AND METHODS: Standardized full-coverage crowns were fabricated from four CAD/CAM materials: resin nano-ceramic (CM), extra-translucent zirconia (VXT), translucent zirconia (VT), and zirconia-reinforced lithium silicate (VS). Two occlusal thicknesses (1.0 mm and 1.5 mm) were designed. Specimens from each material-thickness combination were divided into non-aged and thermocycled subgroups (n = 10 each). The thermocycled subgroups were subjected to 10,000 cycles between 5 °C and 55 °C. Fracture resistance was measured using a universal testing machine. Color measurements (L*, a*, b*) were obtained using a spectrophotometer before and after thermocycling, and color differences (ΔE₀₀) were calculated. Data were analyzed using two-way and three-way ANOVA with post hoc Tukey tests (α = 0.05). RESULTS: Material type (P < .001; ηp² = 0.52) and occlusal thickness (P = .003; ηp² = 0.11) had significant effects on fracture resistance. Increasing occlusal thickness resulted in higher fracture resistance values across all tested materials. VXT demonstrated the highest fracture resistance, whereas VS exhibited the lowest values. Thermocycling did not have a significant main effect on fracture resistance (P = .178; ηp² = 0.02). Color differences (ΔE₀₀) varied significantly among materials (P < .05); however, all values remained below the clinically acceptable threshold. CONCLUSIONS: Occlusal thickness and material type significantly influence the mechanical performance of monolithic CAD/CAM crowns. Although thermocycling had a limited effect on fracture resistance, all materials maintained clinically acceptable color stability. These findings support the use of reduced-thickness restorations with appropriate material selection.