Effect of water storage time and composite cement thickness on fatigue of a glass-ceramic trilayer system

Aim. Static Hertzian contact tests of monolayer glass-ceramics in trilayer configurations (glass-ceramic/cement/composite) have shown that thick cement layers lower strength. This study sought to test the hypothesis that thick resin cement layers lower mouth motion fatigue reliability for flat glass-ceramic/cement/composite trilayer systems and that aging in water reduces reliability. Methods. Dicor plates (n ≥ 12 per group) (10 x 10 x 0.8 mm ³) were aluminum-oxide abraded (50 μm), etched (60 s), silanized, and bonded (Rely X ARC) to water aged (30 days) Z100 resin blocks (10 x 10 x 4 mm³). Four groups were prepared: (1) thick cement layer (>100 μm) stored in water for 24-48 h, (2) thick cement layer stored for 60 days, (3) thin cement layer (≤100 μm) stored for 24-48 h, and (4) thin cement layer stored for 60 days. The layered structures were fatigued (2 Hz) utilizing mouth motion loading with a step-stress acceleration method. A master Weibull distribution was calculated and reliability determined (with 90% confidence intervals) at a given number of cycles and load. Results. The aged group (60 d) with thick cement layer had statistically lower reliability for 20,000 cycles at 150 N peak load (0.11) compared with both nonaged groups (24-48 h) (thin layer = 0.90 and thick layer = 0.82) and aged group with thin cement layer (0.89). Conclusion. Trilayer specimens with thick cement layers exhibited significantly lower reliability under fatigue testing only when stored for 60 days in water. The hypothesis was accepted. These results suggest that diffusion of water into the resin cement and also to the glass-ceramic interface is delayed in the thick cement specimens at 24-48 h.