The influence of low-temperature degradation and cyclic loading on the fracture resistance of monolithic zirconia molar crowns.

The present study analyzed the kinetics of low-temperature degradation (LTD) in zirconia, and evaluated the influence of LTD and cyclic loading on the fracture resistance of monolithic zirconia molar crowns. Bar-shaped zirconia specimens were divided into nine groups and autoclaved at 134°C for 0-200h to induce LTD. The surface fraction and penetration depth of the monoclinic phase were examined using X-ray diffraction and scanning electron microscopy. Monolithic zirconia molar crowns were prepared for crown fracture testing. The crowns were autoclaved for 0-100h (n=6) and cemented to dies. Six crown-die samples that were not autoclaved and six samples that were autoclaved for 100h were subjected to cyclic loading with a load of 300N for 240,000 cycles. All samples were tested in a load-to-failure test. The monoclinic fraction on the surface increased with autoclaving time and reached a plateau after 50h. The depth of the monoclinic phase increased without reaching a plateau. The fracture load of the crowns significantly decreased from 5683N (SD: 342) to 3975N (SD: 194) after 100h of autoclaving. Cyclic loading did not significantly affect the fracture resistance of the crowns in all cases. Kinetic analysis showed no linear correlation between the surface fraction and depth of the monoclinic phase after 50h of autoclaving. Even though LTD increased the monoclinic phase, resulting in lower strength, the fracture resistance of the monolithic zirconia crowns was still sufficient to withstand the loading conditions in the molar regions.