Off-axis sliding contact fatigue reliability of veneered alumina and zirconia

Objective: To compare the reliability and failure modes of veneered alumina and zirconia (four-layer structures) under 30^o off-axis sliding contact cyclic loading.Methods:  Seventeen zirconia (Y-TZP) and twenty alumina plates (10x10 mm) polished to 0.5 mm were porcelain veneered and polished to a total thickness of 1.2mm. All veneered plates were cemented (Rely X ABC Luting Cement) to aged (30 d in water) 10x10x4 mm composite blocks (Z-100) then samples were incubated 7 days in water prior to testing. Uniaxial single load to failure was performed (n=3 for each group) to determine 3 step-stress profiles (ratio 3:2:1). Remaining specimens were fatigue at 30^o off-axis with a sliding contact (translation of ~ 0.7-1.0 mm depending upon load). All specimens were inspected with specular refection polarized light microscope at regular intervals during the fatigue to visualize failure modes. Step-stress results were analyzed software (Alta Pro 7, Reliasoft Tucson, Arizona) a probability Weibull curve established and reliability for a mission of 50,000 cycles at 150 N (two-sided 90.0% confidence bounds) calculated.

Results: Table shows reliability for a mission of 50k cycles @ 150N (two-sided 90% confidence bounds).

Groups	Alumina	YTZ-P
Upper Limit	0.84	0.98
Reliability	0.74	0.91
Lower Limit	0.45	0.67

Failure modes for alumina specimens were deep penetrating partial cone cracks and cementation internal surface radial cracks. Y-TZP specimens showed significant surface damage with inner and deeper penetrating partial cone cracks, while no cementation surface radial cracking was observed. Conclusion: Although no statistical difference between groups was found, a tendency to higher reliability was observed for Y-TZP compared to alumina. Failure modes differed between groups, being mainly inner and deep partial cone cracks for Y-TZP, and deep partial cone and radial cracks for alumina which agrees with clinical finding. Angled sliding contact appears to be a good simulation of oral function.   Supported by NIDCR DE10976.