A testing protocol combining shocks, hydrothermal ageing and friction, applied to Zirconia Toughened Alumina (ZTA) hip implants

Ceramics are materials of choice for hip joint implants because of their excellent biocompatibility and mechanical properties. Wear of the bearing couple (femoral head and cup) remains one of the main concerns of hip implants. Although ceramics are known for their good tribological properties, shocks due to micro separation, friction and hydrothermal ageing in physiological environment remain the three main sources of wear. It has been recently suggested that shock effects dominate but the three degradation mechanisms were so far simulated separately. We developed a procedure that combines sequences of-shocks, hydrothermal ageing in an autoclave and friction on hip-walking simulator to investigate their combined effects on Zirconia Toughened Alumina (ZTA) implants. Our results confirm that shocks can be considered as the key phenomenon causing wear, and that their effect is independent of friction and hydrothermal degradation. The analysis of retrieved femoral heads reveals wear features comparable to the ones created experimentally by shocks. Standards (ASTM or ISO) could be improved by including shock tests, which are more relevant than wear tests currently performed on hip simulators at least for Ceramic-on-Ceramic couplings.