Ageing, Shocks and Wear Mechanisms in ZTA and the Long-Term Performance of Hip Joint Materials.

The surface morphologies and microstructures of Zirconia Toughened Alumina (ZTA) femoral heads were analyzed following in vitro tests aiming to simulate in vivo degradation. Three phenomena potentially leading to degradation were investigated: shocks, friction and hydrothermal ageing. Shocks due to micro-separation created the main damage with the formation of wear stripes on the femoral head surfaces. Atomic Force Microscopy (AFM) images suggested the release of wear debris of various shapes and sizes through inter- and intra-granular cracks; some debris may have a size lower than 100 nm. A decrease in hardness and Young's modulus was measured within the wear stripes by nanoindentation technique and was attributed to the presence of surface and sub-surface micro-cracks. Such micro-cracks mechanically triggered the zirconia phase transformation in those worn areas, which in return presumably reduced further crack propagation. In comparison with shocks, friction caused little wear degradation as observed from AFM images by scarce pullout of grains. The long-term resistance of the ZTA composite material against hydrothermal ageing is confirmed by the present observations.

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.