Effects of in vitro shocks and hydrothermal degradation on wear of ceramic hip joints: Towards better experimental simulation of in vivo ageing

Abstract

In an ageing world population, increasing the lifetime of orthopedic devices is of vital importance for reducing surgical risks for patients and reducing medical costs. Ceramic materials are gaining interest because of their hardness and chemical inertia with respect to metal prosthesis. Wear of the prosthetic couple (femoral head and cup) is one of the main rationales for using ceramic hip implants, because they induce much lower wear debris. The present study investigates the combined effects of shocks due to microseparation and hydrothermal ageing on the wear of Zirconia Toughened Alumina (ZTA) bearings, which have been under strong development these past ten years. Wear stripe patterns are formed on femoral heads tested on a shock device. 3D profilometry is used to evaluate the volume of worn material released from the head that averages at 0.46mm3/year during simulation mimicking about 15 years of ageing. This volume is larger than formerly estimated with hip walking simulators with microseparation, an effect that we tentatively attribute to a larger force (9000N) applied here than on hip simulators where the force was not specified. The present experiment should allow refining the effect of force on the wear of ceramic femoral head, hence to simulate different patient conditions (weight, activity, accidents,…). Micro Raman spectroscopy reveals significant zirconia phase transformation within wear stripes. ZTA components show minor phase transformation during hydrothermal degradation in an autoclave when compared to changes induced by shocks. Finally, five clinically retrieved bearings are characterized for in vivo comparison, and also reveal phase transformation within wear track areas. The association of phase transformation enhancement with wear areas on in vivo samples indicates that wear is induced by shocks, rather than by hydrothermal ageing. Shock simulations are therefore essential for in vitro experimentation aiming at reproducing wear of ceramic implants and in vivo environment. On a material׳s point of view, it is shown that the ZTA ceramics can exhibit a beneficial phase transformation under stress, which may limit wear stripes, without suffering from significant ageing.