Characterisation of the wear mechanisms in retrieved alumina-on-alumina total hip replacements

Due to their superior wear performance and biocompatibility compared to alternative polymer/metal prostheses, alumina-on-alumina total hip replacements (THRs) are extensively used for young and more active patients. However, the understanding of the wear mechanisms of alumina in vivo remains relatively poor, and there remains little quantitative understanding of the structural and chemical changes at the articulating surface. In the current study, the surface and sub-surface microstructures of retrieved in vivo alumina THRs are presented. Severe wear, also called stripe wear, was observed in all cases. The transition between the stripe wear and the mild wear was very sharp. Site-specific cross-section TEM specimens were prepared by Focused Ion Beam (FIB) at the stripe boundary region. The results suggest predominantly intergranular fracture occurred that was restricted to the outer layer of grains below the surface, with transgranular fracture also occurring in the stripe wear region. Cracking was believed to be initiated by extensive dislocation slip. A thin layer of hydroxide was also observed at the extreme surface of the mild wear region by aberration-corrected high resolution transmission electron microscopy (HRTEM). The wear mechanisms are discussed.