With its high wear and corrosion resistance, CoCrMo alloy has been widely used for metal-on-metal total hip replacements (THRs). However, the use of the metal-on-metal implants has dropped substantially as a result of several alerts issued by the Medicines and Healthcare products Regulatory Agency (MHRA) due to concern on metal ion release [1]. However, some of the first generation of metal-on-metal THRs have lasted for more than 20 years [2]. It is far from clear why some MoM joints have survived, while other failed. It is known that dynamic changes occur at the metal surface during articulation. For example, a nanocrystalline layer has been reported on the topmost surface of both in vivo and in vitro CoCrMo THRs [3, 4] but it is not known whether this layer is beneficial or detrimental.

The current work focuses on the sub-surface damage evolution of explanted MoM hips, which is compared to in vitro tested CoCrMo hip prostheses. Site-specific TEM cross-section of both in vivo and in vitro CoCrMo samples were prepared by focused ion beam (FIB) in situ lift-out method (Quanta 200 3D with Omniprobe, FEI, the Netherlands). TEM of the FIB specimens was performed on various microscopes. Routine bright field imaging was performed on a Tecnai 20 (FEI, the Netherland) operating at 200 kV, while high resolution transmission electron microscopy (HRTEM) of the nanocrystalline layer and other surface species was undertaken on a Jeol 2010F (Jeol, Japan) operating at 200 kV.

A nanocrystalline layer (which was not present on the starting surfaces) was observed on both explanted in vivo and in vitro tested materials. For the explanted joints, the nanocrystalline layer was thin (a few 100 nm) and the extent did not appear to correlate with the local wear rate. For in vitro samples, the nanocrystalline layer was thicker (up to micron). HRTEM from this layer are shown in Fig. 1 and Fig. 2. The nanocrystallite size was ∼5 nm and appeared to be a mixture of face centred cubic and hexagonal close packed phases. The formation of the nanocrystalline layer and its correlation with wear behaviour are discussed.

Retrieved alumina-on-alumina hip joints frequently exhibit a localised region of high wear, commonly called ‘stripe wear’. This ‘stripe wear’ can be replicated in vitro by the introduction of micro-separation, where the joint contact shifts laterally reproducing edge loading during the simulated walking cycle. While the origin of stripe wear is clearly associated with the micro-scale impact resulting from micro-separation, the wear processes leading to its formation and the wear mechanisms elsewhere on the joint are not so well understood. The purpose of this study was to compare the surface microstructure of in vivo and in vitro alumina hip prostheses, and investigate the origins of the damage accumulation mechanisms that lead to prosthetic failure.

The in vivo alumina hip prosthesis was Biolox (Ceram-Tec, AG, Plochingen, Gemany) implanted for 11 years [1]. The in vitro alumina hip prosthesis was Biolox-forte (CeramTec, AG, Plochingen, Gemany), which had been tested in a hip joint simulator under micro-separation at Leeds University using the procedures given in [2]. The worn surfaces of the alumina hip prostheses were investigated using a Scanning Electron Microscopy (SEM). Similar worn surfaces were seen for both in vivo and in vitro samples. Focused ion beam (FIB) microscopy was used to determine the sub-surface damage across the stripe wear. Samples were subsequently removed for Transmission Electron Microscopy (TEM). Sub-surface damage was found to be limited to a few μm beneath the surface; ~ 2μm for in vivo samples and ~1μm for in vitro samples. The transition from mild wear to more severe (stripe) wear was entirely triggered by intergranular fracture. The first stages of fracture lead to the liberation of surface grains which act as 3rd body abrasives. The TEM showed that abrasive grooves are associated with extensive surface dislocation activity, which leads to further grain boundary fracture.

This allows the cycle to be repeated and accelerated, thus yielding the stripe wear region.

The conclusions are: 1. In vitro hip simulation with micro-separation can produce similar microstructure to in vivo alumina hip prostheses; 2. To extend the life of the joint through the avoidance of severe wear, material and design solutions can be investigated using ceramic materials that have an increased surface inter-granular fracture toughness and component designs with reduced contact stress under edge loading.