Abstract
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.
