CoCrMo Metal on Metal (MoM) hip implants were designed to be durable, targeting a better quality of life for young, active patients. However, evidence suggests that such implants can release wear particles and metal ions due to a bio-tribocorrosion process. Metallic particles, produced by wear, at articulating interfaces of the joint surface have been linked with the formation of pseudotumors and lymphatic circulation. Whilst the ion release mechanism from the bulk alloy is the focus of many studies, it is not unreasonable to expect further corrosion of particles once they become detached from the surface of the implant. Wear debris is inherent to the process because it is an inevitable consequence of the relative motion between two opposite surfaces in contact. By this means, the removal of material due to mechanical action can generate millions of nanoparticles for each cycle of movement, which can then migrate into the tissues surrounding the implant. Several studies have indicated that metal ion levels rise after surgery and persist. Specifically metal particles spread by lymphatic circulation can continue to release ions even after removal of the source of wear. The mechanisms of ion release at this scale are still unclear along with the subsequent interaction between metal-ions and biological media (e.g. bovine serum albumin (BSA) proteins).
Statistically significant toxicological studies require relatively large amounts of wear debris because most studies are based on the dose- response of triplicate assays, which could require more than a hundred milligrams of nanoparticles. However, producing large volumes of wear debris particles which match the composition, size and morphology of those generated in actual hip replacements remains extremely difficult. This study focuses on the synthesis and characterization of CoCrMo nanoparticles, which mimic metal-on-metal (MoM) wear debris from hip implants. For the synthesis process, we have used a hitherto unexplored approach employing mechanochemical milling to produce a large amount of CoCrMo mimetic wear debris over short time scales. The nanoparticles produced were found to be similar in size, shape and composition to real implant wear debris from CoCrMo hip implants. In addition, pure metals that compose the alloy were compared in terms of their electrochemical corrosion, static corrosion and protein binding.
Dissolution studies indicated a much lower dissolution of cobalt than previously reported. This was attributed to the efficient separation of nanoparticles prior to solute analysis through the use of centrifugation combined with ultrafiltration. The data suggest that the previously accepted route for the release of cobalt ions may need revision. The study showed that after tribocorrosion of the CoCrMo alloy in BSA, the level of Co and Mo ions in solution increased dramatically. The increase in Mo ions could be linked with the preferential binding of Mo to the BSA proteins which results in the formation of a hard protein corona on CoCrMo and Mo particles and does not readily desorb even after washing. These findings are important as it highlighted the interaction of Mo-rich surfaces with amide groups in serum proteins and the possible formation of metal carbonyl complexes both of which can modify biological molecules, altering their ability to function properly.
Electrochemical corrosion in the presence of a realistic concentration of BSA suggested that proteins play an important role in Mo dissolution from CoCrMo. Mo presumably reacts with amino acid residues present in protein molecules, initiating preferential dissolution even at low potentials, where there is no disruption of the Cr passivation layer on the CoCrMo alloy. Co and Mo samples showed active dissolution in all electrolyte solutions, whereas degradation of CoCrMo and Cr samples were controlled by a passivating oxide layer. The use of PBS accelerated the corrosion for all samples, increasing the metal ion concentration in the electrolyte and there was a high incidence and growth of pitting corrosion on Co samples.
This work suggests that the role of Mo as well as Co ions should be accounted for in the tribocorrosion of CoCrMo implant alloys, particularly in terms of inflammatory and toxilogical responses.
