CoCrMo-based hip implants experienced high failure rates despite the high wear and corrosion resistance of the bulk material. Although they exhibit a lower volumetric wear compared to other implant materials, CoCrMo-based implants produced a significantly larger number of smaller wear particles. Simulation studies show that up to one trillion wear particles could be produced in one patient annually. Implant wear particles can trigger an immune system inflammatory response which is associated with eventual failure of the implants. In light of these failures, many devices have been retracted from the market, and class-action lawsuits were filed in the US. Additionally, one of the largest product liability hearings is currently ongoing in the UK as patients sue implant manufacturers.
CoCrMo is nominally an extremely stable material with high Cr content providing passivity. However, despite the Co:Cr ratio in the original alloy being 2:1, chemical analyses of wear particles from periprosthetic tissue have found the particles to be composed predominately of Cr species, with only trace amounts of Co remaining. The immune system’s inflammatory response leads to a rise in local electrochemical potential via the production of reactive oxygen/nitrogen species by activated macrophages as well as local acidification. However, the mechanism eliciting the inflammatory response is uncertain.
In this work, the dissolution of CoCrMo in bulk, thin film and nanoparticle form was studied in a simulated biological environment. Electrochemical potentials were applied to simulate the increasingly oxidising environment found within the macrophage cells. A multimodal in situ spectroscopy and microscopy approach was used, at high chemical and spatial resolution, to elucidate the dissolution mechanism. The results reveal that CoCrMo dissolution occurs via a non-stoichiometric, geometrically inhomogeneous mechanism similar to de-alloying, and previously unreported for this material. Taken together, the results suggest that the in vivo dissolution of CoCrMo wear debris can be understood in terms of the local environment and specific corrosion mechanisms that come into play under these conditions: not observed in conventional testing for implant safety.