Understanding the reactivity of CoCrMo-implant wear particles

CoCrMo-based metal-on-metal hip implants experienced unexpectedly 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. 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. Here a correlative spectroscopy and microscopy approach has shown that these particles dissolve via a non-stoichiometric, and geometrically inhomogeneous, mechanism similar to de-alloying. This mechanism is previously unreported for this material and was not apparent in any of the regulatory required tests, suggesting that such tests are insufficiently discriminating. Biomaterials: understanding implant wear A study of CoCrMo particles, like those produced during the lifetime of hip implants, reveals insights into their dissolution behaviour. Although bulk CoCrMo is nominally stable, its use in hip implants has resulted in high failure rates that are understood to be caused by the generation of many small ‘wear’ particles of CoCrMo that can trigger an inflammatory immune response. Now, a team led by Mary Ryan at Imperial College London, has shown, that CoCrMo particles dissolve in oxidising conditions that mimic inflammatory response through a process akin to de-alloying. Their multifaceted approach uses both spectroscopy and microscopy, and combines in situ and ex situ studies. Bulk CoCrMo does not undergo such de-alloying and it was thus not observed during bulk studies required for clinical approval—suggesting that current methods for material assessment are insufficient.