Understanding the reactivity of CoCrMo-implant wear particles

Mohamed A. Koronfel, Angela E. Goode, Johanna Nelson Weker, Stephen E. R. Tay, Camilla A. Stitt, Thiago A. Simoes, J. Frederick. W. Mosselmans, Paul Quinn, Rik Brydson, Alister Hart, Michael F. Toney, Alexandra E. Porter, Mary P. Ryan

Index: 10.1038/s41529-018-0029-2

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Abstract

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.