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.
Highlights
CoCrMo alloys have been used as a biomaterial for metal-on-metal (MOM) hip implants because of their high corrosion resistance and significantly lower volumetric wear compared to metal-onpolyethylene (MOP)implants.[1]
A complementary spectroscopy and microscopy approach was used to develop an understanding of the dissolution of the particles based on chemical and morphological changes
To understand how Co:Cr ratio varied as a function of simulated environment, CoCrMo particles were analysed by ex situ spectroscopy using TEM-EDX
Summary
CoCrMo alloys have been used as a biomaterial for metal-on-metal (MOM) hip implants because of their high corrosion resistance and significantly lower volumetric wear compared to metal-onpolyethylene (MOP)implants.[1] This made them less likely to fail due to osteolysis (bone resorption due to an inflammatory response[2,3] induced mainly by polyethylene and Ti particles and leading to the aseptic loosening of the implant), the most common hip implant failure mechanism.[4] CoCrMobased hip implants suffered an unexpectedly high failure rate, with many patients experiencing unexplained pain, leading to revision surgery. Simulation studies have since revealed that, despite the lower 'volumetric' wear, CoCrMo alloys produce a much higher number of smaller wear particles: up to 'one trillion' nanoscale particles per patient annually.[5] These wear particles trigger an inflammatory immune response thought to be linked to implant failure,[6] the mechanistic details remain poorly understood. Using scanning electron microscopy–energy dispersive X-ray (SEM-EDX) and Fourier transform infrared (FTIR)
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