Research has recently been focused on the development of hip replacements with alternative bearing surfaces with cobalt chrome alloy, to avoid the production of polyethylene wear particles in hip replacements and polyethylene wear debris mediated bone lysis. Cobalt chrome on cobalt chrome bearing surfaces are being reevaluated. Characterization of wear particles and studies on the reaction of the body to these particles, have played an important role in the determination of the factors that cause aseptic loosening and will therefore play an important role in the comparison of metal on polyethylene and metal on metal hip prostheses. In this paper, a comparison between the different aspects of metal and polyethylene wear particles is made using data from the literature and the authors’ experience. The authors conclude that techniques need to be optimized to isolate and characterize individual metal wear particles from periprosthetic tissues and they advocate the performance of in vitro studies with these in vivo generated wear particles or comparable particles.

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