Biological reactions to wear debris in total joint replacement

The vast majority of total hip prostheses currently implanted consist of a hard metal or ceramic femoral head articulating against an ultra-high molec ular weight polyethylene (UHMWPE) acetabular cup. Over the last 10 years, e vidence has accumulated to show that these prostheses are prone to failure due to late aseptic loosening and few survive beyond 25 years. With an incr easing need to implant hip prostheses in the younger, more active patient t he need to understand the mechanisms of failure and to develop artificial h ip joints using alternative materials have become major issues in the ortho paedic community. This review focuses initially on our current understandin g of the biological reactions to UHMWPE prosthetic wear debris in vivo and in vitro since this is believed to be the main cause of late aseptic loosen ing. While the precise mechanisms of osteolysis induced by UHMWPE wear debr is have not been elucidated, the major message to emerge is that it is not the wear volume that determines the biological response to the debris, but the concentration of the wear volume that is within the critical size range (0.2-0.8 mu m) for macrophage activation. The review then considers whether the problem of wear-debris-induced osteol ysis may be overcome with the use of new generation metal-on-metal or ceram ic-on-ceramic prostheses. For metal-on-metal prostheses, the prospects for increasing the osteolysis free life of the implant are good but additional biological problems associated with the nanometre size and reactivity of th e wear particles in vivo may emerge. For the ceramic-on-ceramic prostheses, although initial prospects are encouraging, more data are needed on the ch aracteristics of the wear particles generated tn vivo before predictions ca n be made. It is concluded that the pre-clinical testing of any new materia ls for joint replacement must include an analysis of the wear particle char acteristics and their biological reactivity in addition to the usual assess ment of wear.