The size and shape of particulate polyethylene wear debris in total joint replacements.

Osteolysis induced by wear particles has been recognized as one of the major causes of long-term failure in total joint replacements. However, little is currently known about the exact nature of particles, as the particles are too small to be characterized by light microscopy. In this study, ultra-high molecular weight polyethylene (UHMWPE) particles retrieved from ten cases (six cemented and four uncemented) for Freeman type conforming tibiofemoral total knee replacements (TKRs), three Charnley total hip replacements (THRs) and five Imperial College/London Hospital double cup surface hip replacements for aseptic loosening were extracted using a high-performance method with ultracentrifugation and characterized by scanning electron microscopy. The equivalent circle diameter (ECD) of all 18 cases ranged from 0.40 to 1.15 microns (Mean +/- SE = 0.70 +/- 0.05 micron, median = 0.67 micron). The aspect ratio was 1.50 to 2.04 (Mean +/- SE = 1.75 +/- 0.04, median = 1.73), and roundness was 1.24 to 2.34 (Mean +/- SE = 1.61 +/- 0.07, median = 1.65). The numbers of particles were 5.2 x 10(8) to 9.17 x 10(10)/g tissue (Mean +/- SE = 1.42 x 10(10) +/- 5.41 x 10(9)/g tissue, median = 7.04 x 10(9)). The number of polyethylene (PE) particles/g tissue in TKRs was significantly larger than that in THRs (1.04 x 10(10)/g tissue and 2.16 x 10(9)/g tissue respectively, median, p = 0.03, Mann-Whitney U test). Unstable fixation of the tibial PE component might account for the accumulation of a large number of PE particles in the interface tissue.