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The wear of acetabular cups made from conventional gamma-sterilized, and electron-beam cross-linked ultra-high-molecular-weight polyethylene was studied with a biaxial hip wear simulator. The femoral heads were either polished or roughened so that they represented the type of roughening and the value of surface roughness (R(a) = 0.14-0.18 micro m) observed in the roughest explanted femoral heads. The lubricant was diluted calf serum, and the test length 3 million cycles. The mean wear rate and standard deviation of conventional polyethylene cups against polished and against roughened heads was 11.6 +/- 0.07 and 64.4 +/- 10.1 mg per 1 million cycles, respectively. The latter value closely corresponds to that measured from explanted Charnley prostheses. Against polished heads, cross-linked polyethylene cups showed net weight gain, whereas against roughened heads, their mean wear rate was 2.4 +/- 0.3 mg per 1 million cycles. The mean equivalent circle diameters of polyethylene wear particles produced in the above four categories were: conventional/roughened 0.32 micro m, cross-linked/roughened 0.29 micro m, conventional/polished 0.28 micro m, cross-linked/polished 0.23 micro m. The size ranges and shapes were close to those seen in particles isolated from tissue samples. In conclusion, the tests indicated that electron-beam irradiation effectively reduces the harmful polyethylene wear particle production in total hip arthroplasty.

The effect of counterface roughness on the wear of conventional gamma-sterilized, and electron-beam-crosslinked ultrahigh molecular weight polyethylene was studied with a circularly translating pin-on-disk device. The counterfaces, CoCr disks, were either polished, or roughened so that they represented the type of roughening and the range of surface roughness values (R(a) = 0.014-0.24 microm) observed in explanted femoral heads of total hip prostheses. The lubricant was diluted calf serum, and the test length 3 million cycles. A total of 24 tests were done. With both types of polyethylene, there was a strong correlation between R(a) and wear factor k. The power equations were k = 5.87 x 10(-5)(R(a))(0.91) for conventional polyethylene (R(2) = 0.94), and k = 7.87 x 10(-5)(R(a))(2.49) for crosslinked polyethylene (R(2) = 0.82). Crosslinking improved wear resistance significantly. The wear of crosslinked polyethylene against the roughest counterfaces was lower than the wear of conventional polyethylene against the polished counterfaces. Against rough counterfaces, the wear of crosslinked polyethylene was an order of magnitude lower than that of conventional polyethylene. On the crosslinked polyethylene pins that were tested against polished counterfaces, remains of original machining marks were still visible after the test. The average size of wear particles produced by both types of polyethylene against rough counterfaces was similar, 0.4 microm, whereas that produced by conventional and crosslinked polyethylene against polished counterfaces was significantly smaller, 0.2 and 0.1 microm, respectively.

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