Effect of counterface roughness on the wear of conventional and crosslinked ultrahigh molecular weight polyethylene studied with a multi-directional motion pin-on-disk device.

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.