Polyethylene cross-linking by two different methods reduces acetabular liner wear in a hip joint wear simulator

Abstract

Advances in cross-linking have led to the development of wear resistant ultrahigh molecular weight polyethylene for total joint replacement. This study compared wear reduction by two different cross-linking methods as measured in a hip wear simulator. One highly cross-linked polyethylene was treated with 7.5 Mrad gamma irradiation with post-irradiation annealing and a sterilization dose of 2.5 Mrad (10 Gamma), while the other used 9.5 Mrad warm irradiation with 10 MeV electron-beam (9.5 EB). Liners of the same design, made from nominally cross-linked (gamma sterilized) polyethylene were also tested. Gravimetric wear analysis was performed every 500,000 cycles for 5,000,000 cycles. After correcting for weight gain due to water absorption, the nominally cross-linked liners demonstrated mean wear rates of 15.7 (+/-1.7) and 12.5 (+/-1.0) mg/million cycles. Both highly cross-linked polyethylene liners demonstrated significantly less wear than their respective controls (with mean wear rates of 1.5 (+/-1.2) and -1.4 (+/-1.5) mg/million cycles). The 9.5 EB liners gained weight presumably due to increased fluid absorption, in addition to that measured in loaded-soaked control implants. Any wear occurring was therefore assumed to have been more than offset by weight gain. Highly cross-linked polyethylene was significantly more wear resistant than non- or nominally cross-linked polyethylene. The differences in wear rates between the two highly cross-linked polyethylene designs (9.5 EB or 10 Gamma) are probably too small to be clinically significant.