The in-vitro wear behavior in the presence of abrasive particles was determined for two highly crosslinked ultrahigh molecular weight polyethylenes (HXPE), one in clinical application for hips and the other for knees. The wear studies were performed in joint simulators and were largely comparative, with conventional ultrahigh molecular weight polyethylene (UHMWPE) gamma irradiated 37 kGy in nitrogen used as the control. The test methodology used for these three-body wear tests was developed in-house. It was found that the wear advantage of the HXPEs relative to the conventional UHMWPE observed under clean conditions is largely preserved in the presence of the abrasive particles used (alumina and bone cement for hips, bone cement for knees) under the test conditions. These results suggest that the surface molecular chain orientation-inhibition mechanism proposed to account for the increased wear resistance of highly crosslinked polyethylenes undergoing micro adhesive/abrasive wear is still operational even when a thicker surface layer is disturbed in the presence of abrasive particles. Therefore, the wear of the UHMWPE is not simply dependent on the bulk mechanical properties of the UHMWPE. The higher than expected wear of the 22 mm hip liners compared to the 32 mm liners in the presence of abrasive particles suggests that the wear rate of the UHMWPE becomes stress dependent rather than load dependent for sufficiently high stresses.

Recently, a knee prosthesis containing an electron beam irradiated (58 – 72 kGy, nominal dose of 65 kGy), melt-annealed, highly crosslinked UHMWPE (HXPE) tibial insert has been developed. In the present study, the wear and delamination resistance of the HXPE tibial insert and its fatigue performance under a posterior loading condition have been evaluated against its conventional gamma-sterilized UHMWPE counterpart (37kGy, in nitrogen). The test methodologies used were newly developed with the aim to evaluate this new material under severe testing conditions. In comparison to the gamma controls, the HXPE inserts: (a) wore significantly less (achieving wear reductions of 81% and 73% over 5 and 20 million cycles, respectively); (b) exhibited significantly improved delamination resistance; and (c) exhibited significantly improved posterior loading fatigue resistance.

Crosslinking has been shown to improve significantly the wear resistance of ultra high molecular weight polyethylene (UHMWPE), both in joint simulator tests and in clinical use. High-energy ion radiation, when used to induce crosslinking in UHMWPE, produces free radicals. In the presence of oxygen, the free radicals have been shown to be responsible for oxidative degradation of polyethylene. A melt-anneal treatment of highly crosslinked UHMWPE substantially eliminates the free radicals to an undetectable low level. The aim of this study was to determine the effect of accelerated oxidative aging on the mechanical and fracture properties of melt-annealed highly crosslinked UHMWPE. Compression molded GUR 1050 UHMWPE was crosslinked by electron-beam irradiation at 100 ± 10 kGy and was followed by a melt-anneal treatment. Both melt-annealed highly crosslinked and conventional gamma irradiated in nitrogen UHMWPE were exposed to an accelerated oxidative challenge in the laboratory. The tensile mechanical properties and crack growth resistance curves (J-R curves) determined for oxidative aged melt-annealed highly crosslinked UHMWPE were either equivalent or superior to the oxidative aged conventional gamma irradiated UHMWPE. The difference in tensile mechanical properties and J-R curves for non-aged and oxidative aged melt-annealed crosslinked UHMWPE was insignificant, indicating that oxidative aging had no detrimental effect on mechanical and fracture properties of melt-annealed highly crosslinked UHMWPE.