‘severe’ Wear Challenge to 36 Mm Mechanically Enhanced Crosslinked Polyethylene Hip Cups

Introduction Recent studies have shown that sequential irradiation/annealing processes improves the wear performance and oxidation resistance of crosslinked polyethylene, while maintaining mechanical properties [1,2]. Although these early results showed great promise for mechanically enhanced XLPE, we still know little about the variables that can lead to high-wear, particularly surface roughness and alternative modes of failure [3]. Analysis of retrieved Co-Cr heads has revealed roughness up to 500 nm (Ra) [4]. This factor becomes important as XLPE has been shown to be sensitive to femoral roughness [5]. A study combining highroughness and an aggressive gait model reported run-away wear for 5 Mrad UHMWPE-on-Co-Cr hip bearings, creating greater concentrations of submicron-sized wear particles [6]. This result suggests that combined failure modes may have an exaggerated influence. An alternative wear mode is micro-separation [3]. Although studies have reported less polyethylene wear with micro-separation [7], its overall influence under combined wear modes is unknown. Therefore, the current study developed a ‘severe’ wear protocol combing femoral roughening, joint micro-separation and high gait forces. The test hypothesis was that mechanically enhanced XLPE would show superior wear performance compared to conventional UHMWPE. As a secondary aim, this study considered the contribution of a large femoral head diameter (36 mm) to the failure of XLPE liners. Materials and Methods Twelve PE-on-Co-Cr hip bearings (36 mm) were investigated (Table 1). Group 1 (control) were UHMWPE liners: gamma sterilized at 3 Mrad in argon. Group 2 were XLPE liners (mechanically enhanced): irradiated at 5 Mrad, pre-heated, deformed (2.23:1), stress-relieved, and ETO sterilized. All liners were coupled with Co-Cr femoral heads. To simulate a worst-case scenario, six of the heads were roughened using glass bead blasting. The rough heads had a Ra and Rp comparable to the most damaged regions of retrieved femoral components (Table 2).