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Wear Debris Generation in Joint Simulator Testing of Crosslinked UHMWPE
Jani, S., Scott, M. L.
Abstract
In 1994 the NIH Consensus Statement on Total Hip Replacement concluded that late term implant failure was related to the inflammatory reaction to particulate matter and subsequent periprosthetic bone resorption. Generation of wear debris during articulation, therefore, is a critical technical issue in modifications applied to UHMWPE materials.
Although radiation crosslinked UHMWPE (XPE) has been used clinically since the 1970s, there has been recent interest in providing oxidative stability to XPE via thermal annealing/remelting treatments. This interest has arisen primarily because oxidation and consequent embrittlement occur over time after irradiation sterilization. The new class of irradiated/thermally treated XPE materials, as expected, have been shown to lower wear rates and improve resistance to oxidative degradation. The focus of this study was the characterization of wear debris generated from these materials.
Non-crosslinked (C-PE), 5 Mrad gamma irradiated/annealed (5-XPE), and 10 Mrad gamma irradiated/annealed (10-XPE) UHMWPE liners were tested against 32-mm CoCr heads on an AMTI hip simulator to 20 Mcycles. All liners (n=3 per group) were EtO sterilized. Serum was digested in hydrochloric acid, and particles were isolated by 0.05 micron filtration. Particle number and diameter were characterized by scanning electron microscopy.
Both gravimetric wear and mean particle diameter decreased as radiation dose increased. Accordingly, the number of particles generated per unit wear volume increased with radiation dose. The 5-XPE liners, which showed a 75% reduction in gravimetric wear, generated 82% more particles than C-PE. The 10-XPE liners showed negative (undetectable) gravimetric wear and generated 65% fewer particles than C-PE.
The gravimetric technique is not capable of detecting the wear volume of extremely low wearing XPEs and cannot reliably rank materials in terms of particle generation. This necessitates the isolation and characterization of wear particles in order to predict the in vivo biological response to the particulate form of XPE.
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