Summary:Ostcolysis induced by ultra high molecular weight polyethylene wear debris is one ol thc primary factors limiting the lifespan of total hip replacements. Crossliiiking polyethylcne is known to improve its wear resistance in certain industrial applications. and crosslinked polyethylene acetabular cups have shown improved wear resistance in two clinical studies. In the present study, crosslinked polyethylene cups wcre produced by two methods. Chemically crosslinked cups were produced by mixing a pcroxide with ultra high molecular weight polyethylene powder and then molding the cups directly to shape. Radiation-crosslinked cups were produced by exposing conventional extruded ultra high molecular weight polycthylene bar stock to gamma radiation at various doses from 3.3 to 100 Mrad (1 Mrad = 10 kGy), remelting the bars to extinguish residual free radicals (i.e., to minimize long-term oxidation), and then machining the cups by conventional techniques. In hip-joint simulator tests lasting as long as 5 million cycles. both types of crosslinked cups exhibited dramatically improved resistance to wear. Artificial aging of the cups by heating for 30 days in air at 80°C induced oxidation of the chemically crosslinked cups. However, a chemically crosslinkcd cup that was aged 2.7 years at room temperature had very little oxidation. Thus, whether substantial oxidation of chemically crosslinked polyethylene would occur at body temperature remains unclear. The radiationcrosslinked remelted cups exhibited excellent resistance to oxidation. Becausc crosslinking can reduce the ultimate tensile strength, fatigue strength, and elongation to failure of ultra high molecular weigh1 polyethylene, the optimal crosslinking dose provides a balancc between these physical properties and the wear resistance of the implant and might substantially reduce the incidence of wear-induced osteolysis with total hip replacements.

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