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.
Sixty 10-mm bone-patellar tendon-bone allografts from young human donors were placed into four test groups, a control fresh-frozen group and three fresh-frozen irradiated groups. The irradiated groups were exposed to 2.0, 3.0, or 4.0 Mrad of gamma irradiation. The specimens were tested to tensile failure. The initial biomechanical strength of fresh-frozen allografts was reduced up to 15% when compared with fresh-frozen controls after 2.0 Mrad of irradiation. Maximum force, strain energy, modulus, and maximum stress demonstrated a statistically significant reduction after 2.0 Mrad of irradiation (P < 0.01). Stiffness, elongation, and strain were reduced but not with statistical significance. A 10% to 24% and 19% to 46% reduction in all biomechanical properties were found after 3.0 (P < 0.005) and 4.0 (P < 0.0005) Mrad of irradiation, respectively. After irradiation with a 4.0 Mrad dose, the ultimate load was below that of reported values for the human anterior cruciate ligament. It is clinically important to observe and document changes in human ligaments that result from currently used doses of gamma irradiation. The results from this study provide important information regarding the initial biomechanical properties of fresh-frozen human bone-patellar tendon-bone allografts after bacterial sterilization with gamma irradiation. The current accepted dose for sterilization is between 1.5 and 2.5 Mrad. There appeared to be a dose-dependent effect of irradiation on all the biomechanical parameters studied. Four of seven parameters were found to be reduced after 2.0 Mrad of irradiation. Reductions were found in all parameters after 3.0 and 4.0 Mrad of irradiation.
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