Improvement of the wear and oxidation resistance of irradiation sterilized ultrahigh molecular weight polyethylene via environmental manipulation

Wear debris generated from UHMWPE bearing surfaces in total joint arthroplasties is known to contribute to osteolysis. UHMWPE components are commonly sterilized by gamma radiation, which also introduces unstable free radicals. The mechanism by which these radicals react can have beneficial or adverse effects on wear performance. Chain scission through reaction of radicals with oxygen diffusing from the surrounding environment can reduce UHMWPE wear resistance. Crosslinking of radicals instead enhances the wear performance of UHMWPE.; In this work, various environmental manipulations were developed to promote free radical consumption through crosslinking, stabilizing the UHMWPE against oxidative degradation. Specimens were artificially aged in order to evaluate the long-term performance of these materials. Wear of these UHMWPE specimens was initially characterized on a unidirectional pin-on-disk tester, using dry roughened steel countersurfaces to efficiently generate measurable wear. Initial testing indicated that a 9-week 70°C vacuum post-irradiation storage environment is effective in preventing any loss of wear resistance that otherwise would occur due to aging. A 9-week 70°C H₂ or C₂H₄ post-irradiation storage environment prior to aging produced a further improvement in wear resistance indicating that these environments have stronger potential to enhance crosslinking of irradiated UHMWPE.; A new multi-station pin-on-disk tribometer was constructed to evaluate the wear of these variously treated materials under more clinical relevant conditions, namely, multi-directional, lubricated sliding, against polished CoCr. Subjecting the UHMWPE to 70°C pre-irradiation vacuum had a beneficial effect on wear measured after aging for both 4 and 10 Mrad of irradiation dose. Further benefit on wear resistance was not realized with the addition of a 70°C temperature post-irradiation storage as would otherwise be expected based on initial testing under unidirectional, unlubricated sliding, against roughened stainless steel. As is discussed, this behavior may be a result of the use of a commonly employed accelerated aging condition which instead promotes additional crosslinking.; This work also contains the results of a mathematical model used to predict oxidation, which accounts for a depth dependent irradiation dose. New appropriately simulative accelerated aging conditions of shelf-aging are predicted, and the potential effectiveness of several post-irradiation storage environments in preventing long-term oxidation is evaluated.