Effect of accelerated aging conditions on resultant depth-dependent oxidation and wear resistance of UHMWPE joint replacement bearing materials

Oxidative scissioning is known to occur in ultra-high molecular weight polyethylene (UHMWPE) hip and knee joint replacement bearings sterilized by 60Co gamma radiation and stored in room temperature 1 atm air (shelf aging). To determine the effects of this aging, gamma irradiated UHMWPE samples were exposed to several environments, including shelf aging and combinations of increased temperature and/or oxygen pressure commonly used to accelerate aging. It was found that each aging environment created a unique oxidation profile with a corresponding wear profile. For example, 680 days of shelf aging produced a subsurface oxidation maximum at approximately 0.5 mm from the free surface, while the oxidation maximum for 5 atm O2 at 25 °C after 271 days had greater intensity and occurred at a greater depth of roughly 2 mm. The commonly used accelerated aging environment of 1 atm air at 70 °C after 271 days produces an oxidation maximum essentially at the free surface with lower levels in the bulk. The two ‘accelerated’ aging conditions share a common wear rate of 0.6×10−6 mm3/N m at the free surface for the 4Mrad dose used in this study, which is approximately the same as that of the material before irradiation, but the values for the two samples diverge in the subsurface. In order to simulate the oxidation and wear profiles created by shelf aging within a shorter accelerated time scale, a proper combination of increased temperature and increased oxygen pressure must be used.