Numerical oxidation model for gamma radiation-sterilized UHMWPE: consideration of dose-depth profile.

Gamma sterilization of UHMWPE hip and knee joint replacement components secondarily creates free radicals along the polymer chains. Though crosslinking between radicals may improve mechanical properties, typical post-irradiation environments (air shelf storage or in vivo service) may instead favor scission reactions with oxygen from the surroundings. As such aging of irradiated UHMWPE joint replacement components has important consequences such as osteolysis, increased insight has been sought through descriptive models of this oxidation process. The quantitative numerical model presented here accounts for a free radical concentration that varies with position (because of irradiation dose-depth profile) and time (because of free radical decay through crosslinking). A moving front of diffusing O(2) is allowed to traverse the UHMWPE medium containing depth- and time-dependent free radical concentration, and these diffusing molecules react with available free radicals persisting at the front. This model's capabilities are illustrated in three examples of irradiated UHMWPE aging behavior: In room-temperature air (shelf-aging), in atmospheres of augmented oxygen partial pressure and temperature intended to accelerate aging while otherwise remaining simulative of real-time aging; and following post-irradiation vacuum storage intended to consume free radicals through complete crosslinking, but often performed to an incomplete extent.

A novel constrained acetabular liner design with increased range of motion.

One treatment strategy for recurrent dislocation after total hip arthroplasty is the use of a constrained acetabular component. A major drawback of currently available constrained components is the limited range of motion (ROM). We present a novel constrained component that employs a monopolar, highly cross-linked polyethylene liner with cutouts oriented to allow increased ROM, while maintaining constraint against dislocation. ROM and lever-out tests comparing this novel design with a conventional constrained component showed that the cutaway monopolar component allowed ROM substantially greater than a currently available design. The lever-out torque for the cutaway monopolar constrained component was 243% higher than the conventional constrained component. This novel constrained acetabular component offers promise for providing excellent constraint against dislocation, while maintaining a wide ROM.