Effect of fluid absorption on the wear resistance of UHMWPE orthopedic bearing surfaces

Abstract

Wear evaluations of ultra-high molecular weight polyethylene (UHMWPE) materials for orthopedic bearings are typically conducted within a bovine serum solution environment, with the wear rate being characterized via mass loss measurements. The mass change of UHMWPE consists of not only mass loss due to wear but also mass increase due to fluid absorption. Indeed, with the highly crosslinked UHMWPE materials recently introduced for orthopedic implants, hip simulator evaluations often produce mass increases of the cup liners. This observation is due to the wear rate being lowered so significantly that it is outpaced by the rate of absorption of the surrounding fluid. Even when this fluid absorption is taken into consideration, the corrected rate of wear is still negative. However, a recent study by the present authors showed that, by lowering the load during the swing phase of the gait cycle, highly crosslinked UHMWPE liners exhibited a positive wear rate after correcting for the fluid absorption. In addition, the surface morphology of the liners indicated increased wear and there was also a concomitant, significantly increased fluid absorption. It was postulated that increased hydration might lead to increased wear of both conventional and highly crosslinked UHMWPE. In the present study, this hypothesis was tested by comparing the wear performance of UHMWPE on a serum-lubricated multidirectional pin-on-flat wear tester as a function of previously imposed fluid absorption. The UHMWPE pin samples were: non-irradiated; gamma irradiated in air (25–37 kGy); gamma irradiated in nitrogen (25–37 kGy); and electron-beam irradiated (100 kGy). It was found that hydration prior to wear testing did significantly increase wear of UHMWPE. Furthermore, highly crosslinked UHMWPE wore significantly less than conventional UHMWPE, regardless of the prior hydration condition.