Alternative bearing surfaces: the good, the bad, and the ugly.

Abstract

This article discusses current bearing-surface alternatives for long-term total hip articulations involving metal-polyethylene, ceramic-polyethylene, metal-metal, and ceramic-ceramic couples. The enduring success of the low-friction arthroplasty advanced by Sir John Charnley as a solution for painful hip problems can be appreciated by the fact that, in 1999, more than 270,000 hip arthroplasties were performed in the United States. Over the last three decades, patient profiles have changed substantially, resulting in demands for a greater service life of ultra-high molecular weight polyethylene hip components. Material failure, often leading to an osteolytic response, is increasingly associated with younger, more active patients. In this context, the low-friction solution has become a problem, limiting in vivo system longevity (Figs. 1 and 2). Fig. 1: A marked osteolytic response in a fifty-year-old patient. Fig. 2: Corresponding intracellular polyethylene debris viewed under polarized light. Previous attempts to improve the performance of ultra-high molecular weight polyethylene have included carbon-fiber reinforcement (Poly-2) and, more recently, polymer reprocessing to enhance mechanical properties (Hylamer). The former was withdrawn from the market because of excessive inflammatory response, whereas the latter has been linked to debris-induced osteolytic responses in early reports. Laboratory simulation has demonstrated that the resistance of ultra-high molecular weight polyethylene to wear is improved with increased cross-linking of the carbon-hydrogen polymer chains. A number of thermal and chemical processing solutions have been described. One such approach involves component storage at elevated temperatures in an oxygen-depleted environment. This is done following irradiation and encourages kinetic recombination of the carbon-hydrogen free radicals created by the radiation process. Other techniques deliver increased radiation doses to the component material followed by remelting to quench free radicals. While this results in dramatic wear reduction in laboratory simulations (Fig. 3), it also changes the amorphous and crystalline regions of the polymer, affecting mechanical properties and potentially reducing fatigue strength. …