Lubrication and wear of ultra-high molecular weight polyethylene in total joint replacements

This paper reviews recent advances in the tribology of artificial joints. Emphasis is placed on the latest developments in understanding wear mechanisms and in wear testing of ultra-high molecular weight polyethylene (UHMWPE) materials and components. Two major advances have been made. One is the discovery and recognition of the importance of multidirectional motion in wear mechanisms and wear testing of UHMWPE. The other is the development of an orientation-softening wear concept. The significance of joint kinematics in wear of acetabular and tibial components is discussed. New ways of improving the wear resistance of UHMWPE are proposed based upon the theory of orientation softening. Crosslinking of UHMWPE by ionizing radiation has been shown to significantly improve the wear resistance. The degree of improvement is greater with higher doses of irradiation and is much more effective for the hip than for the knee. A starved lubrication mechanism is proposed for the conforming contact between an UHMWPE acetabular cup and a CoCr or alumina ceramic head. Soluble proteins are found not to be an effective boundary lubricant for UHMWPE. Criteria for validating wear and joint simulator testing are discussed. The paper focuses on the choice of lubricant, the degree of motion and the positioning of components in joint simulator testing. (C) 1998 Elsevier Science Ltd. All rights reserved.