Wear of ultra high molecular weight polyethylene in total artificial joints

Abstract

In the last 3 years, the wear and generation of wear debris has been recognised as one of the major causes of failure in total joint replacement. In the vast majority of total joint replacements, one of the bearing surfaces consists of a hard very smooth metal or ceramic material, while the other surface is manufactured from ultra high molecular weight polyethylene (UHMWPE). The UHMWPE is used as the concave bearing surface; the acetabular cup in the hip and the tibia1 tray in the knee. UHMWPE was first introduced in the early 1960s and has, over the past 30 years, been one of the most widely used and perhaps successful biomaterials. Indeed, a review of the clinical orthopaedic literature published during the 1980s would indicate that wear of artificial joints and in particular wear of UHMWPE was not considered a major problem. During this period methods of fixation of implants, interfaces between biomaterials and bone, and loosening of implants were considered to be of major clinical importance. However. during the last 3 years it has been recognised that the generation of wear debris and in particular the wear of UHMWPE in artificial joints is a very important determinant of the long term clinical outcome of joint replacement.’ ’ The clinical problems associated with the wear of UHMWPE can be considered in three groups. Firstly, structural failure or fatigue of the bulk pothyethylene has been extensively reported in non-conforming knee joints with high contact stresses.” Secondly, high wear and penetration rates of UHMWPE in acetabular cups in Charnley joints causes adverse biomechanics. impingement of the neck of the femoral stem on the rim of the cup which produces loosening of the acetabular cup.” Thirdly. and most importantly. UHMWPE wear particles generated at the articulating surfaces, are released into the tissues, surrounding the joint. cause adverse cellular reactions which lead to bone resorption and loosening.’ !’ There is a clear indication that it is necessary to reduce the volume and number of UHMWPE wear particles in order to improve long-term clinical performance of total artificial joints. In this review. the fundamental processes that generate UHMWPE wear particles are introduced, clinical evidence of the contribution made by the different wear processes is presented. and the results of laboratory studies that provide insight into the tribological factors that control the wear processes are described. Finally, methods of reducing wear and the number of wear particles are discussed.