Wear of polyethylene in artificial hip joints: superolateral wear of the acetabulum.

The bearing couple of a polyethylene acetabular cup and a polished metal or ceramic femoral head has been a successful combination in hip arthroplasty for over 30 years, but it is now recognised that fine micron and submicron polyethylene wear particles can cause adverse cellular reactions which may lead to osteolysis and loosening. In prostheses showing high wear, the penetration of the femoral head into the acetabular cup often exceeds 2 mm after 10 to 15 years. Although the head does not wear through the cup, which may be over 8 mm thick, there is a significant amount of debris, typically over 600 mm. Most of these particles are less than a micron, and therefore in the size range for phagocytosis, resulting in inflammatory reactions and osteolysis. Recent studies of failed Charnley prostheses at 10 to 20 years have estimated that well over one thousand billion wear particles (10) are released into the tissue during this period. There is much interest in the wear of polyethylene since it can lead to the failure of hip arthroplasties. Wear, unlike elastic modulus or hardness, is not a specific property of a material but is a result of the system and conditions in which the materials articulate. The simplest laws of wear state that the volume produced is proportional to the load and the sliding distance, but many other factors dramatically affect the wear rate. These include: 1) Local damage or roughening of the polished femoral head. 2) Oxidative degradation after sterilisation by gamma irradiation in the presence of oxygen (p. 340 and Besong et al). 3) Multidirectional movement. Polyethylene molecules are orientated by the forces of friction in the primary direction concern. Until there is good evidence that the intra-articular procedure alone is as effective as a combined reconstruction there must be doubt about its continued use. In this respect, the longer-term results of the cohort described by Webb et al are of particular interest.