[UHMW-polyethylene as the substance for articulating components of joint prostheses]

In recent years there has been an increase in the incidence of problems with UHMWPE used in implants. In particular the risk of osteolysis and loosening resulting from the body's reactions to polyethylene wear particles must be eliminated. A variety of investigations with UHMWPE provided hints on how to improve the quality of the original material, machining and reduction of contaminants, which have since been implemented. The knowledge gained of the visco-elastic behaviour of this polymer makes it possible to optimize the design of implant components. The need to sterilize the component parts with ionizing radiation results in changes in the structure of UHMWPE. Via oxidation and acceleration of the ageing process of the material, irradiation in air reduces the molecular weight, which is important for the tribological properties. The use of a nitrogen atmosphere during irradiation and for storage makes it possible to take advantage of the positive effects of this method of sterilization via cross-linking of the polyethylene. Positive ductile properties are thus maintained and the resistance to wear is increased. An investigation of 134 components explanted during re-operation revealed a wide scatter in the results of wear. One of the reasons for increased polyethylene wear is the additional abrasive effect of particles of bone cement that get into the space between the articulating components; this can be minimized by a meticulous cementing technique.