Comparison of gas plasma and gamma irradiation in air sterilization on the delamination wear of the ultra-high molecular weight polyethylene used in knee replacements.

Early failure of knee replacements is thought to be due to the combination of sterilization by gamma irradiation in air and the high cyclic stresses that they endure during use. Such failures are shown through delamination and permanent deformation of the ultra-high molecular weight polyethylene (UHMWPE) component. This study investigated whether gas plasma sterilization, as an alternative to gamma irradiation in air, would give better performance after ageing in a knee replacement using a metal pin on polymer plate wear test. Fourier transform infrared (FTIR) analysis was performed on the components to assess oxidation levels and a finite element stress analysis model is presented to estimate strain at failure in the UHMWPE. Delamination occurred in the majority of the gamma-irradiated plates but did not occur in any of the gas-plasma-sterilized plates. The FTIR analysis showed that the plates gamma irradiated in air were highly oxidized when compared with the gas-plasma-sterilized plates. Plastic strain at failure was determined for the gamma-irradiated plates and found to be less than 2.4-14 per cent.

A two-dimensional model of cyclic strain accumulation in ultra-high molecular weight polyethylene knee replacements.

As new methods of sterilization of the ultra-high molecular weight polyethylene (UHMWPE) component in knee replacements are introduced, reported incidents of delamination will decrease. The prediction of plastic strain accumulation and associated failure mechanisms will then become more important in knee replacement design. The finite element analysis reported in this paper aims to advance the modelling of strain accumulation in UHMWPE over repeated gait cycles and seeks to determine the effects of the knee replacement design variables of geometry and kinematics. Material testing was performed under cyclic and creep conditions to generate the elastic, viscoplastic material model that has been used in this time-dependent analysis. Non-conforming geometries were found to accumulate plastic strains at higher rates than conforming geometries. The anatomical motion known as rollback initially produced lower strain rates, but predictions of the long-term response indicated that designs which allow rollback may produce higher strains than static designs after only about a week of loading for a knee replacement patient.

Comparison of the wear of aged and non-aged ultrahigh molecular weight polyethylene sterilized by gamma irradiation and by gas plasma.

The wear of ultrahigh molecular weight polyethylene in artificial joints is a major cause of long-term osteolysis and loosening. The wear rate of aged and non-aged ultrahigh molecular weight polyethylene sterilized by both gamma irradiation in air and gas plasma has been studied in simple configuration wear tests. Fourier transform infrared analysis (FTIR) showed marked oxidative degradation of the irradiated and aged material compared to the gas plasma sterilized and aged material. The wear rate of the irradiated and aged material was significantly (three times) higher than the gas plasma sterilized, gas plasma sterilized and aged, and non-sterilized materials. Alternative sterilization procedures such as gas plasma, when used instead of gamma irradiation in air, are likely to reduce the incidence of long-term osteolysis.

The influence of scratches to metallic counterfaces on the wear of ultra-high molecular weight polyethylene.

A number of studies of explanted metallic femoral heads have shown scratches or damage caused by bone cement, bone or metallic particles. This damage has been cited as a cause of increased wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. In this laboratory study, small scratches 2 mu m deep were made on smooth stainless steel surfaces at a spacing of 10 mm. These individual scratches were found to increase the wear rate of UHMWPE by a factor of 30 in unidirectional sliding and a factor of 70 in reciprocating motion. It is of particular concern that a single small scratch, which is not detected by the average surface roughness measurement Ra can cause such a dramatic increase in the wear of ultra-high molecular weight polyethylene.