Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints

Abstract A casting technique for assessing the extent of the penetration of femoral heads into polyethylene acetabular cups in explanted Charnley prostheses is described and the observed penetration rates are related to laboratory wear studies carried out in the Institute of Tribology at Leeds over some 16 years. Some 32 explanted high molecular weight polyethylene (RCH 1000) acetabular cups provided by Wrightington Hospital were examined, and 25 were found to be suitable for penetration measurements. Relevant patient details for the 25 explanted acetabular cups were provided and 15 of the subjects were male whilst 10 were female, with 14 of the cups coming from the right hip and 11 from the left. The ages of the subjects at the time of the removal of the prostheses ranged from 18 to 68 years, the average being 54.4 years. The average period of residence of the prosthesis in the body was 8 years 11 months. The correlation between the measured volumes of polyethylene removed and the penetration was found to be linear and therefore entirely in accord with the view developed in part I that the femoral head tunnels into the acetabular cup with a circle of penetration having a diameter comparable with that of the femoral head. This observation enabled the expression derived in part I for a clinical wear factor to be adopted and results are recorded for all 25 prostheses on the assumption that creep played little part in the total penetration process. The average clinical wear factor was found to be 2.9 × 10−6mm3N−1m−1, with the results ranging from 0.09 × 10−6 to 7.2 × 10−6mm3N−1m−1. It was observed that the femoral heads from the explanted prostheses exhibited a number of fine scratches and attention was thus drawn to the influence of counterface roughness on the wear of polyethylene. Measurements of surface roughness indicated that for a number of the femoral heads the roughness of the metal surface was greater than that normally associated with the highly polished finish of new prostheses. The average value of the surface roughness Ra was found to be 0.054 μm. For this roughness laboratory tests revealed a wear factor of 1.2 × 10−6mm3N−1m−1. The agreement between this laboratory wear factor and the deduced clinical wear factor is much better than has been recognized previously. The clinical observations indicate an average penetration range of about 0.19 mm year−1, whereas X-ray studies have previously suggested a mean penetration rate of only 0.07 mm year−1. The powerful role of counterface roughness on the wear rate of polyethylene has been noted elsewhere by Dowson et al. The present study indicates that when full account is taken of counterface roughness the agreement between laboratory and clinical observations is in much closer accord than had been recognized previously. Particles of acrylic cement were detected in the explanted acetabular cups and the role of such particles in determining the effective roughness of the metallic femoral heads and hence the wear rate of the polyethylene acetabular cups is discussed. It is concluded that every effort should be made to minimize the presence of acrylic cement particles between the articulating surfaces in total replacement joints. The major factors thought to be responsible for the remaining discrepancies between the average clinical and laboratory wear factors and the wide spread of results are considered. The results presented in this paper demonstrate the close accord now achieved between laboratory and clinical wear studies. They indicate that although there is undoubtedly a wide range of wear factors and effective penetration rates in both the laboratory and the clinical situations, the mean values are sufficiently close to promote confidence in the understanding of the wear processes involved in both situations. It thus appears that most of the penetration of femoral heads into acetabular cups can be attributed to wear rather than to creep of the polyethylene.