Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints: III: The measurement of internal volume changes in explanted Charnley sockets after 2–16 years in vivo and the determination of wear factors

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 −6 mm 3 N −1 m −1, with the results ranging from 0.09 × 10 −6 to 7.2 × 10 −6 mm 3 N −1 m −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 R a was found to be 0.054 μm. For this roughness laboratory tests revealed a wear factor of 1.2 × 10 −6 mm 3 N −1 m −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