Improving the metal-on-metal hip prosthesis

Abstract

Hip arthroplasty is an increasingly prevalent intervention, aimed at reducing pain and restoring function to patients suffering from common musculoskeletal diseases such as arthritis. Metal-on-metal (MoM) hip replacements were intended to be low-wear alternatives to conventional metal-on-polyethylene prostheses. Recent data has shown that revision rates for most MoM prostheses have not been as low as predicted and are not consistent across all models. Many failures result from complications arising from wear debris. Accurately quantifying wear has proved difficult. Using a co-ordinate measuring machine, a method for measuring the wear of ex-vivo MoM hip prostheses was developed and validated as accurate to within 0.5mm3. The method was applied to bearing surfaces and, where available, the internal tapers of femoral heads. Overall, 143 MoM hip explants were measured (95 resurfacings, 48 total hip replacements). Median total wear rates were 4.17mm3/year (mean=11.52, range=0.30-87.28mm3/year), notably higher than most simulator estimates of 1-2mm3/million cycles. Large differences were noted between different models of MoM hip. Time in vivo correlated with wear volume (SRCC=0.387, p<0.001) but not wear rate (SRCC=-0.086, p=0.169) suggesting that the prostheses wore at an unchanging rate through their lifetime. Acetabular cup inclination and anteversion correlated with wear volume (SRCC=0.414 and 0.233) and wear rate (SRCC=0.353 and 0.231, all p<0.001). Wear scars were consistently seen at the rim of the acetabular cup. The distance between these scars and the rim inversely correlated with volume (SRCC=-0.387, p<0.001) and rate (SRCC=-0.357, p<0.001). Patient blood metal ion levels were elevated (median 10.20μg/ml Cr, 9.73μg/ml Co) and correlated with wear volume (p<0.001). Surface roughness measurements were taken on the bearing surfaces and theoretical lubrication regimes (λ-ratio) calculated. There was an inverse correlation between worn λ-ratio and wear volume (p=0.038). Through these findings, recommendations are made for optimising future designs of hip prosthesis to minimise wear.