Metal-on-metal hip prostheses and systemic health: a cross-sectional association study 8 years after implantation

Abstract

There is public concern over the long term systemic health effects of metal released from hip replacement prostheses that use large-diameter metal-on-metal bearings. However, to date there has been no systematic study to determine which organs may be at risk, or the magnitude of any effect. We undertook a detailed cross-sectional health screen at a mean of 8 years after surgery in 35 asymptomatic patients who had previously received a metal-on-metal hip resurfacing (MoMHR) versus 35 individually age and sex matched asymptomatic patients who had received a conventional hip replacement. Total body bone mineral density was 5% higher (mean difference 0.05 g/cm², P = 0.02) and bone turnover was 14% lower (TRAP 5b, mean difference -0.56IU/L, P = 0.006; osteocalcin, mean difference -3.08 ng/mL, P = 0.03) in the hip resurfacing versus conventional hip replacement group. Cardiac ejection fraction was 7% lower (mean absolute difference -5%, P = 0.04) and left ventricular end-diastolic diameter was 6% larger (mean difference 2.7 mm, P = 0.007) in the hip resurfacing group versus those patients who received a conventional hip replacement. The urinary fractional excretion of metal was low (cobalt 5%, chromium 1.5%) in patients with MoMHR, but creatinine clearance was normal. Diuretic prescription was associated with a 40% increase in the fractional excretion of chromium (mean difference 0.5%, P = 0.03). There was no evidence of difference in neuropsychological, renal tubular, hepatic or endocrine function between groups (P>0.05). Our findings of differences in bone and cardiac function between patient groups suggest that chronic exposure to low elevated metal concentrations in patients with well-functioning MoMHR prostheses may have systemic effects. Long-term epidemiological studies in patients with well-functioning metal on metal hip prostheses should include musculoskeletal and cardiac endpoints to quantitate the risk of clinical disease.

Figure 1. Recruitment flow chart.

MoMHR  =  metal-on-metal hip resurfacing, THA  =  conventional hip replacement using a non-metal-on-metal bearing.

Figure 2. Bone endpoints in MoMHR versus THA patients.

A) Mean difference in bone mineral density at various body sites, and B) differences in bone turnover markers between the patient groups. TRAP-5b = Tartrate-resistant acid phosphatase 5b, CTX-I  =  C-telopeptide of type I collagen, OC =  osteocalcin, PINP =  N-terminal propeptide of type-I procollagen, BAP =  bone-specific alkaline phosphatase. Comparison is the difference in endpoint in MoMHR versus THA patients by paired t-test.

Figure 3. Urinary fractional excretion of cobalt and chromium versus plasma metal level in MoMHR patients.

Line represents regression slope and dotted line represents 95% confidence interval. Comparison is fractional excretion of cobalt versus fractional excretion of chromium by linear regression analysis (P<0.0001).