The Effect of Obesity on Blood Metal Ion Levels after Hip Resurfacing and Metal-on-Metal Total Hip Replacement

There has been concern regarding potentially higher failure rates of metal-on-metal (MoM) bearings in obese patients, and possible adverse reaction to metal debris (ARMD). Elevated chromium (Cr) and cobalt (Co) levels in the blood indicate excess wear, and may predict secondary soft-tissue damage.

We investigated the effect of obesity on blood Cr and Co levels in patients with MoM bearings. We reviewed data on 75 patients with hip resurfacing and MoM total hip arthroplasty (THA) at an average of 4 years follow-up. We excluded patients with factors that can affect metal ion levels including other metal implants, bilateral MoM bearings or during the ‘running-in’ phase. The patients were classified into two groups; obese (n=27) and control (n=48) depending on a body mass index (BMI) ≥ 30 kg/m². There was no significant difference between the groups with regards to age, gender, renal function, cup inclination, implant type, femoral head size, time interval between operation and metal ion levels or the number of symptomatic patients.

There was a trend for lower Cr and Co levels in the obese group but this was not statistically significant (p= 0.17 & 0.07, respectively). There was no correlation between the BMI and Cr or Co levels (Spearman’s correlation coefficient r = − 0.13/p = 0.26 & − 0. 2/p = 0.1, respectively). There was no evidence of higher metal ion levels, suggestive of increased wear, in obese patients with MoM bearings.