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The exercise-related rise in plasma cobalt levels after metal-on-metal hip resurfacing arthroplasty

¹ Institute of Orthopaedics The Robert Jones and Agnes Hunt Orthopaedic Hospital, Gobowen, Shropshire SY10 7AG, UK.

Correspondence should be sent to Professor J. B. Richardson; e-mail: Janet.Morris{at}rjah.nhs.uk

Wear of metal-on-metal bearings causes elevated levels of cobalt and chromium in blood and body fluids. Metal-on-metal bearings have two distinct wear phases. In the early phase, the wear rate is high. Later, it decreases and the bearing enters a steady-state phase. It is expected that as the wear rates decline, the level of cobalt detected in plasma will also decrease. We studied the baseline and exercise-related cobalt rise in 21 patients (13 men and eight women) with a mean age of 54 years (38 to 80) who had undergone successful hip resurfacing at a mean of 44 months (10 to 96) earlier. Our results showed that circulating baseline cobalt levels were not significantly correlated with the time since implantation (r = 0.08, p = 0.650). By contrast, the exercise-related cobalt rise was directly correlated with the inclination angle of the acetabular component (r = 0.47, p = 0.032) and inversely correlated with the time since implantation (r = –0.5, p = 0.020).

Inclination of the acetabular component should be kept less than 40° to decrease the production of wear debris.