Current in vivo wear of metal‐on‐metal bearings assessed by exercise‐related rise in plasma cobalt level

Baseline metal ion levels are elevated in patients with metal‐on‐metal (MOM) hip arthroplasty. Interpretation of baseline levels is difficult as measurements are influenced by wear, corrosion, and metal ion release from stored metal in the body. Schmalzried et al. demonstrated that “wear is the function of use, not time.” The specific research question we asked was: Does physiological exercise increase the wear of metal‐on‐metal articulation which can be measured from the plasma metal ion levels? Patients with three different well functioning MOM bearings [two types of resurfacing (BHR 46.8 mm and Cormet 48 mm) and Metasul 28 mm] were included. Blood samples were taken immediately before, immediately after, and 1 h after exercise to determine cobalt and chromium levels. A significant increase (p < 0.005) in serum cobalt and chromium of 13% and 11%, respectively, was noticed after the exercise. Rise of cobalt levels in patients with a resurfacing MOM was 8.5 times (BHR group) or 6.5 times (Cormet group) larger than in those with a Metasul MOM (p = 0.021 and p = 0.047). Neither rise of metal levels nor baseline levels correlated with any other factor (p > 0.27). Exercise‐related elevations of plasma cobalt level provides information on current in vivo wear production that cannot be inferred from a baseline measurement of cobalt levels. Chromium levels cannot provide reliable information on the in vivo wear of the devices. Diameter was the important feature of the implant in determining exercise‐related elevations of plasma cobalt level. Exercise‐related elevations of plasma cobalt level is a potential in vivo tool to understand and improve the tribology of metal–metal bearings.