In a retrospective review of 100 consecutive patients undergoing primary THA using a modular dual mobility prosthesis (Modular Dual Mobility; Stryker Orthopaedics, Mahwah, NJ), Matsen Ko et al noted 21% of patients to have a serum cobalt level above the normal range, with 9% significantly above normal (> 1.6 ug/L), at a mean of 27.6 months postoperatively. In addition, a recent prospective cohort study of 26 patients receiving the same prosthesis showed elevated whole blood mean cobalt levels in patients receiving a modular dual mobility prosthesis versus patients receiving a conventional bearing (0.23 + 0.39 vs. 0.15 + 0.07ug/L, p<0.001) at 1 year postoperatively. Four patients in the dual mobility cohort had a whole blood cobalt level outside the reference range (maximum 1.81 ug/L).