“Our findings of a greater incidence of metal ion outliers and of increasing cobalt ion levels over time in the 36-mm MoM group are of concern and warrant further patient follow-up,” the researchers wrote in the study.

In the study, researchers randomly assigned 105 patients with unilateral noninflammatory degenerative joint disease to receive a 28-mm metal-on-polyethylene, 28-mm metal-on-metal (MoM) or 36-mm MoM bearing surface. They performed metal ion measurements and clinical evaluations at regular intervals.

At the 5-year follow-up, the cobalt and chromium ion levels in all blood sample types, with the exception of chromium in erythrocytes, were lower for patients in the metal-on-polyethylene group as compared to patients in the two MoM groups. Study results showed significant increases of cobalt in serum and erythrocytes from 2 years to 5 years in the 36-mm MoM group, but not in the 28-mm MoM group. Five patients in the 36-mm MoM group and no patients in the 28-mm MoM group had cobalt or chromium levels greater than 7 ppb at 5 years, according to study results. Correlations among levels in serum, erythrocyte and whole blood were stronger for cobalt than for chromium.

“Because we have previously seen adverse local tissue reactions that were not expected, remain difficult to diagnose and are not fully understood, both of the institutions participating in the present study have avoided further use of metal-on-metal total hip replacements,” the researchers wrote in the study.

Disclosures: Engh receives royalties from DePuy and has stock options with DePuy, Alexandria Research Tech and Stryker. Engh, Jr is a consultant for and received royalties from DePuy. MacDonald is a consultant for and receives royalties from DePuy. Naudie is a consultant for and receives lecture payments from Smith & Nephew and Stryker, and receives royalties from Smith & Nephew.