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Researchers surprised by metal ion levels in 5-year follow-up of MoM THA implants.

The 5-year postoperative metal ion levels plateaued in cases having 28-mm metal-on-metal hip bearings but continued to increase for 36-mm metal-on-metal bearings, according to a recently presented prospective study involving 120 patients.

“This was a prospective randomized trial with three arms: a 28-mm polyethylene liner, 28-mm metal-on-metal (MoM) and a 36-mm [MoM],” C. Anderson Engh Jr., MD, said at the American Academy of Orthopaedic Surgeons 2012 Annual Meeting, in San Francisco. “The same stem and cup were utilized. We looked at serum, erythrocyte and, at 5 years, whole blood ion levels,” he said.

Of the 120 patients enrolled, 105 patients were eligible for the study at the time of surgery and randomized into three groups. The first group underwent metal-on-polyethylene (MoP) total hip arthroplasty (THA), while the second and third groups underwent 28-mm MoM THA and 36-mm MoM THA, respectively, Engh reported.

Engh and colleagues collected blood samples from the patients preoperatively and postoperatively at 6 months, 1 year, 2 years, 3 years and 5 years.

Based on the results, investigators found the MoP group’s chromium and cobalt ion levels at the 5-year follow-up were significantly lower than the two MoM groups for all blood samples.

Erythrocyte chromium was the lone exception in this case.

In the 36-mm MoM group, Engh reported significant increases for serum and erythrocyte cobalt, while the 28-mm MoM group did not display similar increases. At the 6-month mark, the abstract noted one patient in the 36-mm MoM group had serum or erythrocyte ion levels greater than 7 parts per billion, a development noted in four more patients by the 5-year follow-up. No patients in the 28-mm MoM group exhibited these findings.

Engh noted the findings ran counter to his group’s expectations, demonstrating a continual increase of cobalt serum and erythrocyte ion levels in the 36-mm MoM group and eventual leveling off in the 28-mm MoM group.

“These median ion levels are similar to what we see in other studies,” Engh said. “The cobalt levels are well correlated. The surprising thing was that the MoMs were not performing as well both anecdotally — with adverse tissue reaction, osteolysis and the higher ion levels — and also the 36 mm having an increasing ion level with time.”

“Why might this have occurred when it is the opposite of what we expected?” he added. “It may be a difference in the bearing, it may be taper corrosion, which would be my pick, or it may be other unintended sources such as impingement.”

Reference:

  • Engh CA, MacDonald SJ, Sritulanondha S, et al. Five-year metal ion levels after metal-on-metal total hip arthroplasty (THA): A prospective, randomized trial. Paper #310. Presented at the American Academy of Orthopaedic Surgeons 2012 Annual Meeting. Feb. 7-11. San Francisco.
  • Disclosure: Engh is a paid consultant for, receives royalties from and has stock or stock options in DePuy. He also receives research or institutional support from Johnson & Johnson, Stryker and the Alexandria Research Society.
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