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Ion levels elevated with metal-on-metal implants

Erythrocyte cobalt, urine cobalt and urine chromium levels were all elevated in patients with metal inserts.

Issue: September 2004

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BOSTON — Metal-on-metal implants have demonstrated excellent wear rates; however, defining the consequences of the metal ions released by these implants remains an issue.

“The metal-on-metal bearing, when retrieved in various studies, has a 20 to 25 times reduced wear rate over conventional metal-on-polyethylene bearings,” said Steven J. MacDonald, MD, London Health Sciences Center, London, Ontario.

The reason this technology has not become more popular is related to metal ion production, but the impact of those metal ions is unknown. “A lot of it is theoretical, but there are some concerns with metal-on-metal implants and production of those ions,” he said.

Compared metal, poly bearings

MacDonald and colleagues performed a prospective randomized trial comparing metal and polyethylene bearing surfaces. MacDonald is associate professor, University of Western Ontario and chief of orthopedics, London Health Sciences Centre. Twenty-three patients received a metal insert and 18 received a polyethylene insert with identical femoral and acetabular components.

“To our knowledge, this is the first prospective, randomized clinical trial comparing these two bearing surfaces specifically assessing all patients with validated outcome measures and complete blood and urine metal ion analysis,” the authors wrote in their abstract.

Patients were examined preoperatively and at three, six, 12 months and annually. Patients were evaluated for erythrocyte and urine cobalt, chromium, titanium, outcome measures and radiographs. No patients were lost to follow-up.

Patients doing well at latest follow-up

“As you would expect, the clinical scores — with the Harris Hip Score, the WOMAC and SF-12 — demonstrated no difference,” MacDonald said during a presentation at the 117th Annual Meeting of the American Orthopaedic Association. He added, however, that this study was underpowered to show slight clinical differences. “All patients are doing well at latest follow-up. So clinically, there is nothing to be seen.”

MacDonald found that patients with metal inserts had significantly elevated metal ion levels. Erythrocyte cobalt levels were seven times elevated (P<.001), urine cobalt levels were 41 times elevated (P<.001) and urine chromium levels were 14 times elevated (P<.001).

“When we look at metal on polyethylene, we’ve created levels of acceptable wear. And our feeling would be that it’s quite reasonable to expect the same sort of data on metal-on-metal implants,” MacDonald said. “We should have data on their ions. Metal-on-metal bearings are extremely good for wear properties, but we just don’t know their ion profile.”

The safe level of metal ions in metal-metal patients has not been defined. “Current implants need evaluation. And clearly, a future design focus should be on minimizing ion production because it is a good bearing for wear purposes."

For more information:

• MacDonald S. Metal on metal versus metal on polyethylene in total hip arthroplasty — a prospective randomized clinical trial. #9. Presented at the 117th Annual Meeting of the American Orthopaedic Association. June 22-24, 2004. Boston.