Metal-on-Metal Hip Implants Should Be Banned

A new study suggests that metal-on-metal (MOM) hip implants are more likely to fail than devices made from other materials, and should be banned.

Researchers at the University of Bristol (United Kingdom) analyzed data from the National Joint Registry of England and Wales, which includes information on more than 400,000 hip replacements, implanted between 2003 and 2011, and tracked them for up to seven years after surgery; of those, 31,171 were stemmed MOM implants. The researchers estimated failure rates for the implants based on different head sizes and compared them with implants made from ceramic and polythene.

The researchers found that stemmed MOM implants failed much more quickly than other types of bearing surface, with a five-year revision rate of 6.2%. Failure was related to head size, with larger heads failing earlier, corresponding to a 2% increase in the risk of failure for each one mm increase in head size. By contrast, ceramic-on-ceramic implants did better with larger head sizes. In women, failure rates for stemmed MOM implants were up to four-times higher than those of other bearing surfaces, and were also higher compared with men, even with the same head size. The study was published early online on March 13, 2012, in the Lancet.

“Metal-on-metal stemmed articulations give poor implant survival compared with other options and should not be implanted,” concluded lead author Professor of orthopedic surgery Ashley Blom, MD, PhD, and colleagues. “All patients with these bearings should be carefully monitored, particularly young women implanted with large diameter heads.”

The leading cause of total hip replacement (THR) failure is aseptic loosening secondary to polyethylene wear. A second major cause of failure is dislocation, affecting about 4% of THRs, most within six months.

Surgeons and engineers have tried to address these problems by investigating alternative bearing surfaces that have lower wear and allow larger head sizes, which theoretically should lead to lower dislocation rates and improved implant survival. This, however, does not seem to be the case.

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University of Bristol