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By John Gever, Senior Editor, MedPage Today

Published: March 12, 2012

Revision of total hip replacement surgery within five years was significantly more common in British patients who received metal-on-metal implants versus other types, results of a large registry study indicated.

Data from the National Joint Registry of England and Wales showed that the cumulative risk for revision within five years was 6.2% for metal-on-metal implants compared with 3.3% for ceramic-on-ceramic and 1.8% for metal-on-polyethylene devices (P<0.05), using a flexible parametric survival model to adjust for the competing risk of death, reported Alison J. Smith, MSc, of the University of Bristol in Bristol, England, and colleagues.

The analysis covered more than 402,000 total primary hip replacements from 2003 to 2011, including 31,171 involving metal-on-metal implants, the researchers wrote in The Lancet online.

Younger age in women was a risk factor for early revision with metal-on-metal implants. A 55-year-old woman receiving a device with a 28-mm head had a 4.8% risk for revision at five years versus 2.6% for the same device in a 70-year-old woman.

Women tended to have higher revision rates overall than men. “With a head size of 36 mm, women age 60 years had a five-year revision rate of 5.1% compared with 3.7% for men age 60 years,” Smith and colleagues wrote.

Larger head diameters also appeared to increase the risk of metal-on-metal device failure. Each 1-mm increment in head diameter was associated with an approximately 2% greater risk of revision at any given time, the researchers calculated.

As a result, according to their model, a 60-year-old man receiving a metal-on-metal implant with a 28-mm head had a five-year risk for revision of 3.2% versus 5.1% for a 52-mm head.

Ceramic-on-ceramic bearings, on the other hand, were less likely to fail with increasing head diameters. Smith and colleagues estimated that a 28-mm head was associated with a 3.3% five-year failure rate, compared with 2% for a 40-mm head placed in a 60-year-old man.

Why the larger head diameter in metal-on-metal devices should be more prone to failure remains unclear, the researchers stated. One possibility is that larger heads increase torque in the stem, leading to early loosening, Smith and colleagues wrote.

Alternatively, metal debris resulting from trunnion wear (the post that inserts into the head) or uneven lubrication across the bearing surface may promote local inflammation and soft tissue reactions.

Metal ions leaching from the devices might provoke systemic reactions as well, the researchers noted.

“In conclusion, analysis of National Joint Registry data provides unequivocal evidence that metal-on-metal stemmed prostheses are associated with higher failure rates than other types, and that use of large head sizes, and use of these prostheses in younger women, is associated with a particularly high rate of early revision surgery,” they wrote.

“We therefore recommend that metal-on-metal bearing surfaces are not used in stemmed total hip replacements,” they added, suggesting that all patients who have received such devices undergo clinical and radiographic exams every year.

The U.K.’s Medicines and Healthcare Products Regulatory Agency recently issued a similar recommendation, calling for annual clinical exams with blood testing for metal ions.

Finally, Smith and colleagues indicated that ceramic bearings would be most appropriate for patients needing large-head implants.

In an accompanying commentary, Art Sedrakyan, MD, PhD, of Weill Cornell Medical College in New York City, argued that the metal-on-metal implant story had revealed the inadequacy of medical device regulation in the U.S. as well as in Britain and elsewhere.

Sedrakyan called the FDA‘s 510(k) pathway “outdated and low-threshold” and suggested that it and its European counterparts “have created an environment that makes regulators vulnerable to errors.”

He criticized the lack of rigorous requirements for pre- and postmarketing studies of most devices, including most hip implants, which meant that many thousands of metal-on-metal devices were placed before their propensity to fail became apparent.

An additional problem in the U.S., Sedrakyan suggested, is a political climate that pressures the FDA “not to stifle innovation.”

As a result, he wrote, the U.S. device market now boasts “tens of thousands of so-called innovative hip-device components … it would not be surprising if metal-on-metal hip prostheses were just the tip of a device-safety iceberg yet to be revealed.”

And even just considering the problems with metal-on-metal devices, Sedrakyan concluded, the U.S. can look forward to “the burden of further surgical treatment as well as billions of dollars in costs to taxpayers.”

The study was funded by the National Joint Registry of England and Wales.

One study co-author reported receiving royalties from DePuy for a hip prosthesis. Other authors and Sedrakyan declared they had no relevant financial interests.