According to the agency, the problems with metal-on-metal implants are about the same as those seen with polyethylene and ceramic implants, except for the specific risks caused by the metal itself.
But others say that when it comes to complications, metal-on-metal implants are in a class of their own. Writing earlier this month in the New England Journal of Medicine, Dr. Joshua Rising, of the Pew Charitable Trusts, and colleagues said that "there is now compelling evidence that these implants fail at a higher rate than hip prostheses made of other materials; indeed, one type of metal-on-metal hip has a failure rate of nearly 50 percent at six years."
Of the estimated 400,000 hip replacements done in the United States each year, 27 percent involve metal-on-metal devices, according to the FDA.
Critics, including an independent panel of experts at the Institute of Medicine, have noted that metal-on-metal implants may have been approved too quickly under the FDA's "fast-track" program for medical devices.
Responding to concerns, in May 2011 the FDA ordered manufacturers to carry out "post-market" studies, tracking the safety of metal-on-metal hip implants in recipients. But in the NEJM article, Rising and colleagues noted that most of these studies have yet to begin and their first results will take years to arrive.
Metal-on-metal devices have already been recalled twice. In 2008, a device from manufacturer Zimmer, the Durom Acetabular Component, was recalled because instructions were not clear.
And in 2010 a device from DePuy Orthopaedics Inc. was recalled because more patients than expected had to have new surgeries, according to the FDA.
Earlier this month, a device called the R3 Acetabular System from Smith & Nephew was taken off the market because the company said it was "not satisf
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