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Delays and difficulties in assessing metal-on-metal hip implants.

More than 500,000 U.S. patients have received metal-on-metal hip prostheses, most of which were implanted between 2003 and 2010. These prostheses entered the market through the 510(k) pathway at the Food and Drug Administration (FDA), whereby manufacturers need only demonstrate substantial equivalence to a device already on the market to gain approval. Unfortunately, there is now compelling evidence that these implants fail at a higher rate than hip prostheses made of other materials1; indeed, one type of metal-on-metal hip has a failure rate of nearly 50% at 6 years.2 Moreover, a number of unresolved questions related to these devices . . .