Metal hip replacements implanted in a half-million Americans appear to be failing earlier than expected, but it could be years before U.S. health regulators have a clear picture of the scope of the problem.

The Food and Drug Administration holds a two-day meeting this week to scrutinize the safety of metal-on-metal hip implants, following years of patient reports of pain and swelling.

For decades nearly all orthopedic implants were made from plastic or ceramic. But in the last 10 years some surgeons began to favor implants made of metal. Laboratory tests suggested the devices would be more resistant to wear and reduce the chances of dislocation. But recent data gathered by surgeons in the U.K. and other countries appears to show just the opposite.