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

The Food and Drug Administration holds a two-day meeting starting Wednesday to scrutinize the safety of metal-on-metal hip implants, following years of patient reports of pain and swelling that sometimes requires removal of the devices.

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 with metal stems and sockets. 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. appears to show just the opposite. In March, British experts at the world’s largest artificial joint registry told doctors to stop using metal-on-metal hip replacements, citing an analysis showing they have to be replaced more often than other implants.

Hip replacements are supposed to last between 10 to 15 years, but more than 6 percent of patients with metal hips needed them replaced after less than five years. That compared with just 2 percent of people who had ceramic or plastic joints. Both types of devices are prescribed for people suffering hip pain and limited movement due to arthritis or injury.

British regulators now recommend that people who have the implants get yearly blood tests to make sure no dangerous metals are seeping into their bodies as the components rub against each other.

The FDA has not made any recommendations of its own for the estimated 500,000 American patients with the devices. FDA scientists say they want to consider all available information before making their recommendations — not just the U.K. data.