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 scope 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. It is a challenging, but familiar, predicament for the FDA: reviewing the safety of a device that was expected to be superior, but which may actually be more dangerous than what came before.

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 United Kingdom 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.

U.S. regulators have not made any similar recommendations for the estimated 500,000 American patients who have received the devices.

“Why look at a single registry when there’s data from around the world?” said Dr. William Maisel, FDA’s chief scientist for medical devices. “This is an opportunity for us to look at all the available information so that we can have a thoughtful conversation about what clinical recommendations can be made.”

Maisel said the FDA is working to combine data from foreign countries and the U.S. to determine which groups of patients and implants are most problematic. The FDA will ask a panel of experts to recommend the best practices for monitoring patients with the devices. Panelists will consider blood tests, medical imaging and laboratory tests.

But some U.S. orthopedic specialists say they have already reached their own conclusions about metal hips.

“In my personal opinion there is very little room, if any, for metal-on-metal implants because the alternatives we have on the market are likely safer and as effective,” said Dr. Art Sedrakyan, professor of public health at Weill Cornell Medical College in New York.

The FDA’s deliberative approach to tackling the hip implant issue is in some ways a necessity. Unlike other countries, the U.S. has no national registry to track the performance of implants over time.

The FDA has received 16,800 event reports about metal hips between 2000 and 2011, but regulators stress that number is not very useful. Many doctors do not report problems to the FDA, and the volume of reports is influenced by news reports on safety issues.

A registry set up by Minnesota’s HealthEast Care System recorded four times as many replacement surgeries for patients with metal-on-metal hips as those with other implant types. However, a similar registry set up by health care provider Kaiser Permanente found no difference between the two groups.

With little definitive data on U.S. hip implants, the agency has asked manufacturers like Johnson & Johnson, Zimmer Holdings Inc. and Biomet Inc. to conduct long-term, follow-up studies of more than 100 metal-on-metal hips on the U.S. market. But Sedrakyan and others say it could be a decade or more before that information is available.

The prospect of safety findings arriving in eight or 10 years is little comfort to patients like Mary Weaver, 48, who had both hips replaced with a metal implant from Johnson & Johnson in 2007 and 2008. J&J recalled the ASR hip replacement in 2010, after reports that it was failing in some patients after only a few years after implantation.

Due to increasing pain and elevated metal levels in her blood, Weaver had both implants removed in 2011. She was let go from her job due to the time needed to undergo both surgeries, and is currently unemployed.

“I hope that no one has to go through this — it’s frustrating, it’s emotionally draining,” said Weaver, who lives in Mt. Jackson, Va. “It’s not just hard on you, it’s hard on your whole family because your quality of life is not what it used to be.”