The U.S. Food and Drug Administration (FDA) has proposed new rules that would require makers of metal-on-metal hip-replacement implants to either prove their products are safe or stop selling them. Serious problems associated with the all-metal implants – including high early failure rates and the shedding of microscopic metal particles in the body – have led to calls for stricter regulation of the devices.

Under the proposed rules, manufacturers would have 90 days to submit data showing that an implant is safe and effective. All-metal implants that don’t meet these requirements would be removed from the market.

Manufacturers, the public and others can weigh in on the proposals during a 90-day comment period, followed by FDA review of the feedback before the rules are finalized – a process the FDA estimates will take about a year.

Medical devices receive FDA approval in one of two ways. Devices considered high risk must be tested in human clinical trials to prove their safety and effectiveness. But for other devices, clinical testing can, and often is, waived if manufacturers can prove that their new device is roughly similar to one already in use – a fast-track process known as 510(k) clearance. Metal-on-metal hips – those with a ball and socket made of an alloy that includes chromium and cobalt – were approved via the 510(k) route.

Timothy Wright, PhD, a biomedical engineering expert at the Hospital for Special Surgery in New York, says the troubled history of all-metal hips highlights the flaws in the 510(k) clearance process. “It didn’t start out that way, but [the 510(k) process] has become a huge pipeline for [untested] devices to get on the market through the similar-equivalence route,” he says.

Indeed, between 2000 and 2011, more than 600 devices received 510(k) clearance – including all-metal hips, implantable defibrillators and transvaginal mesh – some of which later developed serious problems. In 2010, one all-metal hip implant, the DePuy A.S.R., was recalled when it failed in nearly 40 percent of patients within five years.   

Metal-on-metal implants aren’t equivalent to hips made of other materials, such as plastic or ceramics, or even to each other, says Nicolas O. Noiseux, MD, an orthopaedic surgeon and assistant professor in the College of Medicine at the University of Iowa in Iowa City. “Not all implants perform the same way or have the same design issues. There is a wide spectrum of metal-on-metal implants. Devices from different companies can be as different as night and day in terms of tolerance, engineering and metallurgy,” he says. 

Many all-metal hips had serious problems from the start, including high early failure rates. Designed to last longer than other implants, they have been shown in international studies to wear out sooner – in some cases, much sooner, especially in women and in patients with larger implants. Compared with other implants, all-metal hip replacement failure rates at seven years are about three times higher in women and two times higher in men.

Hips that fail must be removed and replaced in a costly and complex revision surgery that is often less successful than the original hip replacement.

Other problems associated with all-metal implants are caused by cobalt and/or chromium ions – minute particles produced when metal parts rub together. The ions can trigger inflammation that destroys tissue and bone in the hip joint, resulting in severe damage and long-term disability for some patients.

Metal ions can also enter the bloodstream and circulate throughout the body, and they can damage the heart and brain. There is no consensus about how much metal in the bloodstream is harmful, and the FDA has said it does not have enough data to specify a dangerous blood-ion level.