For many people with severe hip arthritis, total joint replacement brings pain relief and improved mobility for the life of the implant – typically 15 years or more. But a new study shows that some metal-on-metal implants are likely to fail much sooner, especially in women. The study adds to a growing list of problems linked to metal-on-metal implants, including bone and tissue destruction and high levels of metal ions in the blood.

For the study, published online in 2012 in The Lancet, researchers used data from the National Joint Registry of England and Wales – the world’s largest joint replacement registry – to track more than 400,000 patients who underwent primary hip replacement from 2003 to 2011. Of these, 31,171 received metal-on-metal implants – hips with a ball and cup made of a cobalt and chromium alloy.

Compared with implants made of other materials, all-metal joints had a substantially higher overall failure rate. After five years, 6.2 percent of metal-on-metal hips had failed, whereas only 3.2 percent of ceramic hips and 1.7 percent of metal-on-plastic implants had. Failure rates were highest for younger women and for implants with larger heads – 36 millimeters or more – in both men and women. The head is the ball of the device that fits into the hip socket.

Joel Buchalter, MD, a clinical assistant professor of orthopaedic surgery at New York University Medical Center, explains that manufacturers developed metal-on-metal implants with larger heads to prevent the problems that lead to early failure. 

Hip replacements can fail for several reasons, but the most common is loosening, in which components detach from the bone. Friction from normal wear produces debris that causes inflammation in the tissues around the joint. Over time, bone erodes and the implant loosens, leading to pain and decreased function. Implants can also dislocate – meaning the metal ball slips out of the socket – often within the first few weeks after hip replacement. Failed hips must be removed and replaced in a complex revision surgery. This operation is often less successful than the original hip replacement because bone loss makes the new implant harder to anchor.

Dr. Buchalter says the metal-on-metal devices were targeted at active younger adults who needed a hip that lasted decades. “In mechanical models, metal-on-metal implants with large-diameter heads showed extremely low wear and better lubrication, so they seemed an excellent solution,” he says.

The Lancet study authors thought so, too, until they looked at the data.

“We were surprised by the results, as studies suggest that larger heads should have … lower wear and a lower risk of dislocation. However, we have seen an increase in revisions for pain and loosening, particularly in women,” says Alison Smith, a study author and statistician at the University of Bristol in the U.K.

Depending on patient age and implant head size, the revision rate for all-metal hips is three to four times higher in women and two to three times higher in men compared with other implants.

Ashley Blom, MD, a study author and professor of orthopaedic surgery at the University of Bristol, suggests that hip anatomy or the presence of osteoporosis (which is more common in women) might account for the greater number of problems in women. He adds that several small studies show women also are more sensitive to the metal ions shed by the implants.