Metal Hips Fail Faster, Raise Other Health Concerns
Two large studies question the safety of metal-on-metal hip joints.
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.
Why all-metal hips fail more often still isn’t entirely clear, and the British researchers conclude that more research is needed. “We do not fully understand the mechanism of failure of these hips,” they say. “We need more … studies to shed some light on this.”
In 2010, device maker DePuy recalled its articular surface replacement hips, or ASR hips, after they failed in about 12 percent of patients who received them. The recalled ASR devices were excluded from the current study.
Metal-on-metal implants have raised other concerns in addition to early failure rates, including potential harm from cobalt and chromium ions released into the bloodstream when metal parts rub together. In February, the journal BMJ published results of a comprehensive investigation of metal implants. Among other findings, investigators concluded that systemic metal ions can cause cardiovascular problems and may damage DNA.
However, Art Sedrakyan, MD, an associate professor at Weill Cornell Medical College in New York, points out that no good studies of the short- and long-term effects of ions exist. “We don’t know what the consequences of even moderately elevated ion levels might be over time, and there is no clear evidence that they lead to cancer or cardiomyopathy or other complications. We just don’t know,” he says.
Dr. Sedrakyan, an authority on comparative effectiveness research, adds that in May 2011, the FDA ordered 21 makers of metal-on-metal hip implants to provide safety studies, including information on metal ion levels. “Once that data is available and independent experts can analyze it, we can make better decisions,” he says.
David Lewallen, MD, an orthopaedic surgeon at Mayo Clinic in Rochester, Minn., and board chairman of the newly launched American Joint Replacement Registry (a national center for data collection and research on total hip and knee replacements), says it’s also important to put current findings in context.
“Hip replacement is one of the most successful surgeries in the history of medicine, and the improvements it makes in the lives of patients are dramatic,” he says. The majority of people, including those with all-metal hips, do extremely well, he adds.
Still, many physicians and leading orthopaedic institutions, such as Rush University Medical Center in Chicago and the Mayo Clinic, have halted or scaled back use of all-metal implants.
But what about the 500,000 Americans who already have metal hips? Unlike Britain, no formal guidelines or protocols exist in the United States for dealing with metal-on-metal implants, so doctors are developing their own.
Dr. Lewallen urges all hip replacement patients to see their doctors at regular intervals, even if they have no symptoms. And he recommends surveillance of metal ion levels for patients with metal-on-metal implants, especially if they have kidney disease or other health problems. Ultimately, he says, care should be individualized to the needs of each patient.
Nicolas Noiseux, MD, an assistant professor of orthopaedics at the University of Iowa in Iowa City, also endorses a patient-specific approach.
“I see patients who have large-head metal implants once a year, and at the slightest complaint, I suggest testing for blood cobalt and chromium. If the numbers are seven parts per billion or above, I follow patients more closely than a yearly interval and may recommend further imaging, such as an MRI, to assess fluid accumulation around the joint. And certainly in that case, or in the face of metal ions above seven parts per billion, some patients have elected to have [the implant removed],” he explains.
Dr. Buchalter endorses a similar program for patients with metal hips, but believes annual monitoring for the life of the implant – now required in Britain – is essential. He says regular blood tests and MRIs can alert doctors and patients to impending implant failure.
Dr. Sedrakyan agrees that all options should be discussed in order to decide the best course for each patient. “But,” he says, “as more information accumulates, we will be able to frame these recommendations based on better evidence.”
As for the future, Dr. Lewallen says metal hips may even have a silver lining. “Patients are now aware that being seen regularly is crucial, and that awareness may help prevent more serious problems,” he says.
He adds that concerns about metal implants have helped increase interest in an American Joint Replacement Registry. Until recently, the United States lacked a large, nationwide registry to track long-term outcomes of joint replacments. Dr. Lewallen says that by uncovering problems sooner, registry data should improve patient safety and overall quality of care.