March 12, 2012 -- Data from the world’s largest joint replacement registry show that troubled metal-on-metal hip implants have early failure rates that are two to four times higher than implants made with other kinds of materials.
The new study, which is published in the Lancet, shows that high early failure rates for all-metal implants are not related to a single design or manufacturer, but may be related to the entire class of devices.
“We thought it was very important to get the message across that this is not a single-brand problem,” says researcher Ashley W. Blom, MD, PhD, a professor of orthopedic surgery at the University of Bristol in the U.K.
Metal-on-metal implants are under scrutiny in both the U.K. and the U.S., though they remain on the market.
Blom, who is also a practicing orthopedic surgeon, says: “I would not implant stemmed metal-on-metal implants on my patients now with the knowledge that I have. There are other options that do better.”
Last month, the British Medicines and Healthcare Products Regulatory Agency advised patients with all-metal implants that they will need to be closely monitored for problems, getting annual exams for as long as 15 to 20 years.
Last May, the FDA ordered 21 makers of all-metal hips sold in the U.S. to conduct new safety studies.
One all-metal hip, the ASR system, was recalled by its manufacturer, DePuy, in 2010 because of high failure rates. The recall affected about 40,000 patients in the U.S. The company continues to monitor those patients and is picking up the cost of some expenses related to testing and further surgeries.
Metal Implants vs. Non-Metal
As many as one-third of patients who got hip implants in the U.S. in recent years received an all-metal replacement, in which both the ball of the joint and the socket are made from metal, typically chromium or cobalt.
When the metal parts rub together, they can release tiny particles into the blood and surrounding tissue. The particles can damage the area around the joint, causing pain and loosening of the implant, according to the FDA.
The new study, which is published in the Lancet, tracked more than 31,000 metal-on-metal replacement hips implanted in patients in the U.K. between 2003 and 2011.
The registry also includes information on more than 371,000 artificial hips made from other materials.
The study excluded the recalled ASR implants.
Compared to patients who got hips made with ceramic and plastic weight-bearing surfaces, patients who got all-metal hip replacements were significantly more likely to need revision surgery within the first few years of use.
The study shows 6.2% of all-metal implants required corrective surgery, usually to replace the implant, within five years.
“The [revision rate for] metal-on-metal in women is an order of three to four times higher, and in men, it’s an order of two to three times higher, depending on their [implant] head size and the age of the patient,” Blom says.
Blom says it’s not clear why women seem to be having more problems with metal implants, but there are some working theories.
“Women do seem to be more sensitive to the metal,” he says.
Anatomy might also be a factor.
“It might be that their implants might be implanted at slightly different angles,” creating more wear on the joint surfaces, says Blom. And because more women than men suffer from osteoporosis, they often start with weaker bones to support their new joints. That bone may break down more quickly if the new joints cause inflammation.
The problems also appear to be worse for patients who got all-metal implants made around 2004, when many manufacturers changed their implant designs to increase the size of the head. The head is the ball of the joint that fits into the hip socket.
Companies hoped larger heads would make the implants safer for patients since mechanical models showed they could reduce wear and make it less likely that the joints would dislocate. Dislocation is a major reason people need corrective surgeries.
Instead, however, the new study finds implants with the largest heads are failing faster than those that aren’t as big.
Researchers said they were particularly shocked by that finding. “The head-size effect, we weren’t expecting it to be going in almost the opposite direction as what was predicted,” says researcher Alison J. Smith, MSC, who works with Blom in the department of orthopedic surgery at the University of Bristol.
Keep It in Perspective
Experts who were not involved in the study had mixed reactions to the results.
Joshua J. Jacobs, MD, chairman of the department of orthopedic surgery at Chicago’s Rush University, and a vice president of the American Academy of Orthopedic Surgeons, said the findings should be kept in perspective.
“This clearly is a higher [failure] rate in some metal-on-metal large head-hip replacements compared to other bearing combinations,” Jacobs says.
But, he says, the vast majority of patients still have good results with their metal implants after five years.
“It’s not like half of the implants are failing or even a quarter or even 10%. It’s in the area of 6%. That’s not what we as orthopedic surgeons want. We would like a 0% failure rate at five years. That’s what we strive for in our patients, and we know we can’t get that just because of the nature of the surgery,” says Jacobs, who has consulted for orthopedic manufacturers in the past. He says he hasn’t received any personal support from device manufacturers in more than a year.
But other studies have shown that failure rates for metal-on-metal implants rise the longer a patient has the joint.
Some studies have shown nearly twice as many patients with metal-on-metal implants need corrective surgeries as patients who get joints made from other materials.
Jacobs says patients with metal-on-metal implants should return to their surgeon for regular checkups, even if they aren’t having any problems.
Patients who are having pain or difficulty walking or moving should see their doctors immediately.
Other experts think the issue is more urgent.
“We have half a million patients who have metal-on-metal hips in the U.S. We need to focus investigation on this issue,” says Art Sedrakyan, MD, PhD, an orthopedic surgeon at Weill Cornell Medical College in New York.
Sedrakyan wrote a commentary on the study but was not involved in the research.
Based on this study and mounting evidence against metal-on-metal designs, Sedrakyan says use of the implants should stop.
“The risk is too high to continue to use metal-on-metal implants,” he says.
WebMD contacted Zimmer and DePuy, two manufacturers of metal-on-metal implants, for comment on the study. WebMD also reached out to an industry group that represents medical device manufacturers, the Medical Device Manufacturers Association. They didn't respond to requests in time for publication.