A study funded by the National Institutes of Health found that lubrication in metal-on-metal hip implants has more in common with the lubrication in a combustion engine than a natural joint.
The study, conducted by U.S. and German researchers, found that a lubrication layer that forms in metal-on-metal hip replacement devices is actually graphite carbon, not proteins as researchers previously suspected.
"Graphite has been used as a lubricant for over a century," study co-author Laurence Marks told Reuters. "It is a classic lubricant, and it appears to form naturally."
When they discovered the graphite lubricant, researchers from Northwestern, Rush University Medical Center and the University of Duisburg-Essen were analyzing 7 retrieved implants to find a way to stop erosion between their metal surfaces.
"This finding opens new avenues of investigation to help scientists understand how joint implants function, and to develop strategies to make them function better," Dr. Stephen Katz, director of the National Institute of Arthritis & Musculoskeletal & Skin Diseases, said in prepared remarks. "The results of such research could have important implications for several 100 thousand Americans who undergo hip replacement each year – as well as those who could benefit from the procedure, but have been advised by their doctors to delay surgery until they are older."
Hip replacements are one of the most common orthopedic surgeries in the U.S. More than 500,000 metal-on-metal implants have been performed so far, according to the FDA.
All-metal joints were designed to be more sturdy and accommodate larger femoral heads than the metal-polyethylene implants they replaced, but several studies found that patients who received the metal-on-metal implants were more likely to need repeat surgeries.
Due to the natural friction of the hip’s movement, metal particles can wear off the implants and enter patients’ bloodstreams, according to the federal watchdog agency.
"Problematic devices have tended to release more metal debris through wear and corrosion than devices that have performed well," lead investigator Dr. Joshua Jacobs said in prepared remarks. "This debris can cause a local tissue response involving the bone, ligaments, tendons and muscles around the hip."
Lawsuits over the implants have skyrocketed in recent months, topping 5,000 in 2011 – more than in the past 4 years combined.
The NIH researchers seemed optimistic about the lubricant discovery.
"Nowadays we can design new alloys to go in racing cars, so we should be able to do this for implants that go into human beings," Northwestern University professor Laurence Marks said in a prepared statement.
If the research doesn’t pan out, growing concern could put the devices on track to be the biggest and most expensive medical implant quagmire since Medtronic (NYSE: MDT) recalled its Sprint Fidelis lead in 2007.
All-metal joint replacements haven’t just been a target for the federal watchdog agency. The implant woes have garnered the attention of lawmakers as well.
Three U.S. senators are calling for increased safety measures for medical devices, spurred by the attention given to the recalls. Their proposed legislation would beef up the FDA’s safety regulations, allowing the federal watchdog agency to conduct safety studies of devices after they’ve been approved and to grant conditional approvals contingent on further trials.
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