Such "metal-on-metal" hips were developed to be more durable than traditional implants that combined metal and polyethylene for its ball-in-socket structure.
But a recent study sponsored by the U.S. Food and Drug Administration showed that patients who received all-metal hips were more likely to need repeat surgery than those who got implants made of other materials. Last year, Johnson & Johnson's DePuy Orthopaedics Inc unit issued a massive recall of a metal-on-metal hip that failed at a higher-than-expected rate.
In a new study, U.S. and German scientists have found graphitic carbon in a lubricating layer that forms on the surfaces of the ball and socket in implanted metallic hips as a result of friction when the two surfaces slide together. Previous research had identified the existence of the layer, but it was assumed to be made of proteins.
The substance is more like the lubricant in a combustion engine than a natural joint and paves the way for new approaches to designing the devices that could lead to implants that work better and last longer, the researchers said.
"Graphite has been used as a lubricant for over a century," said study co-author Laurence Marks, a professor of materials science and engineering at Northwestern University near Chicago. "It is a classic lubricant, and it appears to form naturally."
The researchers included engineers and physicians from Northwestern, Rush University Medical Center in Chicago and the University of Duisburg-Essen in Germany. Using tools including electron and optical microscopes, they studied seven implants that were retrieved from patients for various reasons.
Their findings could help scientists design future metal hips that are less vulnerable to wear and corrosion, Marks said in an interview. For example, device makers could try to enhance the formation of the graphite or develop a way to make it adhere better to the metal, he said.
"Now that we have a handle on how they are working and why they are working well, we can start to design them to make them better," Marks said.
The study, funded by the National Institutes of Health, was published in the journal Science.
Metal-on-metal hips were designed to wear longer and can accommodate larger femoral heads, which lowers the risk of hip dislocation. But they produce debris in the body that can release metal ions into the blood stream.
More than 450,000 Americans, most with severe arthritis that limits their ability to walk, undergo hip replacement each year. The devices, made of combinations of metals, polymers and ceramics, typically last more than a decade, but their failure rates rise beyond 10 years.
Manufacturers include Zimmer Holdings Inc, Wright Medical Group Inc, Smith & Nephew Plc and DePuy.
[The study will be published Dec. 23 by the journal Science. the coauthors are Josh Jacobs and Laurence Marks
Prosthetic materials for hips, which include metals, polymers and ceramics, have a lifetime typically exceeding 10 years. However, beyond 10 years the failure rate generally increases, particularly in young, active individuals. Physicians would love to see that lifespan increased to 30 to 50 years. Ideally, artificial hips should last the patient’s lifetime.
“Metal-on-metal implants can vastly improve people’s lives, but it’s an imperfect technology,” said Laurence D. Marks, a co-author on the paper who led the experimental effort at Northwestern. “Now that we are starting to understand how lubrication of these implants works in the body, we have a target for how to make the devices better.”]