Researchers uncover clues related to metal-on-metal hip implants

A new study, bringing together an interdisciplinary team of physicians and engineers from the United States and Germany, made a surprising finding about implants used in hip replacement surgery: Graphite carbon is a key element in the lubricating layer that forms on metal-on-metal hip implants. The lubricant has more in common with the lubrication of a combustion engine than that of a natural joint. The study was funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), part of the National Institutes of Health.

Made possible by an American Recovery and Reinvestment Act grant and to be reported online in the Dec. 23 issue of Science, "This finding opens new avenues of investigation to help scientists understand how joint implants function, and to develop strategies to make them function better," said NIAMS Director Stephen I. Katz, M.D., Ph.D. "The results of such research could have important implications for several hundred 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."

Touted as one of the greatest advances in arthritis treatment in history, hip replacement involves removing the damaged hip and replacing it with a prosthesis to mimic the natural ball-in-socket joint.
"For most people, the procedure brings relief from pain and a return to normal function for the life of the prosthesis, typically more than 10 years," said Joshua J. Jacobs, M.D., lead investigator and chair of the Department of Orthopaedic Surgery at Rush University Medical Center in Chicago. But for younger, more active people, the prostheses' limited longevity often means postponing surgery—often for a number of years, or having the surgery and facing the prospect of a more difficult repeat surgery at some point when the prosthesis fails. For that reason, scientists have sought ways to improve the materials used.

One such way has been to design components with only metal-bearing surfaces (so called metal-on-metal implants) rather than a combination of metal- and polyethylene-bearing surfaces that were used almost exclusively prior to the 1990s, and tended to break down over time. But metal-on-metal implants, too, have issues.

"We know there are metal-on-metal systems that have not performed well," said Jacobs. “Problematic devices have tended to release more metal debris through wear and corrosion than devices that have performed well. This debris can cause a local tissue response involving the bone, ligaments, tendons and muscles around the hip.”

To better understand what happens in the artificial joints—and consequently what might be improved upon—the scientist turned to metal joint components that had been removed in revision surgeries and a science called tribology, which focuses on the phenomenon of friction, lubrication and wear.
Earlier research by team members Alfons Fischer, Ph.D., professor of materials science and engineering at the University of Duisburg-Essen, Germany, and Markus Wimmer, Ph.D., associate professor of orthopaedic surgery also at Rush, revealed that a lubricating layer forms on metallic joints as a result of friction.

"There is good reason to believe that those layers form a barrier to wear and corrosion on the surfaces of these implants, so it certainly would behoove us to understand the nature of these tribological reaction layers — what they are made of, how they form, etc. — so that we may be able to use this information to design metal-on-metal bearings going forward that are far less susceptible to corrosion and wear," said Wimmer.

While researchers knew little about the layer, they assumed that it was from proteins in the body that entered the joint and somehow adhered to the surface of the implant. As such, it would be, similar to lubrication in natural joints.

Instead, the scientists found that the layer actually consists, at least in part — and perhaps in large part — of graphitic carbon, a solid lubricant with industrial applications. "This was quite a surprise, but the moment we realized what had happened, many things suddenly started to make sense," said Laurence Marks, Ph.D., professor of materials science and engineering at Northwestern University, whose team led the experimental effort. "Knowing that the structure is graphitic carbon really opens up the possibility that we may be able to manipulate the system in such a way as to produce graphitic surfaces. We now have a target for how we can improve the performance of these devices," said Fischer. Marks is equally optimistic. "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."

The next phase, Jacobs said, is to relate that finding with clinical outcomes — by examining the surfaces of retrieved devices and correlating the observations with the reason for removal. Marks also hopes to learn how cells are affected if the graphite flakes off.
"As good as hip replacements are for people in their 60s and 70s, for people who are younger, and more active, there are still question marks," said Jacobs. "We are making a lot of demands on the materials we are using if we want them to last 30 or 40 years."