Lubricant Research Could Lead to Improved Metal-on-Metal Implant Alloys

If you're among the many Americans that have metal-on-metal implants, you have a vested interest in knowing what's under the hood, so to speak. According to a study funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), part of the National Institutes of Health (Bethesda, MD), what's under the hood of metal-on-metal hip implants is graphitic carbon lubricant.

Metal-on-metal implants were developed as a substitute for combination metal- and-polyethylene implants, which were used almost exclusively before the 1990s but tended to deteriorate over time. The problem is that metal-on-metal implants are also not without problems. Some metal-on-metal systems have not performed well, says Joshua J. Jacobs, lead investigator and chair of the department of orthopedic surgery at Rush University Medical Center (Chicago). "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." The study was initiated in an effort to stop erosion between the metal surfaces of hip implants.

Earlier research by team members Alfons Fischer, professor of materials science and engineering at the University of Duisburg-Essen (Germany), and Markus Wimmer, associate professor of orthopedic surgery at Rush, had already revealed that a lubricating layer forms on metallic joints as a result of friction. To better understand this layer and improve artificial joints, the scientists relied on the science of tribology, which studies friction, lubrication, and wear.

"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," Wimmer notes.

While the researchers knew little about the layer, they assumed that it consisted of proteins in the body that entered the joint and somehow adhered to the implant surface--in other words, that it was similar to the lubrication present in natural joints. However, they discovered that the layer actually consists at least in part of graphitic carbon, a solid lubricant used in industrial applications. Knowing that the lubrication is graphitic carbon could enable scientists to manipulate the system to produce graphitic surfaces. "We now have a target for how we can improve the performance of these devices," Fischer says. Wimmer adds, "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 researchers' next task is to relate their findings with clinical outcomes by examining the surfaces of retrieved devices and correlating their observations with the reasons why the implants were removed. They also hope to learn how cells are affected if the graphite flakes off.

"This finding opens new avenues of investigation to help scientists understand how joint implants function and to develop strategies to make them function better," remarks NIAMS director Stephen I. Katz. "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."