NIH researchers uncover clues related to metal-on-metal hip implants

Lubricant in devices found to be graphite, not proteins

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.

X-ray of the hip region with a metal-on-metal implant superimposed.

"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