Researchers were recently surprised to find that friction between moving parts of implanted metal-on-metal hip replacements creates a surface coating that contains graphite carbon, a material more commonly used in industrial lubricants.
In a study funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH), researchers used the science oftribology to study the friction, lubrication and wear of metal-on-metal hip implants removed from patients who needed a revision surgery.
Previous research by several of the study’s authors found that friction of metallic joints creates a layer of lubrication on their surface. They assumed the proteins in the body entered the artificial joint and developed a layer of lubricant much like that of a natural joint. However, when the team of researchers in the current study analyzed the lubricant layer found on hip implants removed from patients, they found graphite carbon is a main ingredient.
Graphite, a form of carbon occurring naturally in the earth and made synthetically, is used in industries such as the steel industry and as a lubricant in applications where wet lubricants such as oil cannot be used because graphite particles slide over each other easily.
“This was quite a surprise, but the moment we realized what had happened, many things suddenly started to make sense,” said study author Laurence Marks, Ph.D., of the Department of Materials Science and Engineering at Northwestern University in Evanston, IL.
In the last few years, there have been problems with several metal-on-metal hip replacement devices, including a recall of the DePuy ASR hip implant and removal of the Zimmer Durom Cup from the market due to higher than expected failure rates.
“We know there are metal-on-metal systems that have not performed well,” said Joshua J. Jacobs, M.D., lead investigator and chair of the Department of Orthopaedic Surgery at Rush University Medical Center in Chicago, IL. “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.”
Researchers will now compare reasons why these patients needed their hip replacement devices removed to the examination of implant surface. They also hope to determine the potential side effects any flaking graphite debris could cause.
“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 study author Markus Wimmer, Ph.D., associate professor from the Department of Orthopedic Surgery at Rush University Medical Center.
The study Graphitic Tribological Layers in Metal-on-Metal Hip Replacementsappears in the December 23, 2011 issue of Science.