A new study, bringing together an interdisciplinary team ofphysicians and engineers from the United States and Germany, made asurprising finding about implants used in hip replacement surgery:Graphite carbon is a key element in the lubricating layer thatforms on metal-on-metal hip implants. The lubricant has more incommon with the lubrication of a combustion engine than that of anatural joint. The study was funded by the National Institute ofArthritis and Musculoskeletal and Skin Diseases (NIAMS), part ofthe National Institutes of Health. Made possible by an American Recovery and Reinvestment Act grantand to be reported online in the Dec. 23 issue of Science, "Thisfinding opens new avenues of investigation to help scientistsunderstand how joint implants function, and to develop strategiesto make them function better," said NIAMS Director Stephen I. |
Katz,M.D., Ph.D. "The results of such research could have importantimplications for several hundred thousand Americans who undergo hipreplacement each year - as well as those who could benefit from theprocedure, but have been advised by their doctors to delay surgeryuntil they are older." Touted as one of the greatest advances in arthritis treatment in history, hip replacement involves removing thedamaged hip and replacing it with a prosthesis to mimic the naturalball-in-socket joint. "For most people, the procedure brings relief from pain and areturn to normal function for the life of the prosthesis, typicallymore than 10 years," said Joshua J. Jacobs, M.D., lead investigatorand chair of the Department of Orthopaedic Surgery at RushUniversity Medical Center in Chicago.
But for younger, more activepeople, the prostheses' limited longevity often means postponingsurgery - often for a number of years, or having the surgery andfacing the prospect of a more difficult repeat surgery at somepoint when the prosthesis fails. For that reason, scientists havesought ways to improve the materials used. One such way has been to design components with only metal-bearingsurfaces (so called metal-on-metal implants) rather than acombination of metal- and polyethylene-bearing surfaces that wereused almost exclusively prior to the 1990s, and tended to breakdown over time. But metal-on-metal implants, too, have issues.
"We know there are metal-on-metal systems that have not performedwell," said Jacobs. "Problematic devices have tended to releasemore metal debris through wear and corrosion than devices that haveperformed well. This debris can cause a local tissue responseinvolving the bone, ligaments, tendons and muscles around the hip." To better understand what happens in the artificial joints - andconsequently what might be improved upon - the scientist turned tometal joint components that had been removed in revision surgeriesand a science called tribology, which focuses on the phenomenon offriction, lubrication and wear. Earlier research by team members Alfons Fischer, Ph.D., professorof materials science and engineering at the University ofDuisburg-Essen, Germany, and Markus Wimmer, Ph.D., associateprofessor of orthopaedic surgery also at Rush, revealed that alubricating layer forms on metallic joints as a result of friction. "There is good reason to believe that those layers form a barrierto wear and corrosion on the surfaces of these implants, so itcertainly would behoove us to understand the nature of thesetribological reaction layers - what they are made of, how theyform, etc.
- so that we may be able to use this information todesign metal-on-metal bearings going forward that are far lesssusceptible to corrosion and wear," said Wimmer. While researchers knew little about the layer, they assumed that itwas from proteins in the body that entered the joint and somehowadhered 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, atleast in part - and perhaps in large part - of graphitic carbon, asolid lubricant with industrial applications.
"This was quite asurprise, but the moment we realized what had happened, many thingssuddenly started to make sense," said Laurence Marks, Ph.D.,professor of materials science and engineering at NorthwesternUniversity, whose team led the experimental effort. "Knowing thatthe structure is graphitic carbon really opens up the possibilitythat we may be able to manipulate the system in such a way as toproduce graphitic surfaces. We now have a target for how we canimprove the performance of these devices," said Fischer. Marks isequally optimistic. "Nowadays we can design new alloys to go inracing cars, so we should be able to do this for implants that gointo human beings." The next phase, Jacobs said, is to relate that finding withclinical outcomes - by examining the surfaces of retrieved devicesand correlating the observations with the reason for removal.
Marksalso hopes to learn how cells are affected if the graphite flakesoff. "As good as hip replacements are for people in their 60s and 70s,for people who are younger, and more active, there are stillquestion marks," said Jacobs. "We are making a lot of demands onthe materials we are using if we want them to last 30 or 40 years." For more information on hip replacement, visit www.niams.nih.gov/Health_Info/Hip_Replacement/default.asp . Additional References Citations.
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