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Lubricant in metal-on-metal hip implants found to be graphite

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Graphite carbon is a key element in the lubricating layer that forms in metal-on-metal hip implants, according to a study recently published in Science. The study was funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases, part of the National Institutes of Health (NIH).

“We know there are metal-on-metal systems that have not performed well,” Joshua J. Jacobs, MD, lead investigator and chair of the Department of Orthopaedic Surgery at Rush University Medical Center in Chicago, stated in a NIH news release. “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.”

Jacobs and his fellow researchers took a tribological approach to looking at metal joint components that had been removed in revision procedures.

According to the release, previous research had revealed that a lubricating layer forms on metallic joints as a result of friction. Although researchers knew little about this lubricating layer, they assumed it was from proteins entering the joint and adhering to the surface of the implant — recreating, in a way, the lubrication seen in natural joints. The researchers found, however, was that the layer actually consists in part of graphitic carbon.

“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,” co-author Alfons Fischer, PhD, stated in the release. “We now have a target for how we can improve the performance of these devices.”

The next phase, Jacobs noted in the release, is relating the new findings with clinical outcomes by examining the surfaces of retrieved devices and correlating observations with the reason for removal.

Reference:

• Liao Y, Pourzal R, Wimmer MA, et al. Graphitic tribological layers in metal-on-metal hip replacements. Science. 2011. doi: 10.1126/science.1213902

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