Discovery could improve hip replacement

U.S. researchers say a lubricant that builds up on metal-on-metal hip implants in the body has been found to be graphite, not proteins. Earlier studies found a lubricating film called a tribological layer forms on metallic joints as a result of their rubbing together, that reduces friction as well as wear and corrosion, but until now researchers did not know what the layer was. It had been assumed it was made of proteins or something similar in the body that got into the joint and adhered to the implant's surfaces. But researchers at Northwestern University, with American and European colleagues, studied implants retrieved from patients and discovered the layer actually consists primarily of graphitic carbon, a well-established solid lubricant, not the proteins of natural joints. "This was quite a surprise," Northwestern researcher Laurence D. Marks said, "but the moment we realized what we had, all of a sudden many things started to make sense." Metal-on-metal implants have advantages over other types of implants, principle investigator Joshua J. Jacobs said, providing a lower-wear alternative to metal-on-polymer devices. Prosthetic materials for hips -- which include metals, polymers and ceramics -- have a lifetime typically exceeding 10 years. Beyond that the failure rate generally increases, particularly in young, active individuals. "Now that we are starting to understand how lubrication of these implants works in the body, we have a target for how to make the devices better," Marks said. "Nowadays we can design new alloys to go in racing cars, so we should be able to design new materials for implants that go into human beings."