A lubricating layer made of graphitic carbon naturally forms in the joints of metal-on-metal hip implants, a new study shows. This solid layer, produced within the body, is more like an industrial lubricant than joint fluid. The finding may help researchers design longer-lasting metal-on-metal hips for treating arthritis and other joint disorders. The most common hip implants are made of metal and polyethylene, a form of plastic. Over time, however, metal and plastic joints wear down, and broken-off bits can damage the remaining bone and tissue of the hip. Over the last 10 years, all-metal joints have become increasingly popular, as they are generally more stable and durable. In some cases, however, metal-on-metal implants can still shed damaging metal debris through wear and corrosion. Metal-on-metal joints aren't designed with lubrication but, with use, a thin layer appears in the joint between the ball and socket. This layer, which forms between the 2 rubbing metal faces, is known as a tribological layer. Researchers initially thought that it was made of proteins and other biological material, like the lubrication in a normal joint.
The researchers examined the tribological layer on 7 all-metal joints that had been removed from patients. They scraped off a bit of the layer and analyzed it by electron energy loss spectroscopy, a method that reveals the type of atoms present in a material. To their surprise, they discovered that the layer was made up in large part of graphitic carbon, with very little, if any, protein.
