Hip replacement surgery has become a common procedure for people in advanced stages of arthritis, but the implants that are used continue to suffer from relatively short lifetimes that often lead to further surgeries. A central mystery of their failure has been the nature of the lubricating layer that forms on both the ball and socket of the joint. Metal-on-metal implants have demonstrated lower wear than metal-on-polymer devices and can be made with larger femoral heads to prevent dislocations, but the reason for the extended lifetime has not been clear.
Now researchers at Northwestern University, Rush University Medical Center, Chicago, and the University of Duisburg-Essen Germany have reported in the journal Science that, unlike previously thought, graphitic carbon seems to be a key ingredient in the lubricating layer that forms around the implants. This surprising discovery should help implant designers develop longer lasting devices that take advantage of the graphitic carbon lubricant.
Earlier research by team members Alfons Fischer at the University of Duisburg-Essen and Markus Wimmer at Rush University Medical Center discovered that a lubricating layer forms on metallic joints as a result of friction. Once formed, the layer reduces friction as well as wear and corrosion. This layer is called a tribological layer and is where the sliding takes place, much like how an ice skate slides not on the ice but on a thin layer of water.
But, until now, researchers did not know what the layer was. (It forms on the surfaces of both the ball and the socket.) It had been assumed that the layer was made of proteins or something similar in the body that got into the joint and adhered to the implant’s surfaces.
The interdisciplinary team studied seven implants that were retrieved from patients for a variety of reasons. The researchers used a number of analytical tools, including electron and optical microscopies, to study the tribological layer that formed on the metal parts. (An electron microscope uses electrons instead of light to image materials.)
The electron-energy loss spectra, a method of examining how the atoms are bonded, showed a well-known fingerprint of graphitic carbon. This, together with other evidence, led the researchers to conclude that the layer actually consists primarily of graphitic carbon, a well-established solid lubricant, not the proteins of natural joints.
Press release: Hips that Function Better and Last Longer
Abstract in Science: Graphitic Tribological Layers in Metal-on-Metal Hip Replacements