Prosthetic materials for hips, which include metals, polymers, and ceramics, have a lifetime typically exceeding 10 years. However, beyond 10 years the failure rate generally increases.

Engineers and physicians from Northwestern University; Rush University Medical Center, Chicago; and the University of Duisburg-Essen, Germany, discovered that graphitic carbon is a key element in a lubricating layer that forms on metal-on-metal hip implants. The lubricant is more similar to the lubrication of a combustion engine than that of a natural joint.

The discovery offers a target for designing new materials for hip implants that are less susceptible to the joint's normal wear and tear.

Metal-on-metal implants are a lower wear alternative to metal-on-polymer devices, and allow for larger femoral heads, which can reduce the risk of hip dislocation. Metal-on-metal also is the only current option for a hip resurfacing procedure, an alternative to total hip replacement.

Earlier research 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.

Researchers did not know what the layer—which forms on the surfaces of both the ball and the socket—was. 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 retrieved from patients and used analytical tools, including electron and optical microscopes, to study the tribological layer. The electron-energy loss spectra, a method of examining how the atoms are bonded, showed a fingerprint of graphitic carbon. This, together with other evidence, led the researchers to conclude that the layer consisted primarily of graphitic carbon, a well-established solid lubricant, not the proteins of natural joints.

Knowing that the structure is graphitic carbon can help developers produce graphitic surfaces to improve the performance and longevity of future implants. "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," says principal investigator Joshua J. Jacobs, the William A. Hark, MD/Susanne G. Swift Professor of Orthopedic Surgery and professor and chair of the Department of Orthopedic Surgery at Rush.

Next, the researchers will examine the surfaces of retrieved devices and correlate the observations of the graphitic layer with the reason for removal and the overall performance of the metal surfaces. They hope to learn how graphitic debris from the implant might affect surrounding cells.

Northwestern University,