Transmission electron micrograph shows a cross-section through the tribofilm and the
Metal-on-metal (MoM) hip-implant failures and recalls have increasingly occupied the attention of regulatory agencies, patients, the mass media, and a bevy of personal-injury lawyers. But orthopedic researchers have been no less busy, endeavoring to uncover how such implants work and why they fail. Centered at Rush University Medical Center (Chicago), Northwestern University (Evanston, IL), and the University of Duisburg-Essen (Germany), one such group of researchers has made the surprising discovery that a lubricant present in MoM hip implants is a carbonaceous material akin to industrial lubricants used in engines. This discovery, they hope, could eventually lead to the development of new alloys that can withstand the mechanical stresses experienced by metal orthopedic implants.
Over the last decade, the scientists have learned that the contact surfaces of cobalt-chromium-based MoM implant components develop a carbonaceous film that sometimes contains metallic and oxidized nanocrystals from the implant's cobalt-chromium matrix. "Originally, we identified this carbonaceous material as a protein, but we have since learned that it contains graphite," remarks Markus Wimmer, associate professor of orthopedics at Rush University Medical Center. "Because of the amount of this material, the carbon can originate only in the synovial fluid--the fluid that is present around the joint. It cannot originate in the metal implant, and it is neither carbides nor carbon released from the alloy itself."
Known as a tribofilm, this carbonaceous layer is generated as a result of tribochemical reactions that are assisted by the mechanical interactions that occur in the joint. While the researchers do not yet understand all the details of how this layer forms out of body fluid, it could arise from very high temperatures and shear stresses that are generated by the contacting asperities--roughnesses--in the artificial joint. "If you have roughness asperities, you can get considerably higher temperatures as a result of the high shear rates, and that can trigger reactions that wouldn't be possible otherwise," Wimmer says. "However, based on our analytical calculations, it appears that the heat caused by the contacting asperities is not sufficient to generate graphite out of proteinaceous material."
Another theory, according to the researchers, is that the graphitic material results from the tribological effects of friction, lubrication, and wear on the joint. When metal particles are shed off the surface of the implant during tribological contact, catalytic reactions can occur in which the metal assists in removing water from proteins, Wimmer explains. This process would leave a hydrogenated carbon film--a classic solid lubricant. With further sliding of the implant, graphitic material would form, a well-known occurrence in other applications in which carbons are used as solid lubricants. Although the scientists have not demonstrated this possibility yet, they say that these tribochemical reactions might be responsible for generating the film. Nevertheless, they are certain that without the tribofilm, metal-on-metal implants could not survive in the body.
Manufacturers of metal-on-metal implants were not initially aware that this tribofilm would develop in the body, Wimmer says. In fact, they originally turned to the use of cobalt alloys because such materials can resist abrasion caused by wear. But, as it turns out, the reason the alloys work has nothing to do with their abrasion resistance, according to Wimmer. Instead, they are successful because they can form a stable tribofilm on the implant surface; abrasion resistance is merely an added benefit.
"In the late 1990s, we knew that tiny cobalt-chromium particles ranging in size from 30 to 150 nm were present in the body of MoM implant patients," Wimmer notes. "But the grain size of the bulk was much larger, measuring in some cast materials up to 1 mm. Thus, we asked: How is it possible for such small particles to be shed if the grain size is so big?" Eventually, the scientists discovered that cobalt-chromium recrystalizes during tribological contact, forming nanocrystals on the metal surface, which are a prerequisite to forming the tribofilm. "Now we know why cobalt-chromium works in implants and why other metals do not," Wimmer adds.
Understanding that the tribofilm enables metal-on-metal implants to achieve ultramild wear rates--defined as the shedding of less than 1 cu mm of material per year--the researchers believe that other alloys could perhaps be developed that can minimize particle shedding. "Today, cobalt-chromium with molybdenum is used in metal-on-metal implants," Wimmer says, "but other metals such as newly developed high-nitrogen stainless steels may have even better properties." While the medical device industry is often reluctant to work with new materials, the understanding that tribofilm formation is linked to specific metallurgical and chemical properties could induce scientists to aid medical device manufacturers in redefining their goals to improve metal-on-metal orthopedic implants.