Carbon Layer Lubricates All-Metal Hips

The lubricant that forms to lessen friction in metal-on-metal hip replacements is made of a thin, graphitelike layer of carbon, not protein as researchers suspected (Science, DOI: 10.1126/science.1213902). That surprise might pave the way for long-lasting implants.

Ball-and-socket-shaped hip prostheses can be made from several different materials, including metal, plastic, and ceramics. The types thought to wear the least with time are metal-on-metal implants, with both ball and socket made from an alloy. But certain metal-on-metal implants trigger pain and inflammation and must be replaced.

To better understand what happens in an implanted metal-on-metal hip, materials scientist Laurence D. Marks of Northwestern University, orthopedic surgeon Joshua J. Jacobs of Rush University Medical Center, and colleagues focused on the joint’s surface, which is a hot spot for friction and wear.

Marks’s team scraped some of the lubricating layer from seven implants that had been removed from patients and took a close look with electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy. The researchers concluded that the lubricant’s major component is graphitic carbon. It’s not clear how the graphitic material forms, they say.

Matthew P. Kelly, an orthopedic surgeon at Kaiser Permanente Los Angeles Medical Center, thinks the work could lead to artificial hips that take longer to deteriorate. However, he’s not convinced the results add much to what’s known about why some metal-on-metal implants fail.

Jacobs and his team plan to examine many more implants to see whether they can correlate observations with patients’ outcomes after surgery.

Historically, analyzing implant wear and tear has been the domain of pathologists with light microscopes, says Timothy Wright, a biomechanical engineer at New York’s Hospital for Special Surgery. “What Marks and his team are saying is that we’ve got to think orders of magnitude smaller” to understand implants’ interactions with the body, he says.

“This report raises a lot more questions than it answers,” Wright adds. “But that’s usually what good science does.”