A new, noninvasive, and low-cost method for the early detection and monitoring of osteoarthritis (arthritis caused by wear and tear) may be on its way, thanks to research by UC Santa Barbara scientists from the Department of Chemical Engineering and the Department of Materials.
By studying patterns of friction between cartilage pads, the researchers discovered a different type of friction that is more likely to cause wear and damage. Their work suggests ways to detect this friction, and points to new research directions for getting to the root cause of arthritis. The findings are published in the recent issue of the Proceedings of the National Academy of Sciences.
Imagine going to the doctor for your aching knees. For some, this may involve uncomfortable needle sticks to draw blood for lab tests, or the extraction of the fluid surrounding the aching joint. But what if your doctor could actually listen to your body, monitoring the way your knees sound as they bend and flex? Research by Jacob Israelachvili, UCSB professor of chemical engineering and materials science; grad student researcher Dong Woog Lee; and postdoctoral researcher Xavier Banquy says that it's possible.
For their work, they used an instrument called a Surface Forces Apparatus (SFA), a device that measures the adhesion and friction forces between surfaces, in this case cartilage -- the pad of tissue that covers the ends of bones at a joint. The degeneration of cartilage is the most common cause of osteoarthritis -- the pads wear away, leaving bone grinding against bone.
What the researchers found is that it isn't just any kind of friction that leads to the irreversible wear and tear on the material. "It is currently believed that a high-friction force, or 'coefficient of friction,' is the primary factor in surface wear and damage," said Israelachvili. "What we found is that this is not the case."
The critical feature is not a high-friction force, but what is known as "stick-slip" friction, or, sometimes, "stiction." Both are characterized by surfaces that initially stick together, and then accelerate away quickly once the static friction force is overcome. With stick-slip friction, the surfaces eventually pull slightly apart and slide across each other, stick again, and pull apart, causing jerky movements.
"That's when things get damaged microscopically," said Israelachvili.
Stick-slip is a common phenomenon. It is responsible for everything from computer hard drive crashes and automobile failures, to squeaking doors and music.
"The same thing happens with a violin string: Even if you're pulling the bow steadily, it's moving in hundreds or thousands of little jerks per second, which determine the sound you hear," Israelachvili said. Each little jerk, no matter how submicroscopic, is an impact, and over time the accumulation of these impacts can deform surfaces, causing irreparable damage -- first microscopically, then growing to macroscopic (large, visible) dimensions. That's when you have to change the string.