DURHAM -- Arthritic knees, it turns out, try hard to repair themselves.
That finding by a team of Duke and UNC-Chapel Hill researchers, published Friday in the Journal of Biological Chemistry, suggests that if a method can be found to halt the breakdown of knee tissue, then the body itself may be able to reverse damage from osteoarthritis. Such treatments are already under study.
The cartilage in knees can wear down significantly over time, and obesity and common sports-related injuries can speed the process. An unrelated study released Friday shows that more than 4 million Americans have had at least one knee replacement, and about 500,000 of those have had more than one.
That data, from a study led by a Harvard researcher, is the first national estimate of the number of knee replacements.
Arthritis is the main reason for knee replacement, which cost on average about $40,000. Modern knee replacements, which date to the 1970s, have been improving and can greatly enhance a recipient's quality of life.
But they don't always work well and can need replacement themselves after wearing out.
That is particularly troubling for the increasing number of people in their 40s and 50s who are getting the surgery, often after sports-related injuries or obesity accelerate wear on the joint. Many could live long enough to need a second or even third replacement of the same joint.
The Duke-led study also looked at hip joints, which did not show a similar robust repair response. That could mean that arthritic hip joints would have to be treated differently, with some method to stop degeneration and another technique to trigger a response that would help fix the joint.
"At least with the knee you've got an ongoing repair response that we didn't appreciate until now," said Dr. Virginia Kraus, a professor of Rheumatology and Immunology at Duke, who led the research team. "If you could capitalize on that and turn off the degradative (breakdown) processing, you might have some good effects."
The researchers used new tools they developed to analyze knees and hips and found that osteoarthritic knee joints are in a constant state of repair, but hip joints are not.
The knee is easily accessible for injections, so if agents are found that can turn off the processes that erode the joint, they perhaps could be delivered periodically with close monitoring, Kraus said.
The researchers now plan to investigate the reasons for the difference between how knees and hips handle degeneration, and also look at the ankle joint's approach to repairing itself.
