“Cartilage doesn’t heal.” Doctors often refer to this as osteoarthritis, which damages the flexible tissues of the hips, knees, and shoulders, causing joint pain and movement. I’ve heard this myself from orthopedic surgeons who explain that the damaged cartilage lacks the blood supply to regenerate cells and nutrients. However, it always amazes me that living tissue cannot replace damaged cells. Recent studies have shown that our ability to repair articular cartilage, a type of joint, is limited. New insights into this ability hold promise for treatments that can improve healing and protect damaged cartilage from further deterioration.
To visualize articular cartilage, imagine a hard, white coating on the end of a chicken bone. Most of it is a spongy material called the extracellular matrix, a mixture of water and fibrous proteins secreted by cells called chondrocytes. “Like all tissues except tooth enamel, there is internal regeneration, where new tissue is formed and old tissue is chewed up and removed,” explains Virginia Kraus, MD, a rheumatologist at Duke University School of Medicine. But he pointed out that cartilage regeneration is slow. It is true that adult cells do not have a blood supply. Instead, the cartilage receives support from joint loading, or weight, in what experts call dynamic loading, which forces nutrient-rich synovial fluid to leak out. “That’s why exercise is so important for joint health,” notes Kraus. “Nutrition reaches cartilage through movement.”
Kraus is one of the few scientists studying the slow turnover of this tissue. In a surprising discovery, he and his team reported in 2019 that the synthesis of proteins involved in repair and regeneration varies by joint: It’s higher in the ankle than in the knee, and higher in the knee than in the hip. Kraus called this gradient “our inner salamander,” explaining that the ability of salamanders and other animals to regenerate lost limbs is stronger in the lower legs than in the upper legs.
His research showed that there is more genetic material associated with joint damage than in healthy joints. Kraus suspects that just as a limb injury initiates a rehabilitation program in salamanders, osteoarthritis may trigger the disease in humans, but “the program we have is not good enough.” However, the recovery process can occur in the ankle, which is much less common than in knee and hip arthritis, he notes.
There is other evidence that human cartilage can regenerate. Knee distraction is being tested in patients with osteoarthritis of the knee as a way to promote healing in patients who are too young to undergo total knee replacement. (Artificial knees last 15 to 20 years and then require complex surgical replacement.) The procedure involves placing needles above and below the knee and using an external device over six weeks to separate the upper and lower leg bones by five millimeters. . It opens the joint space. Patients are encouraged to walk, but the device reduces stress by bathing the knee in a nutrient-rich fluid without overstressing it.
Dutch researchers say the procedure leads to a slight increase in joint cartilage and pain relief, which lasts for at least two years, and up to 10 years in some patients. Philip Conaghan, a rheumatologist at the University of Leeds in England, said the technique would require larger clinical trials, but was a promising model.
Conaghan is researching new anti-arthritis drugs, including growth factors called sprifermin, which slow cartilage loss in some patients. He’s also studying the anti-inflammatory canakinumab, which has been tested as a cardiovascular drug with surprising side effects: recipients had significantly fewer joint replacements than the placebo group. But Conaghan cautions that finding a drug that thickens cartilage is difficult because of the slow and uncertain nature of repair: “The change is so small that even with the best imaging, it’s hard to pick up.”
Currently, strength training remains the best strategy for people with joint injuries. A walk to Conaghan Reservoir is recommended. “A strong quad can reduce knee pain no matter what,” he says. “All life consists of strong muscles.”