Cartilage

Cartilage is the smooth, resilient connective tissue that covers the ends of bones inside joints, forms the discs between vertebrae, and shapes structures such as the nose and ears. Articular cartilage, the joint surface kind, is what most sports discussion means.

Its function is remarkable and easy to underestimate. It provides a surface with less friction than ice on ice, and it distributes load across the joint so that bone is not concentrated onto a small area.

It has no blood supply of its own. Nutrients diffuse in from the joint fluid, driven by the compression and release of movement, which is why joints stiffen when they are not used and why gentle motion is part of managing almost every joint problem.

That absence of blood supply is also why it heals so poorly. Damage to articular cartilage does not repair the way a muscle tear does, and this is the central difficulty in treating cartilage injury and in osteoarthritis.

Running does not wear it out, contrary to persistent belief. Recreational runners have lower rates of hip and knee osteoarthritis than sedentary people in cohort studies, and cartilage appears to adapt to habitual loading like other tissues.

Supplements marketed for it, glucosamine and chondroitin in particular, have been tested extensively and produce small effects at best on pain, with no convincing evidence of structural benefit.

In practice, the interventions with evidence in osteoarthritis are exercise, strength training around the joint, weight management where relevant, and appropriate medical care, which is a less satisfying list than a supplement and a considerably more effective one.