Joints: how one actually works, and what wears

A joint is where two bones meet, and rather more is going on than the bones. The ends are capped with cartilage, a smooth, slippery surface that lets them glide. Around the whole thing is a capsule holding a small amount of thick fluid that lubricates and feeds it. Ligaments hold the bones in place, and muscle moves them and takes a good deal of the load.

When a joint hurts, any of those can be the source, which is why joint pain is rarely one simple thing.

Cartilage has no blood supply of its own. Nothing is delivered to it in the usual way. It is fed by the fluid in the joint, and that fluid only circulates through it when the joint moves and is loaded a little.

That is the whole reason resting a sore joint beyond a day or two backfires. A still joint is an underfed joint, and it stiffens. The old phrase that motion is lotion is a fair description of the mechanism: the fluid is thicker and works better when the joint has been used.

Muscle. Strong muscle around a joint absorbs force that would otherwise pass through the cartilage, and it holds the joint in better alignment while it moves.

This is why leg strength matters so much for sore knees and hips, and why the most useful thing for many painful joints is not a treatment for the joint at all but work on the muscle around it.

Cartilage thins gradually, the fluid becomes less effective, ligaments lose some elasticity, and muscle is lost unless it is used. Several small changes together, rather than one thing wearing out.

That matters because it means several of them can be worked on. This is general guidance on how joints work, not medical advice; persistent or severe joint pain deserves a proper diagnosis.