Cartilage has no blood supply and heals accordingly

The surface that lets a joint glide is fed by movement itself, and once damaged the body has no good way to replace it.

Cut skin and it bleeds, clots, and rebuilds within a fortnight. Damage the cartilage on the end of a femur and almost none of that happens, because there is no blood vessel anywhere near the injury to start the process.

Articular cartilage is an unusual tissue. It has no direct blood supply, no nerves through most of its thickness, and very few cells, and those cells sit in a dense matrix they produced themselves and largely cannot travel through. Nutrition arrives by diffusion from the joint fluid, and the pumping action that drives that diffusion is compression and release. The tissue is fed by being used. Which is a neat arrangement until something goes wrong, because none of the standard repair machinery can get to the site.

Damage that stays within the cartilage layer therefore tends not to heal at all. Damage that penetrates into the bone underneath does bleed, and the body fills it, but it fills it with fibrocartilage, a coarser tissue with different collagen and worse mechanical properties. It works. It does not work as well, and it wears faster under the same loading.

The absence of nerves explains the other confusing part, which is why a knee can have visible surface damage and no symptoms, or terrible symptoms and a tidy looking surface. What hurts is usually the bone underneath, the synovium, or the capsule reacting to debris. The cartilage itself is silent while it goes.

Two consequences I would argue for. Immobilisation is worse for cartilage than almost any other tissue, because you have removed its feeding mechanism entirely, and the thinning that follows a few weeks in a cast is not a minor cosmetic issue. And loading is not the enemy people assume it is. Moderate, repeated, varied compression is what maintains the matrix. It is high impact on a joint that is already damaged, or on one with an incongruent surface after a poorly healed fracture, that accelerates the loss.

This is also why the injuries that end careers are rarely the dramatic ones.

A torn ligament can be replaced. A worn joint surface cannot, and everyone in sports medicine knows which of those two they would rather see on a scan.