Cartilage repair

Cartilage heals poorly, and the reason is simple: it is avascular, meaning it has no blood vessels running through it. Its cells, the chondrocytes, do not receive blood directly; they feed by diffusion, taking up whatever reaches them from the neighboring tissue. That lack of blood supply is the main reason a cartilage injury takes so long to heal.

Everything else follows from that. Feeding by diffusion means feeding slowly, so the extracellular matrix, the material the cells build around themselves that gives cartilage its firmness, renews at a slow pace and barely repairs itself at all. On top of that, the chondrocytes barely divide once the cartilage has formed. Tissue fed drop by drop repairs at the pace of those drops.

In practice this shows up when the defect goes through the full thickness: self-repair is minimal. One technique used for small defects is microfracture, which does fill the gap, but it fills it with fibrocartilage, a different kind of cartilage that does not hold up well to repeated weight bearing and compression.