Avascular cartilage: nutrition by diffusion and slow repair

Mature cartilage is avascular. Its matrix (the material surrounding the cells) contains no blood vessels, lymphatics (vessels that drain tissue fluid) or nerves, so the chondrocytes have no direct blood supply.

Nutrients, oxygen and waste therefore have to travel by diffusion through a dense, water-rich matrix. Where a perichondrium (the fibrous sheath around many cartilages) covers the cartilage, vessels in it feed the outer matrix. Articular cartilage, the smooth tissue covering a joint surface, has no perichondrium on its free surface. In a synovial joint it is fed mainly by synovial fluid, the lubricating fluid made by the vascular lining of the joint.

Movement helps. Loading (repeated compression and release of the joint) helps fluid mix at the surface, so a joint that moves regularly promotes exchange. The deep zones, though, still depend on long diffusion paths.

Few cells make this worse. Chondrocytes make up only about 2% of the volume of articular cartilage, yet they must maintain and replace the whole matrix. Avascularity therefore links three facts: matrix turnover (how fast the matrix is maintained and replaced) is slow once growth ends, inflammatory cells cannot arrive quickly through intact matrix, and repair stays limited unless the injury reaches vascular tissue.

If a defect reaches the blood-supplied bone beneath, the repair tissue tends to be fibrocartilage, a scar-like tissue that is mechanically inferior to the glassy hyaline cartilage that it replaces (hyaline cartilage is rich in type II collagen).