Physiology and nutrition of the cartilage

Cartilage tissue is hypotrophic (of low consumption). As only 1-2 % of the total volume is taken up by the cells for the communication and transfer of all materials (oxigen, sugars, lipids, amino acids, carbon dioxide, etc.,) must be through the extracellular matrix by diffusion.  Human cartilage is completely avascular and alymphatic (as well as aneural) hence all material must be delivered through the synovial fluid (maybe a small fraction through the subchondral bone).

Being the fact, that cartilage is avascular, in case of any cartilage damage (in contrast with the damage in other vascular tissues) the repairing cells, the mesenchymal stem cells are unable to travel to the site of the injury. Since the extracellular matrix is fairly dense the cell shifting is very restricted even for the cartilage cells themselves. (some recently utilized cartilage repair methods – e.g., microfracture- are based on these facts,)

Having no nerve endings in the cartilage raises the question why cartilage injury is painful. One explanation would be that the inner layer of the joint cavity – the synovium is very sensitive to any inflammatory changes in the synovial fluid; the other would be that the subchondral bone is already full of sensory nerve endings and any cartilage defect causes changes in the load transmission, thus induces micro fractures in the bone itself.

As cartilage cells nourish on the synovial fluid any prolonged change in its composition (extensive bleeding, infection) may induce the cell death in the cartilage and ultimately may end up in cartilage degeneration.