Expansion
Avascular, aneural cartilage nourished by synovial fluid
Expansion
Articular (hyaline) cartilage
- Avascular, aneural and alymphatic
- Composed of type II collagen and proteoglycans (aggrecan), holding water
- Nourished by diffusion from synovial fluid, driven by cyclical loading, which is why immobilisation damages cartilage
- Chondrocytes have very limited replicative capacity, so healing is poor and repair tissue is inferior fibrocartilage
Synovial fluid
- An ultrafiltrate of plasma plus hyaluronan and lubricin from synoviocytes
- Thixotropic: viscosity falls with increasing shear rate, so it is thick at rest and thin during rapid movement
- Normal volume in the knee is only 1 to 2 ml
The pain of osteoarthritis cannot come from cartilage itself, which is aneural. It arises from subchondral bone, synovium, capsule and periarticular structures, which is why radiographic change correlates poorly with symptoms.
Joint aspiration is diagnostic: infection gives a very high neutrophil count and low viscosity, crystal arthropathy shows negatively birefringent needles in gout and positively birefringent rhomboids in pseudogout.