Articular Cartilage

For Young Learners

Structure and Location

Look at the end of a chicken bone and you will see a smooth, pale, slightly shiny covering. That is articular cartilage, and your knee has the same thing.

It is only a few millimetres thick, which is about the thickness of a couple of stacked coins. It covers the end of the thigh bone, the top of the shin bone and the back of the kneecap.

Function

Two jobs. It is extraordinarily slippery, more slippery than ice on ice, so the bones glide over each other with almost no friction. It also squashes very slightly under load and spreads the force out.

Here is the catch. Cartilage has no blood supply. Cut your skin and blood arrives to repair it. Damage cartilage and almost nothing arrives, so it heals very poorly. That is why people care so much about protecting it rather than fixing it later.

For Patients

Structure and Location

Articular cartilage, sometimes called hyaline cartilage, coats the ends of the femur and tibia and the back of the patella. In the knee it is between two and six millimetres thick, thickest behind the kneecap.

It is mostly water, held in a framework of collagen fibres and large molecules that attract and hold that water. The arrangement is what gives it its properties.

Function

It provides a near frictionless surface. The coefficient of friction in a healthy joint is lower than ice sliding on ice, which is why a healthy knee moves so freely.

It also distributes load. Under pressure, water is squeezed slowly through the framework, which cushions the bone beneath and spreads the force over a wider area.

Clinical Relevance

Cartilage has no blood supply, no nerves and no lymphatics. It is fed by fluid inside the joint, and that fluid only circulates when the joint moves. A knee that is not used is a knee whose cartilage is poorly nourished, which is one reason prolonged rest is rarely the right answer.

Having no nerves means cartilage damage itself is painless. Pain in a worn joint comes from the bone beneath, the lining and the surrounding tissues rather than from the cartilage.

Having no blood supply means repair is very limited. Damage tends to persist, which is why strength, weight and activity, which change how much load reaches the surface, matter more than any treatment aimed at the cartilage itself.

Specific cartilage injuries and the operations used for them are covered elsewhere.

For Medical Students

Structure and Location

Hyaline articular cartilage covers the femoral condyles, the tibial plateaus and the patellar facets. Composition is approximately seventy to eighty per cent water by wet weight, with type two collagen forming the bulk of the dry weight and proteoglycans, chiefly aggrecan, retained within that network.

It is organised in zones. The superficial zone has collagen orientated parallel to the surface and resists shear. The middle zone is more randomly arranged. The deep zone has collagen perpendicular to the surface, anchored across the tidemark into the calcified layer and the subchondral bone.

Chondrocytes occupy roughly two per cent of the tissue volume and maintain the matrix. There is no vascular, neural or lymphatic supply.

Function

Behaviour is biphasic. Load pressurises the interstitial fluid, which bears the majority of the applied stress and flows slowly through the low permeability matrix. This produces time dependent deformation and underlies both load distribution and lubrication.

Proteoglycans carry a high fixed negative charge, drawing in water and generating a swelling pressure resisted by the collagen network. The balance between the two determines stiffness in compression.

Clinical Relevance

Avascularity limits intrinsic repair. Chondral defects that do not breach the subchondral plate have minimal healing response; those that do may fill with fibrocartilage, which is mechanically inferior.

Aneural tissue means chondral damage is not itself a source of pain, and symptoms arise from subchondral bone, synovium and capsule. This accounts for the imperfect correlation between imaging appearance and symptoms.

Nutrition by diffusion driven by cyclical loading gives the physiological basis for early mobilisation after injury and surgery, and for avoiding prolonged immobilisation.

Cartilage lesions, their classification and their surgical management are covered in the conditions and surgery sections.