Articular cartilage damage
What is articular cartilage?
Articular cartilage is the layer of very smooth shiny white tissue inside a joint, which covers the ends of the bones. Articular cartilage varies in thickness from just a millimetre up to several millimetres; the heavier the loads across any specific joint, the thicker the cartilage.
Articular cartilage is extremely smooth and hence is very low friction, allowing the ends of the bones to move freely against each other without catching or rubbing. Indeed, articular cartilage has a lower coefficient of friction than any other known material - natural or man-made.
Knee joint opened up and viewed from the front/top. The articular cartilage is a layer of smooth shiny white tissue covering the end of the femur, the top of the tibia and the back of the patella.
What is cartilage damage?
Articular cartilage can be damaged by trauma, degenerative wear and tear or other specific joint diseases.
Causes of articular cartilage damage:-
- Trauma, eg fractures into the joint
- Degenerative, eg osteoarthritis
- Inflammatory, eg rheumatoid arthritis
- Others, eg Osteochondritis Dissecans
Unfortunately, articular cartilage, once damaged, does not heal itself with new normal tissue, as bone does. The structure of articular cartilage is complex, and it is referred to as 'hyaline cartilage'. At best, areas of damage or cartilage loss may heal up with what is known as 'fibrocartilage', which is half scar tissue and half like cartilage. However, fibrocartilage does not have the same biological or mechanical properties as normal hyaline cartilage. At the extreme end of the spectrum, damaged cartilage may lead to patches of bare bone developing within a joint, with all the usual painful symptoms and signs of arthritis.
Surgeons often grade cartilage damage according to varying scoring systems. Probably the simplest and most commonly used is a four-point score based on the visual appearance of the cartilage on inspection at arthroscopy:-
- Grade I - Softening of the cartilage
- Grade II - Roughening of the surface of the cartilage
- Grade III - Fissures / cracks in the cartilage, going down to bone
- Grade IV - Cartilage loss down to bare bone
Intra-operative arthroscopic view of a large patch of Grade IV articular cartilage damage. Central V-shaped pink area = bare bone.
In addition, small pieces or even large chunks of cartilage (or bone + cartilage) can break off inside the knee, becoming what is referred to as a 'loose body'. Loose bodies can cause pain, swelling, locking and intermittent giving way of the knee.
What are the symptoms of articular cartilage damage?
Cartilage damage can cause quite a wide variety of potential symptoms, depending on the severity of the damage. Symptoms may include:-
- Pain in the joint
- Swelling of the joint
- Catching or giving way
- Locking (where one is unable to fully straighten the joint)
If the area of cartilage damage is small, then a patient may feel a quite specific, localisable, intermittent sharp pain within the joint. If the cartilage damage is extensive and widespread, then the symptoms are more likely to be those of generalised arthritis.
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