- Articular cartilage is the smooth white shiny glistening layer of tissue covering the surfaces of the ends of the bones in the knee joint.
- The articular cartilage is a few millimeters in thickness, and is highly specialized tissue. Its function is to make the joint surfaces very smooth and low friction, to allow smooth joint movements without wear and tear.
- Articular cartilage does not have any blood supply. Therefore, if the tissue is damaged it cannot repair itself or regenerate and grow back.
- Articular cartilage damage can occur spontaneously, from conditions such as Osteochondritis Dissecans (OCD), from direct trauma/impact to the knee or from wear and tear. The damage from wear and tear can occur secondary to other issues in the knee such as meniscal cartilage loss or ligament damage/instability. Alternatively, articular cartilage damage may develop gradually from simple wear and tear.
- If the articular cartilage wears away completely, exposing bare bone surfaces in the joint, and if this is widespread then this is simply called osteoarthritis.
- Patches of articular cartilage damage can cause pain, swelling, clicking, catching, giving way or locking.
- The best imaging investigation to look for cartilage damage is an MRI scan.
- The most accurate and precise tool for assessing cartilage damage in full detail is a knee arthroscopy.
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The articular cartilage is the smooth white shiny layer of tissue that is a few millimeters thick, that covers the surfaces of the ends of the bones inside the joint. Articular cartilage has the lowest coefficient of friction of any substance known to man, and its function is to make the bone surfaces in the joint smooth and slippery, to allow the joint to move freely.
Articular cartilage is highly specialized tissue, and is made up of layers of cells (that are mainly in the deeper layers) and various molecules such as collagen, glycosaminoglycans and proteoglycans. The articular cartilage sits on the surface of the bone. The deeper parts of the bone are like a honeycomb; however, the surface layer of the bone (the subchondral bone plate) is like a hard egg shell … It can be pictured as being like a Crunchie bar – with a honeycome centre (the cancellous bone), a hard outer layer (the subchondral bone plate) and a chocolate coating layer on the outside (the articular cartilage).
The subchondral bone plate supports the articular cartilage but acts as a barrier. Although the underlying bone has a very good blood supply, no blood vessels actually get through the subchondral bone plate, and so the articular cartilage has no blood supply (it is ‘avascular’). The cells in articular cartilage are not very metabolically active, and as they have no blood supply they get their oxygen and their nutrition by diffusion from the joint fluid (synovial fluid) in the knee. The relevance of all this is that articular cartilage has very little self-healing potential, and therefore once it is damaged or worn, it tends not to be able to heal up or repair itself on its own.
Articular cartilage damage can occur in three main ways:-
- From trauma
- From wear and tear (degeneration)
If the knee is hit hard, by a sharp hard object, from falling onto the knee onto a hard surface, or by twisting the knee badly, e.g. from a tackle or a fall, then chuncks of cartilage can be knocked off the surface of the joint. This can cause:-
- mechanical symptoms, such as painful clicking, catching, giving way and/or locking
Wear and tear / degeneration
Some people are just naturally prone (genetically predispositioned) to developing generalized wear and tear in their joints, and the knee joint is one of the most common joints to be affected. If the articular cartilage layer begins to wear down then it becomes thin and eventually patches of the underlying bare bone may become exposed. ‘Wear and tear’, ‘degeneration’ and ‘arthritis’ all really mean the same thing, but tend to be different ways of expressing the severity of the damage – if the damage is minor then most doctors tend to refer to it as “a bit of minor wear and tear”; if the damage is severe (for example if there is bare bone rubbing on bare bone) then this is fully blown arthritis.
Spontaneous articular cartilage damage
Spontaneous problems with articular cartilage tend not to be very common; however, the most frequent cause for this happening is ‘Osteochondritis Dissecans’. Nobody really knows exactly what the definite cause of osteochondritis dissecans (OCD) is, but it’s been proposed that it could be due to either repetitive microtrauma, or a poor blood supply to the underlying bone, or possibly some kind of autoimmune reaction.
In OCD what happens is that initially a smallish (often about 1cm across) island of bone starts to break away on the surface of the bone. The overlying articular cartilage covering this bone island will then start to crack and split. The chunk of bone with its surface layer of cartilage can then become unstable and start to break away and detach.
The severity of symptoms from OCD will depend on how big the chunk of bone and cartilage (osteoarticular fragment) is, which specific part of the joint surface is being affected (i.e. whether it is on the main weight bearing surface), and how stable the fragment is (or whether it has actually detached, and is floating around in the knee causing problems such as locking).
CLICK HERE for further information about Osteochondritis Dissecans
CLICK HERE for information about articular cartilage repair
CLICK HERE for information about articular cartilage grafting (replacement)
Written by Mr Ian McDermott
Consultant Knee Surgeon London
Last updated 23-6-15