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ARTICULAR CARTILAGE

Articular cartilage – “The Joint Surface”

Articular cartilage is a vital structure in any joint. It is comprised of layers of “matrix” a tough, smooth and flexible substance which is maintained by cells called chondrocytes which live in the matrix. The superficial layer of the cartilage is incredibly smooth, the deep layers are anchored to the underlying bone. This combination allows bones to slide and rotate against each other, allowing the joint to move. The articular cartilage can be damaged in injuries, it can also deteriorate (degenerate) over time. Damage to the articular cartilage is the key issue in the development of osteo-arthritis. As the joint surface becomes split or broken the ability of the joint to glide and flex slowly deteriorates. The damaged joint surface also causes the release of complex chemicals which stimulate inflammation in the joint. This can be a cause of pain and swelling. Another major cause of pain is the overload of bone under damaged articular cartilage surfaces. As damage worsens, the ability of the cartilage to distribute load decreases, this causes the bone to be under increasing pressue. The bone responds by the development of oedema (swelling) which causes a deep, persistent pain. This is often described as being like a toothache.

The different types of articular cartilage damage are treated in different ways.

Chondroplasty

Early splitting and cracking of cartilage layers can be treated by a technique called chondroplasty. This is using mechanical shavers or radio-frequency “wands” to smooth the cracked surface. This only works in smoothing the superficial layers of cartilage. If the deeper layers are affected the technique becomes less useful.

Bone Marrow Stimulation

If the full thickness of the cartilage layer is damaged then there is no point in smoothing the surface. The cartilage has been lost and new cartilage is needed to fill the defect. A useful technique is bone marrow stimulation, often called “microfracture”. The cartilage attaches to the strong surface of the underlying bone called the sub-chondral plate. Under this plate are the softer areas of woven bone which contain the bone marrow cells. These cells are able to create a variety of tissue types including a type of cartilage called fibrocartilage. Bone marrow stimulation is the creation of small channels in the bone plate which allow the healing cells to move into the cartilage defect and fill it with fibrocartilage. It is very useful in small full thickness defects. In larger defects it is less effective, the reason is that fibrocartilage is not as strong or stable as the original articular cartilage. It does not effectively distribute the load and may deteriorate with time. This may mean the symptoms of the injured knee do not fully resolve.

Cartilage Repair

For large full thickness cartilage defects the best option is some form of cartilage repair. This has been an area of great interest over recent years. New techniques are emerging which have the potential to change treatment of these large defects. One promising treatment is augmented bone marrow stimulation. The concept is that once bone marrow stimulation has been performed a covering layer is placed on top of the area. This covering layer contains biological material “ scaffold” that contains and supports the bone marrow cells as they move into the defect. The bone marrow cells populate the scaffold and this allows the cartilage they create to be stronger and more like the original cartilage. As the new cartilage is created it slowly takes the place of the scaffold material. One technique uses a scaffold material called BST Cargel which is a mixture of chitosan protein, a stabilising buffer and a small amount of the patients blood. This technique has great potential and is being used in a large international study into its results.

Re-alignment

Sometimes cartilage damage is so severe it cannot be repaired. In other circumstances repair can be performed but the knee structures are such that the repaired surface will be overloaded when the knee is working. In this circumstance re-alignment can be very useful. Re-alignment can be a soft tissue procedure or one involving bone. Soft tissue realignment is most useful in pain from the patella. When the patella is not balanced well on the front of the femur then re-balancing the soft tissues on either side of the patella can help with pain.

Bone re-alignment procedures can be used for the more severe forms of patella pain and instability. This involves cutting and re-positioning the bone at the tibial tubercle where the patellar tendon attaches. This is a more powerful form of re-alignment surgery than the soft tissue procedure but the two procedures are sometimes combined. Bone re-alignment can also be used when the cartilage between femur and tibia is overloaded. Making a cut in either tibia or femur and changing the angle of the bone close to the joint changes the direction of forces across the joint and decreases the pressure on the damaged or reconstructed cartilage surface.

Knee Joint Replacement

In circumstances where the cartilage damage is so severe that no other options are likely to work joint replacement can be considered. Joint replacement involves removing part or all of the joint surfaces in the knee and replacing them with metal and plastic components. Total joint replacement involves all of the joint surfaces in the knee. Partial joint replacement involves just replacing one area of the joint surface. This can be very useful when one part of the knee is beyond repair but other areas of articular cartilage are still in good condition. Partial joint replacement can be an attractive option for younger patients as the knee feels more natural than after a total knee replacement. Recovery time is also quicker and function generally better in the partially replaced knee joint than the total replacement.

Usually patients will stay around four days in hospital for a total knee replacement or two to three days for a partial knee replacement.

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