Avascular necrosis (AVN) occurs due to the temporary or permanent loss of the blood supply to an area of bone. As a result, the bone tissue dies and the bone collapses. If AVN involves the bones of a joint (e.g.: the talus) it often leads to destruction of cartilage, resulting in arthritis and pain. In the case of the talus, three joints can be affected:
- Ankle joint
- Talonavicular joint (a joint in the middle of the foot)
- Subtalar joint (the joint below the ankle)
The ankle joint allows up and down movement of the foot, while the subtalar and talonavicular joints allow in and out movement of the foot. The normal function of the subtalar joint is to allow walking on uneven surfaces, inclined surfaces, ladders, etc. without falling.
AVN can be caused by two large categories:
- Trauma – a fracture (breaking) of the bone disrupts the blood supply to the bone
- Nontraumatic – include idiopathic (no cause is ever found), steroids (e.g.: anabolic and high dose corticosteroids (prednisone) given for such diseases as rheumatoid arthritis, lupus, and cancer), excess alcohol consumption, sickle cell anemia, radiation treatments, and chemotherapy
Avascular necrosis of the talus can be quite devastating and lead to total loss of the ankle joint with arthritis, deformity and pain. The development of AVN is determined to a large extent by the type of the talus fracture.
There are those fractures that are not very severe (they do not shift or displace much), and in these fractures, the incidence of AVN is lower. However, when the talus dislocates out of the ankle socket, the incidence of AVN is very high, almost 100%.
The development of AVN is related to the type of the fracture and not the manner in which it is treated. This is because of the blood supply to the talus, which is torn with certain fracture types, and not with others, and regardless of how the talus is put back together, the blood supply cannot change. Interestingly, however, the presence of AVN does not change the rate of healing of the fracture.
The healing of the fracture is called a “union”. If the fracture does not heal at all, this is a “non-union”, and if the fracture heals in a poor position, this is called a “mal-union”.
Even in fractures where AVN does develop, the fractured bone invariably goes on to union. There seems to be just enough blood supply left coming across the fracture to heal it, but not enough to maintain the blood supply for a totally viable talus. This is important when planning treatment following treatment of the fracture.
The care of the limb after any fracture in the foot and ankle is based upon the premise that a limited amount of standing, walking and bearing weight on the foot is permissible. This makes sense, since pressure on the fracture with walking before the fracture has healed will lead to a shift in the bones resulting in a non-union, or a malunion.
This has particular relevance with the fracture of the talus where one is concerned about the development of AVN, since the surgeon is understandably concerned about the consequences of bone healing if AVN occurs.
If AVN does occur, the talus can break up into small pieces, fragment and collapse. This is not predictable. The majority of fractures that develop AVN do not go on to collapse, and the AVN is limited to small segments of the talus.
Once the fracture has healed, bearing of weight on the leg is actually permissible. There is no evidence to suggest that the patient has to remain off the foot using crutches for an indefinite period of time to prevent the talus from collapsing further. The foot may need to be protected, using a boot or a brace, and certain activities with impact on the leg may need to be restricted, but walking should be acceptable.
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