Avascular necrosis (also osteonecrosis, bone infarction, aseptic necrosis, ischemic bone necrosis, and AVN) is a disease where there is cellular death (necrosis) of bone components due to interruption of the blood supply.
Without blood, the bone tissue dies and the bone collapses.
There are many theories about what causes avascular necrosis. Proposed risk factors include:
Alcoholism, excessive steroid use, post trauma, caisson disease (decompression sickness), vascular compression, hypertension, vasculitis, arterial embolism andthrombosis, damage from radiation, bisphosphonates (particularly the mandible), sickle cell anaemia, Gaucher's Disease, and deep diving.
In some cases it is idiopathic (no cause is found).
Prolonged, repeated exposure to high pressures (as experienced by commercial and military divers) has been linked to AVN, though the relationship is not well-understood.
Cell death and repair
The hematopoietic cells are most sensitive to anoxia and are the first to die after reduction or removal of the blood supply, usually within 12 hours. Experimental evidence suggests that bone cells (osteocytes, osteoclasts, osteoblasts etc.) die within 12-48 hours, and that bone marrow fat cells die within 5 days.
Upon reperfusion, repair of ischemic bone occurs in 2 phases; First, there is angiogenesis and movement of undifferentiated mesenchymal cells from adjacent living bone tissue grow into the dead marrow spaces, as well as entry of macrophages that degrade dead cellular and fat debris.
Second, there is cellular differentiation of mesenchymal cells into osteoblasts or fibroblasts. Under favorable conditions, the remaining inorganic mineral volume forms a framework for establishment of new, fully functional bone tissue.
The disease may affect just one bone, more than one bone at the same time, or more than one bone at different times.
Avascular necrosis usually affects people between 30 and 50 years of age; about 10,000 to 20,000 people develop avascular necrosis of the head of the femur in the US each year. When it occurs in children at the femoral head, it is known as Legg-Calvé-Perthes syndrome.
X-ray images of avascular necrosis in the early stages usually appear normal. In later stages it appears relatively more radio-opaque due to the nearby living bone becoming resorbed secondary to reactive hyperemia.
Late radiographic signs also include a radiolucency area following the collapse of subchondral bone(crescent sign) and ringed regions of radiodensity resulting from saponification and calcification of marrow fat following medullary infarcts.
Avascular necrosis is especially common in the hip joint. A variety of methods are now used to treat avascular necrosis,--- the most common being the total hip replacement, or THR.
However, THRs have a number of downsides including long recovery times and short life spans.
THRs are an effective means of treatment in the geriatric population, however doctors shy away from using them in younger patients due to the reasons above.
Avascular Necrosis, its suitability depends on how much damage has occurred to the femoral head of the patient, bone is always undergoing change or remodelling. The bone is broken down by osteoclasts and rebuilt by osteoblasts
Some doctors also prescribe bisphosphonates (e.g. alendronate) which reduces the rate of bone breakdown by osteoclasts, thus preventing collapse (specifically of the hip) due to AVN but it is not proven.
Other treatments include core decompression, where internal bone pressure is relieved by drilling a hole into the bone, and a bone graft / bone marrow can also be placed, again not fully established.
The free vascular fibular graft (FVFG), in which a portion of the fibula, along with its blood supply, is removed and transplanted into the femoral head.
Progression of the disease could possibly be halted by transplanting nucleated cells from bone marrow into avascular necrosis lesions after core decompression, although much further research is needed to establish this technique.
The amount of disability that results from avascular necrosis depends on what part of the bone is affected, how large an area is involved, and how effectively the bone rebuilds itself. The process of bone rebuilding takes place after an injury as well as during normal growth.
Normally, bone continuously breaks down and rebuilds—old bone is reabsorbed and replaced with new bone. The process keeps the skeleton strong and helps it to maintain a balance of minerals.
In the course of avascular necrosis, however, the healing process is usually ineffective and the bone tissues break down faster than the body can repair them.
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