Avascular necrosis (also osteonecrosis, bone infarction^[1], aseptic necrosis, ischemic bone necrosis^[2], and AVN) is a disease where there is cellular death (necrosis) of bone components due to interruption of the blood supply.^[3] Without blood, the bone tissue dies and the bone collapses.^[2] If avascular necrosis involves the bones of a joint, it often leads to destruction of the joint articular surfaces (see Osteochondritis dissecans).

Causes

There are many theories about what causes avascular necrosis. Proposed risk factors includealcoholism,^[4] excessive steroid use,^[5] post trauma,^[6][7] caisson disease (decompression sickness),^[8][9] vascular compression,^[10] hypertension, vasculitis, arterial embolism andthrombosis, damage from radiation, bisphosphonates (particularly the mandible),^[11] sickle cell anaemia,^[12] Gaucher's Disease,^[13] and deep diving.^[14] In some cases it is idiopathic (no cause is found).^[15] Rheumatoid arthritis and lupus are also common causes of AVN. 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.^[1] 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.^[1]

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.^[1] Second, there is cellular differentiation of mesenchymal cells into osteoblasts or fibroblasts.^[1] Under favorable conditions, the remaining inorganic mineral volume forms a framework for establishment of new, fully functional bone tissue.^[1]

Presentation

While it can affect any bone, and half of cases show multiple sites of damage, avascular necrosis primarily affects the joints at the shoulder,knee, and hip.

Clinical avascular necrosis most commonly affects the ends (epiphysis) of long bones such as the femur (the bone extending from the knee joint to the hip joint). Other common sites include the humerus (the bone of the upper arm),^[16][17] knees,^[18][19] shoulders,^[16][17] ankles and the jaw.^[20] The disease may affect just one bone, more than one bone at the same time, or more than one bone at different times.^[21]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.^[22]

Diagnosis

Front X-ray of right knee of anadolescent (epiphyseal plates are open): arrows point to avascular necrosis and developing osteochondritis dissecans in the outer medial condyle of femur

Orthopaedic doctors most often diagnose the disease except when it affects the jaws, when it is usually diagnosed and treated by dental and maxillofacial surgeons.

In the early stages, bone scintigraphy^[23] and MRI^[24] are the diagnostic modalities of choice.

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.^[1] The necrotic bone itself does not show increased radiographic opacity, as dead bone cannot undergo bone resorption which is carried out by living osteoclasts.^[1] 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.

[edit]Treatment

Avascular necrosis is especially common in the hip joint. A variety of methods are now used to treat avascular necrosis,^[21] 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. A new, more promising treatment is hip resurfacingor metal on metal (MOM) resurfacing. It is a form of a THR, however in this procedure, only the head of the femur is removed as opposed to a THR in which the entire neck is removed. MOM resurfacing is still experimental in America but has been endorsed in Great Britain as an excellent alternative to a THR. A MOM Resurfacing may not be suitable in all cases of 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.^[25] The bone is broken down by osteoclasts and rebuilt by osteoblasts.^[25] 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.^[26]

Other treatments include core decompression, where internal bone pressure is relieved by drilling a hole into the bone, and a living bone chip and an electrical device to stimulate new vascular growth are implanted; and 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.^[27]

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.^[28]

Prognosis

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.^[25]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.^[25] 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. If left untreated, the disease progresses, the bone collapses,^[2] and the joint surface breaks down,^[15] leading to pain and arthritis.^[15]

Notable individuals affected

Avascular necrosis cut short the football and baseball careers of star athlete Bo Jackson.^[29]

Other sports stars with this condition are former NFL running back Garrison Hearst, cyclist Floyd Landis, NFL quarterback Brett Favre, professional wrestler "Superstar" Billy Graham, wrestler Joe Heat, Number one draft pick for the Minnesota Lynx: Ben Dvorak, NBA playerJorge Garbajosa, NHL goaltender Ray Emery and gymnast Jade Barbosa.

In addition to the athletes listed, AVN has affected Edward Van Halen, lead guitarist for the rock band Van Halen, Ed Graham, drummer of the bands The Darkness and Stone Gods, and Micky Dolenz, the drummer/singer of the band The Monkees.

Recent research into the cause of death of Tutankhamun suggests that it may have been a factor in the pharaoh's death.

Bill Shannon, despite being born with degenerative hip condition, developed a way to express himself through dance and skateboarding on crutches, and became prominent in Disabled dance.

The Staging of Avascular Necrosis

Stage	Clinical	Surgical potential
Stage 0	Theoretical stage:  Assymptomatic.  All diagnostic tests are normal.  Biopsy from histology - is the only  abnormality.	Nil Required :  Theoretical stage only Not a clinical problem.
Stage 1	X-Rays and CT-scan are normal  MRI and Tc99 and histology are abnormal.  Symptoms may or may not be present.	Surgery is possible / desirable. Core - decompression possible.
Stage 2	X-Rays are abnormal -  With linear sclerosis or cysts.  There is no Subchondral lucency  Head of the femur is still spherical.	Surgery is possible / desirable. Core - decompression possible.
Stage 3	The femoral head starts to fail  mechanically -  With Trabecular collapse.  The radioluscent crescent sign is visible  at the Subchondral endplate.  The femoral head itself is still spherical	Surgery is possible  Core - decompression possible. Success depends on degree of changes.
Stage 3a.	Crescent < 15%	Surgery still possible.
Stage 3b	Crescent 15-30%	Increasing risk of failure of preventative surgery.
Stage 3c	Crescent>30%	Increasing risk of failure of preventative surgery.
Stage 4	Flattening of the femoral head is now seen	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 4a	<15% of the surface is collapsed  Depression <4mm	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 4b	15-30% collapse or 2-4mm depression	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 4c	>30% Collapse or >4mm depression.	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 5	Any or all Xray features may be present  There is a decrease in joint space.  Secondary Osteoarthritis is present with:  Sclerosis / Cysts / Osteophytes	Preventative surgery NO HELP Medical Management. Hip Replacement Required.
Stage 6	Extensive destruction	Total Hip Replacement -  Where available / Possible.

See also

Dysbaric osteonecrosis

References