Avascular necrosis (AVN, osteonecrosis, aseptic necrosis, ischemic necrosis, femoral head necrosis)

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What is Avascular necrosis?

Avascular necrosis (AVN) is cellular death of the components of bone, including the bone marrow, due to the impairment of the bones own blood supply. AVN is a component of a number of conditions and usually the epiphysis of long bones, such as the femoral and humeral heads and the femoral condyles, but small bones have also been known to be afffected.

Who gets Avascular necrosis?

Frequency depends on the site involved. The most common site is the hip; other locations include the carpals, talus, and humerus. In most countries, exact figures on incidence and prevalence are unknown. One Japanese survey estimated that 2500-3300 cases of AVN of the hip occur each year; of which, 34.7% were a result of corticosteroid abuse, 21.8% to alcohol abuse, and 37.1% to idiopathic mechanisms. A French study reported AVN in 4.3% of allogenic bone marrow transplant recipients.

Race: No racial predilection exists except for AVN associated with sickle cell disease and hemoglobin S and SC disease, which predominantly are diagnosed in people of African and Mediteranean descent.

Sex: The male-to-female ratio depends on the underlying cause, although primary AVN is more prevalent in men. The overall male-to-female ratio is 8:1.

Age: Age at onset depends on the underlying cause. Primary AVN most often occurs during the fourth or fifth decade and is bilateral in 40-80% of cases. On average, women present almost 10 years later than men.

Predisposing Factors

Avascular necrosis has several causes. Loss of blood supply to the bone can be caused by an injury (trauma-related avascular necrosis or joint dislocation) or by certain risk factors (nontraumatic avascular necrosis), such as some medications (usually steroid basesd), steroid abuse in general, blood coagulation disorders, or alcohol abuse. Increased pressure within the bone also is associated with avascular necrosis. The pressure within the bone causes the blood vessels to narrow, making it hard for the vessels to deliver enough blood to the bone cells.

Progression

The natural history of osteonecrosis is directly linked to the size and level of the necrosis. Very small lesions (involvement of less than 15% of the femoral head) may resolve without any further treatment. Conversly, lesions involving greater than 50% of the femoral head progress to collapse, and ultimately require in total hip arthroplasty.

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