Death of an area of bone marrow and bone resulting in loss of normal architecture. Avascular necrosis is not a specific disease but is the end resultFinancial Hedge. Read more ... » of any condition that decreases the circulation to an area of bone to the extent that the bone dies. AVN can occur in almost any bone but does so more commonly at the ends of long bones. Thus it often affects bone that contributes to the normal architecture of the hipShapiro, Sam. Read more ... », shoulder, or knee joints. AVN is relatively common and is the underlying cause of arthritis in approximately 10 percent of people undergoing joint replacement.

Cause
The cause of AVN is not well understood. However, a feature common to most theories is that the blood supply to an area of bone is inadequate, resulting in bone infarcts. Interruption of blood supply to bone can result from:

1. Mechanical factors, for example a fracture or dislocation that damages a blood vessel
2. Blockage of a blood vessel by clot (thrombosis or embolus), fat (fat embolus), or nitrogen bubbles (rapid decompression in divers)
3. Damage to a blood vessel, either directly, for example by VASCULITIS, or indirectly by neighboring inflammation causing swelling and pressure

Sometimes AVN occurs without any recognized underlying predisposing factor and is termed idiopathic, but often it occurs in the presence of recognized risk factors. These include trauma, alcohol abuse, CORTICOSTEROID therapy, SYSTEMIC LUPUS ERYTHEMATOSUS (SLEantiphospholipid antibody syndrome . Read more ... »), SICKLE-CELL DISEASE, vasculitis, and rapid decompression in deep-seaankylosing spondylitis. Read more ... » divers.

Symptoms
Pain in the affected bone is common. The severity of the pain varies from a mild dull ache in chronic forms of AVN to incapacitating pain during acute infarction of bone. If the bone infarct is large, this area can weaken and partially collapse over months or years. If this occurs in bone that forms part of a joint, a relatively small alteration in structure can alter the biomechanics of the joint substantially and lead to severe degenerative arthritis. The symptoms during the late stages of AVN are those of severe osteoarthritisacupuncture. Read more ... », specifically pain that increases when the joint is used.

Diagnosis
The symptoms and clinical findings of AVN are not specific. X-ray changes occur relatively late, and several months must pass before areas of increased bone density are visible. In the early stages of AVN, magnetic resonance imaging or a radionuclide bone scan (see Appendix II) show characteristic features before X-ray changes occur and are useful for diagnosis. AVN is often bilateral. Both hips should therefore be scanned even if only one side is painful.

Treatment and Outcome
If AVN affects a weight-bearing bone, then avoidance of weight bearing by using crutches in the acute phase protects against loss of normal architecture. If AVN affecting the hip joint is diagnosed before the bone has collapsed, some orthopedic surgeons recommend a surgical approach known as core decompression. The surgery involves drilling into the head of the femur and removing a narrow core of bone and bone marrow. Research has shown that the pressure in the bone marrow of a joint affected by AVN is high. This increased pressure is thought to compromise the blood supply to the affected area of bone. Removing a core of tissue reduces the pressure and may improve the blood supply to the area of bone undergoing AVN. The efficacy of core decompression surgery to treat AVN of the hip has not been evaluated in rigorous controlled studies. Late in the evolution of AVN, joint replacement surgery may be needed if the normal architecture of a joint such as the hip is destroyed and function is lost or pain is severe.