Pathophysiology and natural history of avascular necrosis of bone.

Avascular necrosis of bone (AVN) occurs as two main variants, local and systemic. Local AVN is usually caused by trauma or microtrauma; examples include primary osteonecrosis of the medial condyle, vertebral osteonecrosis, necrosis after meniscectomy, and osteonecrosis of the mandible or small bones. Systemic AVN manifests as epiphyseal necrosis or bone infarction, which is often multifocal. Little is known about the factors that trigger AVN. One possible mechanism is intraluminal obliteration of blood vessels by microscopic fat emboli, sickle cells, nitrogen bubbles (caisson disease), or focal clotting due to procoagulant abnormalities. Extraluminal obliteration may result from elevated marrow pressure or increased marrow fat. Cytotoxicity and genetic factors may be involved also. Many factors are probably capable of inducing AVN, and combinations of factors may be required, although the final mechanism is always critical ischemia. The natural history of AVN is better understood than the early triggering factors. AVN becomes detectable 1-6 months after exposure to an easily identifiable risk factor such as high-dose glucocorticoid therapy or femoral neck fracture. Later on, AVN is uncommon even when the patient remains exposed to the risk factor. The turning point in the natural history of AVN is subchondral plate fracture, which leads to collapse of the necrotic segment of the epiphysis, usually within the first 2 years. The risk of collapse depends chiefly on the initial size and location of the necrotic segment, which can be determined accurately by magnetic resonance imaging (MRI). This natural history has obvious clinical implications.