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AVN: Pathogenesis

- Discussion: (See: Blood Supply to Femoral Head)
    - there are certain areas of bone in which vascular supply is precarious;
    - these areas are subject to infarction and avascular necrosis;
    - three such regions are head of femur, body of talus, & scaphoid;
    - they have the common feature that a very large portion of their total surface is covered with articular cartilage through which vessels do not penetrate;
    - blood supply for these bones enters through very restricted spaces, & there is limited collateral circulation;
    - when principal route of the circulation to these bones is interrupted by trauma or disease, the collateral circulation is inadequate and necrosis ensues;
    - avascular necrosis occurs in other bones, particularly body of talus, scaphoid, the lunate, and the head of the second metatarsal;
    - occasionally, it develops in multiple areas in the same patient;
    - this condition has been found to be frequent complication of prolonged steroid therapy used in patients with kidney transplants;
    - interference w/ blood supply to femoral head results in infarction that may involve the entire head; more frequently, it is limited to a segment of the head;
    - in infarcted area, there is death of marrow elements & of osteocytes;
    - bone death is recognized histologically by disappearance of osteocytes from their lacunae;
    - following infarction, there is an attempt at revascularization that extends inward from the adjacent viable bone;
    - this revascularization process is very slow in the femoral head;
    - hyperemia of surrounding bones causes osteoporosis of living bone, while infarcted bone retains its density and thus appears whiter or denser on roentgenogram;
    - as vessels approach necrotic area, osteoclastic resorption of dead bone may so weaken the femoral head that a portion of it will collapse, resulting in
             an irregular articular surface that sets the stage for painful degenerative arthritis
    - references:
             - Role of impairment of blood supply of the femoral head in the pathogenesis of idiopathic osteonecrosis.
             - A microangiographic study of idiopathic osteonecrosis of the femoral head.
             - Intraosseous arterial architecture in nontraumatic avascular necrosis of the femoral head. Microangiographic and histologic study.
             - Selective digital subtraction arteriography in necrosis of the femoral head.
             - Intraosseous pressure, oxygenation, and histology in arthrosis and osteonecrosis of the hip.
             - Intra-osseous pressure and oxygen tension in avascular necrosis and osteoarthritis of the hip.

Current concepts on the pathogenesis of osteonecrosis of the femoral head.