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Stage III

- Discussion:
    - this stage is characterized by pathognomonic appearance of sequestrum on the radiograph;
    - this appearance is often preceded by changes representing a transition between stage II and III;
    - these are crescent line due to subchondral frx, & segmental flattening of femoral head;
    - sequestrum later becomes manifest by a break in the articular margin extending from one end of the affected area to the other, followed
         by collapse of the sequestrated area into the femoral head;
    - paradoxically, and because of the limited size of the sequestrum, joint space is preserved or even increased;
    - necrosis does not follow subchondral frx as several authors have reported, but frx (which manifests radiographically as crescent or
         rim sign) follows necrosis;
    - this region of resorbed subchondral bone acts as a stress riser for development of small frx produced by continued wt bearing forces;
    - frx begins at stress riser & propagates along subchondral region roughly parallel to the articular cartilage and entirely through dead 
         cancellous bone;
    - subchondral frx is evident radiographically as subchondral radiolucent line and is called the crescent or rim sign;
    - this sign is pathognomonic for osteonecrosis and is only demonstrable before collapse of the articular cartilage and attached necrotic
         cancellous bone