- 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;
