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Crescent sign of femoral AVN

Definition :
The crescent sign that is associated with avascular necrosis (AVN) is seen on conventional radiographic films and is recognized as a curvilinear subchondral radiolucent line . It is typically seen along the anterolateral aspect of the proximal femoral head, which is optimally depicted on the frog-leg radiographic view "obtained with the patient’s thigh abducted and flexed ".

A Conventional radiograph of the right femur in the frog-leg position shows subchondral area of hyperlucency (arrows) in the anterolateral aspect of the proximal femoral head. (Courtesy of Clyde A. Helms, MD, Department of Radiology, Duke University Medical Center, Durham, NC.)

The crescent sign is explaned by Inadequate perfusion in the articular ends of bones that leads to the processes of osteonecrosis and repair . Repair begins at the interface between necrotic and viable bone.
Reactive new bone is laid down over dead trabeculae, which produces a sclerotic margin. An advancing front of fibrosis, hyperemia, inflammation, and bone resorption extends into the necrotic segment of bone as repair is attempted. Mechanical failure of trabecular bone at this interface results in progressive microfracture (as seen below ) and collapse of the adjacent dead subchondral cancellous trabeculae, which leads to the development of a subchondral radiolucent area along the fracture line, or the crescent sign
Specimen radiograph of a coronally sectioned femoral head segment reveals a subchondral fracture (arrows), which manifests as the crescent sign. Note the fragmentation and compaction of the subchondral cancellous trabeculae, which weakens the articulating surface.