The crescent sign refers to a linear cleft due to a subchondral fracture in the setting of osteonecrosis.
It was originally described as a radiographic finding, however, it can also be seen on cross-sectional imaging.
In the setting of Legg-Calve-Perthes disease, it heralds the beginning of the fragmentation phase (stage 2).
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Pathology
The cause of the crescent sign is bone trabeculae failure, leading to subchondral fracture. The fracture may propagate through the subchondral bone plate to the joint space, with a formation of an unstable osteochondral flap.
Location
The crescent sign is most often seen in the femoral and humeral head, as well as the scaphoid, lunate and talus.
Radiographic features
Plain radiograph
The crescent sign is seen as a curvilinear lucent subchondral line. If articular surface flattening is present, it suggests an impending articular collapse.
In the femoral head, it is present usually in its anterior or anterolateral part. Both frontal and frog-leg views should be obtained for assessment.
CT
A subchondral fracture line is seen. Additionally, a presence of gas may be detected within the fracture cleft.
MRI
The subchondral fracture cleft may be filled with fluid. The presence of bone marrow edema distally from the fracture line indicates a stress reaction and a very high likelihood of trabeculae collapse.
Radiology report
Various classification systems of osteonecrosis use the presence of the crescent sign in the assessment of adult femoral head osteonecrosis 5.
Its presence (without more advanced findings) indicates:
grade III of Ficat and Arlet classification
grade III of Steinberg classification (mentioning the extent of involvement as mild, moderate or severe)
grade III of ARCO classification
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