Half-moon sign (femoral neck)

Last revised by Joshua Yap on 3 Oct 2022

The half-moon sign describes the morphology of bone marrow edema at the femoral neck on fluid-sensitive MRI sequences, which can be seen in osteoid osteoma or stress fractures 1-3

Differential diagnosis

  • intra-articular osteoid osteoma

    • in patients without a history of overuse, it is highly specific and sensitive for the diagnosis of osteoid osteoma 1-3

    • identification of the sign on MRI should prompt CT examination for the identification of a nidus 1

  • stress reaction/fracture of the femoral neck in cases with a history of lower limb overuse (e.g. athletes, armed forces members) 4,5

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Cases and figures

  • Case 1
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  • Case 2
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