What is the MRI appearance of hip osteonecrosis?
The MRI appearance depends on the stage of disease. In the early stage, AVN presents on MRI as diffuse femoral head bone marrow edema, identical to that seen in ITOH, which can create a diagnostic challenge. Subsequently, the edema becomes more focal in the subchondral anterolateral femoral head, with a characteristic serpentine low signal intensity rim surrounding an area of fatty marrow, which is the most commonly seen geographic pattern. In the later stage, there is loss of the spherical shape of the femoral head due to subchondral fracture, with collapse and sclerosis appearing as an area of low T1-weighted and variable T2-weighted signal intensity. This corresponds to the “crescent sign” seen on radiographs.