MRI of fibrous cortical defect of the femur

The MR imaging findings of 10 cases of fibrous cortical defect of the femur are presented. Although surgical biopsy was not available in the 10 cases, clinical follow-up confirmed the diagnosis. Most of the lesions were located on the posteromedial aspect of the distal femur, corresponding to the site of attachment of the extensor tendon of the adductor magnus muscle or the medial head of the gastrocnemius. They were oval in shape, and their diameter ranged from 29 x 15 mm to 8 x 6 mm, with an average of 14 mm. With MR, most of the fibrous cortical defects were imaged with iso-signal intensity to muscle on T1-weighted images and high signal intensity on T2*-weighted images. They were invariably surrounded by low signal intensity rims on both sequences, representing marginal sclerosis. The appearance seemed to be sufficiently diagnostic to obviate the need for biopsy.