Radiology for the surgeon: musculoskeletal case 44.

A woman was referred to an orthopedic surgeon with a complaint of vague deep-seated pain in her right knee, which had been ongoing for a period of 3 months. There was no history of a specific inciting traumatic event. Physical examination of her knee revealed no specific signs of internal derangement, and plain radiographs were unremarkable. The patient was referred for an MRI examination. T1-weighted sagittal imaging demonstrated an isointense bulbous expansion of the anterior cruciate ligament (ACL) near its femoral attachment (Fig. 1, arrow). This bulbous expansion returned a high signal on T2-weighted imaging, which showed the lesion to be sited within the apical fibres of the ligament (Fig. 2). Axial fat-saturated T2 imaging confirmed the presence of the lesion (Fig. 3). What is your diagnosis?