A 38-year-old Man with Left Knee Pain

Abstract

A 38-year-old man presented with a 2.5-month history of spontaneous onset of dull, aching left knee pain localized over the proximal tibia and anteromedial knee. The pain was aggravated by activity, improved by rest, and absent at night. It was not relieved by nonsteroidal antiinflammatory drugs or mild narcotic analgesics, and he subsequently was referred for orthopaedic evaluation. The patient denied any constitutional symptoms such as fever, chills, night sweats, fatigue, or recent weight loss. Physical examination of the left lower extremity revealed no skin abnormalities, palpable masses, or lymphadenopathy. Neurologic examination was normal. The left knee was stable with no joint line tenderness but had decreased range of motion from 0 to 70 and a moderate effusion. There was mild tenderness to palpation over the proximal medial tibia. Imaging studies included radiographs (Fig. 1) and MRI (Fig. 2). Based on the history, physical examination, and imaging studies, what is the differential diagnosis?