Multiple bone infarcts with intra-articular extension

A woman in her 30 s had complaints of pain over her right distal thigh and knee for 3 months. The pain was diffuse, dull aching and non-radiating. Rest pain was present. It was aggravated with activity and decreased partially with analgesics. She had no other joint pains. No history of trauma, fever, loss of weight or appetite was noted. She had no previous medical conditions. On examination, she was found to have medial joint tenderness. Her knee range of movement was upto 110° of flexion. No other significant finding was noted. Standing radiographs of the knee (figure 1) at first glance showed medial joint space narrowing. On careful inspection, abnormal areas of lucency were noted in the medullary cavity of the distal end of femur, surrounded by inconspicuous sclerosis. No periosteal reaction or soft tissue changes were noted. This radiograph was seen earlier by two orthopaedic surgeons who the patient had consulted, and she was told to have arthritis changes only. We performed an MRI (figure 2), which showed a serpiginous lesion involving the medullary cavity of the distal end of femur and proximal end of tibia. It was noted to involve the metaphyseal and epiphyseal regions, extending till the articular surface. The lesion was hypointense on T1 and hyperintense on T2 and STIR sequences. The classical ‘smoke up the chimney’ appearance was present. All her blood investigations were found to be within normal limits. She was diagnosed to have bone infarct of the distal femur and proximal tibia. Concomitant arthritis was medial tibiofemoral joint was present.