An 11-year old boy presented to the pediatric emergency department with left knee pain after tripping in the school yard and striking his knee on concrete. He denied having left knee pain before his fall. There was no history of recent skin or other infections, fever, or constitutional symptoms or signs. His medical history and family history were noncontributory.On physical examination, his skin was intact except for a small superficial abrasion over his left patella. A small knee effusion was present and the patella was tender to palpation. He was able to actively extend the knee, but flexion was painful and limited to 30°. The remainder of the physical examination was unremarkable. Radiographs were interpreted as showing a minimally displaced transverse patella fracture (Fig. 1). He wore a cylinder cast for 6 weeks and radiographs were obtained after cast removal (Fig. 2). Three months after cast removal, he was prescribed physical therapy for persistent quadriceps weakness. This consisted of quadriceps strengthening, active assisted knee range of motion, and activity as tolerated. When seen 8 months after injury for routine followup, he had normal knee motion but complained of pain with maximal knee flexion and prolonged walking.Radiographs were repeated 8 months after injury (Fig. 3) and the prior radiographs were rereviewed. MRI of the knee was performed 10 months after injury (Fig. 4). Complete blood count, erythrocyte sedimentation rate, and C-reactive protein were normal.Based on the history, physical examination, and imaging studies, what is the differential diagnosis?Imaging Interpretation