We report the case of a 15-year-old girl who suffered an indirect knee trauma. The standard X-rays revealed a tract of ivory-like bone partially obstructing the medullary canal of the femur and the tibia. Magnetic resonance imaging produced a hypointense signal on the T1 and T2 sequences. The radiographic diagnosis was melorheostosis which was confirmed on the biopsy specimen which ruled out other diagnoses. The MRI also revealed a tear of the anterior cruciate ligament, treated conservatively. Clinical and radiological surveillance were proposed for the melorheostosis.