A 15-year-old girl with a right leg mass

A l!%year-old white girl noticed pain in the right knee and a lump medial to her right knee after a field hockey injury. Over the course of several months, physical therapy relieved most of the pain, and the lump began to shrink but did not completely resolve. Six months after the initial injury occurred, the mass became more tender and swollen. She referred herself to an orthopedic surgeon at the State University of New York (SUNY) Health Science Center at Syracuse. On examination the patient walked without a limp and was able to toe walk, heel walk, and jump without discomfort in the right knee. A firm, tender mass approximately 4 cm in diameter was palpated on the medial aspect of the right knee, just posterior to the medial collateral ligament. There was no evidence of ligamentous instability or joint effusion. Plain films and magnetic resonance (MR) images of the knee were obtained. A bone scan appeared normal.