We present a case of intraarticular myositis ossificans in the right knee of a child. Myositis ossificans (MO), though relatively rare in childhood and even more uncommon within a joint, should be included in the differential diagnosis of an intra-articular mass when indicated by the typical clinical, radiographic, and histologic findings. An 11-year-old male presented with a history of trauma to his right knee. Four weeks after the initial injury, an MRI demonstrated evidence of an ACL rupture with a "cystic mass" within the intercondylar notch along the anterior surface of the torn ligament. At subsequent arthroscopy, the mass noted on MRI was removed. The histology was consistent with MO. The authors believe this to be the first case of MO in the intercondylar notch detected by MRI, treated by arthroscopy, and confirmed by histology.
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