Myositis ossificans circumscripta of the knee improved by alendronate.

Myositis ossificans circumscripta (MOC) is a rare benign disorder characterized by heterotopic ossification of soft tissues. We report a case in a 21-year-old male who presented with knee pain after strenuous physical activity. Evidence of inflammation of the knee and a hard swelling in the subquadricipital area were the main physical findings. The initial plain radiographs showed a subpatellar soft tissue mass containing flocculent calcifications. Over the next 2 months, the calcific deposits organized into a calcified mass. Magnetic resonance imaging findings were nonspecific, with a mass within the vastus medialis muscle generating low signal on T1 images and high signal on T2 images, without postgadolinium enhancement. Computed tomography showed several calcific deposits arranged in a ring, strongly suggesting MOC. Histological examination of a biopsy specimen confirmed this diagnosis. Alendronate therapy given for 6 months was followed by marked clinical improvement, obviating the need for surgery.