Myositis Ossificans of the Soleus Muscle

Myositis ossificans (MO) is a rare condition in which abnormal calcification and bone formation develop within a muscle. It appears most often in adolescents but can also occur in younger children. Although it usually affects a single muscle, it can sometimes involve different parts of the body. We report a case of post-traumatic MO. A 16-year-old girl presented with progressive left knee pain and swelling 15 days after minor trauma, with a visual analog scale (VAS) for pain of 8/10 and nocturnal exacerbation. Examination showed a soft, mobile, tender medial knee mass with reduced range of motion. Laboratory tests revealed elevated erythrocyte sedimentation rate and serum alkaline phosphatase with normal C-reactive protein and creatine phosphokinase. Ultrasound demonstrated heterogeneous thickening of the semimembranosus tendon with a 20×17 mm hypoechoic peritendinous area, consistent with peripheral myo-aponeurotic avulsion. Positron emission tomography imaging showed a hypermetabolic calcified extraosseous lesion adjacent to the medial tibial plateau without cortical disruption, raising concern for MO, organized hematoma, or malignancy (notably osteosarcoma). Computed tomography confirmed mixed calcifications (26×24 mm) with heterogeneous enhancement. Magnetic resonance imaging revealed an ovoid calcified soft-tissue mass within the soleus/paratibial region, marked surrounding edema, and mild periosteal reaction, supporting an intermediate-stage juxtacortical MO and making a primary bone tumor unlikely. Management included nonsteroidal anti-inflammatory drugs, ketoprofen, then indomethacin, zoledronic acid infusion, and physical therapy emphasizing pain-free mobilization and ultrasound therapy.