summary Myositis Ossificans is a reactive soft tissue bone-forming process that commonly occurs following a traumatic event to soft tissues. The condition typically presents in patients between ages 15 and 35 who develop pain, tenderness, swelling, and decreased range of motion following localized trauma. Diagnosis is made with radiographs showing peripheral bone formation with central lucent area within soft tissues. Treatment is usually observation as lesions typically resolve over time. Surgical excision is indicated for lesions that remain persistently symptomatic. Epidemiology Demographics most common in young active males (15 to 35 years old) Anatomic locations quadriceps, brachialis and gluteal muscles Etiology A form of heterotopic ossification that is the result of direct trauma intramuscular hematoma most common location is the diaphysis of long bones Associated conditions Must differentiate from tumors Fibrodysplasia ossificans progressiva (FOP) is a rare subtype of heterotopic ossification involves mutation of the ACVR1 gene (activin A type I receptor gene, a BMP type-1 receptor) Genetics almost always a posttraumatic condition Presentation Symptoms pain, tenderness, swelling and decreased range of motion that usually presents within days of the injury pain and size of the mass decrease with time mass increases in size over several months (usually 3 to 6 cm) after the mass stops growing, it becomes firm Physical exam palpable soft tissue mass restricted range of motion Imaging Radiographs peripheral bone formation with central lucent area may appear as "dotted veil" pattern MRI with gadolinium rim enhancement is seen within the first 3 weeks CT scan lesion has an eggshell appearance Histology Characteristic histology shows zonal pattern periphery of lesion mature trabeculae of lamellar and woven bone calcification seen on xray center of the lesion irregular mass of immature fibroblasts cartilage component may be present (no calcification seen on xray) no cellular atypia seen Treatment Nonoperative rest, range of motion exercises, and activity modification passive stretching is contraindicated (makes it worse) physical therapy utilized to maintain range of motion radiographic monitoring obtained to confirm maturation of the lesion Operative surgical excision indicated only if it remains a problem after it matures do not operate in acute phase, wait at least six months excision of the lesion within 6 to 12 months predisposes to local recurrence Prognosis Usually self limiting mass usually begins to decrease in size after 1 year