Myositis Ossificans

• Heterotopic bone formation often occurs after injury or surgery and can occur in any collagenous supportive tissue of skeletal muscles, tendons, ligaments, and fascia.

There are four clinical types; three may be seen in injured patients:

o Myositis ossificans progressiva is rare and can be genetic. It usually occurs between the ages of 5 and 10 years (younger than age 20) and proceeds relentlessly to progressive ossification of skeletal muscles. It is often present in the shoulders and neck as firm subcutaneous masses, which can be hot and tender and can undergo ossification. Often associated are microdactyly of the great toes and thumbs, ankylosis of the interphalangeal and metatarsophalangeal joints, and bilateral hallux valgus. Minor trauma often causes exacerbations. Treatment may include diphosphonate combined with surgery for severe joint malpositioning and functional impairment.

o Myositis ossificans paralytica occurs in proximal paralyzed muscles. The ossification occurs 1 to 10 months after a spinal cord injury. This process causes decreased passive range of motion. The three classic sites are in the vastus medialis, the quadratus femoris, and the hip abductors. Surgical treatment is indicated only if the position and function of the extremity are unacceptable and when the ossification has matured. After excision, the dead space created must be drained by closed suction and the wound carefully observed for a hematoma.

o Myositis ossificans circumscripta can be idiopathic but is more commonly caused by focal trauma and is common as a sports injury in the contact setting. It is more common in teenage or young adult males. It presents as an uncomfortable, indistinct mass that shows local induration and a local increase of temperature. The lesion occurs 80% of the time in the arm (biceps brachialis) but also occurs in the thigh (abductors and quadratus femoris). Roentgenograms show fluffy calcification 2 to 4 weeks after injury. In 14 weeks, the calcification has matured, and in 5 months, ossification has occurred. The differential diagnosis includes osteosarcoma and periosteal osteogenic sarcoma. Treatment is by excision, only if the lesion is unusually large or painful and after ossification is mature.

o Myositis ossificans traumatica, the most common type of hetertopic ossification presents the same way as the circumscripta type except for a clear history of trauma, with ossification of a single muscle group in the traumatized area (17). Treatment is controversial but generally is aimed at the prevention of ossification by immediate application of cold and compression to the area of muscle injury. Later, heat is applied. An operation is indicated only when the ossification causes permanent impairment and only after the process has stabilized, often as soon as 6 to 8 months after injury.

• The precise pathophysiology of myositis ossificans is not known. Preventive treatment should be designed to stop the sequence of osteogenesis

Treatment

o Rest to the part.

o Pharmacologic treatment is generally prophylactic and has historically included bisphosphonates to inhibit hydroxyapatite crystallization, mithramycin to interfere with mobilization of calcium, and cortisone to decrease bone formation at the site of injury. None of these drugs, however, has proved to be an extremely beneficial therapeutic agent. Indomethacin and Naprosyn have been shown to help minimize posttraumatic heterotopic ossification associated with acetabular fractures and arthroplasty. Similarly, low-dose irradiation with 800 to 1,000 rad has been shown to be very effective at preventing heterotopic ossification.

o When surgical treatment is indicated, traditional teaching has been to wait until the ossification is mature that is, when the bone scan is negative and the alkaline phosphatase level is decreasing. Many authors have recently advocated earlier resection before these tests have returned to normal.