Pathophysiology: Myositis ossificans manifests in 2 forms.

Myositis ossificans circumscripta can develop either in response to soft tissue injury (eg, blunt trauma, stab wound, fracture/dislocation, surgical incision) or can occur without known injury. Proposed mechanisms for atraumatic myositis ossificans include nondocumented trauma, repeated small mechanical injuries, and nonmechanical injuries caused by ischemia or inflammation.

Myositis ossificans progressiva is an autosomal dominant genetic disorder with complete penetrance and variable expression. Overexpression of bone morphogenetic protein 4 and its messenger ribonucleic acid (RNA) occurs; this protein has been mapped to chromosome band 14q22-q23.

Frequency:

• Internationally: The point prevalence of myositis ossificans progressiva in the UK is 0.61 X 10^-6.

Race: Myositis ossificans progressiva occurs in all races. More cases are reported in Europe and North America because of increased survival.

Sex: Myositis ossificans progressiva demonstrates no definitive sexual predilection because the slight male predominance overall probably relates to differences in physical activity levels between the genders.

Age:

• Nonhereditary myositis ossificans is rare in children; fewer than 6.7% of cases occur in the first decade of life.

• In myositis ossificans progressiva, average age of onset for ossification is 5 years, with a reported onset range of birth to 25 years.

History:

• Nonhereditary myositis ossificans

• Pain, tenderness, focal swelling, and joint muscle reduction occur.

• The condition rarely is asymptomatic and may be diagnosed from radiographs obtained for unrelated problems.

• The history of trauma causing the condition may be difficult to elicit.

• Most (ie, 80%) ossifications arise in the thigh or arm. Other sites include intercostal spaces, erector spinae, pectoralis muscles, glutei, and the chest.

• Hereditary myositis ossificans progressiva

• The condition produces painful lumps and stiffness in the adjoining joint. Lumps decrease in a few weeks, but joint mobility reduction persists.

• Exacerbating factors for ossifications at new sites include venipuncture, biopsy of lumps, IM injections, dental treatments, and excision of masses.

• The most common sites are the sternocleidomastoid muscle, paraspinal muscles, the jaw's masticatory muscles, and shoulder and pelvic girdle muscles. Spared are the abdominal muscles, extraocular muscles, and GI tract and tongue muscles.

• Ossification progresses from proximal to distal and cranial to caudal.

Physical:

• Nonhereditary myositis ossificans

• Tender mass, with or without cutaneous erythema

• Associated joint motion abnormalities

• Fever (rare)

• Hereditary myositis ossificans progressiva

• Extremities - Microdactyly or adactyly of thumb and great toe, hallux valgus or hallux rigidus, webbed toes, progressive reduction defects in all digits, fifth digit clinodactyly

• Head and neck - Torticollis, trismus (involvement of masticators), deafness, baldness, mental retardation

• Scoliosis - Kyphosis, restricted shoulder and pelvic girdle movements

Lab Studies:

• Routine laboratory results may be within reference ranges. Erythrocyte sedimentation rate (ESR) and WBC count rarely are elevated.

• In myositis ossificans progressiva, ECG findings may be abnormal, while spirometry may demonstrate restrictive pattern.

Imaging Studies:

• Plain radiography of nonhereditary myositis ossificans traumatica

• Early examination may be unremarkable.

• Floccular calcified density is observable in soft tissues at 2-6 weeks from onset.

• By 6-8 weeks, calcification becomes sharply circumscribed.

• Half of all ossifications adhere to the periosteum.

• Obtain serial radiographs to distinguish between myositis ossificans and osteosarcoma. In osteosarcoma, calcification extends from center to periphery. In myositis ossificans, calcification first occurs in the periphery of the soft tissue mass. Myositis ossificans' calcification occurs in association with the bone's diaphysis, unlike osteogenic sarcoma's association with the metaphysis.

• Plain radiography of myositis ossificans progressiva

• Short metacarpals and metatarsals

• Phalangeal synostosis (eg, monophalangeal great toe)

• Vertebral fusions, vertebral anomalies (ie, small bodies), pedicle thickening

• Thick, short femoral neck

• Variations in bone maturation sequence

• Increased incidence of enchondromas

• CT scan demonstrates fascial plane edema and swelling, even before ossification has occurred.

• MRI depends upon the age of the lesion.

• In immature lesions, T2-weighted spin-echo images are associated with a homogeneous soft tissue mass with increased signal intensity. Surrounding edema may be seen in lesions less than a few months old. In T1-weighted images, only mass effect may be noted with displacement of fascial planes.

• Mature lesions appear as inhomogeneous masses with fatlike signal intensity on both T1- and T2-weighted images.

• Bone scintigraphy reveals enhanced uptake in immature lesions. Once maturation occurs, uptake becomes comparable with normal bones.

• Ultrasound examination shows echogenic and shadowing mass.

Procedures:

• Once diagnosis is established, further biopsy of additional lesions is contraindicated in myositis ossificans progressiva.

• In nonhereditary myositis ossificans, perform a biopsy primarily to exclude osteosarcoma.

Histologic Findings: Ossification has 3 distinct zones: the central undifferentiated zone, the surrounding zone of immature osteoid formation, and the peripheral zone with mature bone. At least 10 days are required following onset of symptoms for these zones to become apparent. If the biopsy is performed before 10 days have elapsed, or if a biopsy sample is obtained from the central region, the specimen yields undifferentiated tissue resembling an osteosarcoma.

In contrast to osteosarcoma, myositis ossificans exhibits a zonal pattern, the lesion has viable muscle fibers, and myositis ossificans does not invade surrounding tissue.

Biopsy performed after ossification maturation reveals primarily mature lamellar bone. Biopsy of lesions from patients with myositis ossificans atraumatica may lack the typical histological appearance of myositis ossificans.

Medical Care: Immediately immobilize the patient, and keep the patient immobile 2-4 weeks. Follow immobilization with a regime of gradually increased exercise to promote a greater range of motion. While myositis ossificans progressiva has no proven medical therapy, patients with this condition may be administered cortisone and adrenocorticotropin during acute episodes. Pain medications may be indicated, as are other supportive measures, especially occupational therapy, to facilitate functioning.

Deterrence/Prevention:

• Patients with mild features of myositis ossificans progressiva can reproduce; their offspring have a 50% probability of inheriting the condition.

Prognosis:

• The prognosis for patients with nonhereditary myositis ossificans includes the following:

• Possible spontaneous resorption

• Typical lesions that decrease in size and persist

• Patients with myositis ossificans progressiva experience progressive ossification.