Myositis ossificans is a lesion of heterotopic ossification, that is, onein which extra-skeletal bone forms within soft tissue. The target tissue is typicallymuscle (as the name of the condition suggests), though heterotopic ossificationcan occur tendon, ligament and fascia, among other locations. Myositisossificans usually forms after an injury. The typical patient is a young male who develops focal pain, tenderness and swelling after trauma. Myositisossificans is a benign, self-limiting process, characterized by three distinct stages:early, intermediate, and mature. Each stage exhibits different clinical and radiographic findings. Importantly, myositis ossificans can be confused withvarious malignancies, and thus attention to the radiographic findings,especially as a function of time, is key. Treatment is usually non-operative,with surgical resection limited to mature lesions that are persistently painful.

Epidemiology

Young males betweenages 15 and 35 are most commonly affected. This pattern of incidence is likelydue to the greater frequency of blunt soft-tissue trauma (e.g., sports injuries)in this population. Likewise, because direct trauma is the usual cause, thethigh, arm, and buttocks are the most commonly affected regions. Thedevelopment of myositis ossificans is also associated with repetitive minortrauma. Examples include so-called rider’s bone (myositis ossificans within theadductor muscles from horseback riding) and shooter’s bone (myositis ossificanswithin the upper arm muscles from the recoil of a rifle butt).

Clinical Features

Patients present with variable symptoms based on the stage of myositis ossificans.

• In the early stage (within4 weeks), patients will present with pain, swelling, and associated jointstiffness.
• In the intermediatestage (4-8 weeks), the pain will improve but joint stiffness will remain, and patientsmay report a palpable mass.
• In the mature stage(greater than 8 weeks), the pain will resolve. However, a persistent soft-tissuemass and permanent decreased range of motion of the nearby joint may be present.Chronic pain may occur if there is soft-tissue irritation.

Blood tests tend tobe not helpful when evaluating for myositis ossificans, but there have been reportsof alkaline phosphatase increasing in the intermediate stage and normalizing duringthe mature stage.

Radiologic Features

In the early stage, anormal xray reading is common if no fracture is visible. In the intermediatestage, peripheral calcifications can be seen within the soft tissue (Figure 1).The center of the calcifications remains lucent. In the mature stage, the massis fully ossified and very visible on x-ray. There is little to no adjacentperiosteal reaction.

Figure 1: Lateral radiograph of the left femur in a skeletally immature patient, demonstratingperipheral calcifications within the anterior soft tissue consistent withmyositis ossificans.

Magnetic resonanceimaging (MRI) is the best modality for evaluating myositis ossificans. In theearly stage when hematoma and inflammation remain the hallmark features, MRI showsa heterogenous mass within the soft tissue with little internal enhancement. Inthe intermediate stage, peripheral areas of calcification may start to show,and the mass may become isointense to muscle. Surrounding edema may or may not be present. In the mature stage, the mass is hypointense on both T1- and T2-weightedsequences since mature bone is now the dominant tissue. Computed tomography (CT)scans can be helpful in the intermediate stage of myositis ossificans toclearly delineate the peripheral rim of calcifications.

Pathology

The histologicappearance of myositis ossificans is based on the stage of the lesion. In theearly stage, low power reveals my ofibroblastic cells within a hemorrhagic background. Some of the cells may appear atypical in nature. As a result, earlymyositis ossificans can be misdiagnosed as a soft-tissue sarcoma. In the intermediate stage, peripheral calcifications and a zonal pattern of differentiation will be present. Immature woven bone surrounded by mature lamellarbone will also be visible (Figure 2). Finally, in the mature stage, most of themass will be mature lamellar bone.

Figure 2: H&Estain demonstrating a zonal pattern of differentiation in the intermediatestage of myositis ossificans.

DifferentialDiagnosis

Heterotopic ossification can also occur after total hip arthroplasty(especially if the hip arthritis was associated with ankylosing spondylitis andPaget's disease) and after fractures, particularly those of the acetabulum andelbow. In addition, patients with spinal cord and traumatic brain injuries(TBI) are well known to develop heterotopicossification — caudal to the spinal cord injury, if that is the cause, oraround any large joint if a TBI is responsible. There are also rare geneticcauses of heterotopic ossification including fibrodysplasia ossificansprogressiva (FOP) and progressive osseous heteroplasia (POH). [Both FOP and POHare beyond the scope of this chapter.]

Disease Course: Treatmentand Prognosis

The first line treatment of myositis ossificans is non-surgical:immobilizing the affected extremity for a brief period of time alongside “RICE”(Rest, Ice, Compression, Elevation). Early physical therapy to restore joint rangeof motion and strength training have been shown to be effective in limiting thedisease. No further treatment is warranted if a mature myositis ossificanslesion remains asymptomatic.

Surgical interventionis reserved for symptomatic myositis ossificans. Pain can be caused by localirritation of the soft tissues or compression of neurovascular structures. In suchcases, marginal excision is indicated and should be performed after the lesionhas reached the mature stage and the patient has failed non-operative management. At the time of surgery, all bleeding vessels should be controlled tominimize the risk of developing a hematoma. Although local recurrence afterexcision has been reported, the magnitude of this risk is not known withcertainty.

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