Myositis ossificans is a lesion of heterotopic ossification, that is, one in which extra-skeletal bone forms within soft tissue. The target tissue is typically muscle (as the name of the condition suggests), though heterotopic ossification can occur tendon, ligament and fascia, among other locations. Myositis ossificans usually forms after an injury. The typical patient is a young male who develops focal pain, tenderness and swelling after trauma. Myositis ossificans 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 with various 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.
Young males between ages 15 and 35 are most commonly affected. This pattern of incidence is likely due to the greater frequency of blunt soft-tissue trauma (e.g., sports injuries) in this population. Likewise, because direct trauma is the usual cause, the thigh, arm, and buttocks are the most commonly affected regions. The development of myositis ossificans is also associated with repetitive minor trauma. Examples include so-called rider’s bone (myositis ossificans within the adductor muscles from horseback riding) and shooter’s bone (myositis ossificans within the upper arm muscles from the recoil of a rifle butt).
Patients present with variable symptoms based on the stage of myositis ossificans.
- In the early stage (within 4 weeks), patients will present with pain, swelling, and associated joint stiffness.
- In the intermediate stage (4-8 weeks), the pain will improve but joint stiffness will remain, and patients may report a palpable mass.
- In the mature stage (greater than 8 weeks), the pain will resolve. However, a persistent soft-tissue mass 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 to be not helpful when evaluating for myositis ossificans, but there have been reports of alkaline phosphatase increasing in the intermediate stage and normalizing during the mature stage.
In the early stage, a normal xray reading is common if no fracture is visible. In the intermediate stage, peripheral calcifications can be seen within the soft tissue (Figure 1). The center of the calcifications remains lucent. In the mature stage, the mass is fully ossified and very visible on x-ray. There is little to no adjacent periosteal reaction.
Figure 1: Lateral radiograph of the left femur in a skeletally immature patient, demonstrating peripheral calcifications within the anterior soft tissue consistent with myositis ossificans.
Magnetic resonance imaging (MRI) is the best modality for evaluating myositis ossificans. In the early stage when hematoma and inflammation remain the hallmark features, MRI shows a heterogenous mass within the soft tissue with little internal enhancement. In the 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-weighted sequences since mature bone is now the dominant tissue. Computed tomography (CT) scans can be helpful in the intermediate stage of myositis ossificans to clearly delineate the peripheral rim of calcifications.
The histologic appearance of myositis ossificans is based on the stage of the lesion. In the early stage, low power reveals myofibroblastic cells within a hemorrhagic background. Some of the cells may appear atypical in nature. As a result, early myositis 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 lamellar bone will also be visible (Figure 2). Finally, in the mature stage, most of the mass will be mature lamellar bone.
Figure 2: H&E stain demonstrating a zonal pattern of differentiation in the intermediate stage of myositis ossificans.
- Parosteal osteosarcoma
- Extraskeletal osteosarcoma
- Soft-tissue sarcoma
Heterotopic ossification can also occur after total hip arthroplasty (especially if the hip arthritis was associated with ankylosing spondylitis and Paget's disease) and after fractures, particularly those of the acetabulum and elbow. In addition, patients with spinal cord and traumatic brain injuries (TBI) are well known to develop heterotopic ossification — caudal to the spinal cord injury, if that is the cause, or around any large joint if a TBI is responsible. There are also rare genetic causes of heterotopic ossification including fibrodysplasia ossificans progressiva (FOP) and progressive osseous heteroplasia (POH). [Both FOP and POH are beyond the scope of this chapter.]
Disease Course: Treatment and 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 range of motion and strength training have been shown to be effective in limiting the disease. No further treatment is warranted if a mature myositis ossificans lesion remains asymptomatic.
Surgical intervention is reserved for symptomatic myositis ossificans. Pain can be caused by local irritation of the soft tissues or compression of neurovascular structures. In such cases, marginal excision is indicated and should be performed after the lesion has reached the mature stage and the patient has failed non-operative management. At the time of surgery, all bleeding vessels should be controlled to minimize the risk of developing a hematoma. Although local recurrence after excision has been reported, the magnitude of this risk is not known with certainty.
Commonly Tested on Exam
- Three stages of myositis ossificans: early, intermediate, and mature.
- Each stage has a unique clinical, radiographic, and histologic appearance.
- Peripheral calcifications with central lucency is a hallmark radiographic finding.
- Myositis ossificans can be easily mistaken for a sarcoma.