Myositis ossificans comprises two syndromes characterized by heterotopic ossification (calcification) of muscle.
- In the first, and by far most common type, nonhereditary myositis ossificans (commonly referred to simply as "myositis ossificans", as in the remainder of this article), calcifications occur at the site of injured muscle, most commonly in the arms or in the quadriceps of the thighs.
- The second condition, myositis ossificans progressiva (also referred to as fibrodysplasia ossificans progressiva) is an inherited affliction, autosomal dominant pattern, in which the ossification can occur without injury, and typically grows in a predictable pattern. Although this disorder can be passed to offspring by those afflicted with FOP, it is also classified as nonhereditary, as it is most often attributed to a spontaneous genetic mutation upon conception.
Most (i.e. 80%) ossifications arise in the thigh or arm, and are caused by a premature return to activity after an injury. Other sites include intercostal spaces, erector spinae, pectoralis muscles, glutei, and the chest. On planar x-ray, hazy densities are sometimes noted approximately one month after injury, while the denser opacities eventually seen may not be apparent until two months have passed.
Pathophysiology of myositis ossificans traumatica
The specific cause and pathophysiology are unclear - it may be caused by an interaction between local factors (e.g., a reserve of available calcium in adjacent skeletal tissue or soft tissue edema, vascular stasis tissue hypoxia or mesenchymal cells with osteoblastic activity) and unknown systemic factors. The basic mechanism is the inappropriate differentiation of fibroblasts into bone-forming cells (osteoblasts). Early edema of connective tissue proceeds to tissue with foci of calcification and then to maturation of calcification and osifications
Calcification is typically depicted 2 weeks earlier at US when compared to radiographs. The lesion develops in two distinct stages with different presentations at US. In the early stage, termed immature, it is depicts a non-specific soft tissue mass that ranges from a hypoechoic area with an outer sheet-like hyperechoic peripheral rim to a highly echogenic area with variable shadowing. In the late stage, termed mature, myositis ossificans is depicted as an elongated calcific deposit that is aligned with the long-axis of the muscle, exhibits acoustic shadowing, and has no soft tissue mass associated. US may suggest the diagnosis at early stage, but imaging features need to evolve with successive maturation of the lesion and formation of the characteristic late stage changes before they become pathognomonic.
The differential diagnosis includes many tumoral and nontumoral pathologies. A main concern is to differentiate early myositis ossificans from malignant soft-tissue tumors, and the latter is suggested by a fast-growing process. If clinical or sonographic findings are dubious and extraosseous sarcoma is suspected, biopsy should be performed. At histology, detection of the typical zonal phenomenon is diagnostic of myositis ossificans, though microscopic findings may be misleading during the early stage.
Radiation therapy subsequent to the injury or as a preventive measure of recurrence may be applied but its usefulness is inconclusive.
Treatment is initially conservative, as some patients' calcifications will spontaneously be reabsorbed, and others will have minimal symptoms. In occasional cases, surgical debridement of the abnormal tissue is required, although success of such therapy is limited.
Treatment of myositis ossificans:
- Anti-inflammatory drugs
- Physiotherapy management
Surgical removal of the myositis ossificans is rarely warranted. If the myositis ossificans is excised before its maturation, it will likely reoccur. Consequently, most healthcare providers wait 6–12 months before considering excision. There is a chance of relapse even when removed in a mature state. In general, myositis ossificans is removed surgically if it limits the range of motion of the joint, irritates a nerve or lowers quality of life.
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