Myositis ossificans comprises two syndromes characterized by heterotopic
ossification of muscle. Classification
In the first, and by far most common type, nonhereditary myositis ossificans,
calcifications occur at the site of injured muscle, most commonly in the
arms or in the quadriceps of the thighs. The term myositis ossificans traumatica
is sometimes used when the condition is due to trauma. Also Myositis ossificans
circumscripta is another synonym of myositis ossificans traumatica refers to
the new extraosseous bone that appears after trauma.
The second condition, myositis 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 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 and unknown systemic factors. The basic
mechanism is the inappropriate differentiation of fibroblasts into
bone-forming cells. Early edema of connective tissue proceeds to tissue
with foci of calcification and then to maturation of calcification and
osifications. Sonographic diagnosis
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. Radiologic diagnosis
The radiological features of myositis ossificans are ‘faint soft tissue
calcification within 2–6 weeks, separated from periosteum by lucent zone
and on CT, the characteristic feature is peripheral ossification’.
Prevention Radiation therapy subsequent to the
injury or as a preventive measure of recurrence may be applied but its
usefulness is inconclusive. If the surgery performed next step in
accordance with literature postoperative single low-dose radiation with 3 weeks
of oral indomethacin regimen will be preventive for recurrence.
Treatment 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:
Rest Reduction
Immobilization 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. References
External links Radiograph showing myositis ossificans