Discussion
MOC is a benign entity and yet it can mimic more aggressive pathology
(15). It can be traumatic or nontraumatic (1). Some conditions share
similar names or are related to MO – MO circumscripta, fibro-osseous
pseudotumor of the digits, panniculitis ossificans, and MO progressiva
(MOP) (16). MOC occurs following traumas or surgical operations such as
an abdominal scar, orthopedic operation, and dislocation (17). MOP
mainly occurs in early life, progressively affects all skeletal muscles,
and leads to death (18,19). Nontraumatic MOC is the least common type
and can easily be confused with malignant tumors, due to the lack of
history of trauma (20). The presented case had no history of recorded or
memorable trauma. The most common locations of MOC are the arm,
shoulder, thigh, and hand (21), in our case, it arose from the left
external oblique muscle, which is extremely rare.
MOC may occur at any age but rarely occurs in children and older
patients (14,22). The youngest documented case of MOC in the literature
was a five-month-old girl (22) and the oldest was an 81-year-old woman
(14). Males are more commonly affected than females (4). The presented
case was a 7-year-old male child.
Even though up to three-quarters of patients had a history of trauma,
there are also nontraumatic MOC cases reported (23), which are similar
to our case.
Radiological investigations are very important in the detection,
diagnosis, and follow-up of MOC (18). Moreover, it is a useful tool for
differentiating MOC from infections and malignancies (24). However, the
radiological findings are typically vary depending on the lesion
maturity (25,26).
MOC maturity has three phases which are; active (early), subacute
(intermediate), and chronic (mature) (27). The presented case was in its
mature or chronic phase during presentation.
Plain radiographs are mostly normal in the active phase and
mineralization is seen in the chronic phase (13). Ultrasonography and
magnetic resonance imaging (MRI) are important in the acute phase of
symptomatology even before the appearance of calcifications (2,13,28).
Ultrasound scan of the mass showed predominantly hypoechoic mass in the
left hypochondriac with the increased flow on color Doppler.
The findings of myositis ossificans on MRI scans depend on the maturity
phase (2,24). MRI is not sensitive in soft-tissue calcification
detection, as a result, it can be difficult to differentiate from
adjacent muscles with marked surrounding edema in the early phases
(12,18). In the subacute phase, a hypointense border corresponding to
peripheral calcification is observed, whereas the intensity decreases in
the chronic phase (29).
Computed tomography is the preferred imaging modality for detecting and
diagnosing myositis ossificans since it is very good at showing the
zonal patterns of the calcification that are expected at the different
phases of MOC (20,26). The differentials of MOC in the acute or subacute
phases are muscle abscess, rhabdomyolysis, focal myositis, and
soft-tissue sarcoma (12,20). Bone scintigraphy is the very sensitive
imaging modality in the early detection of myositis ossificans, shows
increased uptake in damaged muscle (15). In the presented case the
lesion has lasted 3 years and the entire mass was made up of mature
compact bone.
The CT scan revealed a large micro-lobulated calcified hyperdense mass
in the left external oblique muscle of the abdomen.
Histopathology in the acute phase of some MOC cases could be
misinterpreted for fibromatosis or sarcoma, but once it reaches the
chronic phase, the diagnosis is clear (30). Our case was in the chronic
phase, and histopathological examination showed tissue composed of
mature bone trabeculae.
There are different management approaches reported in the literature,
that includes immobilization, elevation, ice application, cold laser,
acetic acid iontophoresis, extracorporeal shockwave therapy, and
surgical excision (23,31). Among the different management approaches,
surgical excision is the most common treatment for non-traumatic
myositis ossificans seen in an unusual site (32). The presented case had
surgical excision of the mass.