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.