What are the histological findings in the early phase of myositis ossificans?

Histological Findings in the Early Phase of Myositis Ossificans

The early phase of myositis ossificans is characterized by an inflammatory pseudotumor with proliferating fibroblasts and myofibroblasts, immature osteoid formation, and a lack of peripheral ossification that can mimic malignancy. 1, 2

Histopathological Features

Early Phase Findings

• The earliest histological feature is an inflammatory reaction with infiltration of polymorphonuclear cells (acute infection) or mononuclear cells (chronic inflammation) within the affected tissue 3
• Proliferating fibroblasts and myofibroblasts are present, creating a pseudosarcomatous appearance that can be mistaken for malignancy 2
• Immature osteoid formation begins without the characteristic zonal pattern seen in later stages 4
• Muscle fibers show evidence of degeneration and necrosis with preservation of muscle fascicles 2

Zone Phenomenon

• The characteristic "zone phenomenon" begins to develop but is not fully formed in the early phase 1
• In early stages, the lesion lacks the peripheral ossification that defines mature myositis ossificans 4, 2
• CT is superior to radiography in detecting the early zonal pattern of mineralization, which is essential for radiologic diagnosis of early myositis ossificans 3

Diagnostic Challenges

Mimicking Malignancy

• Early myositis ossificans can be mistaken clinically and histologically for malignant soft tissue tumors due to its pseudosarcomatous appearance 1, 2
• The absence of calcification or ossification on imaging in early stages makes diagnosis particularly challenging 4
• Histologically, the proliferative fibroblastic component can resemble sarcoma, requiring careful pathological assessment 2

Imaging Correlation

• Histopathological findings should be correlated with imaging studies for accurate diagnosis 3
• MRI in early stages may show an intramuscular lesion with preserved muscle fascicles 2
• Ultrasound can reveal the early "zone phenomenon" before calcification is visible on radiographs 1

Biopsy Considerations

• Core needle biopsies using 14-16G needles are the standard approach for diagnosis 3
• Frozen-section technique for immediate diagnosis is not recommended as it may not allow complete assessment of the lesion 3
• Biopsy material should be fixed in 4% buffered formalin (not Bouin fixative) to allow for molecular analysis if needed 3
• Collection of fresh frozen tissue and tumor imprints is encouraged to enable additional molecular pathology assessments 3

Distinguishing Features from Other Conditions

• Unlike inflammatory myopathies (dermatomyositis, polymyositis), myositis ossificans lacks the characteristic lymphocytic infiltration and perifascicular atrophy 3
• Unlike malignant tumors, myositis ossificans shows a more organized pattern of cellular proliferation even in early stages 2
• Absence of nuclear atypia and abnormal mitotic figures helps differentiate from sarcomas 2

Early diagnosis of myositis ossificans is crucial as it can prevent unnecessary aggressive treatments. While waiting for complete maturation of the lesion has traditionally been recommended, early surgical excision may be beneficial in cases with severe symptoms, even before radiological evidence of calcification appears 4.