What are the histopathological findings of fibrous dysplasia?

Histopathological Findings of Fibrous Dysplasia

The classic histopathological finding of fibrous dysplasia is irregularly shaped bony trabeculae in a fibrous stroma of variable cellularity without osteoblast rimming, resembling "Chinese characters" or "alphabet soup" pattern. 1

Key Microscopic Features

Bone Component

• Woven bone trabeculae:
  • Irregularly shaped and haphazardly arranged
  • Lack normal lamellar structure
  • Absence of osteoblastic rimming (critical diagnostic feature)
  • Resemble "Chinese characters" or "alphabet soup" pattern 1, 2

Fibrous Component

• Fibrous stroma:
  • Variable cellularity (can range from hypocellular to moderately cellular)
  • Composed primarily of spindle-shaped fibroblasts
  • Arranged in a storiform or whorled pattern
  • Background of collagenous matrix 1, 3

Other Characteristics

• Absence of cartilaginous differentiation (important for differential diagnosis)
• Minimal inflammatory component
• Occasional cystic degeneration or hemorrhagic areas
• Rare mitotic figures (always typical, never atypical) 4, 5

Histological Variants

Three major histological patterns may be observed:

1. Pagetoid pattern: Characterized by thickened, disorganized trabeculae with a mosaic pattern
2. Sclerotic pattern: Dense, compact bone with minimal fibrous stroma
3. Cystic pattern: Predominant fibrous component with scattered small islands of bone 4

Diagnostic Pitfalls

• Sampling error: Small biopsies may not be representative of the entire lesion
• Misdiagnosis: Can be confused with other fibro-osseous lesions:
  • Ossifying fibroma (which typically shows osteoblastic rimming)
  • Low-grade osteosarcoma (which shows cellular atypia)
  • Osteochondroma (which has a cartilage cap) 1
• Radiologic-pathologic correlation: Essential for accurate diagnosis as radiographic appearance (ground-glass) should match histologic findings 1, 4

Special Considerations

• Malignant transformation: Extremely rare but possible, characterized by sudden change in growth pattern, cellular atypia, and increased mitotic activity
• Polyostotic vs. monostotic: Histologically identical, differentiation requires clinical and radiological correlation
• McCune-Albright syndrome: Same histological features as isolated fibrous dysplasia, but associated with endocrinopathies and café au lait spots 2

The diagnosis of fibrous dysplasia requires integration of clinical, radiological, and histopathological findings, as the histological appearance alone may sometimes be inconclusive or overlap with other fibro-osseous lesions.