Fibrous dysplasia
Pathology
- Developmental bone anomaly (not a true neoplasm)
- Normal bone replaced by immature woven bone in a fibrous stroma
- Caused by postzygotic GNAS mutation
- No periosteal reaction or pain (unless complicated)
- Lesion stabilizes after skeletal maturity
Epidemiology
- Onset in childhood or adolescence
- Monostotic: discovered in teens–20s, often incidental
- Polyostotic: earlier onset, more severe
- No sex predilection (monostotic); female bias in syndromic forms
Common Locations
| Type | Sites |
|---|---|
| Monostotic | Ribs, femur, tibia, jaw, humerus |
| Polyostotic | Femur/pelvis most frequent, often asymmetric |
| Skull/facial bones | Extensive cases = Leontiasis Ossea |
Subtypes
| Subtype | Features |
|---|---|
| Monostotic | Most common (~70–80%), single site, often asymptomatic |
| Polyostotic | Multiple bones, often unilateral |
| McCune-Albright syndrome | - Polyostotic + café-au-lait spots + endocrine dysfunction (precocious puberty) - Mostly girls |
| Mazabraud syndrome | - Polyostotic + intramuscular myxomas - Middle age women - ↑ risk malignant transformation |
Complications
- Pathological fracture (esp. femoral neck)
- Deformity (e.g. “Shepherd’s crook” of proximal femur)
- Malignant transformation (rare; ↑ risk with polyostotic disease, prior radiation)
- Usually → Osteosarcoma, Fibrosarcoma, Chondrosarcoma
Histology
- Irregular woven bone trabeculae in bland fibrous stroma
- No osteoblastic rimming
- Classic: "alphabet soup"/"Chinese characters"
Imaging Features
X-ray
- Ground-glass/hazy matrix (hallmark)
- Intramedullary, expansile lesion, thinned cortex
- No periosteal reaction
- Well-defined sclerotic border ("rind sign"): often much thicker than sclerotic border in other lesions.
CT
- Better evaluation of matrix density, cortical detail, and craniofacial involvement
MRI
- T1: Low signal (fibrous)
- T2: Variable (often low-intermediate)
- May enhance (non-specific)
- Less specific than X-ray/CT
Bone Scan
- Increased uptake (can mimic metastasis in polyostotic)
- Useful for mapping disease extent
Differential Diagnosis
| Diagnosis | Key Differences |
|---|---|
| NOF | Metaphyseal, eccentric (cortical-based), younger patients |
| Enchondroma | Chondroid matrix (rings and arcs) |
| Low-grade Osteosarcoma | Aggressive features, periosteal reaction, older patients |
| Paget disease | Older age, cortical thickening, coarsened trabeculae |
| Osteofibrous dysplasia | Anterior tibia, cortical-based, Cytokeratin -ve |
Management
- Observe if asymptomatic
- Bisphosphonate for pain control (polyostotic)
- Surgery for fracture or deformity
- Avoid radiation (sarcoma risk ↑; same as Paget disease)
Exam Tips
- Buzzword MSK rad: “Ground-glass matrix”, “Rind sign”, “Shepherd’s crook deformity”
- Craniofacial ± endocrine/café-au-lait = McCune-Albright
- Malignant transformation is rare but high-yield exam trap
- Bone scan may show more extensive involvement than X-ray
Real-Life Reporting Tips
- Confirm matrix = ground-glass; avoid mislabeling as aggressive
- Report:
- Extent, deformity, fracture risk
- Suggest bone scan if polyostotic suspected
- Mention possible syndromic associations if clinical context fits
