Fibrous Dysplasia - Oral Pathology (per Shafer's framework)

Note: Shafer's Textbook of Oral Pathology is not directly available in this library, but the content below is compiled from Robbins & Kumar Basic Pathology, Robbins Cotran & Kumar Pathologic Basis of Disease, Harrison's Principles of Internal Medicine, Cummings Otolaryngology, and Scott-Brown's Otorhinolaryngology - all of which align with Shafer's descriptions of this entity.

Definition

Etiopathogenesis / Molecular Basis

• The mutation constitutively activates Gsα, elevating cyclic AMP (cAMP) levels via the protein kinase A signal transduction pathway
• This promotes mesenchymal cell proliferation and disrupts osteoblast differentiation, causing fibroblast-like cells to produce imperfect woven bone instead of mature lamellar bone
• The phenotype depends on the stage of embryogenesis at which the postzygotic mutation occurs and which mesenchymal cells harbor it - this explains the mosaic (patchy) distribution of lesions

Classification

Oral and Craniofacial Features

Sites of Involvement

• The maxilla is the most commonly affected jaw bone; the mandible is less frequently involved
• Craniofacial involvement occurs in up to 25% of monostotic cases and in >50% of polyostotic cases (skull involvement)
• Other craniofacial bones: frontal, sphenoid, temporal, orbital walls, ethmoid, and zygoma
• The temporal bone may be involved (mostly monostotic form, 70%)

Clinical Presentation

• Painless, slowly progressive facial swelling causing facial asymmetry
• Malocclusion due to jaw expansion
• Migration and mobility of teeth from jaw involvement
• Displacement or impaction of teeth
• In McCune-Albright - melanosis (café-au-lait pigmentation) of gingival tissues and facial skin
• Cranial nerve involvement - conductive hearing loss (most common, ~80% with temporal bone FD) from narrowing of the external auditory canal; rarely optic nerve compression, facial nerve palsy
• Proptosis from orbital involvement
• The disease starts early in life (childhood); the monostotic form tends to become quiescent at puberty, while polyostotic form may continue to progress

Radiographic Features

Histopathology (Microscopic Features)

1. Replacement of normal cancellous bone by a cellular fibroblastic stroma arranged in a whorled pattern
2. Irregular curvilinear trabeculae of woven bone - often described as resembling "Chinese letters" or "alphabet soup"
3. No rimming osteoblasts around the woven bone trabeculae (key distinguishing feature from ossifying fibroma, which has osteoblastic rimming)
4. The stroma is moderately cellular fibroblastic proliferation without significant atypia
5. Additional findings: cystic degeneration, hemorrhage, foamy macrophages, osteoclast-type giant cells

Laboratory Findings

• Serum calcium and phosphorus levels - usually normal
• Serum alkaline phosphatase - elevated in ~30% of polyostotic cases
• In rare widespread polyostotic cases: renal phosphate wasting, hypophosphatemia (due to a phosphaturic factor produced by abnormal fibrous tissue), which may cause rickets or osteomalacia

Differential Diagnosis

Complications

1. Pathologic fractures - particularly in polyostotic/long bone disease
2. Cranial nerve palsies - optic nerve compression, facial nerve palsy, SNHL
3. Sarcomatous transformation - rare but serious; incidence ~0.4-1%; risk increased by radiation therapy; most commonly osteosarcoma or fibrosarcoma; involves facial bones or femur
4. Recurrence after curettage - common

Treatment

• Conservative/watchful waiting - for small, asymptomatic, monostotic lesions
• Bisphosphonate therapy (e.g., pamidronate) - reduces severity of bone pain in polyostotic disease
• Surgical correction - for deformities, pathologic fractures, nerve decompression
• Curettage - for symptomatic lesions (recurrence is common)
• Surgical timing for craniofacial lesions: ideally after puberty when lesions tend to stabilize
• Radiotherapy is absolutely contraindicated - increases risk of malignant transformation

Key Points for Exam (Shafer's Highlights)

• Fibrous dysplasia = woven bone trabeculae WITHOUT osteoblastic rimming (vs. ossifying fibroma = WITH rimming)
• Mutation: GNAS1 gene → Gsα activation → elevated cAMP
• Radiograph: "ground-glass" appearance is pathognomonic
• Maxilla > mandible in jaw involvement
• McCune-Albright triad: polyostotic FD + café-au-lait spots ("coast of Maine") + endocrine dysfunction (precocious puberty in girls)
• Malignant transformation rare but possible; never irradiate as it increases this risk
• Histology classic: "Chinese letter" trabecular pattern in cellular fibrous stroma