fibrous dysplasia of maxilla

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Here is a comprehensive summary of fibrous dysplasia of the maxilla drawn from multiple authoritative textbooks:

Fibrous Dysplasia of the Maxilla

Definition and Background

Fibrous dysplasia (FD) is a benign, slowly progressive developmental anomaly of bone-forming mesenchyme - not a true neoplasm - in which normal cancellous bone is replaced by a fibrous stroma containing immature woven bone. It results from a defect in osteoblastic differentiation and maturation. The maxilla is one of the most frequently involved bones in the craniofacial skeleton.

Cummings Otolaryngology; Scott-Brown's Otorhinolaryngology

Molecular Pathogenesis

All forms arise from somatic gain-of-function mutations in GNAS1 (chromosome 20q13), which encodes the alpha subunit of the stimulatory G-protein (Gs-alpha). The constitutively active Gs-protein elevates intracellular cAMP, promoting cellular proliferation while disrupting osteoblast differentiation. The phenotype depends on when in embryogenesis the mutation is acquired and what proportion of mesenchymal cells carry it.

Robbins & Kumar Basic Pathology

Classification

Form	Frequency	Craniofacial Involvement
Monostotic	~80%	Maxilla and mandible most commonly affected (up to 25% of cases)
Polyostotic	~20%	Craniofacial involvement in 40-50% of cases
McCune-Albright syndrome	~5%	Polyostotic FD + café-au-lait spots + endocrine hyperfunction
Mazabraud syndrome	Rare	FD + soft tissue myxoma

The maxilla is the most commonly affected craniofacial bone in monostotic disease. Importantly, unlike FD in long bones (which does not cross joint lines), craniofacial FD can cross bony sutures - involvement of multiple facial bones is still classified as monostotic craniofacial fibrous dysplasia, not polyostotic.

Scott-Brown's Otorhinolaryngology; Harrison's Principles of Internal Medicine 22E

Clinical Features

Age of onset: First two decades; monostotic form may become quiescent at puberty, polyostotic form can progress into adulthood (and with pregnancy or estrogen therapy)
Sex: Equal frequency in monostotic form; McCune-Albright is 10:1 female predominance
Presentation in the maxilla:
- Painless facial swelling and asymmetry
- Expansion causing nasal obstruction, altered dentition
- Interference with maxillary sinus drainage - pressure/pain
- Aesthetic deformity ("leontiasis ossea" in severe polyostotic involvement)
- Rarely: cranial nerve compression (optic nerve, trigeminal branches)
Monostotic cases are often incidentally discovered; polyostotic forms present earlier with bone pain and deformity

Radiology

Plain X-ray / CT (hallmark): The appearance depends on the degree of mineralization:

Early phase (high fibrous density): Radiolucent/lytic appearance
Mixed phase: Classic "ground-glass" appearance - hazy, homogeneous density from irregularly arranged woven bone spicules (most typical)
Late/sclerotic phase: Dense "cotton-wool" areas

On CT: expansile lesion with intact cortex, well-defined margins, no periosteal reaction.

CT scan showing fibrous dysplasia - expansile sphenoid lesion with ground-glass appearance (arrowheads) and low-density areas (asterisks)

Axial CT showing fibrous dysplasia with a mixed density "T"-labelled expansile lesion - ground-glass areas (arrowheads) and lytic areas (asterisks). - Cummings Otolaryngology

MRI:

T1: intermediate signal (hypointense where sclerotic)
T2: hypointense (distinguishes it from other lesions like bone cysts)
May show non-homogeneous enhancement with gadolinium

Histopathology

The classic microscopic picture (H&E):

Intramedullary lesion with no capsule
Curvilinear/Chinese-letter trabeculae of woven bone (immature, irregular, no lamellar pattern)
Critically: no osteoblastic rimming of the bony trabeculae (this distinguishes FD from ossifying fibroma)
Background of moderately cellular fibroblastic stroma in a whorled pattern
Cystic degeneration, hemorrhage, and foamy macrophages may be present

Histology of fibrous dysplasia - curvilinear woven bone trabeculae without osteoblastic rimming in fibrous stroma

Fibrous dysplasia - curvilinear woven bone trabeculae lacking conspicuous osteoblastic rimming, arising in fibrous tissue. - Robbins & Kumar Basic Pathology

FD vs. Ossifying Fibroma (Key Distinction)

Feature	Fibrous Dysplasia	Ossifying Fibroma
Nature	Developmental/genetic anomaly	True benign neoplasm
Capsule	Absent	Present
Bone trabeculae	Woven, no osteoblastic rimming	More mature, osteoblastic rimming
CT	Ground-glass, no eggshell rim	Well-defined multiloculated, peripheral eggshell rim
MRI T2	Hypointense	Hyperintense
Malignant transformation	Rare (0.5% polyostotic)	None reported
Recurrence after surgery	Less (if quiescent)	High, especially ethmoid

Cummings Otolaryngology

Laboratory Findings

Serum alkaline phosphatase (ALP): elevated in ~30% of polyostotic cases
Serum calcium and phosphorus: usually normal
In extensive polyostotic disease: hypophosphatemia and hyperphosphaturia due to excess FGF-23 production by abnormal fibrous tissue (may cause rickets/osteomalacia)
Bone turnover markers may be elevated

Complications

Pathological fracture (more common in long bones)
Cranial nerve compression - optic nerve at highest risk in craniofacial involvement
Sinus obstruction and mucocele (frontal > maxillary)
Sarcomatous transformation - rare but documented:
- 0.5% in polyostotic forms
- Up to 4% in McCune-Albright syndrome
- Radiation therapy increases this risk significantly

Treatment

Conservative (Preferred)

Asymptomatic lesions should be observed with periodic clinical and radiological follow-up
Lesions typically stabilize after skeletal maturation - surgery should be deferred in young patients when possible

Medical

Bisphosphonates (e.g., IV pamidronate): inhibit osteoclastic activity; reduce bone pain and can partially resolve lesions; particularly useful for extensive disease with pain/disfigurement
Denosumab: effective in reducing bone turnover markers; occasional hypercalcemia on discontinuation

Surgical

Indications:

Optic nerve compression - most urgent; decompression now often achievable endoscopically (endonasal or transorbital)
Severe aesthetic deformity - local controlled resection and recontouring
Sinus obstruction - endoscopic sinus surgery for drainage
Any rapidly growing or painful lesion (raises suspicion for sarcomatous change)

Radiotherapy is contraindicated - increases risk of malignant transformation (to osteosarcoma) and impairs facial growth in children.

Cummings Otolaryngology; Scott-Brown's Otorhinolaryngology; Harrison's Principles of Internal Medicine 22E

Summary Points

Caused by GNAS1 gain-of-function somatic mutation -> excess cAMP -> impaired osteoblast differentiation
Maxilla is the most common craniofacial site in monostotic FD
Classic CT finding: expansile lesion with ground-glass appearance
Histology: woven bone trabeculae without osteoblastic rimming in fibrous stroma - no capsule
Distinguish from ossifying fibroma: FD has no capsule, no osteoblastic rimming, CT shows no eggshell rim
Surgery only for symptoms (optic nerve compression, deformity, sinus obstruction); defer in young patients
Radiotherapy is absolutely contraindicated
Malignant transformation (<1%) to osteosarcoma is a rare but recognized complication

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