Fibrous dysplasia

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"fibrous dysplasia"[MeSH Terms]

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Fibrous Dysplasia

Definition

Fibrous dysplasia (FD) is a benign skeletal lesion arising from a localized developmental arrest in which normal bone and marrow are replaced by fibro-osseous tissue - disorganized, inadequately mineralized woven bone embedded in a cellular fibrous stroma. All skeletal components are present but fail to mature into normal lamellar bone. It accounts for approximately 7% of benign bone tumors.

Robbins & Kumar Basic Pathology; Robbins, Cotran & Kumar Pathologic Basis of Disease

Pathogenesis

All forms result from somatic gain-of-function mutations in GNAS1 (encoding the stimulatory Gs-protein alpha subunit), acquired during fetal development. The same gene is mutated in pituitary neuroendocrine tumors.

The mutation causes constitutive activation of Gs-protein, elevating intracellular cAMP
Elevated cAMP alters Wnt signaling, promoting cellular proliferation and inhibiting osteoblast differentiation
The phenotype depends on:
1. The stage of embryogenesis when the mutation occurs
2. The proportion and location of mesenchymal cells carrying the mutation
Very early embryonic mutation → McCune-Albright syndrome; later mutation in a single osteoblast precursor → monostotic FD
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 1102

Forms / Classification

Form	Description
Monostotic	Single bone involved (~80% of cases)
Polyostotic	Multiple bones involved
McCune-Albright syndrome	Polyostotic FD + café-au-lait spots + endocrine abnormalities (especially precocious puberty in girls)
Mazabraud syndrome	FD (usually polyostotic) + soft-tissue myxomas

Grainger & Allison's Diagnostic Radiology

Epidemiology & Sites

No gender predilection
75% of cases present before age 30; ~25% diagnosed after age 30
Monostotic - most common sites: ribs (28%), proximal femur (23%), craniofacial bones (20%)
Polyostotic - femur, tibia, and pelvis most often affected; may involve up to 75% of the skeleton
Can affect epiphysis, metaphysis, or diaphysis; metadiaphyseal region is typical in long bones
Grainger & Allison's Diagnostic Radiology; Rheumatology 2-Volume Set

Morphology

Gross: Tan-white, gritty intramedullary lesional tissue; may cause cortical expansion, thinning, and bowing.

Microscopy (key features):

Curvilinear ("C"- or "S"-shaped) trabeculae of woven bone - often described as resembling "Chinese letters" or letters of the alphabet, arranged around vessels
No osteoblastic rimming (absent rim of osteoblasts around trabeculae - a hallmark distinguishing FD from ossifying fibroma)
Moderately cellular fibroblastic stroma with loose collagen
Cystic degeneration, hemorrhage, and foamy macrophages may be present
No periosteal reaction (unless fracture is present)
Robbins & Kumar Basic Pathology; Rheumatology 2-Volume Set

Radiological Features

Plain X-ray:

"Ground-glass" appearance - most characteristic; caused by mineralized woven bone spicules within fibrous tissue
Elongated lesion causing symmetric cortical thinning and expansion - classic "long lesion in a long bone"
Geographic lytic lesion; thick sclerotic margin ("rind sign") is characteristic
Purely lytic variants reflect extensive cystic degeneration
Periosteal reaction is absent unless fracture has occurred

X-ray of fibrous dysplasia in the fibula showing a long lesion in a long bone with ground-glass appearance (arrow)

X-ray of fibrous dysplasia in the fibula (arrow) - classic "long lesion in a long bone" with ground-glass appearance (Rheumatology 2-Volume Set)

Polyostotic FD - pelvis and hip:

AP radiograph showing polyostotic fibrous dysplasia of pelvis and proximal femora, femoral neck lesion with sclerotic margin, and shepherd's crook deformity

(A) Polyostotic FD - multifocal bilateral involvement of pelvis and proximal femora. (B) Well-defined left femoral neck lesion with sclerotic margin ("rind sign"). (C) Classic "shepherd's crook" deformity from repeated fractures of the proximal femur. (Grainger & Allison's Diagnostic Radiology)

Clinical Features

Often asymptomatic - discovered incidentally on X-ray
May present with: pain, pathologic fracture, and bone deformity
Monostotic FD frequently stops enlarging at growth plate closure
Polyostotic FD with weight-bearing bone involvement leads to deformity from repeated fractures - the classic "shepherd's crook" deformity of the proximal femur
In severe polyostotic disease, FD may produce FGF-23 causing secondary osteomalacia
Malignant transformation is extremely rare but recognized (with or without prior radiotherapy)

Syndromes Summary

McCune-Albright Syndrome

Polyostotic FD (typically unilateral) + ipsilateral café-au-lait spots + endocrine dysfunction
Most common endocrine abnormality: precocious puberty (girls)
Other endocrine manifestations: hyperthyroidism, Cushing syndrome, acromegaly
Usually symptomatic before age 10
Not inherited - mosaic activating GNAS1 mutation acquired in early embryogenesis

Mazabraud Syndrome

FD (usually polyostotic) + intramuscular myxomas

Treatment

Asymptomatic monostotic FD: observation; often stabilizes after skeletal maturity
Symptomatic lesions: curettage (recurrence is common)
Extensive polyostotic disease: pain management + surgery + bone-strengthening measures
Bisphosphonates (e.g., pamidronate, zoledronic acid): used for pain control and to reduce fracture risk, particularly in polyostotic and McCune-Albright cases
Surgical stabilization for impending or actual pathologic fractures; intramedullary rodding for shepherd's crook deformity
Rheumatology 2-Volume Set; Campbell's Operative Orthopaedics 15th Ed 2026

Key Summary Table

Feature	Detail
Mutation	GNAS1 gain-of-function (somatic, mosaic)
Pathology	Woven bone trabeculae, no osteoblast rimming, fibrous stroma
X-ray	Ground-glass, rind sign, long lesion in long bone
Hallmark deformity	Shepherd's crook (proximal femur)
Associated syndromes	McCune-Albright, Mazabraud
Malignant potential	Extremely rare
Treatment	Curettage, bisphosphonates, surgical stabilization

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