Fibrous dysplasia is a benign developmental bone disorder characterized by replacement of normal bone and bone marrow with fibrous tissue and immature bone trabeculae (woven bone). Somatic activating mutation in the GNAS1 gene disrupts osteoblast differentiation. There are two main types: monostotic form (70-80%, single bone — most commonly rib, craniofacial bones, proximal femur) and polyostotic form (20-30%, multiple bones — usually unilateral distribution). McCune-Albright syndrome (café-au-lait spots + precocious puberty + polyostotic FD) and Mazabraud syndrome (FD + intramuscular myxomas) are associated syndromes. Radiologically, the 'ground-glass' matrix pattern is pathognomonic. Malignant transformation is very rare (<1%, usually after radiation therapy).
Age Range
10-40
Peak Age
20
Gender
Equal
Prevalence
Uncommon
Fibrous dysplasia develops from a somatic gain-of-function mutation in the GNAS1 gene. This gene encodes the G-protein alpha subunit (Gsα), and the mutation constitutively activates adenylate cyclase enzyme, increasing intracellular cyclic AMP (cAMP) levels. Elevated cAMP prevents terminal differentiation of osteoblast precursor cells (preosteoblasts); consequently, disorganized fibrous tissue and immature woven bone trabeculae form instead of normal lamellar bone. This pathologic tissue creates the radiologic 'ground-glass' appearance — because the random arrangement of immature trabeculae and fibrous stroma attenuates X-rays homogeneously but at reduced intensity; showing neither full bone density nor full soft tissue density. Bone expansion is surrounded by periosteal new bone formation, which can lead to deformities (e.g., shepherd's crook deformity of the femur).
The matrix pattern showing homogeneous intermediate density (70-130 HU) on CT is pathognomonic for fibrous dysplasia. The mixture of disorganized woven bone trabeculae and fibrous stroma creates this characteristic appearance that is neither full bone nor full soft tissue density. The term ground-glass differs from ground-glass opacity in lung parenchyma — ground-glass in bone describes the homogeneous but under-mineralized structure of the bone matrix.
The pathognomonic CT finding of fibrous dysplasia is ground-glass matrix density. The lesion shows homogeneous, intermediate density (70-130 HU) — lower than normal cortical bone (~1000 HU), higher than soft tissue (30-60 HU). It is an expansile, well-defined lesion surrounded by cortical thinning. Periosteal reaction is typically absent. Endosteal scalloping (internal cortex erosion) may be seen. The matrix is homogeneous with focal lytic or sclerotic areas being rare. Ring or punctate calcifications may accompany in some cases.
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An expansile, well-defined lesion is seen in the proximal [bone] region demonstrating homogeneous ground-glass matrix density (approximately ___ HU); cortical thinning is present with no periosteal reaction identified.
Shepherd's crook deformity characterized by lateral bowing (varus angulation) of the proximal femur is a classic finding in polyostotic fibrous dysplasia. Coxa vara angulation develops in the femoral neck and proximal shaft. The weakened bone cannot withstand mechanical loading, creating progressive deformity. Repeated microfractures and bone remodeling increase the deformity. 3D reconstruction on CT best evaluates the deformity. Bilateral involvement suggests polyostotic form and McCune-Albright syndrome.
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Varus angulation and lateral bowing of the proximal femur is seen in association with the ground-glass matrix density lesion, consistent with 'shepherd's crook' deformity.
On T1-weighted sequences, fibrous dysplasia demonstrates low to intermediate signal intensity. Normal bone marrow fat is replaced by fibrous tissue and immature bone, so fat signal is not expected. Signal intensity is similar to or slightly lower than muscle. Lesion margins are well defined, separated from surrounding normal bone marrow by lower signal. Homogeneous signal pattern is typical; focal high T1 signal areas suggest cystic degeneration or hemorrhage.
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On T1-weighted sequences, the lesion demonstrates low to intermediate signal intensity with absence of normal bone marrow fat signal.
On T2-weighted sequences, fibrous dysplasia shows variable signal but is typically of low to intermediate signal intensity. Dense fibrous tissue content results in short T2 and low signal — this finding differs from many bone tumors (most tumors show high T2 signal). Cystic degeneration areas show high T2 signal. Sclerotic regions give very low signal. Ground-glass matrix may show intermediate T2 signal. The lesion may show heterogeneous appearance — reflecting the mixture of fibrous, cystic, and bone components.
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On T2-weighted sequences, the lesion demonstrates low to intermediate signal intensity with focal high signal areas that may indicate cystic degeneration; consistent with fibrous dysplasia.
On STIR, fibrous dysplasia shows variable signal. With fat suppression, surrounding bone marrow signal decreases while the fibrous component's signal appears relatively increased. Cystic degeneration areas show markedly high signal. Perilesional bone marrow edema is better evaluated on STIR — edema may accompany active lesions or stress fracture complication. Lesion size and extent are well defined on STIR.
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On STIR, the lesion demonstrates variable signal intensity with cystic degeneration areas showing high signal; perilesional edema is [present/absent].
On post-gadolinium sequences, fibrous dysplasia shows variable enhancement — mild to prominent. Enhancement is expected since the fibrous stroma is vascularized. Enhancement pattern can be homogeneous or heterogeneous. Cystic degeneration areas do not enhance (except peripheral rim enhancement). Presence of enhancement should not be confused with malignancy — enhancement in fibrous dysplasia is regular and does not show prominent aggressive features. Enhancement degree is proportional to lesion vascularity.
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On post-contrast sequences, the lesion shows [mild/moderate/prominent] enhancement without aggressive enhancement pattern or soft tissue component.
On FDG PET-CT, fibrous dysplasia can show variable FDG uptake, with SUVmax potentially reaching 5-10 in some cases. This high uptake may be falsely interpreted as malignancy (especially may be confused with metastasis in patients with known malignancy history). FDG uptake depends on fibroblastic activity and metabolic rate within the lesion. Multiple bone involvement in polyostotic form can mimic metastatic disease. The CT component showing ground-glass matrix density guides toward the correct diagnosis.
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On FDG PET-CT, increased FDG uptake (SUVmax: ___) is seen in the known fibrous dysplasia lesion of the [bone]; CT component with ground-glass matrix density is consistent with fibrous dysplasia, and should be evaluated for malignancy.
Criteria
Single bone involvement, 70-80% of cases; most commonly rib, craniofacial bones, proximal femur; usually diagnosed in 2nd-3rd decade
Distinct Features
Well-defined, expansile lesion in a single bone; no systemic findings. Other foci should be excluded with whole-body bone scintigraphy. Rib involvement shows expansile fusiform lesion, craniofacial involvement may show leontiasis ossea (lion face) deformity. Malignant transformation risk is very low (<0.5%).
Criteria
Multiple bone involvement, 20-30% of cases; usually unilateral or dominant in one extremity; childhood diagnosis (earlier presentation)
Distinct Features
Ground-glass lesions in multiple bones, usually on the same side (unilateral). Pathologic fracture and deformity risk is increased. Shepherd's crook deformity is most commonly seen in this form. May be associated with McCune-Albright syndrome (café-au-lait + precocious puberty) and Mazabraud syndrome (intramuscular myxomas). Malignant transformation risk is higher than monostotic (1-4%, especially after radiation therapy).
Criteria
Polyostotic fibrous dysplasia + café-au-lait pigmentation (irregular borders, 'coast of Maine' pattern) + endocrine disorders (most commonly precocious puberty); GNAS1 mutation with mosaic distribution
Distinct Features
Bone involvement is usually extensive and severe. Café-au-lait spots on the affected side ('coast of Maine' — irregular borders, different from NF1's 'coast of California' — smooth borders). Endocrine disorders: precocious puberty, hyperthyroidism, growth hormone excess, Cushing syndrome. Radiologically severe polyostotic FD with deformities. Whole-body MRI evaluates extent of involvement.
Distinguishing Feature
Simple bone cyst is a unilocular cavity with fluid density (10-20 HU), while fibrous dysplasia shows ground-glass matrix density (70-130 HU). Simple bone cyst gives homogeneous fluid signal on MRI (T1 low, T2 high), while fibrous dysplasia shows low-intermediate T2 signal.
Distinguishing Feature
Non-ossifying fibroma (NOF) is an eccentric, cortically based lytic lesion with sclerotic rim that does not show ground-glass matrix. NOF is smaller, metaphyseal, usually asymptomatic, and tends to regress with age. Fibrous dysplasia is centrally/medullary located, larger, and expansile.
Distinguishing Feature
Enchondroma shows chondroid matrix calcification in an arc-and-ring pattern; fibrous dysplasia shows homogeneous ground-glass matrix without chondroid calcification. Enchondroma shows lobulated high T2 signal (hyaline cartilage) on MRI, while fibrous dysplasia shows low-intermediate T2 signal.
Distinguishing Feature
Osteosarcoma shows aggressive periosteal reaction (Codman triangle, sunburst pattern), soft tissue mass, wide transition zone, and prominent diffusion restriction on DWI. Fibrous dysplasia has no periosteal reaction, narrow transition zone, and soft tissue component is not expected. Biopsy is mandatory when malignant transformation is suspected (rapid growth, increasing pain).
Urgency
lowManagement
Observation for asymptomatic lesions; bisphosphonates for pain; surgical curettage/grafting for symptomatic or high fracture risk lesions; corrective osteotomy for deformitiesBiopsy
Not NeededFollow-up
Annual radiographs for growing children; MRI if rapid growth or pain increase (to exclude malignant transformation); endocrine evaluation in polyostotic formFibrous dysplasia is a benign lesion and observation is sufficient in most cases. Asymptomatic lesions are followed. Bisphosphonates (pamidronate, zoledronic acid) provide symptomatic relief for painful lesions. Surgical intervention (curettage with grafting, cortical bone graft, intramedullary nailing) is planned for lesions with high pathologic fracture risk or causing deformity. Endocrine evaluation (McCune-Albright syndrome screening) is mandatory in polyostotic form. Malignant transformation is very rare (<1%) and usually seen in irradiated cases — biopsy is needed if rapid growth, increasing pain, or soft tissue mass develops. Biopsy is not routine but indicated in atypical cases.
Fibrous dysplasia is generally benign but carries risk of bone deformity, pathologic fracture, and rarely malignant transformation (0.5% monostotic, 4% polyostotic). McCune-Albright syndrome may include endocrine anomalies (precocious puberty, hyperthyroidism). Treatment is symptomatic (fracture fixation, deformity correction).