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Fibrous Dysplasia (FD) is a benign intramedullary fibro-osseous lesion characterized by the replacement of normal bone with immature fibro-osseous tissue, resulting in structurally weak, expansile lesions.

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https://www.youtube.com/watch?v=FnPFD8GiHs8

https://youtu.be/wgtNNlN1aZ4

It is a developmental anomaly arising from a somatic activating mutation in the GNAS gene, affecting bone-forming mesenchyme.

Classification

Paris Nomenclature of Constitutional Diseases of the Bone naming scheme for diseases associated with fibrous dysplasia (FD):

Type	Description
Monostotic FD	Single bone involved; most common form (~70–80%)
Polyostotic FD	Multiple bones involved; more extensive skeletal deformities
McCune-Albright syndrome	Polyostotic FD + café-au-lait spots + endocrine dysfunction (e.g., precocious puberty)
Mazabraud syndrome	Polyostotic FD + intramuscular myxomas

Common Sites of Involvement:

Craniofacial bones (especially maxilla > mandible)
Ribs
Femur (classic “shepherd’s crook” deformity)
Tibia, humerus, pelvis

Pathophysiology

FD is caused by a postzygotic activating mutation in GNAS1, encoding the α-subunit of Gs protein.
This leads to constitutive cAMP activation and impaired osteoblastic differentiation.
Histologically, lesions consist of fibrous stroma intermixed with irregular trabeculae of immature woven bone (without osteoblastic rimming).

Histopathology:

https://www.youtube.com/watch?v=MKo8q0qwHvU