Fibrous dysplasia

Last revised by Rohit Sharma on 20 May 2024

Fibrous dysplasia (FD) is a developmental benign medullary fibro-osseous process characterized by the failure to form mature lamellar bone and arrest as woven bone that can be multifocal. It can affect any bone and occur in a monostotic form involving only one bone or a polyostotic form involving multiple bones. Since the WHO classification of soft tissue and bone tumors (5th edition) it has been titled a benign bony neoplasm.

Fibrous dysplasia accounts for the "F" in the popular mnemonic for lucent bone lesions FEGNOMASHIC.

This article is confined to the monostotic (involves a single bone) and polyostotic (multiple bones are involved) forms. Craniofacial fibrous dysplasia and cherubism are discussed in separate articles.

Terms such as "liposclerosing myxofibrous tumor" and "fibrocartilaginous dysplasia" are no longer recommended 1.

Fibrous dysplasia is uncommon, occurs in children and adults and can affect all age groups 1,2. It is usually first diagnosed in children and young adults. The true incidence is not known but it is estimated to make up for ~5% of benign bone lesions 3,4. There is no gender predilection 1

Although fibrous dysplasia is usually sporadic, a number of associations are well recognized 4,5:

The diagnosis of fibrous dysplasia is mainly based on clinical and typical radiographic features 1 and if the imaging features are characteristic the lesion does not require histology 5,6.

Histological confirmation is indicated in cases with atypical imaging appearance or in isolated monostotic lesions with clinical symptoms or other concerning features 5,6. Typical asymptomatic lesions are followed-up with serial radiographs to rule out significant mechanical implications and assure biological inactivity 5.

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:

Essential features include 1:

  • a bone lesion with compatible imaging characteristics

  • osseous part consisting of irregular curvilinear branching trabeculae of woven bone without apparent osteoblastic rimming

  • fibrous part consisting of bland fibroblasts

The following additional criterion is desirable:

  • evidence of GNAS activating missense mutations

The condition is often an incidental finding and is usually painless. Alternatively, it may present with bony expansion and remodeling or with pain 1,2. Morbidity may arise from or from pathologic fracture 4 or compression and displacement of adjacent structures. The latter is particularly true in craniofacial fibrous dysplasia, where the content of the orbit or cranial nerves may be compressed resulting in loss of vision or hearing loss 4

The monostotic form is far more common accounting for 70-80% of cases and is usually asymptomatic until the 2nd to 3rd decade but can be seen throughout adulthood 7. After puberty, the disease can become inactive. 

The polyostotic form accounts for 20-30% and presents earlier, typically in childhood with about 60% showing symptoms before the age of 10 years 2.

Fibrous dysplasia is characterized by altered osteogenesis leading to an intramedullary fibro-osseous proliferation with fibrous and osseous tissue components being present in varying degrees 1. It comes in a monostotic or polyostotic form depending on whether only one single bone or multiple bones are affected. However, there is no progression from the monostotic to the polyostotic form 5.

Fibrous dysplasia is linked to postzygotic activating missense GNAS mutations that encode the alpha subunit of the stimulatory G-protein 1-5.

  • ribs: 28%, most common 7,8

  • proximal femur: 23%

  • tibia

  • craniofacial bones: 10-25% 9

  • humerus

  • often unilateral and monomelic: one limb 7

  • femur: 91%

  • tibia: 81%

  • pelvis: 78%

  • foot: 73%

  • ribs

  • skull and facial bones: 50% 9

  • upper extremities

  • lumbar spine: 14%

  • clavicle: 10%

  • cervical spine: 7%

There are no officially recognized subtypes 1

Macroscopically, lesions are intramedullary, well-circumscribed, and often expansile with abnormal whitish or tannish-grey color and gritty consistency. Cystic changes might be present, especially in older lesions. Rarely cartilage might be found characterized by a blue-tinged translucent appearance 1,2.

Microscopically fibrous dysplasia is characterized by the following 1,2,4,7:

  • varying proportions of fibrous and osseous tissue

  • fibrous tissue principally made up of bland spindle cells without conspicuous cellular atypia

  • irregular curvilinear branching trabeculae of woven bone with a pattern that has been characterized as “looking like Chinese characters” 2,4

  • absence of osteoblastic rimming

  • uncommon mitoses unless there is a fracture

Other possible histological features include 1,2:

  • cementum-like bone deposition

  • rounded psammomatous calcifications

  • islands of benign hyaline cartilage might rarely be seen

  • possibly aneurysmal bone cyst-like changes

The absence of osteoblastic rimming aids in the differentiation from the cemento-ossifying fibroma.

In about 50-70%, GNAS activating missense mutations can be detected in particular involving pArg201His and pArg201Cys 1,4.

General imaging features of fibrous dysplasia are 7:

  • intramedullary, expansile lesion

  • well-defined borders

  • maintenance of a smooth cortical contour

  • endosteal scalloping might be present

Fibrous dysplasia can generally display the following three main imaging features 5:

  • cystic/lucent

  • sclerotic

  • mixed

Beyond that, appearances are usually smooth and homogeneous with endosteal scalloping and cortical thinning 5. The borders are well defined and the cortex is usually intact but thinned due to the expansive nature of the lesion 5. Other features include:

Ribs are the most common site of monostotic fibrous dysplasia. Fibrous dysplasia is the most common cause of a benign expansile lesion of a rib (see rib lesions)

CT better delineates morphological osseous changes of bone and is considered the modality of choice in the evaluation of fibrous dysplasia, especially in the setting of craniofacial lesions 5. CT imaging features include:

  • ground-glass opacities: 56% 9

  • homogeneously sclerotic: 23%

  • cystic: 21%

  • well-defined borders

  • expansion of the bone, with intact overlying bone

  • endosteal scalloping may be seen 7

The attenuation of lesions usually ranges from 60-140 HU and they usually enhance after the application of contrast media 5.

MRI is not particularly useful in differentiating fibrous dysplasia from other entities as there is marked variability in the appearance of the bone lesions, and they can often resemble a tumor or more aggressive lesions. 

  • T1: usually intermediate to low heterogeneous signal 5

  • T2: variable signal 10

  • T1 C+ (Gd): heterogeneous moderate to avid contrast enhancement 9,10

Demonstrates increased tracer uptake on Tc99 bone scans (lesions remain metabolically active into adulthood).

The radiological report should include a description of the following 5,6:

If features are typical the lesion can be categorized as Bone-RADS 1 on CT or MRI 6.

Management aims to establish the extent of the disease and the maintenance of bone quality via dietary measures and exercise. The prognosis is excellent and usually, no other treatment is required 1.

However monostotic fibrous dysplasia can lead to deformities leg-length differences and impingement or nerve compression syndromes 1. If a mass effect is severe, then surgery excision may be considered 4,5.

Not surprisingly, bone affected by fibrous dysplasia is weaker than normal and thus susceptible to pathological fractures.

Sarcomatous dedifferentiation (most commonly osteosarcoma 12, fibrosarcoma, undifferentiated pleomorphic sarcoma, or rarely chondrosarcoma) is occasionally seen (<1%) and is more common in the polyostotic form. It should be noted that many reported cases may relate to previous treatment with radiation therapy 7.

Fibrous dysplasia was first described by the American bone pathologist Louis Lichtenstein in 1938 and the clinical, radiological and histological spectrum of findings has been later characterized by him and his colleague Henry Louis Jaffe in 1942 13,14

Due to the variability of the appearance of fibrous dysplasia the potential differential is very long but will be significantly influenced by the dominant pattern.

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