Fibrous dysplasia

Last revised by Dr Patrick J Rock on 22 May 2021

Fibrous dysplasia (FD) is a non-neoplastic tumor-like congenital process, manifested as a localized defect in osteoblastic differentiation and maturation, with the replacement of normal bone with large fibrous stroma and islands of immature woven bone. Fibrous dysplasia has a varied radiographic appearance. If asymptomatic, it does not require treatment. 

Fibrous dysplasia can affect any bone and can be divided into four subtypes 8 (although there is some overlap):

The remainder of this article concerns itself with skeletal fibrous dysplasia. For a discussion of craniofacial fibrous dysplasia and cherubism, please refer to the respective articles.

Fibrous dysplasia is found predominantly in children and young adults, with ~75% of patients presenting before the age of 30 years (highest incidence between 3 and 15 years). In polyostotic form, patients usually present by 10 years old. There is no recognized gender predilection 9.

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

The condition is often an incidental finding and is usually painless. Alternatively, it may present due to bony expansion or remodeling. Morbidity may arise from compression and displacement of adjacent structures. This is particularly true in craniofacial fibrous dysplasia, where the content of the orbit or cranial nerves may be compressed. 

Fibrous dysplasia is due to developmental dysplasia and focal arrest in normal osteoblastic activity secondary to a non-hereditary mutation which results in the presence of all of the components of normal bone with a lack of normal differentiation into their mature structures.

Macroscopically, lesions are intramedullary and well circumscribed with abnormal whitish-grey color.

Microscopically it manifests as large fibrous matrix with scattered curvilinear irregularly shaped trabeculae of immature, inadequately mineralized bone 6. There is no rimming by osteoblasts differentiating feature from cemento-ossifying fibroma. Cartilaginous islands are present in 10%, differentiating feature from chondrosarcoma.

This is by far the most common and accounts for 70-80% of cases 6. It is usually asymptomatic until 2nd-3rd decade but can be seen throughout adulthood 6. After puberty, the disease becomes inactive, and monostotic form does not progress to polyostotic form.

In the remaining 20-30% of cases, multiple bones are involved. As expected this presents earlier, typically in childhood (mean age of 8 years) with two-thirds symptomatic by the age of 10. 

  • ribs: 28%, most common 6,7
  • proximal femur: 23%
  • tibia
  • craniofacial bones: 10-25% 4
  • humerus
  • often unilateral and monomelic: one limb 6
  • femur: 91%
  • tibia: 81%
  • pelvis: 78%
  • foot: 73%
  • ribs
  • skull and facial bones: 50% 4
  • upper extremities
  • lumbar spine: 14%
  • clavicle: 10%
  • cervical spine: 7%

The appearance of fibrous dysplasia is usually smooth and homogeneous with endosteal scalloping and cortical thinning 12. The borders are well defined and the cortex is usually intact but thinned due to the expansive nature of the lesion 12. Other features include:

  • ground-glass matrix
  • may be completely lucent (cystic) or sclerotic
  • well circumscribed lesions
  • no periosteal reaction
  • rind sign

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)

  • ground-glass opacities: 56% 4
  • homogeneously sclerotic: 23%
  • cystic: 21%
  • well-defined borders
  • expansion of the bone, with intact overlying bone
  • endosteal scalloping may be seen 6

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: heterogeneous signal, usually intermediate
  • T2: heterogeneous signal, usually low, but may have regions of higher signal
  • T1 C+ (Gd): heterogeneous contrast enhancement 4

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

Usually, no treatment is required as the bone lesions usually do not progress beyond puberty. If a mass effect is severe, then surgical decompression may be considered. The monostotic form does not transform or progress into the polyostotic form 10.

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

Sarcomatous dedifferentiation (osteosarcoma [most common 10], fibrosarcoma, malignant fibrous histiocytoma, 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 6.

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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Cases and figures

  • Figure 1: distribution of monostotic fibrous dysplasia
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  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4: polyostotic
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  • Case 5: left femur with rind sign
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  • Case 6: with shepherd crook deformity
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  • Case 7
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  • Case 8
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  • Case 9
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  • Case 10
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  • Case 11: polyostotic form
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  • Case 12
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  • Case 13
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  • Case 14
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  • Case 15
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  • Case 16
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  • Case 17
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  • Case 18: involving skull base
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  • Case 19: ribs
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  • Case 20: polyostotic
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  • Case 21: polyostotic
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  •  Case 22
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  •  Case 23
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  • Case 24: with pathologic fracture
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  • Case 25: rind sign in fibrous dysplasia
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  • Case 26: involving lumbar spine
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  • Case 27: involving right maxilla
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  • Case 28
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  • Case 29: Mazabraud syndrome with ABC
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  • Case 30: shepherd crook deformity
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  • Case 31: involving sphenoid bone
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  • Case 32: fibrous dysplasia of the rib
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