Fibrous dysplasia

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

Terminology

FD 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.

Epidemiology

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 recognised gender predilection 9.

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

Clinical presentation

The condition is often an incidental finding and is usually painless. Alternatively it may present due to bony expansion or remodelling. 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. 

Pathology

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

Macroscopic appearance

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

Histology

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.

Subtypes
Monostotic form (involving only one bone) 

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.

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. 

Location
Monostotic form
  • ribs: 28%, most common 6,7
  • proximal femur: 23%
  • tibia
  • craniofacial bones: 10-25% 4
  • humerus
Polyostotic form 
  • 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%

Radiographic features

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

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)

Extremities
CT
  • 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

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 tumour 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
Nuclear Medicine

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

Treatment and prognosis

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

Complications

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.

Differential diagnosis

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


Bone tumours

The differential diagnosis for bone tumours is dependent on the age of the patient, with a very different set of differentials for the paediatric patient.

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Article Information

rID: 4915
Section: Pathology
Synonyms or Alternate Spellings:
  • Fibrous dysplasia (FD)
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    Distribution of m...
    Figure 1: distribution of monostotic fibrous dysplasia
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    Case 1
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    Case 3
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    MR-Sag

Fibrous d...
    Case 4
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    Fibrous dysplasia
    Case 5: polyostotic
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    Case 6: left femur with rind sign
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    Fibrous dysplasia
    Case 7
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    Fibrous dysplasia
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    Case 12: polyostotic form
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    Case 17
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    Case 19: involving skull base
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    Case 20: ribs
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    Case 21: polyostotic
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    Case 22: polyostotic
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     Case 23
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     Case 24
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    Case 25: with pathologic fracture
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    Case 26
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    Case 27: involving lumbar spine
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    Case 28: involving right maxilla
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    Case 29
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    Case 30: Mazabraud syndrome with ABC
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