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Fibrous dysplasia

Fibrous dysplasia (FD) is a benign tumour-like congenital process, manifested as a localised defect in osteoblastic differentiation and maturation, with progressive replacement of normal bone with immature woven bone.

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

The remainder of this article concerns itself with run-of-the-mill skeletal fibrous dysplasia. For 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). 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 arises 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 has been linked to localized developmental arrest; all of the components of normal bone are present, but they do not differentiate into their mature structures.

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

Histology
  • fibrocellular matrix of immature collagen contains small irregularly shaped trabeculae of immature, inadequately mineralized bone 6
  • trabeculae not rimmed by osteoblasts (differentiating feature from cemento-ossifying fibroma)
  • cartilaginous islands 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 2/3rds having become symptomatic by the age 10. 

Distribution

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 + facial bones: 50% 4
  • upper extremities
  • lumbar spine: 14%
  • clavicle: 10%
  • cervical spine: 7%

Radiographic features

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

Complications

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

Sarcomatous de-differentiation (osteosarcoma, fibrosarcoma, malignant fibrous histiocytoma or rarely chondrosarcoma) is occasionally seen (less than 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.


Related articles

Bone tumours

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

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