Revision 58 for 'Fibrous dysplasia'

Fibrous dysplasia (FD) is a benign tumour-like congenital process, manifested as a 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 sub types ⁸ (although there is some overlap):

• monoostotic : single bone
• polyostotic : multiple bones
• craniofacial fibrous dysplasia : skull and facial bones alone
• cherubism : mandible and maxilla alone (not true fibrous dysplasia)

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.

Clinical presentation

The condition is 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

Macroscopically the medullary cavity is filled by abnormal whitish fibrous tissue.

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)

Sub types

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 2^nd - 3^rd 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 2/3^rds having become symptomatic by the age 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 + facial bones : 50 % ⁴
• upper extremities
• lumbar spine : 14 %
• clavicle : 10 %
• cervical spine : 7 %

Associations

Fibrodysplasia may be associated with :

• McCune-Albright syndrome : In 2 - 3 % of cases with the polyostotic form
• isolated endocrinopathy without the full McCune-Albright syndrome
  
  • precocious puberty in girls
  • hyperthyroidism
  • hyperparathyroidism : renal stones, calcinosis
  • acromegaly
  • diabetes mellitus
  • Cushing syndrome : osteoporosis, acne
  • growth retardation
• Mazabraud syndrome :  soft-tissue myxomas (rare) ; typically multiple intramuscular lesions in vicinity of most severely affected bone

Radiographic features

Plain film

• ground-glass opacities
• may be completely lucent (cystic) or sclerotic
• well circumscribed lesions

Pelvis + 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)

• bubbly cystic lesions
• fusiform enlargement of ribs
• protrusio acetabuli

Extremities

• may lead to premature fusion of growth plates leading to short stature
• bowing deformities
• shepherd's crook deformity of femoral neck
• discrepant limb length
• Looser zones

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+ (GAD) : heterogeneous contrast enhancement ⁴

Nuclear Medicine

Demonstrates increased tracer uptake on Tc⁹⁹ bone scans (lesions remain metabolically active into adulthood) : ^{Cases 3 and 6}

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 FD bone 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.

• Paget's disease
  • mosaic pattern bone histologically
  • radiographically may be similar
  • different demographics
• neurofibromatosis type I
  • osseous lesions are rare
  • vertebral column is primary target
  • ribbon ribs
  • other features of the disease usually present
• osteofibrous dysplasia
  • almost exclusively in tibia with anterior bowing
  • lesion begins in cortex
  • usually seen in children <10 years
• adamantinoma
  • 80% seen in the tibia
  • may appear indistinguishable
• non-ossifying fibroma
• simple bone cyst
• giant cell tumour
• enchondromatosis