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.
FD can affect any bone and can be divided into four subtypes 8 (although there is some overlap):
- monostotic: single bone
- polyostotic: multiple bones
- craniofacial fibrous dysplasia: skull and facial bones alone
- cherubism: mandible and maxilla alone (not true fibrous dysplasia)
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:
- McCune-Albright syndrome: in 2-3% of cases with the polyostotic form
- isolated endocrinopathy without the full McCune-Albright syndrome precocious puberty in girls
- Mazabraud syndrome: soft-tissue myxomas (rare); typically multiple intramuscular lesions in the vicinity of most severely affected bone
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.
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.
Macroscopically lesions are intramedullary, well circumscribed with abnormal whitish-grey colour.
Microscopically it manifests as large fibrous matrix with scattered curvilinear trabeculae of woven bone with no surrounding osteoblastic rimming.
- 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)
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.
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%
- craniofacial bones: 10-25% 4
- often unilateral and monomelic: one limb 6
- femur: 91%
- tibia: 81%
- pelvis: 78%
- foot: 73%
- skull and facial bones: 50% 4
- upper extremities
- lumbar spine: 14%
- clavicle: 10%
- cervical spine: 7%
- 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)
- bubbly cystic lesions
- fusiform enlargement of ribs
- protrusio acetabuli
- may lead to premature fusion of growth plates leading to short stature
- bowing deformities
- shepherd crook deformity of the femoral neck
- discrepant limb length
- Looser zones
- 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 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
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.
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 appearance of fibrous dysplasia the potential differential is very long but will be significantly influenced by the dominant pattern.
- mosaic pattern bone histologically
- radiographically may be similar
- different demographics
neurofibromatosis type I
- osseous lesions are rare
- vertebral column is the primary target
- ribbon ribs
- other features of the disease usually present
- almost exclusively in tibia with anterior bowing
- lesion begins in cortex
- usually seen in children <10 years
- 80% seen in the tibia
- may appear indistinguishable
- non-ossifying fibroma
- simple bone cyst
- giant cell tumour
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.
- bone-forming tumours
- chondromyxoid fibroma
- fibrous bone lesions
- bone marrow tumours
- other bone tumours or tumour-like lesions
- aneurysmal bone cyst
- benign fibrous histiocytoma
- giant cell tumour of bone
- Gorham massive osteolysis
- haemophilic pseudotumour
- intradiploic epidermoid cyst
- intraosseous lipoma
- cockade sign
- musculoskeletal angiosarcoma
- primary intraosseous haemangioma
- simple bone cyst
- impending fracture risk
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