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)
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:
- 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 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 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.
Macroscopically the medullary cavity is filled by abnormal whitish fibrous tissue.
- 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 2/3rds having become symptomatic by the age 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 + facial bones : 50 % 4
- upper extremities
- lumbar spine : 14 %
- clavicle : 10 %
- cervical spine : 7 %
- 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
- 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
- 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 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
Demonstrates increased tracer uptake on Tc99 bone scans (lesions remain metabolically active into adulthood) : Cases 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.
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.
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 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
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- 7. Kransdorf MJ, Murphey MD. Diagnosis please. Case 12: Mazabraud syndrome. Radiology. 1999;212 (1): 129-32. Radiology (full text) - Pubmed citation9
- 8. Maramattom BV. Leontiasis ossea and post traumatic cervical cord contusion in polyostotic fibrous dysplasia. Head & Face Medicine. 2006;2 (1): 24. doi:10.1186/1746-160X-2-24 - Free text at pubmed - Pubmed citation
- 9. Larheim TA, Westesson P. Maxillofacial Imaging. Springer Verlag. (2008) ISBN:3540786856. Read it at Google Books - Find it at Amazonkill
Synonyms & Alternative Spellings
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