Fibrous Dysplasia is a bone abnormality characterised by the ground-glass appearance on imaging, most commonly affecting the femur, tibia, and craniofacial bones.
Description
Fibrous dysplasia is a non-neoplastic, intramedullary bone lesion resulting from the replacement of normal bone and marrow by fibrous tissue and randomly distributed, irregularly shaped trabeculae of immature woven bone.
Pathogenesis
Fibrous dysplasia is caused by a sporadic mutation in the GNAS1 gene on chromosome 20, leading to constitutive activation of adenylyl cyclase, increased cAMP, and excessive proliferation of osteoblast precursors, resulting in the production of immature, woven bone.
Subtypes
- Monostotic fibrous dysplasia (more common, 70-80%): Involves a single bone, commonly in the femur or tibia
- Polyostotic fibrous dysplasia: Involves multiple bones, often with involvement of the skull and facial bones.
Epidemiology, Risk Factors & Associations
- No identifiable risk factors.
- No gender predilection.
- In polyostotic form, can be associated with;
- McCune-Albright syndrome (unilateral café-au-lait spots, precocious puberty, and ipsilateral polyostotic fibrous dysplasia)
- Mazabraud syndrome (myxomatous tumours)
Clinical Features
- Can be asymptomatic and discovered incidentally.
- Pain and deformity, pathologic fracture.
Complications
- Low risk of malignant transformation to osteosarcoma (<1%).
- May cause premature fusion of growth plates leading to short stature or discrepant leg length
- Pathological fractures
Pathological Features
Histopathology
- Macroscopic: Expanded, “soft” bone which is grey-white and has a “gritty” texture.
- Microscopic: Irregular, curvilinear trabeculae of woven bone forming Chinese characters in a background of cellular fibrous stroma.
Biochemistry
- Hypercortisolism
Radiological Features
General Features
- Well-defined, intramedullary, expansile lesion, with characteristic ground-glass matrix.
- There may be smooth cortical thinning and endosteal scalloping.
- No periosteal reaction.
- Ribs are the most common site of monostotic fibrous dysplasia. May appear as a bubbly cystic lesions causing fusiform enlargement of ribs.
- Pelvic involvement typically indicates polyostotic disease
- Most cases are monostotic (70 – 80%). About 20-30% are polyostotic.
XR
- Ground-glass appearance of the involved bone.
- Bowing deformities and pseudoarthrosis may be present.
- Rind-sign refers to the layer of thick, sclerotic reactive bone surrounding the lesion, commonly seen at the proximal femur.
- Shepherd crook deformity – coxa varus angulation of the proximal femur (also seen in Paget disease of bone and osteogenesis imperfecta)
CT
- Non-contrast: Ground-glass appearance, expansion of the bone.
- Contrast-enhanced: No significant enhancement.
MRI
- T1: Homogenous hypointense signal compared to normal marrow.
- T2: Variable signal intensity.
- T1 C+: No significant enhancement.
- DWI/ADC: No diffusion restriction.
NM
- PET FDG: Variable uptake, generally low to moderate.
Grading and Staging
- Not applicable as it is a benign condition.
Diagnosis
- The diagnosis is primarily radiologic, confirmed by biopsy showing characteristic histopathologic features.
Differential Diagnosis
- Osteosarcoma: Typically demonstrates aggressive features with a permeative pattern of bone destruction, a soft tissue mass, and new bone formation.
- Paget’s disease: Older age group with coarsening of trabeculae and cortical thickening. Increased skull size and hearing loss can be present.
Management
- Observation for asymptomatic lesions.
- Surgical intervention for symptomatic lesions, cosmetic reasons, or prevention/correction of deformity.
- Bisphosphonates may provide pain relief.
- Complete surgical excision is curative but is usually not possible due to the anatomical location and size of the lesions.