What is fibrous dysplasia?

Fibrous Dysplasia

Fibrous dysplasia (FD) is a benign bone disorder characterized by the replacement of normal bone with fibrous tissue and varying degrees of bone metaplasia, resulting in decreased bone strength, pain, and increased risk of fractures. 1

Types and Classification

• FD can manifest as monostotic (single bone), polyostotic (multiple bones), or rarely panostotic (affecting the entire skeletal system) 2
• Monostotic FD is the most common form, typically remains relatively stable throughout life, and is often asymptomatic 3
• Polyostotic FD can exhibit more aggressive growth and place adjacent structures at risk for compressive complications 3

Pathophysiology

• FD arises from activating mutations in the GNAS gene, leading to constitutive activation of the Gsα protein and elevated cAMP levels 1
• The mutation prevents the maturation of osteoprogenitor cells, resulting in the development of abnormal bone matrix, trabeculae, and collagen 3
• The disease is characterized by the replacement of normal bone and bone marrow with structurally unsound fibrous tissue 4

Clinical Presentations

• FD can occur independently or as part of McCune-Albright Syndrome (MAS), which includes café-au-lait skin hyperpigmentation and endocrine abnormalities 1
• Long bones are most commonly affected, and lesions are often identified incidentally 2
• Craniofacial involvement is the most common and challenging form to manage 4
• Clinical manifestations may include bone pain, pathological fractures, and deformities 1

Associated Syndromes

• McCune-Albright Syndrome (MAS): FD with café-au-lait skin pigmentation and endocrine hyperfunction 4
• Mazabraud Syndrome: FD associated with intramuscular myxomas 4

Diagnostic Approach

• Diagnosis is based on clinical, radiographical, and histopathological findings 2
• CT imaging is superior for detecting and quantifying established heterotopic bone formation 5
• CT allows better characterization of mineralization patterns and can distinguish ossification from calcification 5
• FD lesions typically demonstrate a characteristic "ground-glass" appearance on radiographic imaging 4
• FD lesions undergo age-related histological, radiographical, and clinical transformations 4

Management Considerations

• Current management strategies focus primarily on symptom control 1
• Conservative management is usually the mainstay of therapy in asymptomatic cases 3
• Emerging pharmacologic treatments include:
  • Denosumab: A monoclonal antibody showing promise in reducing lesion size and pain 1
  • Bisphosphonates: Used for pain management 1
  • Burosumab: A monoclonal antibody targeting FGF23, which reduces renal phosphate wasting and osteomalacia in FD patients 1
• Surgical management is indicated for:
  • Symptomatic lesions with mass effect 3
  • Prevention or treatment of pathological fractures 1
  • Correction of significant deformities 3

Imaging Considerations

• Careful and gentle positioning during imaging is essential to avoid triggering flare-ups 5
• Low-dose CT protocols should be considered to minimize radiation exposure, especially in pediatric patients 5
• Review of patient photographs prior to imaging and consultation with specialists can help assess positioning feasibility 5

Prognosis

• Monostotic FD typically has a good prognosis with minimal progression 3
• Polyostotic FD may have a more variable course with potential for progressive deformity and functional impairment 1
• Regular follow-up is important to monitor for disease progression and complications 4