Fibrous dysplasia

Introduction

• A benign fibro-osseous lesion of immature mesenchymal cells.
• Commonly affects long bones, ribs, and skull.

Pathophysiology

• Originates from mutated pluripotent embryonic cells leading to skeletal stem cells.
• Lesions form as cancellous bone is replaced with fibrous tissue and immature woven bone.
• Monostotic FD (70% of cases) vs. Polyostotic FD.

Genetic Basis and Related Syndromes

• Genetic mutation: Chromosome 20q13 at the GNAS1 locus.
• Constitutively active mutant of the α subunit of the G protein (Gsα).
• McCune-Albright syndrome: Characterized by polyostotic FD, endocrinopathy, café au lait spots.

Clinical Presentation

• Monostotic FD usually presents by age 30.
• Polyostotic FD presents in early childhood.
• Symptoms: Painless osseous expansion, facial asymmetry.
• Craniofacial involvement: Ethmoid (71%), Sphenoid (43%), Frontal (33%), etc.

Diagnosis

• 

  CT Imaging: Expansile remodeling of bone, ground-glass appearance.
• MRI: For cranial nerve compression.
• Histology: “Chinese figure” formation in bone trabeculae.

Treatment

• Asymptomatic lesions: Conservative management.
• Medications: Alendronate (bone density), Denosumab (reduces pain but concerns over side effects).
• Surgical options: Contouring, total resection, computer-based planning, timing for children.

Considerations

• McCune-Albright syndrome management.
• Orbital apex involvement: Proptosis, diplopia, and options for optic nerve compression.
• Malignant transformation: Rare (0.4 to 4%).