Overview
This lecture covers the definition, pathogenesis, clinical types, features, diagnosis, and treatment of fibrous dysplasia, an uncommon bone disorder characterized by abnormal fibrous and bony tissue replacement.
Definition and Pathogenesis
- Fibrous dysplasia is a rare, non-hereditary, developmental, tumor-like bone disorder.
- Characterized by the replacement of normal bone with cellular fibrous connective tissue and irregular bony trabeculae.
- Caused by a post-zygotic missense mutation in the GNAS1 gene on chromosome 20, leading to increased G protein and cyclic AMP (cAMP) production.
- Increased cAMP causes hyperfunction of endocrine glands, affects odontoblast differentiation, and promotes melanocyte proliferation.
Clinical Types and Features
- Three clinical types: Monostotic (one bone), Polyostotic (multiple bones), and Craniofacial (mainly facial bones).
- Monostotic: affects ribs, femur, tibia, craniofacial bones, or humerus; common in ages 10-19; painless swelling.
- Polyostotic: affects femur, tibia, pelvis, skull, facial bones, etc.; can be unilateral or bilateral; associated with skeletal deformities like Shepherd’s Crook Deformity (curved femoral neck and shaft).
- Extraskeletal features: Café-au-lait spots (pigmented macules with jagged borders, resemble “coast of Maine”, do not cross midline).
- Polyostotic type with café-au-lait spots and endocrine disturbances (precocious puberty, hyperthyroidism, etc.) forms the McCune-Albright syndrome (Albright triad).
- Craniofacial type often affects maxilla and adjacent bones, causing “lion-like” (leontiasis ossea) facial appearance.
Radiographic and Histopathological Features
- Early lesions: uniform unilocular radiolucency; advanced: multilocular radiolucency with increased opacity (ground-glass, cotton wool, orange peel appearance).
- Classic “Rind sign”: radiolucent lesion with a thick, sclerotic border.
- Histology: irregular, immature (woven) bony trabeculae in fibrous stroma; Chinese letter pattern; no capsule or sharp demarcation.
Associated Syndromes
- McCune-Albright Syndrome: fibrous dysplasia, café-au-lait spots, endocrine disturbances.
- Jaffe’s type: fibrous dysplasia with only café-au-lait spots.
- Mazabraud Syndrome: fibrous dysplasia with intramuscular myxomas.
Treatment and Prognosis
- Self-limiting disease; usually ceases at puberty.
- Cosmetic surgery (e.g., optical canal decompression) for monostotic cases.
- Polyostotic type requires multidisciplinary approach; bisphosphonates and vitamin D if surgery is contraindicated.
- Malignant transformation is rare (0.4% monostotic, ~4% polyostotic); may result in osteosarcoma, fibrosarcoma, etc.
Key Terms & Definitions
- Fibrous dysplasia — disorder where normal bone is replaced by fibrous tissue and irregular bone.
- GNAS1 gene — encodes G protein; mutation causes fibrous dysplasia.
- Café-au-lait spots — light brown skin macules with irregular borders.
- Shepherd’s Crook Deformity — femoral neck curvature typical in polyostotic fibrous dysplasia.
- Rind sign — radiographic feature; lucent lesion with sclerotic border.
- Chinese letter pattern — histological appearance of irregular bony trabeculae.
Action Items / Next Steps
- Review radiographic images illustrating “rind sign” and ground-glass appearance.
- Read about other bone lesions that mimic fibrous dysplasia for comparison.
- Prepare a summary table comparing fibrous dysplasia types and associated syndromes.