Fibrous dysplasia is a developmental tumorlike condition that is characterized by replacement of normal bone by an excessive proliferation of cellular fibrous connective tissue intermixed with irregular bony trabeculae.
Cause
- Sporadic condition that results from a postzygotic mutation in the GNAS1 (guanine nucleotide-binding protein, α-stimulating activity polypeptide 1) gene.
- Constitutive activation of G protein signaling impairs osteoblastic differentiation in skeletal progenitor cells, stimulates melanin production in melanocytes, and causes hyperplasia and hyperfunctioning of various endocrine cell types. In addition, mutated osteoblasts overexpress IL-6 which stimulates osteoclastic activity and may contribute to bone lesion expansion.
Clinical Presentation
- Monostotic or Polyostotic
- Can be localized process involving one bone, multiple bones, or multiple bone lesions in conjunction with cutaneous and endocrine abnormalities.
- Severity depends on point in time of mutation of GNAS1.
- Maxillary lesions often involve adjacent bone (e.g., zygoma, sphenoid, occipital) and are not strictly monostotic (termed craniofacial fibrous dysplasia).
| Monostotic Fibrous Dysplasia | Polyostotic Fibrous Dysplasia |
| Limited to a single bone | Involvement of two or more bones |
| Accounts for 80-85% of all cases, with jaw being most commonly affected site. Maxilla > Mandible | Often dominated by symptoms related to long bone lesions. Pathologic fractures resulting in pain and deformity. Leg length discrepancy |
| Diagnosed during 2nd decade of life | Diagnosed before age of 10 |
| No sex predilection | Female predilection |
| Painless swelling. Teeth remain firm but may be displaced. Cortical expansion. Superior displacement of IAN. | Hypophosphatemia caused by renal phosphate wasting |
Associated Conditions
- Jaffe-Lichtenstein Syndrome: polyostotic fibrous dysplasia combine with café au lait pigmentation
- McCune-Albright Syndrome: polyostotic fibrous dysplasia combined with café au lait pigmentation and multiple endocrinopathies, such as sexual precocity, pituitary adenoma, or hyperthyroidism.
- Mazabraud Syndrome: fibrous dysplasia in combination with intramuscular myxoma. Higher risk of sarcoma development.
Radiographic Features
- Poorly calcified bone trabeculae arranged in a disorganized pattern result in a ground-glass opacifications
- Not well demarcated.
- Superior displacement of the IAN is not uncommon.
- narrowing of the periodontal ligament space with ill-defined lamina dura that blends with the abnormal bone pattern.
Histopathology
- Irregular shaped trabeculae of immature (woven) bone in a cellular, loosely arranged fibrous stroma; bone trabeculae are not connected and look like Chinese characters
- In later stages, the woven bone is replaced by lamellar bone with roughly parallel trabeculae. The rather monotonous pattern of calcification in fibrous dysplasia differs from the more haphazard mixture of woven bone, lamellar bone, and spheroid particles characteristics of ossifying fibroma and cemento-osseous dysplasia
Treatment
- Resection of smaller lesions; recontouring of cosmetic deformity
- Disease stabilizes and stops enlarging when skeletal maturation is reached.
- Polyostotic disease can be managed with pamidronate/alendronate (bisphosphonate)
- Radiation contraindicated because of possible sarcoma
