Fibrous Dysplasia of the Jaw

Fibrous dysplasia of the jaw is a benign developmental disorder in which normal medullary bone is replaced by abnormal fibro osseous tissue consisting of fibrous stroma with irregular trabeculae of woven bone. It results from a post-zygotic activating mutation of the GNAS gene (Gs alpha, R201), producing excess cyclic AMP in osteogenic cells. The jaw is affected in 10 to 25 percent of cases of the polyostotic form and 20 to 30 percent of the monostotic form; craniofacial involvement occurs in up to 90 percent of polyostotic disease.

Forms: monostotic (70 to 80 percent, one bone), polyostotic (20 to 30 percent), and McCune-Albright syndrome (polyostotic fibrous dysplasia with café-au-lait macules and endocrine hyperfunction: precocious puberty, hyperthyroidism, growth hormone excess, Cushing syndrome, and phosphate wasting with hypophosphataemia). The maxilla is more often affected than the mandible.

Lesions arise in childhood, enlarge until puberty, and usually stabilise in adulthood. Malignant transformation (osteosarcoma) is under 1 percent and is higher in McCune-Albright and after radiotherapy. Gradual jaw deformity, tooth displacement, sinus obliteration, and in the skull base nerve compression can occur.

• Painless, slow growing, hard swelling of the jaw, facial asymmetry, malocclusion, delayed tooth eruption, and tooth displacement or tipping; overlying mucosa normal.
• Maxillary: nasal obstruction, proptosis, diplopia, epiphora, vision loss (optic canal narrowing in sphenoid involvement), hearing loss.
• Pain is uncommon; sudden pain suggests fracture, cystic change or malignant transformation.
• McCune-Albright syndrome: café-au-lait spots with irregular ("coast of Maine") borders, precocious puberty, hyperthyroidism, acromegaly, Cushing syndrome, bone pain, pathological fractures.
• Mazabraud syndrome: fibrous dysplasia with intramuscular myxomas.
• Red flags: rapid growth, pain, new neurological symptoms, visual loss, hypophosphataemic rickets, hypercortisolism, and sarcomatous change.

• Radiographs: "ground glass" or "orange peel" medullary pattern with poorly defined margins blending into normal bone, expansion with thinned but intact cortex, loss of lamina dura, displacement of teeth, narrowing of the periodontal space, and displacement of the inferior alveolar canal; may be radiolucent (early), mixed, or sclerotic.
• CT: ground glass density with Hounsfield units 70 to 130 and cortical expansion; essential for craniofacial and optic canal assessment; MRI shows variable signal.
• Whole body bone scintigraphy (technetium-99m) for polyostotic involvement.
• Biopsy when diagnosis is uncertain: irregular curvilinear (Chinese character) trabeculae of woven bone without osteoblastic rimming, in a fibrous stroma; GNAS mutation analysis on tissue.
• Laboratory tests: serum calcium, phosphate, alkaline phosphatase (raised), parathyroid hormone, 25-hydroxy vitamin D, fibroblast growth factor 23, urine phosphate; thyroid function, IGF-1 and growth hormone, cortisol, and gonadotropins and sex steroids in McCune-Albright syndrome.
• Exclude ossifying fibroma, cemento osseous dysplasia, Paget disease, hyperparathyroidism (brown tumour), osteosarcoma, juvenile giant cell granuloma, and cherubism.

• Observation for asymptomatic, stable lesions, with clinical and radiographic review every 6 to 12 months until growth stops.
• Cosmetic and functional recontouring (shaving with burs) once skeletal growth is complete or earlier when function is impaired; early surgery in children risks regrowth (recurrence 25 to 50 percent), so it is deferred where possible; radical resection and reconstruction reserved for severe, recurrent or malignant disease.
• Orthodontic treatment can proceed in stabilised lesions; dental implants may be placed in dysplastic bone with caution.
• Optic nerve decompression only for documented progressive visual loss; prophylactic decompression is not recommended.
• Bone pain and progressive lesions: bisphosphonate: pamidronate 1 mg/kg (maximum 60 mg) intravenously over 4 hours daily for 3 days, repeated every 3 to 6 months (children 0.5 to 1 mg/kg per day); or zoledronic acid 5 mg intravenously once yearly; or oral alendronate 70 mg once weekly for 6 to 12 months; ensure calcium 1000 mg and vitamin D 800 to 2000 IU daily, check dental health first and monitor for osteonecrosis of the jaw, hypocalcaemia and renal function. Denosumab 60 mg subcutaneously every 6 months is used in refractory cases, with rebound hypercalcaemia on stopping in children.
• Hypophosphataemia: oral phosphate 20 to 40 mg/kg per day (elemental phosphorus) in 4 divided doses with calcitriol 20 to 30 nanograms/kg per day in 2 divided doses (adults 0.25 to 0.5 micrograms twice daily).
• McCune-Albright syndrome endocrine management: letrozole 2.5 mg orally once daily or tamoxifen 20 mg orally once daily for precocious puberty in girls; methimazole 0.4 mg/kg per day for hyperthyroidism (with definitive thyroidectomy); octreotide or cabergoline for growth hormone excess; adrenalectomy for Cushing syndrome.
• Radiotherapy is contraindicated (risk of sarcoma).
• Pain: paracetamol 1 g every 6 hours, ibuprofen 400 mg every 8 hours; neuropathic agents when needed.
• Follow up: annual review, vision and hearing screening, endocrine screening every 6 to 12 months in McCune-Albright syndrome, and prompt imaging for pain or rapid change.
• Counselling: benign disease that tends to stabilise after adolescence; genetic counselling is not required because the mutation is post-zygotic and not inherited.