A benign developmental condition – replacement of normal medullary bone with abnormal fibrous CT proliferation with the capacity to form new metaplastic bone

Clinical: 3 main presentations, all of which involve the skull and jaws

1. Monostotic disease – Painless swelling
2. Polyostotic non syndromic disease.
3. Polyostotic syndromic type:
   • Jaffe type:
     – Polyostotic FD
     – Cutaneous pigmentation (café au lait spots)
   • McCune Albright syndrome:
     – Polyostotic FD
     – Cutaneous pigmentation (cafe au lait spots)
     – Endocrine hyperfunction – precocious puberty, acromegaly, hyperthyroidism, hyperparathyroidism, hyperprolactinaemia

• Asymptomatic, self limiting, stabilizing at puberty and growing slowly thereafter

Etiology:

• Gs alpha mutation (chromosome 20) – which transcribes transmembrane signaling proteins.
• Resultant dysfunction in cAMP signaling affects multiple systems.
• In the musculoskeletal system:
  – Osteoblasts secrete abnormal bone
  – ↑ IL-6 – ↑ Osteoclastic bone remodeling
• The severity of the condition depends of the timing of the mutation and the size of the affected cell mass:
  • a) Mutations in embryonic life -Polyostotic and syndromic types (20 %).
  • b) Postnatal mutations – Monostotic types (80 %)

Common site: Maxilla > mandible, with jaws being the most common site but ribs, femur and tibia are also commonly affected

Radiology:

• Early lesions – Radiolucent, with a unilateral “ground glass” appearance being acquired in the process of calcification.
• Ill defined margins
• Bucco-lingual expansion pattern
• Loss of lamina dura around root
• No displacement of teeth and no resorption

Lab: Serum laboratory values are normal

Histology:

• Irregularly shaped trabeculae – of immature woven bone – in cellular, loosely arranged
  fibrous stroma.
• Metaplastic deposits of curvilinear bone arranged in ”chinese letter” pattern
• Few osteoblasts, no osteoclasts and no inflammatory cells – except 2⁰ infection

Management:

• Surgical recontouring/bone shaving
• Alleviate pain, pressure and deformity
• Bisphosphonates
• May recur (25%)

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