Chronic condition involving cellular remodelling and deformity of one or more bones

Aetiology

• Cause uncertain
  • Genetic elements that increase activity of Nf ⲕβ
  • Viral infection may play a role
• Rare in people < 50 years
• Predominantly affects long bones, pelvis, lumbar spine and skull

Pathophysiology

• Abnormal osteoclastic activity followed by increased osteoblastic activity results in abnormal bone structure with reduced strength and increased fracture risk
• May be single site (monostotic) or multiple sites (polyostotic)

Three stages:

1. Osteolytic - restoration pits with large osteoclasts
2. Mixed - osteoclasis and osteoblastic activity
3. Osteosclerotic

Effect on bone

• Net result is thick excess bone with abnormal reversal lines - mosaic pattern
• Bone matures but is soft and porous
• Can affect all bones - usually axial, small bones less commonly affected

Clinical features

• Often asymptomatic
• Pain - micro-fracture or nerve compression
• Enlargement and abnormal shape of bone → leontiasis ossea, platybasia, sabre tibia