Paget's disease of bone (osteitis deformans)

Tumor biology and incidence

• Disease of bone with uncertain etiology (although slow virus may be cause)
• 1-3% of individuals in US over age 45 have Paget's disease
• Usually polyostotic; involves cortical and cancellous bone
• Three phases of the disease
  • Osteoclastic (reportive)
    • Active resorption of bone by osteoclasts
    • Endosteal erosion leading to expansion of the bone and subperiosteal new bone formation
    • Increased urinary excretion of collagen degradation products (eg, hydroxyproline)
  • Osteoblastic (sclerotic)
    • Compensatory new bone formation by osteoblasts
    • Increase in serum alkaline phosphatase levels
  • "Burn out"
    • Marked decline in osteoclastic and osteoblastic activity

Presentation

• Most patients asymptomatic
• Those who are symptomatic complain of bone and joint pain
• Compression of neural structures possible
• Pathologic fractures possible

Physical findings

• Often diagnosed incidentally during radiographic investigation of other conditions.
• May have gross bony deformity; may be only physical finding
• Rapid onset of pain and swelling suggests malignant degeneration (rare; less than 1% chance of malignant degeneration, usually to osteosarcoma)

Radiographic appearance

Osteoclastic phase

• Lytic lesions in bone; thinning of trabeculae
• Skull may show zone of rarefied bone (osteoporosis circumscripta)
• In long bones, lysis advances in V- or flame-shaped cutting cone
• Cortices may be rarefied; risk of pathologic fracture

Osteoblastic phase

• Deposition of new bone outpaces bone resorption
• New bone is woven bone; laid down along stress lines
• Thickening of trabeculae along stress lines is radiographically characteristic of Paget's
• Evidence of lysis also present
• New bone formation in skull may have a "cotton wool" appearance.
• May see thickening of bony trabeculae, widening of cortices, and subperiosteal new bone formation, as well as bony enlargement
• Blastic changes in vertebral bodies of spine may have classic "ivory vertebrae' or "picture frame vertebrae" appearance

“Burn out” phase

• Lytic and blastic appearance of bones
• May see fractures in long bones affected by disease

Diagnosis and treatment

• Control osteoclastic activity
  • Diphosphonates
  • Calcitonin
  • Methotrexate