Paget Disease
Osteoporosis Circumscripta

General Considerations

• Multifocal chronic skeletal disease due to chronic paramyxoviral infection
• Found in about 3% of individuals >40 years; 10% of persons >80 years
  • Unusual <40 years
• Strong male to female ratio of 2:1
• Histology
  • Increased resorption and increased bone formation
  • Newly formed bone abnormally soft with disorganized trabecular pattern
• Active or Osteolytic phase
  • Aggressive bone resorption with lytic lesions
• Inactive or Quiescent phase
  • Decreased bone turnover with skeletal sclerosis and thickening of the cortex
• Mixed pattern
  • Lytic and sclerotic phases frequently coexist

Clinical Findings

• Most patients are asymptomatic
• When symptomatic, symptoms may include
  • Fatigue
  • Enlarging hat size
  • Peripheral nerve compression
  • Neurologic disorders from compression of brainstem (basilar invagination)
  • Hearing loss, blindness
  • Facial palsy (narrowing of neural foramina) - rare
  • Pain from the primary disease process is rare so think of
    • Pathologic fracture
    • Malignant transformation
    • Secondary degenerative joint disease aggravated by skeletal deformity
  • High-output congestive heart failure from markedly increased perfusion (rare)
  • Increased alkaline phosphatase (increased bone formation)
  • Hydroxyproline increased (increased bone resorption)
  • Normal serum calcium and phosphorus
• Sites of involvement
  • Pelvis (75%) most common, followed by
    • Lumbar spine
    • Thoracic spine
    • Proximal femur
    • Calvarium
    • Scapula
    • Distal femur
    • Proximal tibia
    • Proximal humerus

Imaging Findings

• Imaging Findings
  • Classical triad
    • Thickening of the cortex
    • Accentuation of the trabecular pattern
    • Increased size of bone
  • Cyst-like areas
  • Skull
    • Inner and outer table involved
      • Leads to diploic widening
    • Osteoporosis circumscripta is well-defined lysis, most commonly in frontal bone producing well-defined geographic lytic lesion in skull
      • Represents early destructive phase of disease active stage
    • "Cotton wool" appearance represents mixed lytic and blastic pattern of thickened calvarium (later stage)
    • Basilar invagination with encroachment on foramen magnum
    • Deossification and sclerosis in maxilla
    • Sclerosis of skull base
• Long bones (almost invariably starts at end of bone)
  • "Candle flame" or "blade of grass" pattern of lysis is the advancing tip of V-shaped lytic defect in diaphysis of long bone originating in subarticular site
  • Lateral curvature of femur
  • Anterior curvature of tibia (commonly resulting in fracture)
• Pelvis
  • Thickened trabeculae in sacrum, ilium
  • Rarefaction in central portion of ilium (looks like a large lytic lesion)
  • Thickening of iliopectineal line
  • Acetabular protrusio with secondary degenerative joint disease
• Spine (upper cervical, low dorsal, midlumbar most common sites)
  • Coarse trabeculations at periphery of bone
  • "Picture-frame vertebra" mimics bone-within-bone appearance
    • Enlarged vertebral body with reinforced peripheral trabeculae and more lucent center, typically in lumbar spine
  • "Ivory vertebra" is a blastic vertebra with increased density
  • Ossification of spinal ligaments, paravertebral soft tissue, disk spaces can occur

• Bone scan
  • Sensitivity
    • Scintigraphy and radiography (60%)
      • Scintigraphy only (27%)
      • Radiography only (13%)
  • Usually markedly increased uptake (symptomatic lesions strikingly positive)
  • Normal scan may occur in some burned-out lesions
  • Marginal uptake can be seen in lytic lesion
• MR
  • Hypointense area / area of signal void on T1WI and T2WI (cortical thickening, coarse trabeculation)
  • Widening of bone
  • Reduction in size and signal intensity of medullary cavity due to replacement of high-signal-intensity fatty marrow by medullary bone formation
  • Focal areas of higher signal intensity than fatty marrow (from cyst-like fat-filled marrow spaces)
  • Areas of decreased signal intensity within marrow on T1WI and increased intensity on T2WI

Complications

• Complications
  • Associated neoplasia (0.7-20%)
    • Sarcomatous transformation into osteosarcoma (22-90%)
    •  Fibrosarcoma /malignant fibrous histiocytoma (29-51%)
    • Chondrosarcoma (1-15%)
  • Sarcomas are usually osteolytic in pelvis, femur, humerus
  • Giant cell tumor occurs in 3-10%
    • Lytic expansile lesion in skull, facial bones
  • Lymphoma or plasma cell myeloma are reported
  • Fracture
    • "Banana fracture" = tiny horizontal cortical infractions (“Looser lines”) on convex surfaces of lower extremity long bones (lateral bowing of femur, anterior bowing of tibia)
    • Compression fractures of vertebrae
  • Early-onset osteoarthritis

Treatment

• Treatment
  • Calcitonin, diphosphonate, mithramycin

Differential Diagnosis

• Differential diagnosis
  • Depends on the bone in which it occurs
  • Skull
    • Osteolytic or osteoblastic metastases
  • Long bones
    • Metastases
    • Chronic osteomyelitis (thickened cortex)
    • Old trauma (thickened cortex)
    • Hodgkin’s disease
  • Spine
    • Hemangioma
    • Metastases

Osteoporosis Circumscripta. Top. Lateral skull radiograph shows a large geographic, lytic lesion in the left frontal bone (blue arrows). Also seen are islands of bone (white arrows) producing a "cotton-wool" appearance.
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