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Paget disease of bone

Paget disease of the bone is a common, chronic bone disorder characterised by excessive abnormal bone remodeling. It frequently affects the pelvis, spine, skull and proximal long bones and has characteristic radiographic features. 

Epidemiology

It is relatively common and can affect up to 4% of individuals over 40 and up to 11% over the age of 80 1. There may a slight male predilection.

Clinical presentation

The majority (approximately three quarters) of patients are asymptomatic at the time of diagnosis. Presenting symptoms include:

  • localised pain and tenderness
  • increased focal temperature due to hyperaemia (due to hypervascularity)
  • increased bone size: historically changing hat size was a give-away
  • bowing deformities
  • kyphosis of the spine
  • decreased range of motion
  • signs and symptoms relating to complications (see below)

Polyostotic disease is more common than the monostotic type 1. The most frequent sites of involvement are:

  • spine
  • pelvis (often asymmetric)
  • skull
  • proximal long bones

Pathology

The aetiology is not entirely known but it is a disease of osteoclasts. Viral infection (paramyxovirus) 6 in association with genetic susceptibility has been postulated.

There are three stages classically described (but is part of continuous spectrum)

  • lytic (incipient active): predominated by osteoclastic activity
  • mixed (active): osteoblastic as well as osteoclastic activity
  • sclerotic/blastic (late inactive)
Markers
  • normal Calcium and phosphorous levels
  • serum alkaline phopatase (ALP) elevated
  • urine hydroxyproline increased

Radiographic features

Radiography

The early phase features osteolytic (lucent) region which is later followed by coarsened trabeculae and bony enlargement. Sclerotic changes occur much later in the disease process.

Skull
  • osteoporosis circumscripta: large well defined lytic lesion
  • cotton wool appearance: mixed lytic and sclerotic lesions of skull
  • diploic widening: both inner and outer calvarial tables are involved, with the former usually more extensively affected
  • Tam o' Shanter sign: frontal bone enlargement, with the appearance of the skull falling over the facial bones, like a Tam o' Shanter hat.
Spine
  • picture frame sign: Paget disease of the spine frequently manifests with cortical thickening  and sclerosis encasing the vertebral margins, which gives rise to the on radiographs in mixed phase disease
  • squaring of vertebrae: on lateral radiographs flattening of the normal concavity of the anterior margin of the vertebral body also adds to the rectangular appearance
  • vertical trabecular thickening: in Paget disease is coarser than the more delicate pattern seen in hemangiomas with which it can be confused
Pelvis
  • cortical thickening and sclerosis of the iliopectineal and ischiopubic lines
  • acetabular protrusio
  • enlargement of the pubic rami and ischium

These findings are often asymmetric and for some reason, may be more commonly seen on the right side.

Long bones
  • blade of grass or candle flame sign: begins as a subchondral area of lucency with advancing tip of V-shaped osteolysis, extending towards the diaphysis
    • in rare cases, the disease is isolated to the diaphysis, most commonly in the tibia, rather than subchondral bone, which can cause diagnostic confusion
  • lateral curvature (bowing) of the femur 
  • anterior curvature of tibia
Signs

Paget disease related signs include:

Bone scintigraphy
  • Tc99m-MDP
    • highly sensitive but not specific
    • traditionally has been said to demonstrate marked increased uptake in all phases of disease, although in the burnt out sclerotic quiescent phase uptake may be normal 1.

Treatment and complications

Complications

Differential diagnosis

For skull lesions consider:

History and etymology

It was first described by Sir James Paget in 1877 in a case report of a patient he had observed over some twenty years 5

The condition was originally named by Paget "osteitis deformans", implying an inflammatory aetiology. The term "osteodystrophica deformans" is now preferred.

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