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Paget's disease of bone

Dr Matt Skalski and Dr Frank Gaillard et al.

Paget’s disease of the bone (also known as osteitis deformans) is a chronic bone disorder characterised by excessive abnormal bone remodelling.

Epidemiology

It is relatively common and can affect upto 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
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, however viral infection 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

serum alkaline phopatase (ALP) elevated
urine hydroxyproline increased

Radiographic features

Plain film

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

In the skull, osteolysis is frequently seen as well-defined, often large areas of radiolucency most commonly affecting the frontal and occipital bones; these areas are referred to as osteoporosis circumscripta or osteolysis circumscripta.

Both inner and outer calvarial tables are involved, with the former usually more extensively affected.

Spine

Paget disease of the spine frequently manifests with cortical thickening encasing the vertebral margins, which gives rise to the picture frame sign on radiographs in mixed phase disease. On lateral radiographs flattening or squaring of the normal concavity of the anterior margin of the vertebral body also adds to the rectangular appearance.

The osteoblastic activity is seen along all four margins of the vertebral body cortices, unlike the rugger jersey vertebrae in renal osteodystrophy, which only involves the superior and inferior endplates.

The vertical trabecular thickening pattern in Paget disease is coarser than the more delicate pattern seen in hemangiomas with which it can be confused.

Pelvis

Paget disease of the pelvis usually manifests with cortical thickening and sclerosis of the iliopectineal and ischiopubic lines. These findings are often asymmetric and for some reason, may be more commonly seen on the right side. Enlargement of the pubic rami and ischium are also often seen.

Long bones

In the long bones, osteolysis begins as a subchondral area of lucency. The advancing wedge of osteolysis often demonstrates a characteristic sharp radiolucent margin without sclerosis likened to a blade of grass or flame.

In rare cases, the disease is isolated to the diaphysis, most commonly in the tibia, rather than subchondral bone, which can cause diagnostic confusion

Signs

Paget's disease related signs include :

Bone scintigraphy

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.

Complications

osseous weakening resulting in deformity and fractures
arthritis
neural compression
- deafness is the most common complication
- cranial nerve paresis may occur
- basilar invagination may occur in advanced cases with hydrocephalus or brainstem compression
secondary development of tumours (e.g. osteosarcoma : ≈ 1 % of cases), which is often highly resistant to treatment
high output congestive cardiac failure
hyperparathyroidism (approximately 10 %)

Differential diagnosis

For skull lesions : consider

hyperostosis frontalis interna : thickening of the internal table of the frontal bone
fibrous dysplasia
- different age group
- fibrous dysplasia usually affects the outer table more prominently

Etymology

First described by Sir James Paget in 1877

References

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1. Smith SE, Murphey MD, Motamedi K et-al. From the archives of the AFIP. Radiologic spectrum of Paget disease of bone and its complications with pathologic correlation. Radiographics. 22 (5): 1191-216. Radiographics (full text) - Pubmed citation
2. Walsh JP. Paget's disease of bone. Med. J. Aust. 2004;181 (5): 262-5. Med. J. Aust. (link) - Pubmed citation
3. Tjon-a-tham RT, Bloem JL, Falke TH et-al. Magnetic resonance imaging in Paget disease of the skull. AJNR Am J Neuroradiol. 6 (6): 879-81. AJNR Am J Neuroradiol (abstract) - Pubmed citation