Paget's Disease of Bone

Paget's disease (osteitis deformans) is a localised disorder of accelerated, disorganised bone remodelling. Excessive osteoclastic resorption is followed by chaotic compensatory osteoblastic formation, producing bone that is enlarged, structurally weak, hypervascular and prone to deformity, fracture and—rarely—malignant transformation.

High-yield: The classic biochemical signature is a markedly raised serum alkaline phosphatase (ALP) with NORMAL serum calcium and phosphate. This single line wins most MCQs.

Definition & basic concept

Paget's disease is a focal metabolic bone disease (it affects one or several bones, not the whole skeleton uniformly—contrast with osteoporosis or osteomalacia, which are generalised). The fundamental defect is in osteoclasts, which become large, multinucleated and overactive. The lesion evolves through phases and the bone laid down is "woven" (immature, mosaic, disorganised) rather than the normal lamellar architecture.

Most commonly affected bones (mnemonic = "PSV-FT": Pelvis, Skull, Vertebrae (lumbar), Femur, Tibia):

• Pelvis (most commonly involved bone overall)
• Femur
• Skull
• Tibia
• Lumbar vertebrae / sacrum

It rarely affects the fibula. The disease is monostotic (single bone) in ~10–35% and polyostotic in the remainder, but it does not "spread" from bone to bone—new sites do not appear over time once diagnosed.

Epidemiology

• Disease of the elderly—rare before age 40; prevalence rises sharply after 55.
• Males slightly more than females.
• Striking geographic clustering: common in the UK, Western Europe, North America, Australia, New Zealand; distinctly uncommon in India, China, Japan and Scandinavia. This Anglo-Saxon predilection is a favourite exam point.
• A significant proportion is asymptomatic, discovered incidentally on an X-ray taken for another reason or on a routine raised ALP.

Etiology & pathophysiology

The cause is multifactorial—a combination of genetic susceptibility and a possible environmental (viral) trigger.

Genetics

• SQSTM1 (sequestosome-1 / p62) gene mutation is the most important and most frequently tested genetic association, affecting the NF-κB signalling pathway that drives osteoclast activation.
• Strong familial clustering; first-degree relatives have a 7–10× increased risk.

Viral hypothesis

• Paramyxovirus inclusion bodies (resembling measles / respiratory syncytial virus nucleocapsids) have been demonstrated within the nuclei of pagetic osteoclasts on electron microscopy. Causation remains unproven but is heavily examined.

The remodelling sequence

Disordered remodelling proceeds through three histological/radiological phases:

Osteolytic (active resorption) → Mixed (lytic + sclerotic) → Osteosclerotic (burnt-out/quiescent)

Phase	Dominant cell	Bone appearance	Classic X-ray sign
Lytic (incipient/active)	Osteoclast	Aggressive resorption	Osteoporosis circumscripta (skull); "blade of grass" / flame-shaped advancing lytic front in long bone
Mixed	Both	Lysis + chaotic formation	Coarsened, thickened trabeculae; cortical thickening begins
Sclerotic (burnt-out)	Osteoblast (declining)	Dense, woven, mosaic bone	Cotton-wool skull, sclerotic dense bone

The hallmark histology is a "mosaic" / "jigsaw" pattern of irregular cement lines in woven bone, with abundant marrow fibrosis and markedly increased vascularity (explains warmth over the bone and the high-output cardiac state).

Clinical features

The majority of patients are asymptomatic. When symptomatic, features stem from bone enlargement, deformity, increased vascularity, and compression of adjacent neural structures.

Bone pain

• The commonest symptom: a deep, boring, constant ache, worse at night and at rest (unlike mechanical/osteoarthritic pain, which is activity-related).
• Overlying skin may feel warm due to hypervascularity.

Deformity

• Bowing of the tibia ("sabre tibia"—anterolateral bow) and femur are classic; the weakened bone bends under load.
• Increasing skull/hat size—the patient reports their hat no longer fits ("my hat got tighter"). Frontal bossing.
• Kyphosis from vertebral involvement.

Neurological & cranial complications

• Sensorineural / mixed deafness (cranial nerve VIII)—the single most common neurological complication, from involvement of the petrous temporal bone / cochlear capsule and ossicles.
• Platybasia / basilar invagination → brain-stem compression, hydrocephalus.
• Cranial nerve compression (II, VII), nerve root or cord compression from vertebral expansion.

Cardiovascular

• High-output cardiac failure in extensive (often polyostotic) disease—the hypervascular pagetic bone acts as a large arteriovenous shunt. Tested repeatedly.

High-yield: Triad worth memorising — rising ALP + sensorineural deafness + bowed warm tibia in an elderly patient = Paget's disease.

Diagnosis & investigation of choice

Biochemistry

Parameter	Paget's disease
Serum ALP	Markedly ↑↑↑ (best marker of activity & monitoring)
Serum calcium	Normal
Serum phosphate	Normal
Bone turnover markers (urinary hydroxyproline, P1NP, urinary N-/C-telopeptides)	↑
PTH, vitamin D	Normal

High-yield: If calcium rises in a known Paget's patient, suspect (a) immobilisation/fracture or (b) coexisting primary hyperparathyroidism / sarcomatous change—the disease itself keeps calcium normal.

Imaging

• Plain X-ray is usually diagnostic—look for cortical thickening, coarse/sclerotic trabeculae, bone expansion, and the named signs below.
• Radionuclide (Technetium-99m MDP) bone scan = investigation of choice to determine the EXTENT/distribution of disease and to identify clinically silent lesions. Pagetic bone shows intense, homogeneous "hot" uptake of the entire involved bone.
• Biopsy is reserved for atypical lesions or to exclude malignant transformation.

Named radiological signs (very high-yield)

Sign	Site	Description
Osteoporosis circumscripta	Skull	Large, well-defined lytic area (early/lytic phase)
Cotton-wool appearance	Skull	Mixed lytic + sclerotic fluffy patches (later phase)
Tam-o'-shanter skull	Skull	Soft skull base lets enlarged skull "flop" over the face
Picture-frame vertebra	Spine	Cortical thickening at all margins of an enlarged, dense vertebral body
Ivory vertebra	Spine	Uniformly dense sclerotic vertebra (differential: lymphoma, metastasis)
Blade of grass / flame sign	Long bone	V-shaped advancing lytic resorption front
Banana fracture	Convex side of bowed long bone	Transverse incremental ("chalk-stick") insufficiency fractures
Sabre tibia	Tibia	Anterolateral bowing

Management & drug of choice

The aims are to relieve pain, suppress disease activity (normalise ALP), prevent deformity/fracture and treat complications.

Indications to treat

Treat symptomatic disease (pain, neural compression, high-output failure) and asymptomatic disease at high-risk sites—skull base, weight-bearing long bones, near major joints, vertebrae, or before elective orthopaedic surgery on pagetic bone.

Bisphosphonates — drug of choice

Bisphosphonates are first-line; they inhibit osteoclast-mediated resorption and durably suppress disease.

High-yield: A single intravenous infusion of zoledronic acid (5 mg) is the treatment of choice—it produces the most rapid, profound and prolonged biochemical remission (ALP normalisation lasting years).

• Oral options: alendronate, risedronate.
• Monitor response with serial serum ALP.
• Give adequate calcium and vitamin D to avoid hypocalcaemia after potent IV bisphosphonate.

Adjuncts

• Calcitonin (salmon)—an older second-line agent, now reserved for those who cannot take bisphosphonates.
• Analgesia / NSAIDs for pain.
• Surgery: joint replacement for secondary osteoarthritis, osteotomy for deformity, fracture fixation, decompression for neural compression.

High-yield: Before elective surgery on pagetic bone, pre-treat with bisphosphonates to reduce the marked intra-operative haemorrhage from hypervascular bone.

Stepwise approach

Suspect (elderly + bone pain/deformity/↑ALP) → confirm with X-ray → assess extent with Tc-99m bone scan → confirm biochemistry (↑ALP, normal Ca/PO₄) → treat with IV zoledronate (+ Ca/Vit D) → monitor ALP → surgery for mechanical/neural complications.

Complications

1. Osteoarthritis of adjacent joints (hip, knee)—the commonest complication overall, from altered joint mechanics.
2. Pathological / insufficiency fractures (banana/chalk-stick fractures of bowed long bones).
3. Deafness and other cranial neuropathies; basilar invagination.
4. High-output cardiac failure (extensive disease).
5. Hypercalcaemia / hypercalciuria with immobilisation → renal stones.
6. Malignant (sarcomatous) transformation — see below.

Sarcomatous transformation (the dreaded complication)

High-yield: Malignant change occurs in roughly <1% of patients; the commonest resulting tumour is OSTEOSARCOMA (others: fibrosarcoma, chondrosarcoma, malignant fibrous histiocytoma). Paget's is the commonest cause of osteosarcoma in the elderly (a bimodal age peak—the second peak is pagetic).

• Suspect when there is sudden new/worsening pain, a soft-tissue mass, a new lytic destructive area, or an unexpected sharp rise in ALP in a stable patient.
• Pagetic sarcoma carries a poor prognosis (worse than conventional osteosarcoma of youth).
• Common sites: humerus, femur, pelvis. Benign giant-cell tumours can also arise in pagetic skull/facial bones.

Key differentials

Condition	Calcium	Phosphate	ALP	Distinguishing point
Paget's disease	Normal	Normal	↑↑↑	Focal, ↑ALP isolated, named X-ray signs
Osteoporosis	Normal	Normal	Normal	Generalised low bone mass, fragility fractures, no ↑ALP
Osteomalacia	Low/Normal	Low	↑	Looser's zones, proximal myopathy, low vitamin D
Primary hyperparathyroidism	↑	↓	↑	↑PTH, subperiosteal resorption, stones/bones/groans
Osteoblastic (sclerotic) metastases (prostate/breast)	Variable	—	↑	Multiple discrete lesions, primary tumour, PSA ↑
Renal osteodystrophy	Low	↑	↑	CKD, ↑PTH, rugger-jersey spine
Fibrous dysplasia	Normal	Normal	±↑	Younger patient, "ground-glass" lesions, café-au-lait (McCune-Albright)

High-yield: Ivory vertebra differential = Paget's, osteoblastic metastasis (prostate), lymphoma (Hodgkin's). In Paget's the vertebra is also enlarged (picture-frame), which others are not.

Recently asked / exam angle

• "Markedly raised ALP with normal serum calcium and phosphate in an elderly patient" → single most repeated stem; answer Paget's disease.
• Investigation of choice to assess extent of disease → Tc-99m MDP bone scan (X-ray for individual lesions; biopsy for suspected malignancy).
• Drug/treatment of choice → bisphosphonate, specifically IV zoledronic acid.
• Commonest malignant transformation → osteosarcoma; incidence ~1%; poor prognosis; commonest cause of osteosarcoma in the elderly.
• Commonest bone involved → pelvis; bone NOT typically involved → fibula.
• Genetic association → SQSTM1 (p62); ultrastructure → paramyxovirus-like nuclear inclusions in osteoclasts.
• Histology → mosaic pattern of cement lines in woven bone.
• Cause of deafness → cranial nerve VIII / petrous temporal bone involvement.
• Mechanism of high-output cardiac failure → AV shunting through hypervascular bone.
• Match the X-ray sign: cotton-wool skull, picture-frame vertebra, blade of grass, banana fracture, osteoporosis circumscripta, sabre tibia, Tam-o'-shanter skull.
• Why does calcium rise in a Paget's patient → immobilisation or fracture (a classic "second-order" question).
• Pre-operative bisphosphonate before orthopaedic surgery → to reduce bleeding from vascular bone.

Rapid revision

1. Paget's = focal accelerated, disorganised remodelling; primary defect in the osteoclast.
2. Phases: lytic → mixed → sclerotic (burnt-out); histology = mosaic cement lines in woven bone.
3. Biochemistry: ALP ↑↑↑, calcium normal, phosphate normal (the defining triad).
4. Commonest bone = pelvis; spares the fibula; rare in India/Asia, common in Anglo-Saxons.
5. Genetics = SQSTM1/p62; EM = paramyxovirus-like inclusions in osteoclast nuclei.
6. Symptoms: bone pain (worse at rest/night), bowed warm tibia, increasing hat size, CN VIII deafness.
7. High-output cardiac failure from hypervascular AV shunting in extensive disease.
8. X-ray signs: cotton-wool skull, osteoporosis circumscripta, picture-frame & ivory vertebra, blade-of-grass, banana fracture, sabre tibia.
9. Investigation of choice for extent = Tc-99m MDP bone scan (intense homogeneous uptake).
10. Treatment of choice = IV zoledronic acid (bisphosphonate); calcitonin is second-line; give Ca + vitamin D.
11. Commonest complication = osteoarthritis; most feared = osteosarcoma (~1%) — suspect on sudden pain, mass, new lytic area or sharp ALP rise.
12. Rising calcium in Paget's → think immobilisation/fracture or coexisting hyperparathyroidism.