Paget's Disease of Bone

Paget's disease of bone is a chronic disorder of excessive and disorganized bone remodeling characterized by elevated alkaline phosphatase with normal calcium and phosphate—distinguishing it from hyperparathyroidism and malignancy. Three radiographic phases exist: lytic (osteoporosis circumscripta in skull), mixed (blade of grass/flame sign in long bones), and sclerotic (cotton wool appearance). Most patients are asymptomatic (70-80%); when symptomatic, zoledronic acid 5mg IV single dose is first-line treatment, normalizing ALP in 90% by 6 months. Orthopaedic complications include pathological transverse stress fractures, secondary arthritis, and sarcomatous transformation (1% risk) presenting as sudden pain increase with soft tissue mass—typically osteosarcoma with extremely poor prognosis.

Paget's disease of bone is a chronic disorder of excessive and disorganized bone remodeling. Second most common metabolic bone disease after osteoporosis.

Epidemiology:

• 3-4% prevalence over age 55
• Male predominance (M:F 3:2)
• 70-80% asymptomatic
• High in UK/Australia/NZ, rare in Asia/Africa
• Familial clustering (15-30% have affected relative)

Orthopaedic relevance: Pathological fractures, deformity, secondary arthritis, increased operative bleeding, implant fixation challenges.

Cellular Mechanism

Etiology

Multifactorial: SQSTM1 gene mutations (15-30% familial cases), chromosome 18q abnormalities. Slow virus hypothesis (measles paramyxovirus) not proven. Geographic clustering and declining incidence suggest environmental trigger.

The Pathognomonic Triad

Paget's disease is defined by three cardinal features:

1. Biochemical Hallmark: Elevated ALP with Normal Calcium

• Alkaline phosphatase reflects osteoblast activity (10-15x normal in active disease)
• Normal calcium distinguishes from malignancy and hyperparathyroidism
• Normal phosphate and PTH confirm isolated bone remodeling disorder

2. Radiological Hallmark: Bone Expansion with Cortical Thickening

• Increased bone diameter (expansion)
• Cortical thickening (widened cortex)
• Trabecular coarsening (thickened trabeculae)
• Mixed lytic-sclerotic pattern in active disease

3. Histological Hallmark: Mosaic Woven Bone

• Disorganized woven bone instead of lamellar bone
• Mosaic cement lines (multiple resorption-formation cycles)
• Giant osteoclasts (up to 100 nuclei)
• Mechanically weak despite increased bone mass

Clinical Principles

Key facts for orthopaedic surgeons:

• 70-80% asymptomatic (incidental finding on X-ray or bloods)
• Geographic variation (high in UK/Australia, rare in Asia/Africa)
• Zoledronic acid single dose: 90% ALP normalization, 2+ year remission
• Preoperative bisphosphonates reduce bleeding 40-50%

Orthopaedic impact:

1. Pathological fractures need long implants bypassing pagetic bone
2. Joint arthroplasty requires cemented long-stem components
3. Hyperemic bone causes significant operative bleeding
4. Deformity correction complicated by altered anatomy

History

Most patients are asymptomatic (70-80% discovered incidentally). When symptomatic:

Examination

Inspection:

• Bone enlargement (skull circumference increased, tibial bowing, femoral bowing)
• Skin warmth over affected bone (hypervascularity)
• Gait abnormality (leg length discrepancy, joint deformity)

Palpation:

• Bone tenderness over active lesions
• Increased temperature over pagetic bone
• Joint effusions if secondary arthritis

Neurological:

• Cranial nerve examination (skull base involvement)
• Spinal cord signs if vertebral disease with stenosis

Algorithm

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Bisphosphonates - First-Line Treatment

Mechanism: Inhibit osteoclast activity, reduce bone turnover, normalize ALP

Indications for Treatment

Symptomatic disease:

• Bone pain attributable to Paget's
• Neurological complications (nerve compression)
• High-output cardiac failure (rare)

High-risk lesions (even if asymptomatic):

• Weight-bearing long bones (risk of fracture/deformity)
• Skull base involvement (risk of cranial nerve compression)
• Spine with stenosis risk
• Juxta-articular disease (risk of secondary arthritis)

Preoperative optimization:

• Planned surgery on pagetic bone (reduce vascularity and bleeding)
• Give bisphosphonate 3-6 months before elective surgery

Calcium and Vitamin D Supplementation

Always coadminister:

• Calcium 1000-1200mg daily
• Vitamin D 800-1000 IU daily
• Prevents hypocalcemia after bisphosphonate treatment
• Optimizes bone health

Monitoring

ALP at 3, 6, 12 months (goal: normalize or 75% reduction). Rising ALP = reactivation. Bone turnover markers (NTX, CTX) more sensitive.

Indications for Surgery

Preoperative Optimization

Bisphosphonate treatment 3-6 months before surgery:

• Reduces bone vascularity
• Decreases operative blood loss (up to 50% reduction)
• Normalizes bone turnover
• Improves fixation quality

Cross-match blood:

• Pagetic bone highly vascular
• Risk of significant intraoperative bleeding
• Have blood products available

Operative Considerations

Total Hip Arthroplasty in Paget's

Special considerations:

• Altered femoral anatomy (bowing, coxa vara)
• Acetabular protrusion common
• Use cemented long-stem femoral component (better fixation in soft bone)
• Consider cementless acetabular component with screw fixation
• Higher rate of heterotopic ossification (consider prophylaxis with indomethacin or radiation)

The combination of an elevated ALP and a sclerotic/expanded bone has several mimics. The discriminators are serum calcium, PTH, the pattern of bone change, and whether multiple bones are involved.

Global epidemiology: Strong geographic and ethnic gradient. Highest prevalence in populations of Anglo-Celtic / Western European descent (UK, Australia, New Zealand, North America, and emigrant communities), where it affects roughly 1-4% over age 55. Rare in Scandinavia, much of Asia and sub-Saharan Africa. Incidence and severity have declined markedly over recent decades across high-prevalence regions, supporting an environmental contribution alongside SQSTM1-related genetic susceptibility.

Side-by-Side Guideline Comparison

Point of difference: older guidance emphasised normalizing ALP in all active disease; the PRISM-EZ RCT showed no clinical benefit from intensive turnover suppression in established disease, so contemporary UK/European practice favours treating symptoms and high-risk sites rather than every elevated ALP.

Registry evidence: Arthroplasty registries (NJR England & Wales, AOANJRR Australia, AJRR US, Swedish/Norwegian/NZ registries) and dedicated pagetic cohorts report 10-year hip implant survival around 90%, with bleeding/transfusion and heterotopic ossification as the dominant complications rather than early loosening.

High- vs limited-resource practice: Where IV zoledronate and DXA/bone scan are available, a single infusion with biochemical monitoring is standard. In limited-resource settings, oral risedronate/alendronate and serial ALP (a cheap, widely available marker) are pragmatic alternatives; plain radiographs alone are usually sufficient for diagnosis and surgical planning.