Paget's Disease of Bone

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

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

History

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

Examination

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.

Algorithm

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Bisphosphonates - First-Line Treatment

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

Indications for Treatment

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

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 one-pager and treatment indications repeatedly invoke high-output cardiac failure as a systemic complication of extensive Paget's, but the mechanism deserves explicit development because it is a classic basic-science viva stem and the reason "disease extent," not just symptoms, can justify treatment.

Why pagetic bone raises cardiac output

Actively remodelling pagetic bone is intensely hypervascular: dilated marrow sinusoids and increased blood flow through the affected bone and overlying skin behave functionally like a low-resistance arteriovenous shunt (clinically felt as warmth over the bone). Where a large fraction of the skeleton is involved, the summed increase in resting blood flow raises cardiac output and preload, producing a high-output state.

The topic names CN VIII deafness, skull-base involvement, basilar invagination (platybasia), spinal stenosis and radiculopathy as complications but leaves them as bullet stubs. Because these are high-yield viva material - and because the mechanism dictates whether medical or surgical treatment comes first - they are developed here.

Hearing loss (the commonest neurological complication)

Deafness in skull Paget's is typically mixed sensorineural and conductive. The dominant contributor is thought to be pagetic remodelling and demineralisation of the cochlear (otic) capsule and cochlear structures, rather than simple compression of the eighth nerve at the internal auditory meatus; a conductive component can arise from involvement of the ossicles and temporal bone. Practically, established hearing loss responds poorly, so treatment aims to slow progression.

Skull base: basilar invagination and platybasia

Softening of the pagetic skull base allows the odontoid to migrate upward into the foramen magnum (basilar invagination / platybasia). This can produce brainstem and upper cervical cord compression, vertebrobasilar symptoms, and obstructive hydrocephalus from aqueduct/outflow compromise. It is a serious, though uncommon, complication of extensive calvarial disease.

Spine: stenosis, myelopathy and the vascular-steal phenomenon

Vertebral Paget's (picture-frame or ivory vertebra with bony expansion) narrows the canal and neural foramina, causing spinal stenosis, myelopathy, radiculopathy or cauda equina syndrome.

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

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.