Paget Disease of Bone Osteitis Deformans

Slide 1 · Paget Disease of Bone

(Osteitis Deformans)

Remodeling • morphology • clinical consequences

Pathology teaching lecture

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain why enhanced bone turnover produces architecturally weak bone.

Distinguish osteolytic, mixed, and osteosclerotic phases morphologically.

Link SQSTM1–NF-κB and RANK–OPG biology to osteoclast activity.

Interpret characteristic distribution, complications, and laboratory findings.

Slide 3 · Disease definition: Excessive, disordered remodeling

Paget disease is characterized by increased but structurally abnormal bone formation.

Excessive resorption and compensatory formation become poorly coordinated.

Affected bone may enlarge and become sclerotic without gaining normal strength.

Density and strength are not equivalent.

Slide 4 · The paradox of pagetic bone

Apparent gain

Functional loss

Thickened trabeculae

Expanded, frequently sclerotic bone

High osteoblastic activity during active remodeling

Disorganized lamellar units

Relatively porous, mechanically weak cortices

Microfractures, deformity, and fracture

Abnormal remodeling explains the coexistence of sclerosis and fragility.

Slide 5 · Three cellular participants in bone remodeling

Osteoclast

Monocyte-derived, multinucleated cell; Dissolves mineral and organic matrix.

Osteoblast

Synthesizes osteoid, builds matrix, and regulates mineralization.

Osteocyte

Mechanosensory matrix cell that influences local remodeling.

Osteoprogenitor

Provides osteoblast precursors; Expands near active pagetic surfaces.

Normal bone-cell functions are summarized from the parent bone biology section.

Slide 6 · Normal remodeling is a coupled sequence

Osteoclast attachment

Surface sealing

Matrix resorption

Resorption lacuna

Osteoblast recruitment

Formation coupled

Matrix deposition

Repair / renewal

Paget disease exaggerates this remodeling program and leaves disorganized architecture.

Slide 7 · RANKL promotes osteoclast differentiation

Osteoblast / stroma

Express RANKL

Osteoclast precursor

RANK receptor

NF-κB pathway

Cell survival / differentiation

Mature osteoclast

Matrix resorption

The axis supplies mechanistic context for Paget-associated increased osteoclast function.

Slide 8 · Osteoprotegerin limits RANKL signaling

Osteoblast

Releases OPG

OPG decoy receptor

Binds RANKL

Less RANK activation

Reduced signaling

Less differentiation

Reduced osteoclastogenesis

This negative control is relevant to the OPG gene defects linked to juvenile disease.

Slide 9 · Woven and lamellar bone have different organization

Woven bone

Lamellar bone

Rapidly formed bone matrix

Randomly oriented collagen

Reduced structural integrity

Orderly layered collagen

Greater structural organization

Normal mature skeletal architecture

Paget disease can contain woven bone in its active phase and mosaic lamellar bone later.

Slide 10 · Three sequential phases define disease evolution

Osteolytic

Dominant resorption

Mixed

Resorption + formation

Osteosclerotic

Mosaic lamellar bone

Activity and microscopic appearances vary across time and between affected bones.

Slide 11 · Osteolytic phase: Osteoclast-driven resorption

The earliest stage contains numerous abnormally large osteoclasts.

Active resorption pits indent the bone surface.

Resorption creates the substrate for subsequent exaggerated formation.

Hallmark: Excessive osteoclast activity.

Slide 12 · Giant osteoclasts can be striking

Pagetic osteoclasts may contain 100 or more nuclei.

This as more than four times the normal nuclear count.

Multinucleation must be interpreted with trabecular and phase-related changes.

Extreme osteoclast size is characteristic of an active lytic lesion.

Slide 13 · Why resorption pits develop

Multinucleated osteoclast

Attaches to bone

Sealed resorption zone

Acid and proteases

Matrix dissolution

Mineral + osteoid

Resorption pit

Localized excavation

This normal cellular mechanism becomes markedly exaggerated in the lytic phase.

Slide 14 · Mixed phase: Both cell populations are active

Osteoclasts remain conspicuous rather than disappearing.

Osteoblastic activity increases alongside resorption.

New bone is deposited rapidly but is architecturally disorganized.

Coexisting activity distinguishes the mixed stage.

Slide 15 · Woven bone reflects rapid deposition

Mixed-phase osteoblasts generate poorly mineralized woven bone.

Collagen architecture lacks the orderly alignment of mature lamellar bone.

Abundant new matrix does not guarantee mechanical competence.

Unorganized deposition is central to functional weakness.

Slide 16 · Fibrovascular marrow accompanies active remodeling

Marrow near bone-forming surfaces becomes loose connective tissue.

Osteoprogenitor cells are present in this region.

Numerous blood vessels accompany the active remodeling interface.

Active marrow change explains increased local vascularity.

Slide 17 · Osteosclerotic phase: Cellular activity wanes

The late “burned-out” stage becomes relatively quiescent.

Prominent cement lines outline disorganized lamellar units.

Thickened trabeculae coexist with mechanically weak cortical bone.

The signature microscopic pattern is mosaic lamellar bone.

Slide 18 · The mosaic pattern: Irregular cement lines

Schematic (not a histologic image)

Prominent cement lines

Intersecting boundaries form an irregular “jigsaw” architecture.

The irregular orientation of lamellar units reflects disordered remodeling.

Slide 19 · What creates the jigsaw-puzzle pattern?

Multiple lamellar bone units have haphazard orientations.

Unusually prominent cement lines demarcate the units.

Irregularly intersecting cement lines generate the mosaic appearance.

The hallmark is architectural—not simply an excess of bone.

Slide 20 · Marrow changes evolve with disease phase

Active mixed phase

Quiescent late phase

Loose fibrovascular connective tissue

Osteoprogenitor cells near deposition surfaces

Many vessels adjacent to bone

Periosseous fibrovascular tissue recedes

More typical marrow returns

Mosaic lamellar architecture persists

A specimen reflects disease timing and sampling site.

Slide 21 · Bone enlargement does not imply strength

Orderly lamellar architecture

Pagetic remodeling

Aligned lamellar units

Thickened, disorganized units

Native vector schematic: Arrangement simplified; Not a specimen

Slide 22 · Why the skeleton fractures despite sclerosis

Newly formed bone is thick but irregular in architecture.

Soft, porous cortices have diminished structural stability.

Normal mechanical loading produces microfracture or gross failure.

Bone mass, radiographic density, and quality must be distinguished.

Slide 23 · Phase-dependent pathologic findings

Feature

Osteolytic

Mixed

Osteosclerotic

Dominant cell

Giant osteoclast

Osteoclast + osteoblast

Relatively fewer active cells

Matrix

Resorption pits

New woven bone

Mosaic lamellar bone

Marrow

Resorptive surfaces

Fibrovascular tissue

More normal marrow

Structure

Bone loss

High turnover

Thick but weak bone

Qualitative phase summary reconstructed; Not a diagnostic scoring system.

Slide 24 · Pathogenesis connects histology to symptoms

Excessive resorption

Giant osteoclasts

Disordered formation

Woven → mosaic bone

Weak enlarged bone

Porosity / deformity

Clinical sequelae

Pain / fracture / compression

Different phase appearances participate in the same mechanical disease process.

Slide 25 · Etiology: Genetic and environmental contributions

The initiating cause is not fully established.

Familial susceptibility supports genetic contributions.

Geographic heterogeneity suggests environmental influences.

No single environmental agent is confirmed by the textbook.

Slide 26 · SQSTM1 mutations: Different familial burden

About 60% of familial cases involve germline SQSTM1 mutations.

About 10% of sporadic cases are associated with SQSTM1 germline mutations.

SQSTM1 encodes the protein sequestosome-1.

Percentages are textbook-specific estimates; Not universal diagnostic probabilities.

Slide 27 · SQSTM1 dysfunction amplifies osteoclast activity

SQSTM1 mutation

Sequestosome-1

Increased NF-κB activity

Downstream signaling

Greater osteoclast activity

Resorption increased

Disordered remodeling

Abnormal bone quality

The source describes this mechanism as an apparent disease pathway, not a complete causal model.

Slide 28 · A negative SQSTM1 result does not exclude disease

Familial cases

Sporadic cases

Higher reported SQSTM1 mutation fraction

Family history can support inherited susceptibility

Mutation is not present in every pedigree

Smaller mutation-associated subset

Absence of known mutation is common

Clinical and morphologic findings remain important

Do not equate a molecular association with a universal diagnostic requirement.

Slide 29 · Juvenile Paget disease: RANK and OPG genes

RANK activating mutation

More RANK signaling

OPG inactivating mutation

Less RANKL blockade

Osteoclast dysregulation

Excess differentiation/activity

These are mechanisms for some juvenile cases, distinguished from typical late-adult onset.

Slide 30 · Geography points to unidentified exposures

The disease shows marked geographic heterogeneity.

A decrease in new cases has been observed in some countries over recent decades.

The responsible environmental factor remains unidentified.

Geographic association alone does not establish causation.

Slide 31 · A viral hypothesis remains unproven

Experimental observation

Evidence boundary

In vitro work proposed chronic viral infection of osteoclast precursors.

Measles and other RNA viruses have been considered.

Firm disease links have not been established.

An etiologic model must not be taught as confirmed infection.

Distinguish experimental plausibility from established etiology.

Slide 32 · Age distribution: Usually late adulthood

Paget disease typically appears in later adult life.

Frequency rises with increasing age.

Many asymptomatic cases leave the exact incidence uncertain.

Age is contextual evidence, not a stand-alone diagnostic feature.

Slide 33 · Population patterns are geographically uneven

The textbook estimates approximately 1% of U.S. Adults over 40 years are affected.

Relatively common: United Kingdom, central Europe, Greece, United States, Australia.

Rare in native populations of Scandinavia, China, Japan, and Africa.

These are textbook estimates and geographic observations, not a current surveillance update.

Slide 34 · Extent of involvement: One bone or many

Monostotic

Polyostotic

One affected skeletal site

Approximately 15% of cases

Limited anatomic distribution

Multiple affected bones

Approximately 85% of cases

Potential for greater skeletal and systemic burden

The supplied percentages describe the distribution reported by Robbins.

Slide 35 · Which bones are most often involved?

Axial skeleton and proximal femur are involved in up to 80% of cases.

A wide spectrum of bones can be involved.

Clinical consequences depend strongly on anatomic site.

Sites of load bearing and neural passage are especially consequential.

Slide 36 · Incidental recognition is common

Most patients are asymptomatic at presentation.

An abnormal radiograph may be found during unrelated clinical workup.

Incidental discovery can occur during staging of another malignancy.

Radiographic recognition must be interpreted within clinical context.

Slide 37 · Radiology: Enlarged, sclerotic, deformed bone

The source figure shows an enlarged bowed tibia.

Cortical and cancellous regions show irregular thickening.

Sclerosis coexists with altered shape and structural vulnerability.

Imaging illustrates morphology; It does not directly measure bone strength.

Slide 38 · Weight bearing transforms weakness into deformity

Relatively straight axis

Anterior bowing

Long-axis load

Abnormally remodeled bone bends under load

Slide 39 · How lower-limb bowing develops

Pagetic bone lacks normal structural organization.

Weight bearing can produce anterior bowing of femurs and tibiae.

Deformity increases abnormal joint loading and mechanical symptoms.

Mechanical consequence follows architectural failure.

Slide 40 · Craniofacial enlargement can be dramatic

Overgrowth can enlarge the craniofacial skeleton.

Severe enlargement may create leontiasis ossea (lion-like facial contour).

Skull mass may increase substantially in advanced cases.

Overgrowth can produce both cosmetic and functional effects.

Slide 41 · Skull-base deformity threatens the posterior fossa

Pagetic weakness may permit invagination of the skull base.

This distortion as platybasia.

Compression of posterior fossa structures may result.

Anatomic location determines neurologic risk.

Slide 42 · How bone overgrowth produces neurologic symptoms

Enlarged pagetic bone

Structural expansion

Narrowed neural passage

Skull or spine

Nerve root compression

Cranial / spinal

Symptoms

Pain / neurologic deficits

Compression is a local mechanical effect—not a mandatory systemic manifestation.

Slide 43 · Vertebral fracture can cause kyphosis and injury

Compression fractures may occur in affected vertebrae.

Multiple or severe collapse can accentuate kyphotic deformity.

The source describes potential spinal cord injury.

Neurologic deficits warrant attention to structural compromise.

Slide 44 · Hip deformity predisposes to osteoarthritis

Pagetic deformation can distort the femoral head.

Abnormal joint geometry increases mechanical stress.

Severe secondary osteoarthritis may follow.

Joint pain need not originate solely from active bone remodeling.

Slide 45 · Chalk-stick fractures reflect brittleness

Long bones of the lower limbs are frequent sites.

Chalk-stick–type fractures.

A hard-appearing but poorly organized bone may fracture under load.

Clinical consequence: Fracture risk persists in radiographically sclerotic bone.

Slide 46 · Sources of pain in Paget disease

Microfracture

Remodeled bone is vulnerable to local mechanical disruption.

Mechanical deformity

Bowing alters the loading of long bones.

Secondary osteoarthritis

Articular distortion produces degenerative joint symptoms.

Neural compression

Bony expansion may affect cranial or spinal nerve roots.

Multiple mechanisms can coexist; Do not ascribe all pain to one process.

Slide 47 · Pagetic bone can become highly vascular

The remodeling-associated connective tissue contains numerous vessels.

In active disease, local hyperemia can warm the skin over the bone.

The finding is uncommon rather than a universal diagnostic sign.

Increased local blood flow provides a pathologic clue in severe disease.

Slide 48 · Rare systemic consequence: High-output failure

Severe polyostotic disease

Extensive vascular bed

Increased skeletal blood flow

Shunt-like circulation

Greater cardiac demand

High-output physiology

Heart failure risk

Rare complication

Possible new high-output failure or exacerbation of existing heart disease.

Slide 49 · Serum biochemistry follows remodeling activity

Frequently observed

Usually preserved

Elevated serum alkaline phosphatase

Osteoblastic response to high turnover

Biochemical clue to active disease

Normal serum calcium

Normal serum phosphate

Does not resemble obligatory systemic mineral imbalance

Interpret ALP with imaging and the clinical picture.

Slide 50 · Laboratory pattern: Contextual interpretation

Clinical vignette

An older adult has abnormal bone enlargement on radiography, increased alkaline phosphatase, and normal calcium and phosphate.

1. Which bone process best unifies these findings?

2. Why can ALP rise when serum calcium remains normal?

Interpretation

Disordered high bone turnover may elevate ALP without systemic calcium or phosphate abnormalities.

Slide 51 · Secondary osteosarcoma is uncommon but serious

Osteosarcoma in fewer than 1% of all individuals with Paget disease.

In severe polyostotic disease, the reported proportion is 5%–10%.

Paget-associated osteosarcoma is described as rapidly fatal.

The subgroup estimate must not be applied to every affected individual.

Slide 52 · Overall prognosis is usually favorable

Without malignant transformation, most cases are not life threatening.

Many patients have mild symptoms or no symptoms.

Clinically important morbidity comes from fracture, deformity, joint disease, and compression.

Population prognosis differs substantially from rare severe complications.

Slide 53 · Therapeutic rationale: Reduce osteoclast activity

Osteoclast hyperactivity

Mechanistic driver

Antiresorptive therapy

Bisphosphonates / calcitonin

Less bone resorption

Turnover dampened

Symptom management

Clinical benefit

The source states the therapeutic class rationale; Detailed drug regimens are outside this lecture scope.

Slide 54 · Distinguish Paget disease from osteoporosis

Paget disease

Osteoporosis

Increased, disordered turnover

Often focally enlarged / sclerotic bone

Abnormal mosaic architecture

Reduced mass of normally mineralized bone

Diffuse loss of bone quantity

Fragility from reduced bone amount

Both predispose to fracture through different structural abnormalities.

Slide 55 · Distinguish Paget disease from osteomalacia

Paget disease

Osteomalacia

Disordered architecture during high turnover

Mosaic lamellar bone in late phase

ALP often increased; Ca/P often normal

Inadequate mineralization of matrix

Undermineralized remodeling bone

A mineralization defect, not simply high bone mass

The distinction centers on disordered remodeling versus impaired mineralization.

Slide 56 · Distinguish Paget disease from hyperparathyroidism

Paget disease

Hyperparathyroidism

Focal or multifocal mosaic remodeling

Abnormally large osteoclasts in early lesions

Local enlargement and deformity

PTH-driven skeletal resorption

Brown tumors / osteitis fibrosa cystica in severe disease

Often different laboratory context

Some osteoclastic features overlap; Mechanism and pattern separate the entities.

Slide 57 · A coherent pathologic interpretation

Age + anatomic pattern

Clinical context

Imaging

Enlargement / thickening

Biochemistry

ALP may be high

Phase-linked morphology

Mosaic / osteoclasts

No isolated feature substitutes for clinicopathologic synthesis.

Slide 58 · Case: Incidental tibial sclerosis

Clinical vignette

A patient undergoing unrelated imaging has enlarged, irregularly sclerotic bone with cortical and trabecular thickening; There is no bone pain.

1. Is the absence of symptoms incompatible with Paget disease?

2. What is the mechanistic explanation for sclerosis?

Interpretation

No. Many cases are incidental; Intense disordered formation produces sclerosis.

Slide 59 · Case: Enlarging skull and neural symptoms

Clinical vignette

An older individual has progressive craniofacial enlargement and symptoms referable to cranial nerve compression.

1. Which structural mechanism could account for neurologic symptoms?

2. Which additional skull-base consequence is?

Interpretation

Expansion may compress nerve roots; Skull-base invagination can compromise posterior fossa structures.

Slide 60 · Case: Bowed limb and hip pain

Clinical vignette

A patient has a visibly bowed lower extremity and progressive hip symptoms alongside abnormal bone remodeling.

1. Why does the shaft bow?

2. Why can degenerative hip disease develop?

Interpretation

Architectural weakness permits bowing; Altered femoral head geometry promotes secondary osteoarthritis.

Slide 61 · Case: Severe polyostotic disease

Clinical vignette

A patient with extensive pagetic skeletal involvement develops new, disproportionate focal symptoms.

1. Which serious bone neoplasm is a recognized rare complication?

2. Why must the risk estimate be stratified by disease severity?

Interpretation

Secondary osteosarcoma; Reported risk is far lower overall than in severe polyostotic disease.

Slide 62 · Integrative morphology: The essential chain

Lytic phase: Giant osteoclasts and resorption pits.

Mixed phase: Osteoblasts, woven bone, and fibrovascular marrow.

Late phase: Mosaic lamellar bone, thickened trabeculae, porous cortex.

Clinical endpoint: Deformity, fracture, compression, and joint effects.

Slide 63 · Five high-yield distinctions

Paget disease increases bone turnover; It does not produce normal bone quality.

Mosaic cement lines are a key late histologic feature.

SQSTM1–NF-κB is an important but not exhaustive pathogenesis model.

Elevated ALP can coexist with normal calcium and phosphate.

Secondary osteosarcoma is rare overall but clinically significant.

Slide 64 · Source and verification boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th ed. (2026).

Chapter 26: Bones, Joints, and Soft Tissue Tumors; Paget disease, pp. 1073–1075.

Normal bone cells/remodeling and RANK–OPG context, pp. 1064–1066.

Original, editable illustrations are conceptual schematics—not diagnostic images.