Osteochondroma

Slide 1 · Osteochondroma

OSTEOCHONDROMA

Cartilage-capped surface lesion of bone

Morphology · Molecular pathogenesis · Clinical correlation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Recognize the defining architecture of an osteochondroma.

Explain the connection to the growth plate and EXT1/EXT2.

Integrate gross, microscopic, and radiographic morphology.

Distinguish solitary disease, hereditary disease, and malignant transformation.

Slide 3 · Osteochondroma: Essential definition

Benign cartilage-capped lesion arising from the bone surface.

A bony projection carries the hyaline cartilage cap.

Both sessile and pedunculated forms are recognized.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 4 · Why “cartilage-capped” matters

Growth plate-associated cartilage

Surface cartilage cap

Endochondral bone formation

Schematic sequence

Slide 5 · Anatomical origin: An endochondral bone

Osteochondromas occur in bones formed by endochondral ossification.

The metaphysis near a growth plate is the characteristic site.

The lesion is topographically related to developing cartilage.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 6 · Two clinically important presentations

Solitary / sporadic

Hereditary multiple exostoses

Approximately 85% of cases

Remaining cases occur with syndrome

Usually recognized in late adolescence / early adulthood

Often apparent during childhood

Single osteochondroma

Multiple lesions may cause skeletal deformity

Slide 7 · Distribution: Long tubular bones

The metaphyses of long tubular bones are the commonest location.

The region around the knee is particularly frequent.

A surface projection may be readily visible on imaging.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 8 · Other locations: Pelvis, scapula, ribs

After long bones, pelvic bones, scapula, and ribs are important sites.

Osteochondromas at these sites often have relatively short stalks.

A broad or short attachment can make the projection less obvious.

Slide 9 · Age patterns reflect skeletal growth

Sporadic osteochondroma

Multiple hereditary exostoses

Usually diagnosed in late adolescence or early adulthood

Typically recognized during childhood

Often one slow-growing lesion

Multiple lesions during skeletal development

Expansion usually ceases after growth-plate closure

Deformity reflects altered epiphyseal growth

Slide 10 · Sex distribution in sporadic disease

Male predominance

Males affected approximately three times more often than females in sporadic osteochondroma.

Slide 11 · External configuration: Sessile or stalked

Schematic

Schematic

Sessile = broad-based projection

Pedunculated = projection with bony stalk

Slide 12 · The decisive structural relationship

Cortex of the lesion merges with host-bone cortex.

Medullary cavity of the lesion continues with host marrow.

This architecture is the key to recognizing osteochondroma.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 13 · Growth plate and direction of the cap

Lesions originate near the metaphyseal growth plate.

On plain radiographs the cap points away from the nearest physis.

Orientation is a clue; Continuity remains decisive.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 14 · Reading an osteochondroma on radiographs

Identify surface projection

Trace cortex into parent bone

Trace marrow into parent bone

Locate cap / nearest physis

Schematic sequence

Slide 15 · Metaphyseal anatomy: A location-based clue

The metaphysis lies next to the epiphyseal growth plate.

Osteochondroma morphology develops in relation to this zone.

Location, outward growth, and continuity reinforce the diagnosis.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 16 · Solitary lesion versus syndromic multiplicity

One lesion

A solitary bone-surface mass is the usual presentation.

Multiple lesions

Multiple exostoses prompt consideration of the hereditary syndrome.

Skeletal effects

Multiple lesions may disturb growth and cause bowing or shortening.

Slide 17 · Hereditary multiple exostoses: Inheritance

Autosomal dominant predisposition

Germline EXT1 or EXT2 mutation

Additional somatic loss in growth-plate chondrocytes

Schematic sequence

Slide 18 · EXT1 and EXT2: Molecular function

EXT1 and EXT2 encode glycosyltransferases.

These proteins participate in heparan sulfate biosynthesis.

Disruption affects growth-plate chondrocyte behavior.

EXT1 / EXT2

Heparan sulfate

Growth signaling

Schematic molecular relationship

Slide 19 · Hereditary disease: The second hit

Germline EXT1/EXT2 variant

Loss of remaining wild-type allele

Affected growth-plate chondrocyte clone

Exostotic outgrowth

Schematic sequence

Slide 20 · Why a growth-plate progenitor matters

Cell of origin

The relevant somatic event occurs in a growth-plate chondrocyte.

Proliferation

The altered progenitor population may proliferate inappropriately.

Polarity

Loss of normal directional organization may favor outward growth.

Slide 21 · Loss of heparan sulfate: Upstream event

EXT-related loss impairs heparan sulfate synthesis.

Heparan sulfate normally contributes to local growth-factor signaling.

Its reduction may disrupt growth-plate organization.

EXT1 / EXT2

Heparan sulfate

Growth signaling

Schematic molecular relationship

Slide 22 · Signaling pathways secondarily perturbed

Reduced heparan sulfate

Hedgehog

BMP

FGFR3

Wnt

Pathway perturbation (not a proven linear cascade)

Slide 23 · Pathogenesis: Integrated proposed sequence

EXT dysfunction

Reduced heparan sulfate

Altered proliferation / polarity

Growth-plate outgrowth

Cartilage-capped lesion

Schematic sequence

Slide 24 · Sporadic osteochondroma: What is known

Reduced expression of EXT1 or EXT2 has been observed in sporadic lesions.

This is distinct from the established inherited two-hit model.

Robbins does not assign one universal mutation pattern to all sporadic cases.

Slide 25 · Molecular mechanism: Evidence versus inference

Established

Mechanistic interpretation

Hereditary lesions associate with EXT1 / EXT2 loss

Growth-plate signaling perturbation is downstream

EXT enzymes produce heparan sulfate

Progenitor proliferation and loss of polarity may result

Several pathways are implicated

Not a single direct linear signaling pathway

Slide 26 · How a surface outgrowth develops

Growth-plate chondrocyte disturbance

Outward cartilage growth

Cartilaginous cap remains

Endochondral bone forms below cap

Schematic sequence

Slide 27 · Osteochondroma and growth-plate closure

Osteochondromas ordinarily stop expanding as the growth plate closes.

Persistent enlargement after skeletal maturity deserves evaluation.

Growth arrest supports dependence on developmental cartilage.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 28 · Gross size and shape

Size

Lesions ranging approximately 1–20 cm.

Attachment

Sessile forms have broad bases; Pedunculated forms have stalks.

Composition

The projecting bone is covered by a cartilage cap.

Slide 29 · Gross anatomy: Cartilage, bone, marrow

Perichondrium

Cartilage cap: Benign hyaline cartilage

Endochondral ossification zone

Bone and continuous marrow

Schematic cross-section; Not a photomicrograph

Slide 30 · Pedunculated osteochondroma: Anatomy

A projecting bony stalk arises from the host cortex.

Marrow continues into the projection.

The cartilage cap covers the head of the outgrowth.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 31 · Sessile osteochondroma: Anatomy

The lesion has a broad attachment to the host bone.

The continuity of cortical and medullary components still applies.

The cartilage cap overlies the surface.

Growth plate

Cartilage cap

Host bone

Schematic

Slide 32 · Cartilage cap: Tissue identity

The cap is composed of benign hyaline cartilage.

It is invested by perichondrium.

It resembles an abnormally organized growth plate.

Perichondrium

Hyaline cartilage

Endochondral bone

Schematic

Slide 33 · Endochondral ossification beneath the cap

Benign hyaline cartilage

Growth plate-like organization

Endochondral ossification

New bone within head and stalk

Schematic sequence

Slide 34 · Microscopic architecture: Interpret by zones

Perichondrium

Cartilage cap: Benign hyaline cartilage

Endochondral ossification zone

Bone and continuous marrow

Schematic cross-section; Not a photomicrograph

Slide 35 · Why the cap looks like a growth plate

Normal growth-plate principle

Osteochondroma cap

Organized cartilage supporting longitudinal growth

Disorganized growth plate-like cartilage

Endochondral ossification at a defined interface

Cartilage undergoes endochondral ossification

Bone extends along skeletal axis

Bone forms within protruding head and stalk

Slide 36 · Perichondrium is part of the cap

The cartilage cap is covered by perichondrium.

Recognition requires considering the surface covering and the underlying cartilage.

It is a structural layer rather than proof of malignancy.

Perichondrium

Hyaline cartilage

Endochondral bone

Schematic

Slide 37 · Histologic cellularity: Interpret in context

Hereditary multiple osteochondromas may be more cellular than sporadic lesions.

Greater cytologic atypia can also occur in hereditary lesions.

Neither finding alone establishes secondary chondrosarcoma.

Slide 38 · Anatomic continuity links imaging and histology

Radiograph: Outward bony mass

Cortex continues with parent bone

Marrow continues with parent bone

Cap of benign cartilage

Schematic sequence

Slide 39 · Clinical presentation: Painless growth

The typical lesion is a slow-growing bony mass.

Many lesions are discovered incidentally.

A symptomatic lesion often produces local mechanical effects rather than systemic illness.

Slide 40 · Mechanical pain: Compression of a nerve

Pain may result when the protrusion impinges on a nerve.

The mechanism is local anatomy, not necessarily aggressive tumor growth.

Correlate lesion position with the symptomatic structure.

Schematic

Nerve

Adjacent nerve impingement

Slide 41 · Mechanical pain: Fracture of the stalk

A pedunculated lesion can sustain fracture of its bony stalk.

A previously painless lesion may then become painful.

A fractured stalk is distinct from malignant transformation.

Schematic

Possible stalk fracture

Slide 42 · Growth disturbance in hereditary disease

Multiple metaphyseal exostoses

Abnormal epiphyseal growth

Bowing of long bones

Shortening / deformity

Schematic sequence

Slide 43 · Sporadic versus hereditary: Clinical phenotype

Solitary sporadic

Multiple hereditary

Single slow-growing surface lesion

Numerous lesions may appear in childhood

Bowing is not a defining feature

Bowing and shortening may occur

Malignant transformation is rare

Secondary malignancy more often arises in this setting

Slide 44 · What happens at skeletal maturity?

Growth usually stops

Osteochondromas commonly cease enlargement with growth-plate closure. This temporal pattern helps interpret subsequent enlargement.

Slide 45 · When is simple excision relevant?

Symptomatic lesions are generally cured by simple excision.

Clinical effects can include neural impingement or stalk fracture.

Management follows symptoms and anatomy; The lecture does not prescribe a universal surgical threshold.

Slide 46 · Secondary chondrosarcoma: Key risk context

Secondary chondrosarcoma is a rare complication.

It usually occurs in association with multiple hereditary exostoses.

Do not treat all benign cellularity or atypia as malignant transformation.

Slide 47 · Differential: Surface versus medullary chondroma

Osteochondroma

Enchondroma

Surface bone projection with cartilage cap

Cartilage nodule within medullary cavity

Cortical and marrow continuity into outgrowth

Circumscribed intramedullary lucent lesion

Usually metaphysis of long bone; Knee common

Often small tubular bones of hands / feet

Slide 48 · Differential: Juxtacortical chondroma

Osteochondroma

Juxtacortical chondroma

Surface growth has cortical–medullary continuity

Cartilage tumor located on bone surface

Contains bone and cartilage cap

Classified as a chondroma rather than a cartilage-capped exostosis

Growth-plate association typical

Not characterized by the osteochondroma continuity pattern

Slide 49 · Chondrosarcoma: What is different?

Typical osteochondroma

Conventional chondrosarcoma

Benign cartilage cap on bone-surface projection

Malignant cartilage-producing tumor

Growth usually stops with skeletal maturity

May produce painful progressively enlarging mass

Cortex and medulla continuous with host bone

Aggressive tumors can destroy cortex and form soft-tissue mass

Slide 50 · Chondrosarcoma arising in exostosis

Preexisting osteochondroma

Rare secondary malignant change

Cartilage-producing chondrosarcoma

Clinical reassessment required

Schematic sequence

Slide 51 · EXT genes across benign and malignant lesions

Hereditary osteochondroma

Associated secondary chondrosarcoma

EXT1 / EXT2 disruption is fundamental to syndrome

Chondrosarcomas in this syndrome can harbor EXT1 / EXT2 variants

Growth-plate clonal outgrowth

Malignancy cannot be inferred from EXT status alone

Usually benign behavior

Requires morphology and clinical–radiologic correlation

Slide 52 · Integrated diagnosis: Information to combine

Clinical

Patient age; Single versus multiple lesions; Symptoms; Growth course.

Radiologic

Surface location and cortical–medullary continuity.

Morphologic

Cartilage cap with perichondrium and underlying endochondral bone.

Slide 53 · Diagnostic exercise: Simple stalked lesion

A late adolescent has a slowly enlarging metaphyseal projection near the knee. Imaging shows uninterrupted cortex and medulla extending into its stalk.

What diagnosis is best supported?

Which structural feature is decisive?

Slide 54 · Case interpretation: Typical osteochondroma

Most consistent diagnosis: Osteochondroma.

The lesion is surface-based with cortex and marrow continuity.

The location and stage of skeletal growth support the interpretation.

Slide 55 · Diagnostic exercise: Multiple deforming lesions

A child has several exostoses of long bones with bowing and shortening. The lesions have cartilage caps and arise near metaphyses.

Which syndrome is most likely?

Which genes and mechanism are involved?

Slide 56 · Case interpretation: Hereditary exostoses

The pattern is compatible with hereditary multiple exostoses.

The inherited predisposition involves EXT1 or EXT2.

Subsequent loss of the wild-type allele in growth-plate chondrocytes is implicated.

Slide 57 · Diagnostic exercise: Pain after a fall

A young person with a pedunculated osteochondroma develops focal pain following minor trauma. The lesion was previously asymptomatic.

What mechanical complication should be considered?

Does pain automatically imply malignant change?

Slide 58 · Case interpretation: Fracture of the stalk

A stalk fracture is a recognized cause of pain.

Pain alone cannot establish malignant transformation.

Clinical and radiologic correlation must distinguish local injury from a new growth pattern.

Slide 59 · Diagnostic exercise: Adult enlargement

An adult with a longstanding hereditary exostosis reports that the mass is growing again after skeletal maturity.

Why is this history important?

Which complication deserves consideration?

Slide 60 · Case interpretation: Re-evaluate growth

Osteochondromas usually stop growing at skeletal maturity.

New growth in an adult warrants reassessment.

Rare secondary chondrosarcoma should be included in that evaluation, especially in hereditary disease.

Slide 61 · Three discriminators that carry the diagnosis

Surface anatomy

Cartilage-capped outward projection.

Internal continuity

Cortex and medulla continuous with parent bone.

Developmental behavior

Growth-plate association and usual cessation at maturity.

Slide 62 · Final synthesis: Mechanism to consequence

EXT / heparan-sulfate dysregulation

Disordered growth-plate outgrowth

Cartilage cap + bone stalk

Local mechanical effects / deformity

Schematic sequence

Slide 63 · High-yield review questions

Which two anatomical continuities define the lesion?

How do EXT1 / EXT2 alterations affect the growth plate?

How does hereditary disease differ from sporadic presentation?

What explains pain and which complication is rare but important?

Slide 64 · References and source scope

Primary teaching source

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

Chapter 26, “Bones, Joints, and Soft Tissue Tumors,” pp. 1080–1083.

Osteochondroma section, including Fig. 26.21 and morphology; adjacent chondroma and chondrosarcoma for comparison.

All explanatory figures in this deck are newly constructed editable schematics. Authentic specimen images and the textbook figure are not reproduced.