Pyogenic Osteomyelitis

Slide 1 · Pyogenic Osteomyelitis

Etiology · pathogenesis · morphology · clinicopathologic correlation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish hematogenous, contiguous and directly implanted bone infections.

Relate age and clinical setting to organisms and sites of infection.

Explain acute suppuration, necrosis, sequestrum and involucrum.

Interpret clinical findings, imaging and microbial sampling.

Anticipate chronic complications and key diagnostic distinctions.

Slide 3 · Osteomyelitis affects bone and marrow

Osteomyelitis is inflammation of bone and marrow, virtually always due to infection.

Pyogenic osteomyelitis is almost always bacterial.

Infection may be part of a systemic process or present as a solitary primary focus.

The local response combines inflammation, tissue necrosis and reactive bone changes.

Slide 4 · Three routes to bone infection

Hematogenous spread: Organisms seed bone from the bloodstream.

Contiguous extension: An adjacent infectious focus enters bone.

Direct implantation: Organisms enter through open injury or an orthopedic procedure.

The route changes the likely patient population and microbiology.

Bloodstream

Contiguous infection

Direct implantation

Three paths to bone infection

Schematic

Slide 5 · Hematogenous spread in children

Most osteomyelitis in otherwise healthy children is hematogenous.

Long bones are frequent targets.

An initiating bacteremia may follow trivial mucosal injury or minor skin infection.

A seemingly minor antecedent infection can precede a serious bone focus.

Epiphysis

Metaphysis

Metaphyseal circulation predominates

Schematic

Slide 6 · Contiguous spread versus implantation

Route

Entry mechanism

Typical context

Contiguous spread

Infection extends from adjacent tissues

Diabetes-associated foot infection

Direct implantation

Organisms reach bone at time of disruption

Open fracture; Orthopedic surgery

Hematogenous spread

Bacteremia seeds osseous vessels

Otherwise healthy children

Slide 7 · Age changes the dominant context

Population

Predominant clinical setting

Important concern

Children

Bloodborne infection; Long bones

Metaphyseal seeding

Adults

Open fractures, operations, diabetes

Direct or contiguous infection

Infants

Epiphyseal infection may reach a joint

Septic arthritis; Cartilage loss

Slide 8 · Diabetes-associated foot infection

Diabetes as an adult predisposition to foot infections involving bone.

The clinical context differs from otherwise healthy childhood hematogenous disease.

Adjacent infected tissue provides a plausible route of extension to bone.

The anatomic site and route guide microbiologic suspicion.

Bloodstream

Contiguous infection

Direct implantation

Three paths to bone infection

Schematic

Slide 9 · Infant epiphyseal infection threatens joints

In infants, infection may involve the epiphysis.

The nearby joint can become infected: Septic or suppurative arthritis.

Destruction of articular cartilage can produce permanent disability.

This is a major age-specific complication of bone infection.

Epiphyseal focus → adjacent joint

Schematic

Slide 10 · The organism depends on the setting

Staphylococcus aureus predominates among culture-positive pyogenic cases.

Neonates have distinct likely organisms.

Sickle cell disease shifts organism risk.

Direct spread, surgery and open injury can yield mixed bacterial infection.

S. Aureus

Neonate: GBS / E. Coli

Sickle cell: Salmonella

Open injury: Mixed flora

Schematic

Slide 11 · Staphylococcus aureus predominates

80%–90% of culture-positive pyogenic osteomyelitis to S. Aureus.

The denominator is culture-positive cases—not all suspected episodes.

Staphylococcal surface proteins help bind the organism to bone matrix.

Clinical setting still matters when considering alternative organisms.

Slide 12 · Staphylococcal adherence to matrix

Staphylococcal cell wall proteins bind bone matrix components.

Collagen is one of the matrix ligands named.

Attachment favors bacterial localization on a permissive bone surface.

This links a bacterial factor with osseous tropism.

Bone matrix collagen

Bacterial surface proteins bind matrix components

Schematic

Slide 13 · Neonatal pathogen spectrum

During the neonatal period, group B streptococci are important pathogens.

Escherichia coli is another likely neonatal cause.

Adult and older-child assumptions should not be transferred automatically to newborns.

Age provides information before culture results are available.

Slide 14 · Older children: Gram-positive pathogens

In older children, gram-positive organisms are the most likely causes.

S. Aureus remains particularly important.

The common hematogenous presentation is often localized to a long bone.

Microbial confirmation should still be pursued when feasible.

Slide 15 · Why sickle cell disease predisposes

Bone infarcts create areas of osteonecrosis susceptible to bacterial seeding.

Impaired splenic function weakens the immune response.

Both damaged tissue and altered host defense contribute.

This risk context also changes the expected microbiology.

Bone infarction

Bacterial seeding

Impaired host defense

Sickle cell anemia combines tissue injury and susceptibility to infection

Schematic

Slide 16 · Sickle cell disease: Organism clue

Salmonella and other gram-negative bacteria as frequent pathogens in sickle cell disease.

S. Aureus is important in osteomyelitis generally, but the risk setting modifies the differential.

A bone pain episode cannot be interpreted from pain alone; Infection and infarction require consideration.

Use clinical assessment and microbiological evidence to discriminate.

Slide 17 · Polymicrobial pyogenic infection

Mixed bacterial infection is common after direct spread.

Orthopedic surgery is another setting in which multiple organisms may be involved.

Open fractures create an opportunity for direct environmental inoculation.

Do not assume every adult case has a single staphylococcal pathogen.

Slide 18 · Host setting directs microbiology

Setting

Organisms emphasized

Mechanistic context

Culture-positive pyogenic disease

S. Aureus (80%–90%)

Matrix adherence

Neonate

Group B streptococci; E. Coli

Age-dependent distribution

Sickle cell disease

Salmonella; Other gram-negative bacteria

Infarction + splenic dysfunction

Open fracture / surgery

Frequently mixed organisms

Direct implantation

Slide 19 · Organism identification is imperfect

Microbiological cultures are obtained whenever possible.

The chapter reports identifiable organisms in approximately half of patients.

Negative or unrevealing culture does not by itself exclude bone infection.

Robbins anticipates that organism-specific DNA testing may improve identification; No performance figures are supplied.

Slide 20 · Age-dependent bone vascular supply

The osseous circulation changes during skeletal growth.

Vascular architecture determines where hematogenous organisms lodge.

Metaphyseal and epiphyseal communication differs before and after physeal closure.

The expected lesion site is consequently age dependent.

Epiphysis

Metaphysis

Metaphyseal circulation predominates

Schematic

Slide 21 · Neonates: Vessels cross the physis

Neonatal metaphyseal vessels penetrate the growth plate.

Both metaphysis and epiphysis may be involved.

The epiphyseal location helps explain risk of adjacent joint infection.

A neonatal focus need not remain confined to the metaphysis.

Epiphysis

Metaphysis

Vessels cross the growth plate

Schematic

Slide 22 · Older children: Metaphyseal localization

After neonatal vascular remodeling, metaphyseal infection is typical in older children.

The growth plate limits the direct route found in the neonate.

This anatomic pattern is particularly relevant to long-bone hematogenous seeding.

Age and site should be interpreted together.

Epiphysis

Metaphysis

Metaphyseal circulation predominates

Schematic

Slide 23 · Adults: Post-physeal communication

Growth plate closure alters metaphyseal–epiphyseal circulation.

Anastomoses provide a route for bacterial spread across these regions.

Epiphyseal and subchondral involvement becomes more common.

Adult anatomy differs from the usual metaphyseal pattern of older children.

Epiphysis

Metaphysis

Vessels reconnect after plate closure

Schematic

Slide 24 · Age–location relationship

Age group

Vascular relationship

Distribution

Neonate

Metaphyseal vessels traverse plate

Metaphysis, epiphysis or both

Older child

Distinct metaphyseal circulation

Metaphysis typical

Adult

Metaphyseal–epiphyseal anastomoses

Epiphyseal / subchondral more common

Slide 25 · Inflammation evolves over time

Acute infection begins with bacterial multiplication.

Neutrophilic inflammation causes early suppurative tissue injury.

Bone and marrow necrosis can develop rapidly.

Chronic inflammation and reactive repair become prominent as the process evolves.

Bacterial growth

Neutrophilic inflammation

Necrosis

Chronic repair

Schematic

Slide 26 · Early suppurative inflammation

Bacterial proliferation evokes a neutrophil-rich inflammatory response.

Purulent exudate accumulates in infected marrow spaces.

Neutrophil-dominated inflammation is an acute-stage clue.

The inflammatory process can extend beyond its initial medullary focus.

Neutrophil-rich infiltrate (schematic)

Schematic

Slide 27 · Bone and marrow necrosis

Necrosis of bone cells and marrow within about 48 hours.

Infection and inflammation damage tissues and impair local viability.

Devitalized bone is a substrate for persistent infection.

Necrosis becomes central to chronic morphologic complications.

Suppurative focus

Schematic

Slide 28 · Spread through Haversian canals

Bacteria and inflammation extend through intracortical Haversian canals.

The process can reach the outer cortex and periosteum.

An intramedullary infection is therefore not necessarily contained within marrow.

Cortical spread links deep infection to periosteal abscess formation.

Spread through cortical canals

Schematic

Slide 29 · Why the pediatric periosteum matters

The periosteum is comparatively loosely attached to the cortex in children.

Suppuration can lift it away from bone.

A dissecting subperiosteal collection may spread along the cortical surface.

Separation jeopardizes a component of cortical blood supply.

Raised periosteum

Schematic

Slide 30 · Subperiosteal abscess formation

Purulent material accumulates beneath detached periosteum.

The abscess may track longitudinally along the bone surface.

The lifted periosteum restricts its normal vascular contribution.

Consequent ischemia aggravates bone necrosis.

Raised periosteum

Schematic

Slide 31 · Vascular injury amplifies necrosis

Cortical spread

Periosteal elevation

Reduced blood supply

Additional necrosis

The inflammatory process and ischemic injury reinforce one another.

Slide 32 · Infection can reach soft tissues

A subperiosteal abscess can extend beyond periosteal confines.

Soft tissue abscesses may form after periosteal rupture.

The extramedullary collection is a consequence of cortical and periosteal extension.

Recognition of spread matters for drainage planning.

Extracortical abscess

Schematic

Slide 33 · Formation of a draining sinus

A deep soft tissue abscess may channel toward the skin.

An external sinus creates a route for chronic drainage.

The tract is a morphologic sign of extension beyond the bone.

Long-standing draining tracts have important late complications.

Drainage toward skin

Schematic

Slide 34 · The first week: Chronic response emerges

Chronic inflammatory cells are recruited as acute inflammation evolves.

Their cytokines promote bone resorption and tissue remodeling.

Fibrous tissue begins to grow into the affected region.

Reactive bone may be laid down at the focus periphery.

Slide 35 · Cytokine-associated bone resorption

Chronic inflammatory cells release cytokines into the infected region.

These signals to bone resorption.

Resorption contributes to osseous structural loss.

Destruction and repair may coexist in a chronic lesion.

Inflammatory mediators → resorption

Bone destruction at infected focus

Schematic

Slide 36 · Fibrous tissue ingrowth

A healing response follows tissue destruction.

Fibrous tissue grows into affected marrow as inflammation persists.

Marrow fibrosis is a common histologic feature of chronic disease.

Fibrosis does not imply eradication of the underlying infection.

Slide 37 · Reactive new bone at the periphery

Repair-related new bone forms around infected, damaged tissue.

Reactive bone formation may accompany ongoing resorption.

The peripheral shell can encircle devitalized bone.

This reaction provides the anatomical basis of an involucrum.

Necrotic cortical bone

Reactive viable bone shell

Schematic

Slide 38 · Sequestrum: Devitalized bone

A sequestrum is a fragment or region of dead bone.

It remains within an infected or previously infected osseous focus.

Necrotic tissue is separated from viable reparative bone.

The finding is characteristic of established chronic osteomyelitis.

Necrotic cortical bone

Schematic

Slide 39 · Involucrum: Reactive viable bone shell

Involucrum is new living bone formed around devitalized infected bone.

It represents reactive peripheral bone deposition.

The sequestrum–involucrum relationship is central to chronic gross morphology.

The shell may contain a route for a draining sinus.

Necrotic cortical bone

Reactive viable bone shell

Schematic

Slide 40 · Sequestrum and involucrum together

The inner sequestrum is devitalized native bone.

The outer involucrum is newly formed viable bone.

The residual drainage path can traverse this shell.

The conceptual cross-section is schematic; It is not a specimen photograph.

Necrotic cortical bone

Reactive viable bone shell

Schematic

Slide 41 · Stage-associated morphology

Stage

Dominant lesion

Supporting tissue response

Acute

Bacterial growth; Neutrophils

Bone / marrow necrosis

Evolving

Periosteal extension; Abscess

Osteolysis; Fibrous ingrowth

Chronic

Sequestrum; Chronic infiltrate

Marrow fibrosis; Reactive bone

Slide 42 · Acute histology: What matters

Neutrophils dominate the acute inflammatory infiltrate.

Acute marrow injury and bone cell necrosis may coexist.

Microscopic findings depend on disease stage and sampling location.

The diagram is illustrative, not an H&E image.

Neutrophil-rich infiltrate (schematic)

Schematic

Slide 43 · Chronic histology: Mixed repair and injury

Marrow fibrosis is a typical chronic change.

Lymphocytes and plasma cells form a chronic inflammatory infiltrate.

Sequestrum may be present alongside reactive new bone.

The histologic pattern varies with location and disease duration.

Lymphocytes / plasma cells + fibrosis

Schematic

Slide 44 · Acute systemic presentation

Hematogenous disease may cause malaise, fever and chills.

Leukocytosis can accompany the systemic inflammatory response.

Intense throbbing pain may localize to the involved region.

A severe systemic syndrome is only one clinical presentation.

Systemic

Fever · malaise · chills

Local

Intense pain

Laboratory

Leukocytosis

Schematic

Slide 45 · Subtle presentations also occur

Some infants present with unexplained fever rather than clear focal complaints.

Adults may report only localized bone pain.

Absence of a dramatic febrile illness does not rule out osteomyelitis.

Clinical interpretation depends on age and site of infection.

Infant

Unexplained fever

Child

Acute fever + bone pain

Adult

Localized pain possible

Schematic

Slide 46 · Age-specific clinical patterns

Population

Possible presentation

High-priority issue

Infant

Unexplained fever

Adjacent joint spread

Child

Acute systemic illness with bone pain

Long-bone hematogenous focus

Adult

Localized pain, sometimes without marked systemic features

Injury / procedure / diabetes context

Slide 47 · Plain radiographs: Destructive response

A lytic focus of bone destruction.

Associated reactive bone formation may be visible.

The film depicts structural consequence, not the microbial species.

Radiographic information must be integrated with clinical and microbiological findings.

Slide 48 · MRI is emphasized for detection

MRI as more sensitive and specific for detecting osteomyelitis changes than plain radiographs.

MRI helps characterize a suspected infection when bone disease is not fully defined by radiography.

Imaging may support suspicion and lesion localization.

MRI alone does not identify the pathogen.

Slide 49 · Imaging and culture answer different questions

Radiographs reveal lysis and reactive bone.

MRI more sensitively detects changes of osteomyelitis.

Biopsy and microbial culture are used to identify the organism.

Structural evidence and etiologic evidence are complementary.

Radiograph

Lysis + reactive bone

MRI

More sensitive / specific

Biopsy / culture

Pathogen identification

Schematic

Slide 50 · Biopsy and microbial culture

Microbiological cultures should be obtained whenever possible.

The chapter emphasizes biopsy and microbial culture for pathogen identification in most instances.

Identification is incomplete in a substantial portion of cases.

The culture result supports etiologic classification rather than merely visual diagnosis.

Slide 51 · Diagnostic reasoning: Integrated approach

Assess age, route of infection and host vulnerability.

Recognize local symptoms, systemic illness or subtle presentations.

Use imaging to characterize osseous changes.

Obtain suitable material for microbiologic identification when possible.

Suspicious clinical context

Imaging of affected bone

Biopsy / microbial culture

Organism-directed care

Schematic

Slide 52 · Antibiotics and surgical drainage

Antibiotics combined with surgical drainage are usually curative.

Management must address both causative bacteria and collections of infection.

Necrotic bone or inadequately drained infection can impede resolution.

The text does not specify antimicrobial regimens or treatment durations.

Slide 53 · Why some acute infections persist

The chapter reports failure of acute infection to resolve in 5%–25% of cases.

Delayed diagnosis increases the risk of persistence.

Extensive necrosis and inadequate antibiotics or debridement are adverse factors.

Weakened host immunity further reduces the chance of uncomplicated resolution.

Delay in diagnosis

Extensive necrosis

Inadequate antimicrobials / debridement

Weakened immunity

Schematic

Slide 54 · Chronic disease may flare after dormancy

Chronic osteomyelitis may remain quiescent for years.

Spontaneous flares can interrupt prolonged clinical dormancy.

Persistent devitalized bone and altered local tissue responses help explain chronicity.

A quiet interval does not establish that a chronic focus is eradicated.

Initial infection

Chronic persistence

Late flare

Schematic

Slide 55 · Pathologic fracture as a complication

Osteolysis, structural destruction and tissue necrosis can weaken bone.

Compromised osseous integrity may culminate in a pathologic fracture.

The complication reflects local mechanical consequences of chronic infection.

Clinical risk depends on the location and extent of damaged bone.

Slide 56 · Sepsis, endocarditis and amyloidosis

Chronic osteomyelitis can have systemic complications.

Sepsis and endocarditis are listed among these complications.

Secondary (reactive) amyloidosis is another recognized late consequence.

These outcomes are clinically distinct from local bone destruction.

Slide 57 · Malignancy in chronic osteomyelitis

Squamous cell carcinoma arising within draining sinus tracts.

Sarcoma may arise in infected bone.

These are complications of long-standing chronic disease, not routine acute findings.

A new destructive or proliferative change in a chronic lesion warrants careful assessment.

Slide 58 · Septic arthritis in infants

Epiphyseal bone infection can spread to the joint.

Suppurative arthritis can destroy articular cartilage.

Permanent disability is a possible consequence.

Neonatal circulation makes this an important anatomic teaching point.

Epiphyseal focus → adjacent joint

Schematic

Slide 59 · Local and systemic consequences

Category

Complication

Pathologic connection

Local

Abscess / draining sinus

Periosteal or soft tissue extension

Mechanical

Pathologic fracture

Loss of viable bone

Articular

Septic arthritis (infants)

Epiphysis-to-joint spread

Systemic

Sepsis; Endocarditis; Amyloidosis

Persistent infectious / inflammatory disease

Neoplastic

Sinus SCC; Bone sarcoma

Long-standing chronic lesion

Slide 60 · Case: Febrile child with focal bone pain

An otherwise healthy school-age child develops fever, malaise and intense localized pain over a long bone after a minor skin infection. Imaging raises concern for osteomyelitis.

What route and typical osseous site are most plausible?

Slide 61 · Case analysis: Pediatric hematogenous focus

A preceding skin lesion can provide an entry point for transient bacteremia.

Hematogenous infection is typical in otherwise healthy children.

The metaphysis is the expected location in an older child.

Biopsy and culture are useful for identifying the pathogen.

Slide 62 · Case: Persistent pain after open fracture

An adult with a previously open tibial fracture has localized pain and repeated purulent drainage. Imaging shows destructive and reactive bone changes.

Which route and chronic morphologic findings should be considered?

Slide 63 · Case analysis: Chronic post-traumatic infection

Direct implantation after an open fracture is an important adult mechanism.

Mixed bacterial infection is possible in this setting.

Sequestrum, involucrum and a draining sinus support chronic osteomyelitis.

Persistence should prompt attention to necrosis and adequacy of local drainage.

Slide 64 · Pyogenic versus mycobacterial disease

Feature

Pyogenic osteomyelitis

Mycobacterial osteomyelitis

Common pattern

Suppuration / neutrophils

Caseating granulomas

Course

May be acute, chronic or recurrent

Often indolent; Can be destructive

Important site

Long bones in healthy children

Spine commonly involved (Pott disease)

Microbiology

Often S. Aureus in culture-positive cases

Mycobacterial infection

Slide 65 · Separate infarction from infection

Osteonecrosis means ischemic death of bone and marrow; It is not synonymous with pyogenic infection.

Osteomyelitis denotes an inflammatory process, usually infectious.

Bone infarction is relevant as a nidus for seeding in sickle cell disease.

The two conditions can overlap clinically, but their initiating injury differs: Ischemia versus infection.

Slide 66 · Integrated mechanism: From seeding to chronicity

Entry / seeding

Neutrophilic injury

Necrosis + extension

Sequestrum + repair

Anatomic route, age-related circulation and host factors shape the sequence.

Slide 67 · Five clinicopathologic conclusions

Entry route and patient age strongly influence pathogen and site.

Acute pyogenic infection is neutrophil-rich and can rapidly necrose bone.

Periosteal elevation worsens vascular injury; Chronic lesions may form sequestrum and involucrum.

Imaging localizes damage, while biopsy and culture guide etiologic identification.

Chronic disease can relapse and cause local, systemic and rare neoplastic complications.

Slide 68 · Primary reference

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

Chapter 26: Bones, Joints, and Soft Tissue Tumors.

Pyogenic Osteomyelitis, pp. 1076–1077; surrounding Osteomyelitis morphology and clinical features.

All anatomical illustrations in this lecture are original editable schematics, not specimen photographs.