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Osteomyelitis: From Basics to Clinical

1. Definition

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2. Classification

By Duration

By Route of Infection

1. Hematogenous - most common in children; bacteremia seeds the metaphysis
2. Contiguous spread - from adjacent soft tissue infection, septic arthritis, or pressure ulcer
3. Direct inoculation - open fractures, surgery, puncture wounds, prosthetic implants

By Host/Anatomy: Cierny-Mader Classification (chronic osteomyelitis)

• Stage 1 (Medullary) - infection limited to the endosteum
• Stage 2 (Superficial) - cortical surface involvement only
• Stage 3 (Localized) - full-thickness cortical sequestrum, stable bone
• Stage 4 (Diffuse) - through-and-through instability; most complex

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3. Pathogenesis & Microbiology

How Hematogenous Infection Starts

• Blood flow is sluggish in the looped sinusoidal vessels
• Phagocytic activity is reduced in this region
• Microtrauma (common in active children) disrupts the vascular endothelium

Pathogens by Clinical Context

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4. Pathology: Stages and Key Terms

Acute Phase (first 48 hours onward)

• Bacteria proliferate → neutrophilic infiltration
• Bone cell necrosis and marrow necrosis occur within 48 hours
• Bacteria spread through Haversian canals → reach the periosteum
• In children: periosteum loosely attached → subperiosteal abscess forms
• Periosteal lifting → impaired cortical blood supply → more necrosis
• Soft tissue abscess → may rupture to skin as draining sinus

Subacute - Chronic Transition

• Chronic inflammatory cells (lymphocytes, plasma cells) recruited
• Cytokines stimulate bone resorption and fibrous tissue ingrowth
• Dead avascular bone fragment = sequestrum
• Reactive periosteal new bone that encases the sequestrum = involucrum
• This creates the classic "bone-within-a-bone" radiographic appearance
• Gaps in the involucrum (called cloacae) allow pus to track to skin
• Subacute variant: Brodie abscess - a well-defined lytic cavity with a sclerotic rim; represents a contained focus of subacute infection

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5. Age-Related Anatomical Differences

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6. Clinical Features

Acute Osteomyelitis

• Fever, rigors, malaise - systemic sepsis signs
• Point tenderness over the infected bone segment (the single most reliable physical finding)
• Localized warmth, swelling, erythema - variable
• Limb guarding / refusal to bear weight - especially in children
• Sympathetic joint effusion - adjacent joint may have sterile effusion even without septic arthritis

Chronic Osteomyelitis

• Systemic signs may be absent or minimal
• Palpable involucrum or sequestrum may be felt
• Draining sinus tracts to skin
• Periodic acute flares after years of dormancy
• Chronic pain and functional limitation

Special Presentations

• Lumbar and cervical vertebrae most common in adults
• Back pain + fever (fever often absent in elderly/immunocompromised)
• Paravertebral or epidural abscess in many cases
• Red flags requiring emergency evaluation: radicular pain, sensory loss, lower extremity weakness, urinary retention, bowel/bladder incontinence (signs of cord compression) - Goldman-Cecil Medicine, p. 3146
• MRI is the modality of choice for showing intraspinal complications

• Extension from contiguous soft tissue infection (ulcer → fascia → bone)
• Insidious; foot may be neuropathic with minimal pain
• "Probe-to-bone" test: if a sterile probe through an ulcer reaches bone, osteomyelitis is highly likely (PPV ~89%)
• Polymicrobial

• Areas of bone infarction from vaso-occlusion serve as nidus
• Loss of splenic function impairs antibody-mediated immunity
• Salmonella is classically the pathogen; however S. aureus remains common

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7. Investigations

Laboratory

Imaging

• First-line, readily available
• Changes lag 10-21 days behind clinical onset - initial X-ray often normal
• Early signs: soft tissue swelling, periosteal reaction
• Late signs: lytic destruction, cortical irregularity, sclerosis, periosteal new bone, sequestrum
• Radiolucency requires 30-50% bone mineral loss to be visible

• Useful in infants and children (simple, no radiation)
• Detects subperiosteal fluid/abscess, periosteal lifting, soft tissue edema
• Guides aspiration/drainage procedures

• Superior for defining cortical destruction and sequestrum
• Guides biopsy/aspiration (especially vertebral)
• Limited soft tissue contrast compared to MRI

• Highest sensitivity and specificity for early osteomyelitis
• T1: decreased signal in infected marrow (fat replacement by edema/pus)
• T2 / STIR: increased signal in infected marrow and adjacent soft tissue
• Gadolinium contrast: highlights abscess walls, reveals devascularized bone
• Penumbra sign in subacute osteomyelitis (Brodie abscess): peripheral high-signal ring (granulation tissue) surrounding low-signal central cavity on T1
• Whole-body MRI or skeletal scintigraphy useful in neonates with possible multifocal disease

• Sensitive but not specific; positive early (before X-ray changes)
• Useful when MRI is unavailable or for whole-body survey
• False-negative in neonates (poor periosteal reaction)

Diagnostic Algorithm

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8. Management

General Principles

• Identify the organism before starting antibiotics whenever possible (culture blood, aspirate, biopsy)
• Drain pus when present
• Appropriate and often prolonged antibiotic therapy: parenteral then oral
• Rest and splintage of the affected limb
• Treat the underlying condition (diabetes, sickle cell disease, malnutrition, immunodeficiency)

Antibiotic Therapy

• Acute uncomplicated (children): historically 4-6 weeks IV; evidence now supports early switch to oral after clinical/CRP improvement (after ~3-5 days IV if responding)
• Adults, hematogenous: 6 weeks total (IV + oral step-down)
• Vertebral osteomyelitis: 6-8 weeks minimum
• Chronic osteomyelitis / prosthetic: often months, especially with biofilm organisms; requires surgery
• MRSA osteomyelitis: vancomycin or daptomycin; consider rifampicin combination in biofilm-associated disease

Note on MRSA: The Panton-Valentine leukocidin (PVL)-positive S. aureus strains significantly increase morbidity. Community-acquired MRSA (CA-MRSA) is now the most common organism in acute hematogenous osteomyelitis in children in many regions. - Bailey & Love, p. 657

Surgical Management

1. Failure to respond to antibiotics within 24-48 hours (especially in children)
2. Abscess (subperiosteal or soft tissue) requiring drainage
3. Chronic osteomyelitis with sequestrum, sinus tract, or Brodie abscess
4. Infected hardware or prosthetic joint
5. Vertebral osteomyelitis with spinal instability or cord compression

1. Radical debridement - remove all sequestrum, infected/necrotic tissue, scar, and biofilm
2. Dead space management - fill the cavity left by debridement:
   • Antibiotic-impregnated PMMA beads (gentamicin or tobramycin)
   • Muscle flap coverage (rotational or free flap)
   • Cancellous bone graft (after infection controlled)
3. Skeletal stabilization - external fixation or internal fixation as appropriate
4. Wound coverage - local or free tissue transfer when soft tissue deficient

• Early (<3 months): debridement, irrigation, prosthesis retention (DAIR) + antibiotics
• Late (>3 months) or biofilm-established: two-stage revision (implant removal → spacer + antibiotics → reimplantation) - Rosen's Emergency Medicine, p. 3854

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9. Special Forms

Tuberculosis Osteomyelitis (Pott's Disease)

• Bloodborne from primary pulmonary focus; approximately 1-3% of all TB cases develop osseous infection
• Typically solitary in immunocompetent; disseminated in immunocompromised
• Most commonly affects the thoracolumbar spine (Pott's disease) - intervertebral discs and adjacent vertebral bodies
• Presentation: insidious - malaise, weight loss, low-grade fever, localized pain
• Classic complication: paraplegia from gibbus deformity and cord compression
• Spina ventosa: TB dactylitis - cyst-like expansion of metacarpal/metatarsal bones ("windy bone")
• Treatment: standard anti-TB regimen (HRZE x 2 months then HR x 10 months); surgery for cord compression or spinal instability - Grainger & Allison's Diagnostic Radiology, p. 1638; Bailey & Love, p. 657

Fungal Osteomyelitis

• Candida (especially in IV drug users, ICU patients, neonates)
• Coccidioides immitis, Blastomyces dermatitidis (endemic mycoses)
• Cryptococcus (immunocompromised)

Chronic Recurrent Multifocal Osteomyelitis (CRMO)

• Rare, non-infectious, autoinflammatory condition mimicking osteomyelitis
• Children and adolescents; symmetrical multifocal lesions
• No pathogen identified; responds to NSAIDs/bisphosphonates

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10. Complications

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11. Key Differentials

• Ewing's sarcoma - can closely mimic acute osteomyelitis radiologically and clinically (fever, pain, onion-skin periosteal reaction)
• Osteosarcoma - periosteal reaction, Codman's triangle; usually no fever
• Septic arthritis - primary joint infection; sympathetic effusion in osteomyelitis can confuse the picture
• Bone infarct (sickle cell) - difficult to distinguish from osteomyelitis on imaging and clinically
• Stress fracture - no systemic signs; localized periosteal reaction

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Summary Table

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