Osteomyelitis

Definition

1. Hematogenous spread (most common in children)
2. Contiguous spread from adjacent soft tissue or joint
3. Direct inoculation - trauma, surgery, open fractures

Robbins & Kumar Basic Pathology

Microbiology

Robbins & Kumar Basic Pathology; Goldman-Cecil Medicine

Pathophysiology

Acute Phase (first 48 hours)

• Bacteria proliferate → neutrophilic inflammation
• Necrosis of bone cells and marrow ensues within 48 hours
• Infection spreads through Haversian systems to reach the periosteum
• Purulent material forms → abscess → pressure-driven bony destruction
• Pus passes through cortical bone and elevates the periosteum, rendering cortical bone avascular

Why the metaphysis?

• Slow blood flow through looped capillary vessels
• Combined microtrauma - both encourage bacterial seeding during bacteremia
• The terminal capillaries lack phagocytic lining cells

Chronic Phase (after 1 week)

• Chronic inflammatory cells release cytokines stimulating osteoclastic resorption
• Dead bone = sequestrum
• New bone forming a living shell around the sequestrum = involucrum
• The involucrum has drainage gaps; pus can track through these into soft tissue and eventually to the skin surface as sinus tracts

Robbins & Kumar Basic Pathology, p. 779; Bailey and Love's Short Practice of Surgery

Classification

By Duration

• Acute: Days to weeks - fever, leukocytosis, elevated inflammatory markers
• Subacute: Slow onset, symptoms less prominent; cultures often negative
• Chronic: Persistent or recurrent after acute episode; sequestrum/involucrum present; sinus tracts may form

By Age Group / Clinical Subset

• Male:female ratio 2-3:1
• Long bones involved ~80% of time (distal metaphysis most common)
• Fever, chills, pain, point tenderness, refusal to bear weight
• Blood cultures positive in ~40%; tissue cultures positive 86%
• MRSA now the most common cause

• Often post-abnormal pregnancy or delivery
• Multiple sites in ~50% of cases
• Septic arthritis often accompanies osteomyelitis (unique neonatal vascular anatomy allows metaphyseal vessels to penetrate the growth plate)
• Leading pathogen: Group B Streptococcus

• Increasing with aging population and chronic diseases
• Risk factors: IV access devices, indwelling lines, urinary infections, IV drug use
• Lumbar (58%) > thoracic (30%) > cervical (11%)
• Only ~10% appear septic/toxic - most present insidiously
• Back pain is the most common symptom (~90%), with spinous process tenderness
• Diagnostic delay up to 4 months is common
• ESR elevated in 98%, CRP in 100% of cases
• Epidural abscess accompanies it in many cases - up to 60% present without fever or leukocytosis

• Most common in adults; polymicrobial
• Often complication of open fractures, surgery, or DM ulcers

ROSEN's Emergency Medicine; Goldman-Cecil Medicine

Clinical Features

Symptoms

• Fever, rigors (may appear toxic)
• Systemic: headache, fatigue, malaise, anorexia
• Localized pain - throbbing in nature, progressive
• Limp or inability to bear weight (children with lower extremity involvement)

Signs

• Point tenderness over the infected bone segment (cardinal finding)
• Palpable warmth, soft tissue swelling, erythema
• In chronic osteomyelitis: palpable involucrum or sequestrum, skin sinus tracts
• Sympathetic effusion in adjacent joint (even without joint infection)

ROSEN's Emergency Medicine

Investigation

Laboratory

• FBC: Leukocytosis (may be absent in chronic cases)
• ESR: Elevated in 98% of vertebral osteomyelitis
• CRP: Elevated in ~100% of vertebral osteomyelitis; most sensitive marker
• Blood cultures: Before starting antibiotics; positive in ~40% of AHO / vertebral cases
• Bone biopsy with culture: Gold standard for identifying causative organism - obtain whenever possible

Imaging

• First-line but has a major limitation: changes lag behind disease by 10-14 days (up to 2 weeks)
• Early signs: soft tissue swelling only
• Later: cortical irregularity (bony destruction), periosteal reaction
• Plain films are often normal in the first 2 weeks

• Highest sensitivity and specificity
• Shows marrow edema, abscess, periosteal reaction early
• Identifies epidural extension in vertebral osteomyelitis
• "Penumbra sign" in subacute infection: peripheral high-signal ring of granulation tissue surrounding low-signal abscess cavity
• Contrast-enhanced fat-suppressed sequences show avid enhancement

• Good for defining cortical destruction and detecting sequestra
• Often used to guide needle biopsy
• Higher radiation dose

• Useful in children and infants
• Detects subperiosteal abscess, guides aspiration
• Normal US does not exclude osteomyelitis

• Useful when MRI is unavailable or contraindicated
• 3-phase scan increases specificity
• Used for multifocal infection screening (whole-body)

Grainger & Allison's Diagnostic Radiology; ROSEN's Emergency Medicine

Diagnostic Algorithm

Treatment

General Principles

1. Blood cultures and bone biopsy BEFORE starting antibiotics (whenever possible)
2. Empiric antibiotics - cover S. aureus (including MRSA in high-prevalence areas) as first priority
3. Surgical drainage when abscess or necrotic bone is present
4. Total duration: typically 4-6 weeks for acute; longer for chronic or vertebral

Empiric Antibiotic Selection

• De-escalate based on culture and sensitivity results
• IV-to-oral switch once clinically improving (oral bioavailability good for many agents)
• Recent evidence supports oritavancin for MRSA osteomyelitis (long-acting lipoglycopeptide) - [2025 meta-analysis, PMID 41224240]

Surgical Indications

• Failure of antibiotics (no improvement in 24-48 hours in children)
• Subperiosteal abscess
• Sequestrum / chronic osteomyelitis
• Spinal cord compression from epidural abscess (emergency)
• Sinus tract formation

Complications

• Chronic osteomyelitis (5-25% of acute cases - risk factors: delayed diagnosis, immunocompromise, extensive necrosis, inadequate treatment)
• Septic arthritis (especially when metaphysis is intracapsular, e.g., proximal femur)
• Pathologic fracture
• Growth disturbance (children - if epiphysis involved)
• Bacteremia and sepsis
• Epidural abscess / spinal cord compression (vertebral)
• Secondary amyloidosis (chronic)
• Squamous cell carcinoma in chronic draining sinus tracts
• Sarcoma developing in chronically infected bone

Robbins & Kumar Basic Pathology, p. 779; ROSEN's Emergency Medicine

Special Types

Brodie Abscess

• Subacute osteomyelitis variant
• Well-defined lytic lesion with a sclerotic rim on imaging
• Usually in metaphysis; most common in adolescents
• Cultures often negative

Chronic Recurrent Multifocal Osteomyelitis (CRMO)

• Autoinflammatory, not infectious
• Multiple skeletal foci, indolent, recurrent episodes
• Culture always negative
• May be associated with psoriatic disease
• Recent 2026 systematic review on treatment ([PMID 41654205])

Mycobacterial Osteomyelitis (Pott's Disease)

• ~1-3% of patients with TB develop osseous infection
• Spine involved in 40% (Pott disease)
• Multi-vertebral involvement, psoas abscess
• Histology: caseating granulomas
• More destructive and treatment-resistant than pyogenic osteomyelitis
• Localized pain, low-grade fever, weight loss

Robbins & Kumar Basic Pathology

Key Points Summary

• Robbins & Kumar Basic Pathology, pp. 779-780
• ROSEN's Emergency Medicine - Concepts and Clinical Practice
• Goldman-Cecil Medicine International Edition
• Grainger & Allison's Diagnostic Radiology
• Bailey and Love's Short Practice of Surgery, 28th Ed.