Acute Pyogenic Haematogenous Osteomyelitis

Definition

Epidemiology & Predisposing Factors

Pathophysiology & Why the Metaphysis?

• Terminal metaphyseal arterioles make sharp loops just beyond the physis
• Blood flow is sluggish at these loops
• Phagocytic activity is poor in this region
• S. aureus cell wall proteins bind directly to bone matrix collagen, facilitating bacterial adherence

Causative Organisms

Pathological Sequence

Acute Phase (first 48 hours)

1. Bacteria proliferate → neutrophilic infiltrate
2. Bone cell and marrow necrosis within 48 hours
3. Bacteria + inflammatory response spread via Haversian canals → periosteum
4. In children, subperiosteal abscesses dissect along the bone surface (loose periosteal attachment)
5. Periosteal lifting → further impairment of blood supply → more necrosis
6. Subperiosteal abscess may rupture → soft tissue abscess → draining sinus tract

Transition to Chronic

• Stimulate osteoclastic bone resorption
• Drive fibrous tissue ingrowth
• Deposit reactive bone at the periphery

• Sequestrum — devitalised dead bone fragment
• Involucrum — shell of new reactive periosteal bone surrounding the sequestrum
• Brodie abscess — chronic walled-off intraosseous abscess with a sclerotic rim (subacute form)

Clinical Features

• Fever, rigors, malaise, anorexia, headache
• Children may appear moderately unwell but not usually "toxic"
• Neonates may have minimal systemic signs — paucity of findings is characteristic

• Point tenderness over the infected metaphysis — the most consistent finding
• Swelling, warmth, erythema (variable)
• Sudden limp or refusal to bear weight in children
• Sympathetic joint effusion (without joint infection)

Investigations

Laboratory

Imaging

Management

Antibiotic Therapy

• IV antibiotics are the best initial treatment when diagnosed early and no abscess is present
• Empirical broad-spectrum cover → narrow once organism and sensitivities identified from aspiration/culture
• MRSA must be covered empirically in communities with high prevalence
• CRP is the preferred marker to monitor antibiotic response
• Duration: typically 4–6 weeks total (IV transitioning to oral once CRP improving and no surgical indication)

Surgical Indications

• Failure to respond to antibiotics within 48–72 hours
• Frank pus on MRI or subperiosteal/intraosseous abscess not amenable to percutaneous drainage
• Sequestered abscess (Brodie abscess not accessible to antibiotics)
• Metaphyseal drilling can assist drainage

Outcome

• When treated promptly: usually curative
• In 5–25% of acute cases, infection fails to resolve → chronic osteomyelitis
• Worse outcomes with: delayed diagnosis, extensive bone necrosis, inadequate debridement, or immunocompromise

Complications

Summary Algorithm (Paediatric AHO)

Clinical suspicion (fever + point tenderness + limited limb use)
       ↓
Blood cultures + WBC/ESR/CRP
       ↓
Plain X-ray (may be normal early)
       ↓
If X-ray equivocal/normal → MRI (preferred) or Bone scan
       ↓
MRI confirms: start IV antibiotics empirically (anti-MRSA cover)
       ↓
Percutaneous aspiration/biopsy → culture-directed therapy
       ↓
CRP monitoring every 48–72 h
       ↓
No improvement → surgical drainage/debridement