Key Facts

Osteomyelitis is infection of bone, most commonly caused by S. aureus. It is classified by route of infection (haematogenous, contiguous, vascular insufficiency) and by duration (acute or chronic).

Epidemiology

• Incidence: ~2–5 per 100,000 per year
• Children: peak <5 years; haematogenous
• Adults: peak >50 years; contiguous spread, diabetic foot

Aetiology

• S. aureus: ~60–80% overall
• Streptococci, coagulase-negative staphylococci: prosthetic/hardware-related
• Pseudomonas: puncture wounds through trainers, IV drug users
• Salmonella: sickle cell disease
• TB: vertebral (Pott disease)

Pathophysiology

• Bacteria reach bone → inflammatory response → pus formation → raised intraosseous pressure → vascular compromise → bone necrosis
• Children: rich metaphyseal blood supply → haematogenous seeding; growth plate is avascular barrier (infection rarely crosses in >18 months)
• Adults: contiguous spread from soft tissue infection, surgery, or trauma
• Chronic: dead bone (sequestrum) acts as nidus for persistent infection; new bone forms around it (involucrum)

Acute Haematogenous (Children)

• Fever, malaise, irritability
• Localised bone pain, tenderness, swelling
• Refusal to weight-bear or use limb
• Long bones: femur, tibia most common

Acute Contiguous (Adults)

• Post-operative wound infection with persistent discharge
• Increasing pain at surgical site weeks–months after procedure
• Erythema, warmth, swelling over affected bone

Diabetic Foot

• Non-healing ulcer overlying bone
• Probe-to-bone test positive
• May have minimal pain (neuropathy)

Vertebral (Adults)

• Back pain (progressive, worse at night)
• Fever (may be absent)
• Neurological deficit if epidural abscess forms

Red Flags

• Neurological deficit with vertebral osteomyelitis (epidural abscess — surgical emergency)
• Sepsis
• Non-healing wound with exposed bone
• IV drug user with back pain and fever

Bloods

• CRP/ESR: elevated (ESR >70 mm/hr is highly suggestive in chronic)
• WCC: may be elevated or normal
• Blood cultures: positive in ~50% of haematogenous cases
• Procalcitonin: may help differentiate

Imaging

• X-ray: may be normal for 10–14 days; later shows periosteal reaction, lytic changes, sclerosis
• MRI: modality of choice (sensitivity ~90–100%, specificity ~80–90%); bone marrow oedema, cortical destruction, soft tissue abscess
• Bone scan (Tc-99m): sensitive but less specific; useful when MRI contraindicated
• CT: chronic osteomyelitis (sequestrum, involucrum detail)

Special Tests

• Bone biopsy and culture: gold standard for definitive diagnosis and antimicrobial guidance
• Probe-to-bone test: diabetic foot — if probe reaches bone through ulcer, ~89% specific for osteomyelitis
• Histopathology: confirms osteomyelitis, excludes malignancy

Pharmacological

Empiric IV antibiotics (after blood cultures and ideally bone culture):

• Flucloxacillin 2g IV QDS (first-line anti-staphylococcal)
• MRSA risk: vancomycin 15–20mg/kg BD
• Penicillin allergy: clindamycin 450mg IV QDS

Duration:

• IV for 1–2 weeks, then oral switch for total of 6 weeks (OVIVA trial: oral switch at 1 week non-inferior to IV for 6 weeks — for many presentations)
• Chronic osteomyelitis: may need months of therapy
• Guided by culture/sensitivity

Vertebral osteomyelitis:

• 6–12 weeks total antibiotic therapy
• Immobilisation/bracing for pain

Surgical/Interventional

• Debridement: chronic osteomyelitis — remove sequestrum and necrotic tissue
• Drainage: abscess formation
• Amputation: severe diabetic foot osteomyelitis with non-reconstructable limb
• Hardware removal: prosthetic-related infection

Referral Criteria

• Orthopaedics: surgical debridement, hardware-related
• Microbiology/infectious diseases: antibiotic guidance
• Vascular surgery: diabetic foot (revascularisation assessment)
• Spinal surgery: vertebral osteomyelitis with epidural abscess/neurological deficit

• Acute haematogenous (children): ~90–95% cure rate with appropriate treatment
• Adult contiguous: ~70–80% cure rate; often requires surgical debridement
• Chronic osteomyelitis: recurrence rate ~20–30%; may require multiple surgeries
• Vertebral osteomyelitis: mortality ~5–10%; neurological deficit in ~10–15%
• Diabetic foot osteomyelitis: ~20–30% lead to amputation
• Risk of secondary amyloidosis with chronic infection (rare in modern era)

Organisms by Clinical Scenario