Osteomyelitis

Overview

•Staphylococcus aureus - most common cause across ALL age groups; expresses surface adhesins (fibronectin-binding proteins) that bind directly to bone matrix, giving it high skeletal affinity

•Haematogenous spread from distant source (UTI, indwelling catheter, skin) most commonly seeds bone with S. aureus - even when the primary source is urinary

•*Escherichia coli* - common UTI pathogen but does NOT express bone-binding receptors; unlikely cause of osteomyelitis despite urinary source

•*Neisseria gonorrhoeae* - associated with septic arthritis in young adults, NOT osteomyelitis

•*Haemophilus influenzae* and *Mycobacteria* - rare causes; far less common than S. aureus

🎯

Even when the apparent source is a UTI or urinary catheter, S. aureus (a skin commensal colonising the catheter) is the most likely bone pathogen - not the Gram-negative urinary organism.

Investigations

🥇 First-line

•CRP (elevated in >97% of acute cases - most sensitive marker), FBC (neutrophilia), ESR, blood cultures x2 (positive in ~60% of haematogenous cases)

•Plain X-ray - normal in first 10-14 days; only shows changes after 30-50% bone mineral lost; used to exclude other diagnoses

•MRI - most sensitive and specific imaging; detects bone marrow oedema within 3-5 days of onset; investigation of choice when osteomyelitis is suspected

🏆 Gold standard

•bone biopsy and culture - positive in ~90% of cases; obtain before starting antibiotics where possible

⚠️

A normal X-ray does NOT exclude osteomyelitis. Always proceed to MRI when there is clinical suspicion.

Routes of Infection

•Haematogenous spread - most common in children and elderly; bacteria seed bone from distant bacteraemia

•Contiguous spread - from adjacent soft tissue (diabetic foot ulcer, pressure ulcer)

•Direct inoculation - open fractures, orthopaedic surgery, penetrating trauma