🔑 Key Learning

  • Staph aureus = most common cause overall
  • Neisseria gonorrhoeae = common in young, sexually active adults
  • Knee is the most frequently affected joint
  • Red, hot, swollen joint with very limited ROM
  • Diagnosis = blood cultures, urgent joint aspiration
  • Management = IV antibiotics (flucloxacillin) + joint drainage

🧬 Pathophysiology

Infection of the joint space via:

  • Haematogenous spread (most common)
  • Direct inoculation (trauma, surgery)
  • Contiguous spread from adjacent infection

Microbiology

  • Staph aureus – most common overall
  • Neisseria gonorrhoeae – common in young, sexually active adults
  • Other causes: Strep species, Gram-negative bacilli

👀 Clinical Features

  • Single red, hot, swollen joint
  • Severe pain with minimal or no ROM (even passive)
  • Pyrexia, rigors
  • Weight-bearing difficulty (if lower limb)
  • May show signs of SIRS

🧪 Investigations

  • Urgent joint aspiration → send for:
    • Gram stain
    • Microscopy
    • Culture & Sensitivities (MCS)
  • Blood cultures
  • Bloods: Raised CRP, ESR, WCC
  • Imaging (US/XR) may help rule out effusion/fracture but not diagnostic

💊 Management

  • Empirical antibiotics (IV) after aspiration:
    • Flucloxacillin – for 4–6 weeks total
    • Clindamycin – if penicillin allergy
    • Vancomycin / Teicoplanin – if MRSA suspected
    • Cefotaxime – if gonococcal cause suspected
  • Joint decompression: needle aspiration ± lavage

📝 Exam Clues & Clinchers

  • Single hot, painful, swollen joint – often knee + fever = septic arthritis until proven otherwise
    • Severe pain + no passive movement
  • Joint aspiration + blood cultures essential for diagnosis
  • Empirical IV abx first (fluclox!) then tailor based on cultures