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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Hot, swollen, painful joint with inability to weight-bear or move = septic arthritis until proven otherwise
- Most common organism: Staphylococcus aureus (>50%). Gonococcus in young sexually active adults
- URGENT joint aspiration is MANDATORY — before antibiotics if possible, but do NOT delay antibiotics waiting for aspiration
- Aspirate: turbid/purulent, WCC >50,000/mm³ (predominantly neutrophils), Gram stain, culture
- IV flucloxacillin 2 g QDS (or vancomycin if MRSA risk). Orthopaedic referral for joint washout
Overview
Septic arthritis is a bacterial infection of the joint space, constituting a surgical emergency. Joint cartilage can be destroyed within hours to days by proteolytic enzymes from neutrophils and bacterial toxins. Infection reaches the joint haematogenously (most common), by direct inoculation (trauma, surgery, injection), or by spread from adjacent osteomyelitis. The knee is the most commonly affected joint (~50%), followed by hip, shoulder, ankle, and wrist. Prosthetic joint infection is an important consideration in patients with joint replacements.
Epidemiology
Incidence is approximately 4–10 per 100,000 per year. Risk factors include pre-existing joint disease (RA, OA, gout — damaged joints are vulnerable), prosthetic joints, immunosuppression (steroids, biologics, diabetes, HIV), IV drug use, skin breach (wounds, cellulitis), recent joint surgery or injection, and extremes of age. S. aureus causes >50% of cases. Neisseria gonorrhoeae should be considered in sexually active young adults.
Clinical Features
Symptoms
Acute onset of severe joint pain — typically monoarticular
Hot, red, swollen joint
Inability to weight-bear or move the joint (pseudoparalysis)
Fever (may be absent in immunosuppressed or elderly)
Rigors and systemic sepsis (septic arthritis can cause bacteraemia)
Signs
Joint held in position of maximal comfort (slight flexion — maximises intra-articular volume)
Exquisite tenderness with any passive or active movement
Large tense effusion
Overlying warmth and erythema
Pyrexia (but absence does not exclude — especially if immunosuppressed)
In hip septic arthritis (especially children): groin pain, leg held in flexion and external rotation, refusal to weight-bear
Investigations
First-line
Urgent joint aspirationBEFORE antibiotics if possible (but do NOT delay antibiotics). Send for: MC&S (Gram stain, culture, sensitivity), crystal microscopy (exclude gout/pseudogout), cell count (WCC >50,000 = highly suggestive)
Blood culturesBefore antibiotics — positive in ~50%
BloodsFBC (raised WCC), CRP (raised), ESR, U&Es, blood glucose
Second-line
X-rayOften normal early. Soft tissue swelling, joint effusion. Later: joint space narrowing, bony erosion, periosteal reaction
USSConfirms effusion — particularly useful for hip (guides aspiration)
Specialist
MRIIf osteomyelitis suspected or complex anatomy (sacroiliac, spine). Shows bone marrow oedema, soft tissue collection
Gonococcal PCR and sexual health screenIn sexually active young adults with migratory polyarthralgia, tenosynovitis, or pustular rash — disseminated gonococcal infection
1
Empirical IV antibiotics — start immediately
- IV flucloxacillin 2 g QDS — first-line (covers S. aureus)
- If penicillin allergy: IV clindamycin 450–600 mg QDS or IV vancomycin
- If MRSA risk: IV vancomycin (guided by local protocol)
- If gonococcal suspected: IV ceftriaxone 1 g OD
- Prosthetic joint infection: IV vancomycin + rifampicin (specialist decision)
- Typical duration: IV for 2 weeks, then oral for 4 weeks (total 6 weeks — guided by response)
2
Surgical management
- Urgent orthopaedic referral for joint washout (arthroscopic or open)
- Joint lavage removes purulent material and reduces bacterial load
- Repeat washout may be needed if not improving
- Hip septic arthritis almost always requires urgent surgical washout (especially in children)
3
Supportive
- Analgesia: paracetamol, opioids. Avoid NSAIDs initially (may mask response to treatment)
- Splint in position of comfort initially, then early mobilisation once improving
- Monitor CRP as marker of response (should fall steadily)
Complications
- Joint destruction: Cartilage destroyed within days — early treatment is critical. Even with treatment, ~25–50% have residual joint damage
- Osteomyelitis: Spread of infection to adjacent bone
- Sepsis and death: Mortality ~10% (higher in elderly and immunosuppressed)
- Need for joint replacement: If severe destruction — may require arthroplasty once infection fully treated
- Chronic infection: Particularly in prosthetic joints — may require implant removal
UKMLA Exam Tips
- 1Hot swollen joint + fever + unable to move = septic arthritis until proven otherwise — ASPIRATE URGENTLY
- 2S. aureus is the most common organism. Gonococcus in sexually active young adults
- 3Joint aspiration: WCC >50,000, turbid/purulent, positive Gram stain/culture. Also check for crystals (gout/pseudogout can coexist)
- 4IV flucloxacillin is first-line. Total antibiotic duration typically 6 weeks
- 5Septic arthritis and gout can COEXIST — always send aspirate for both crystals AND culture
- 6Prosthetic joint infection: lower threshold to suspect. Different microbiology (coagulase-negative staphylococci)
- 7In a child refusing to weight-bear with a fever: septic hip until proven otherwise (paediatric emergency)
practicetest your knowledge on septic arthritisApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — musculoskeletal and beyond.
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