Key Facts

Septic arthritis is an acute infection of a joint, most commonly bacterial. It is an orthopaedic emergency requiring urgent aspiration and antibiotics to prevent permanent joint destruction.

Epidemiology

• Incidence: ~4–10 per 100,000 per year in the UK
• More common in children (hip) and elderly
• Knee is the most commonly affected joint overall (~50%)
• Mortality: ~5–15% in native joint; higher in prosthetic joint infection

Aetiology

• S. aureus: commonest cause overall (~60–70%)
• Streptococci: second most common
• N. gonorrhoeae: young sexually active adults
• Gram-negatives: elderly, immunocompromised, IV drug users
• H. influenzae: children (rare with vaccination)
• Route: haematogenous (most common), direct inoculation (trauma, injection), contiguous spread (osteomyelitis)

Pathophysiology

• Bacteria enter the synovial membrane → intense neutrophilic inflammatory response
• Proteolytic enzymes from neutrophils and bacteria → cartilage degradation within 48 hours
• Raised intra-articular pressure → compromised blood supply → avascular necrosis (especially femoral head)
• Joint destruction can be irreversible if treatment delayed >24–48 hours

Typical Presentation

• Hot, swollen, exquisitely painful joint with severely restricted movement
• Usually monoarticular (>85%) — knee most common
• Fever (but may be absent in immunosuppressed/elderly)
• Patient holds joint in position of maximum comfort (flexion)
• Unable/unwilling to weight-bear

Specific Joints

• Knee: most common in adults
• Hip: most common in children; may present as limp/refusal to weight-bear
• Gonococcal: may be migratory polyarthralgia → monoarthritis; associated with tenosynovitis, skin pustules

Red Flags

• Any acute hot swollen joint must be treated as septic until proven otherwise
• Prosthetic joint infection (persistent pain, loosening, sinus formation)
• Immunosuppressed patient with joint swelling
• Sepsis features (tachycardia, hypotension, altered consciousness)

Bedside

• Joint aspiration (URGENT — before antibiotics): appearance (turbid/purulent), WCC (>50,000 = likely septic), Gram stain, culture, crystals
• Blood cultures: positive in ~50%
• Temperature, observations

Bloods

• FBC: leucocytosis
• CRP/ESR: elevated (CRP >100 mg/L strongly suggestive)
• Blood cultures: before antibiotics
• Urate: if gout differential
• Procalcitonin: may help differentiate (elevated in bacterial infection)

Imaging

• X-ray: may be normal early; soft tissue swelling, joint effusion; later: joint space narrowing, erosion
• USS: confirm effusion and guide aspiration (especially hip)
• MRI: if suspected osteomyelitis extension or deep joint

Special Tests

• Gonococcal: NAAT (urine, genital, throat, rectal) + gonococcal culture of aspirate
• Prosthetic joint: lower WCC threshold (>1,700/mm³ suspicious)

Pharmacological

Empiric IV antibiotics (start immediately after aspiration — do NOT delay):

• Flucloxacillin 2g IV QDS (first-line for suspected S. aureus)
• MRSA risk: vancomycin 15–20mg/kg IV BD (guided by trough levels)
• Penicillin allergy: clindamycin 600mg IV QDS
• Gonococcal: ceftriaxone 1g IV OD
• Gram-negative cover: add gentamicin (if IV drug user, elderly, immunocompromised)

Duration:

• IV for ≥2 weeks (until clinical/biochemical improvement)
• Switch to oral for ≥4 weeks (total 6 weeks minimum)
• Guided by culture/sensitivity results

Prosthetic joint:

• Prolonged antibiotics (often 6 weeks IV + oral suppressive therapy)
• May need surgical debridement, exchange, or removal

Surgical/Interventional

• Repeated aspiration/washout: daily aspiration until effusion resolves
• Arthroscopic washout: if no improvement after 48–72 hours or if loculated
• Open surgical drainage: hip (especially children), prosthetic joints, failed arthroscopic washout

Referral Criteria

• Orthopaedics: all suspected septic arthritis (emergency)
• Microbiology: guide antibiotic therapy
• GUM/sexual health: gonococcal arthritis

• Mortality: ~5–15% for native joint septic arthritis (higher in elderly, immunosuppressed)
• Joint damage: ~25–50% have some residual functional impairment
• S. aureus: worst outcomes
• Delay >48h: significantly increases risk of permanent joint damage
• Prosthetic joint infection: may require implant removal; ~10–20% treatment failure
• Gonococcal arthritis: excellent prognosis with appropriate antibiotics
• Risk of secondary OA: common long-term sequela

Kocher Criteria (Paediatric Hip — Differentiating Septic Arthritis from Transient Synovitis)