Septic Arthritis — Pediatric vs Adult

Septic arthritis is a surgical emergency. Rapid diagnosis and joint washout are essential to prevent irreversible articular cartilage destruction, avascular necrosis, and permanent joint dysfunction. The management principles differ significantly between children and adults, reflecting differences in anatomy, microbiology, pathophysiology, and the consequences of delayed treatment.

• Incidence: approximately 2–10 per 100,000 population; higher in children, elderly, immunocompromised, and patients with prosthetic joints
• Articular cartilage destruction begins within 8 hours of bacterial inoculation — bacterial proteases and inflammatory cytokines (IL-1, TNF-α) degrade proteoglycans rapidly; irreversible damage occurs within 24–48 hours in untreated cases; this is why septic arthritis is a surgical emergency
• Most commonly affected joints: knee (45%), hip (15–20%), shoulder (10%), ankle (9%), wrist (6%)
• Routes of infection: haematogenous spread (most common in children), direct inoculation (joint aspiration, surgery, penetrating trauma), contiguous spread from adjacent osteomyelitis or soft tissue infection
• The paediatric hip (and to a lesser extent the shoulder) has a unique anatomical vulnerability — the proximal femoral growth plate traverses the intra-articular space, making epiphyseal ischaemia and avascular necrosis a direct risk from joint pressure and vascular disruption

• Kingella kingae: emerging recognition in children <5 years — often low virulence; gram-negative coccobacillus; may present with lower inflammatory markers and milder symptoms than S. aureus septic arthritis; PCR on synovial fluid increases detection rate as culture often negative
• Blood cultures: positive in approximately 40–50% of septic arthritis — always collect before antibiotics; essential for directing antibiotic therapy

Septic arthritis of the hip is the most critical diagnosis to make in a febrile child with a painful hip — the consequences of a missed diagnosis are catastrophic. The Kocher criteria (and its modifications) provide a validated clinical prediction tool.

• Presentation: fever, refusal to weight-bear, irritability, holding the affected joint in position of comfort (hip: flexion, ER, abduction); infant may have pseudoparalysis of the limb

• Kocher score: 0 criteria = 0.2% probability; 1 criterion = 3%; 2 criteria = 40%; 3 criteria = 93%; 4 criteria = 99.6% probability of septic arthritis
• CRP is more sensitive than ESR for acute bacterial infection — modified Kocher criteria incorporate CRP >20 mg/L as a fifth criterion, further improving predictive accuracy
• Differentiating transient synovitis from septic arthritis: transient synovitis (irritable hip) is more common than septic arthritis; afebrile or low-grade fever; WCC and CRP less elevated; able to bear some weight; responds to NSAIDs; USS shows effusion in both — aspiration required if doubt
• The only reliable way to distinguish septic arthritis from transient synovitis is joint aspiration — synovial fluid WCC >50,000/mm³ with >75% PMNs, turbid fluid, Gram stain/culture positive = septic arthritis; do not rely on imaging alone

• Presentation: acute monoarticular joint pain and swelling with fever; hot, erythematous, extremely tender joint; reduced range of motion; systemic sepsis in severe cases
• Risk factors: diabetes, IV drug use, immunosuppression (steroids, biologics, HIV), RA (most common underlying joint disease), prosthetic joints, skin infection, recent joint injection/aspiration
• Synovial fluid analysis: WCC >50,000/mm³ is highly suggestive of septic arthritis; >100,000/mm³ is near-diagnostic; PMN predominance (>75%); low glucose; elevated protein; Gram stain positive in approximately 50%; culture positive in approximately 70–80%
• Crystal arthropathy (gout, pseudogout) can mimic septic arthritis — polarised microscopy for crystals; both can coexist; do not exclude septic arthritis because crystals are found
• Reactive arthritis (Reiter syndrome): sterile joint inflammation following genitourinary or gastrointestinal infection; Chlamydia trachomatis, Salmonella, Yersinia; classic triad: arthritis + urethritis + conjunctivitis; treat the primary infection

Management requires two simultaneous interventions: joint washout and antibiotics. Neither alone is sufficient.

• Paediatric hip: surgical washout (open or arthroscopic) is mandatory — needle aspiration alone is insufficient for the hip; open arthrotomy via anterior (Smith-Petersen) or anterolateral approach provides best access; arthroscopic washout increasingly performed in experienced centres
• Adult peripheral joints (knee, shoulder, ankle, wrist): arthroscopic washout preferred — thorough irrigation, synovectomy where indicated, repeated washout if needed; serial aspiration (needle) acceptable for less virulent organisms with early presentation but surgical washout is more reliable
• Empirical antibiotics: start immediately after cultures are taken — flucloxacillin IV (covers S. aureus and Streptococci) ± gentamicin (for Gram-negatives in neonates and high-risk adults); vancomycin for MRSA risk; adjust based on culture and sensitivity results
• Duration of antibiotics: IV for minimum 2 weeks then oral for 4 weeks total (minimum) — prolonged courses for destructive infections or delayed presentation
• Monitoring response: daily WBC, CRP, ESR; clinical improvement; persistent fever or rising inflammatory markers after 48 hours of treatment = re-washout required

• Neonatal septic arthritis: most dangerous form — presents with pseudoparalysis, irritability, and swelling; proximal femoral epiphysis is entirely cartilaginous and not visible on plain X-ray; USS to detect joint effusion; MRI for soft tissue and epiphyseal assessment; blood cultures frequently positive; multi-joint infection can occur; aggressive surgical drainage and IV antibiotics; AVN and growth disturbance frequent sequelae
• Concomitant osteomyelitis and septic arthritis: most common at the hip and shoulder in children — the proximal femoral metaphysis is intracapsular; haematogenous seeding of the metaphysis can directly spread to the joint; assess for bone involvement with MRI or bone scan; treat both concurrently with surgical drainage of joint and bone
• Tuberculous arthritis: insidious onset chronic monoarthritis; imaging shows periarticular osteoporosis, joint space narrowing, and peripheral erosions (Phemister triad); synovial biopsy essential — culture for TB; treat with standard RIPE therapy; surgery for deformity or instability
• Gonococcal arthritis management: two distinct presentations — disseminated gonococcal infection (DGI) with polyarthralgia, skin lesions, tenosynovitis; and septic joint monoarthritis; treat DGI with ceftriaxone IV 1g daily for 7 days; screen and treat sexual contacts; exclude co-existing STI (Chlamydia, HIV)
• Septic arthritis in RA: particular diagnostic challenge — baseline synovitis, elevated baseline CRP, and joint damage mimic infection; RA patients have 10× higher risk of septic arthritis; high index of suspicion required; do not attribute acute flare to RA without excluding infection, particularly in patients on biologics or immunosuppressants

• Cartilage destruction begins within 8 hours; irreversible by 24–48 hours — surgical emergency
• Kocher criteria (4): fever, NWB, ESR >40, WCC >12 — 4/4 = 99.6% probability of septic arthritis
• Hip in child: open arthrotomy or arthroscopic washout — needle aspiration alone is NOT adequate
• S. aureus: most common organism across all age groups; Kingella kingae in <5 years; GBS in neonates; Neisseria in sexually active adolescents/adults
• Synovial WCC >50,000 = highly suggestive; >100,000 = near-diagnostic; PMN >75%; culture positive 70–80%
• Transient synovitis vs septic arthritis: only joint aspiration reliably distinguishes them — do not rely on imaging alone
• Neonatal hip: cartilaginous epiphysis not visible on X-ray — USS + MRI; pseudoparalysis; multi-joint possible; AVN is the devastating complication
• Empirical antibiotics: flucloxacillin IV; vancomycin for MRSA risk; adjust on cultures
• RA + septic arthritis: 10× higher risk; biologic-treated patients especially at risk; always aspirate acutely hot RA joint
• Concomitant osteomyelitis in children: up to 30% of septic hip/shoulder — MRI to assess; treat both