Septic Arthritis

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Etiology & Risk Factors - Joint Under Siege

  • Pathogens:
    • Overall: Staphylococcus aureus (most common).
    • Young, sexually active: Neisseria gonorrhoeae (📌 Mnemonic: "STD" - Synovitis, Tenosynovitis, Dermatitis).
    • Neonates: Group B Strep, S. aureus, Gram-neg bacilli.
    • IVDU, immunocompromised: Gram-neg bacilli (e.g., Pseudomonas), Candida.
    • Sickle cell: Salmonella spp. Septic Arthritis Infographic
  • Risk Factors:
    • Damaged joints (RA, OA, gout).
    • Prosthetic joints.
    • IV drug use.
    • Immunosuppression (DM, HIV, steroids).
    • Age extremes.
    • Joint surgery/injection.
    • Skin infection, bacteremia.

S. aureus is the most common cause of septic arthritis in adults and children; N. gonorrhoeae in sexually active individuals.

Pathogenesis & Symptoms - Fiery Joint Drama

  • Pathogenesis:
    • Bacterial entry: Hematogenous (commonest), direct inoculation, contiguous spread.
    • Synovial inflammation: Rapid bacterial proliferation → intense neutrophilic response → pus.
    • Cartilage destruction: By bacterial toxins (e.g., PVL), host enzymes (MMPs, elastase), ↑ intra-articular pressure, inflammatory cytokines (TNF-α, IL-1β).
  • Clinical Features (Fiery Joint):
    • Acute onset: Severe pain (arthralgia), swelling (effusion), warmth, erythema.
    • Restricted movement: Pseudoparalysis, pain on passive motion.
    • Systemic: Fever, chills, malaise, tachycardia.
    • 📌 Kocher's Criteria (pediatric septic hip):
      • Fever > 38.5°C
      • ESR > 40 mm/hr
      • Non-weight bearing on affected side
      • WBC count > 12,000/mm³

⭐ Irreversible cartilage damage can occur within 24-48 hours if untreated.

Diagnosis - Cracking Joint Case

  • Synovial Fluid Analysis (Arthrocentesis): KEY!
    • Appearance: Turbid, purulent
    • WBC: > 50,000/mm³
    • PMN: > 75%
    • Gram Stain & Culture: ~50-70% positive
    • Crystals: Exclude gout/pseudogout
  • Blood Tests:
    • CBC: ↑WBC
    • Inflammatory markers: ↑ESR (>30 mm/hr), ↑CRP
    • Blood Culture: ~50% positive
  • Imaging:
    • X-ray: Early: normal/swelling. Late: joint narrowing, erosions.
    • USG: Effusion detection, aspiration guidance.
    • MRI: Most sensitive for early changes, osteomyelitis.
  • Kocher's Criteria (Pediatric Hip): 📌 FEN-W
    • Fever > 38.5°C
    • Non-weight bearing
    • ESR > 40 mm/hr
    • WBC > 12,000/mm³
    • (≥3 criteria suggest high risk)

⭐ Synovial fluid: WBC > 50,000/mm³ & PMNs > 75% strongly suggest septic arthritis, even with negative Gram stain.

Synovial fluid microscopy: neutrophils & bacteria

Management & Complications - Dousing Joint Flames

  • Immediate Goals:
    • Eradicate infection: Prompt IV antibiotics (empiric → culture-guided).
    • Remove pus: Urgent joint drainage.
    • Pain relief & preserve function.
  • Treatment Pillars:
    • Antibiotics: IV for 2-4 weeks (e.g., Vancomycin + Ceftriaxone/Cefotaxime), then oral for 2-4 weeks. Total 4-6 weeks.
    • Drainage:
      • Needle aspiration (repeated).
      • Arthroscopic lavage.
      • Arthrotomy (hip, resistant cases).
    • Supportive: Splinting, analgesia, later physiotherapy.
  • Complications (If Delayed/Severe):
    • Cartilage destruction → Degenerative Joint Disease (DJD).
    • Osteomyelitis.
    • Joint stiffness/ankylosis.
    • Growth plate damage (children).
    • Sepsis.

    ⭐ Failure to drain a septic joint within 24-48 hours can lead to irreversible cartilage loss.

High‑Yield Points - ⚡ Biggest Takeaways

  • Most common organism: Staphylococcus aureus.
  • Hematogenous spread is the predominant infection route.
  • Knee is most affected in adults; hip in children.
  • IV drug users: Suspect Pseudomonas aeruginosa, often in sacroiliac joints.
  • Diagnosis: Synovial fluid with WBC >50,000/mm³, >75% PMNs, and positive culture.
  • Treatment: Urgent IV antibiotics and joint drainage (arthrocentesis, arthroscopy, or arthrotomy).
  • Kocher criteria (fever, non-weight bearing, ESR >40, WBC >12,000) aid pediatric septic hip diagnosis.
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Practice Questions: Septic Arthritis

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A child presents with painful limp and restricted hip rotation. ESR and CRP are elevated. Initial plain radiograph is normal. What is the next best imaging study?

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X-ray showing _____ hip are suggestive of tuberculosis (TB) of the hip\

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X-ray showing _____ hip are suggestive of tuberculosis (TB) of the hip\

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