Septic Arthritis

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.
• 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.

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.