What is the management approach for a patient with suspected septic arthritis of the knee?

Management of Suspected Septic Arthritis of the Knee

Joint aspiration is the definitive diagnostic procedure for suspected septic arthritis of the knee and should be performed immediately, prior to antibiotic administration, to maximize the likelihood of identifying the causative organism. 1, 2

Diagnostic Approach

• Septic arthritis is an orthopedic emergency as bacterial proliferation can rapidly cause cartilage damage and permanent joint destruction 2
• Initial evaluation should determine if the case is a surgical emergency based on clinical presentation (presence of sepsis, immunocompromised status, or evidence of extensive joint destruction) 3, 2
• Synovial fluid analysis is essential for diagnosis and should include:
  • White blood cell count and differential (≥50,000 cells/mm³ suggestive of septic arthritis) 2
  • Gram stain and culture 1, 2
  • Crystal analysis to rule out crystalline arthropathy 1, 4

Imaging Considerations

• Plain radiographs should be obtained initially but may only show soft tissue swelling or joint effusion in early infection 1
• Ultrasound is recommended as the initial imaging modality, especially for detecting effusions and guiding aspiration 2
• MRI has high sensitivity for detecting joint effusions, soft tissue involvement, and concomitant osteomyelitis when clinical suspicion remains high despite negative joint aspiration 1, 2

Joint Aspiration Procedure

• Image-guided aspiration (ultrasound or fluoroscopy) is preferred to ensure proper needle placement and reduce complications 1
• Antibiotics should be withheld until after joint aspiration is performed, as prior antibiotic administration significantly reduces the sensitivity of synovial fluid culture 5
  • Sensitivity of microscopy drops from 58% to 12% when patients have received antibiotics 5
  • Sensitivity of culture drops from 79% to 28% when patients have received antibiotics 5
• If sepsis is present, blood cultures should be obtained before starting antibiotics 3

Treatment Algorithm

1. Immediate Joint Aspiration:

   • Perform joint aspiration for diagnosis and as initial therapeutic intervention 1, 2
   • Send fluid for cell count, differential, Gram stain, culture, and crystal analysis 1, 2

2. Empiric Antibiotic Therapy:

   • After obtaining synovial fluid, initiate empiric antibiotics if there is clinical concern for septic arthritis 2, 6
   • For adults, IV vancomycin is recommended as first-line therapy for MRSA coverage 2
   • For children, IV ceftriaxone (25-50 mg/kg/day) is recommended 1, 2

3. Surgical Management:

   • Surgical debridement (arthroscopy or arthrotomy) is indicated for:
     • Surgical emergencies (sepsis, immunocompromised status) 3, 2
     • Persistent infection despite aspiration and antibiotics 2, 7
     • Loculated effusions or compartmentalization 3
   • Arthroscopic irrigation and debridement has a 91% success rate in treating septic arthritis 7

4. Antibiotic Duration:

   • 2-4 weeks for uncomplicated cases 2, 6
   • Longer courses (4-6 weeks) for cases with concomitant osteomyelitis 2
   • Oral antibiotics can be used after initial IV therapy if the patient is clinically improving and the organism is susceptible 6

Special Considerations

• Presence of synovial fluid crystals does not exclude concurrent septic arthritis 4
• Consider specific pathogens based on patient characteristics:
  • Gonococcal arthritis in sexually active young adults 3
  • Kingella kingae in children <4 years 2
  • Salmonella in patients with sickle cell disease 2
• Concomitant osteomyelitis occurs in up to 30% of children with septic arthritis and requires longer treatment 2

Pitfalls and Caveats

• Negative joint aspirate culture does not rule out infection, especially if antibiotics were administered prior to aspiration 3, 5
• Multivariable assessment improves diagnostic accuracy, with the following factors independently associated with septic arthritis:
  • Synovial fluid WBC ≥30,000 cells/mm³ (strongest predictor) 4
  • Positive Gram stain 4
  • Pain duration >2 days 4
  • History of prior septic arthritis 4
  • Clinical effusion 4
• The stage of infection at presentation affects treatment outcomes and the need for repeated interventions 7