Is arthrocentesis of the knee performed in a 3-year-old with suspected septic arthritis?

Arthrocentesis for Suspected Septic Arthritis in a 3-Year-Old Child

Yes, arthrocentesis of the knee should be performed in a 3-year-old child with suspected septic arthritis as it is essential for definitive diagnosis and is considered an orthopedic emergency. 1

Rationale for Arthrocentesis

Septic arthritis in children is considered an orthopedic emergency because bacterial proliferation and metabolites can rapidly result in cartilage damage, leading to significant morbidity and potential long-term functional impairment 1. The definitive diagnosis of septic arthritis is made through arthrocentesis, which allows for:

1. Confirmation of the diagnosis through synovial fluid analysis
2. Identification of the causative organism for targeted antibiotic therapy
3. Therapeutic drainage to reduce intra-articular pressure and bacterial load

Clinical Assessment Prior to Arthrocentesis

When evaluating a 3-year-old with suspected septic arthritis of the knee, consider:

• Fever >101.3°F
• Elevated inflammatory markers (ESR ≥40 mm/hr, WBC ≥12,000 cells/mm³)
• Elevated C-reactive protein >2.0 mg/dL (highly predictive of septic arthritis)
• Inability to bear weight on the affected side
• Presence of joint effusion on physical examination or imaging

The more Kocher criteria present, the higher the likelihood of septic arthritis, with nearly 100% likelihood when all criteria are met 1.

Imaging Before Arthrocentesis

Before performing arthrocentesis:

• Obtain plain radiographs of the knee (initial imaging)
• Ultrasound of the knee to confirm joint effusion and guide aspiration
  • Ultrasound is highly sensitive for detecting joint effusion and can identify effusions as small as 1 mL 1
  • The absence of joint effusion on ultrasound makes septic arthritis unlikely (though a 5% false negative rate exists, particularly with symptoms <1 day) 1

Arthrocentesis Procedure

For a 3-year-old child:

• Procedural sedation is typically required due to the child's age
• Ultrasound guidance is recommended to improve accuracy and success rates 1, 2
• The procedure should be performed under sterile conditions
• Aspirate as much fluid as possible for diagnostic testing
• Send synovial fluid for:
  • Cell count and differential
  • Gram stain
  • Culture and sensitivity
  • Crystal analysis
  • Glucose and protein levels

Post-Arthrocentesis Management

Based on the arthrocentesis findings:

1. If septic arthritis is confirmed:

   • Initiate appropriate intravenous antibiotics
   • Consider further surgical intervention if needed:
     • Arthroscopic drainage may be necessary (4% of cases require additional drainage procedures after initial arthroscopy) 3
     • Arthroscopic treatment has shown excellent outcomes in children with septic knee arthritis 4, 5

2. If arthrocentesis reveals transient synovitis or another non-infectious cause:

   • Manage accordingly with appropriate anti-inflammatory medications and rest

Important Considerations

• Staphylococcus aureus is the most common pathogen in pediatric septic arthritis, but Kingella kingae should be considered in children <4 years of age 1
• Concurrent osteomyelitis and septic arthritis are common in young children and may require additional imaging (MRI) for complete evaluation 1
• Delayed diagnosis and treatment can lead to cartilage destruction, growth disturbance, and long-term functional impairment
• In children <2 years of age, septic arthritis is more common than osteomyelitis 1

Pitfalls to Avoid

• Delaying arthrocentesis while waiting for imaging studies in a clinically suspicious case
• Starting antibiotics before obtaining synovial fluid samples
• Misinterpreting a negative ultrasound in very early disease (symptoms <1 day)
• Failing to consider concurrent osteomyelitis, especially in children under 18 months due to transphyseal vessels 1
• Inadequate drainage of purulent material, which may necessitate repeat procedures

Arthrocentesis remains the gold standard for diagnosis of septic arthritis in children of all ages, including 3-year-olds, and should be performed promptly when septic arthritis is suspected to minimize morbidity and optimize outcomes.