What is the recommended approach for draining a knee effusion with a needle in a patient presenting with knee swelling?

Draining a Knee Effusion: Recommended Approach

Arthrocentesis with synovial fluid analysis should be performed when knee effusion is present to differentiate infectious arthritis, crystal-induced arthropathy (gout, CPPD), and inflammatory conditions—this is essential before any treatment decisions, particularly before administering intra-articular corticosteroids. 1

When to Perform Arthrocentesis

Knee aspiration is indicated in the following clinical scenarios:

• Suspected infection or septic arthritis: Any knee effusion with signs of inflammation (warmth, erythema, fever) requires immediate arthrocentesis to exclude septic arthritis before any corticosteroid administration 1
• Crystal-induced arthropathy: When gout or calcium pyrophosphate deposition (CPPD) disease is suspected based on clinical presentation 1
• Diagnostic uncertainty: When the cause of effusion is unclear after initial evaluation 2
• Chronic knee pain with effusion and chondrocalcinosis on radiographs: Aspiration may be appropriate to confirm CPPD disease 2

When Arthrocentesis is NOT Routinely Indicated

Joint aspiration is usually not indicated in the following situations:

• Osteochondritis dissecans (OCD), loose bodies, or history of cartilage repair: These conditions do not require routine aspiration 2
• Signs of prior chronic osseous knee injury: Aspiration adds little diagnostic value in this setting 2
• Clear traumatic effusion without signs of infection: When the diagnosis is evident from history and examination 2

Technical Approach to Knee Aspiration

Image Guidance Options

• Ultrasound-guided aspiration: Highly accurate for diagnosis of septic arthritis, particularly for hip joints, and can be applied to knee aspiration 2
• Fluoroscopy-guided aspiration: Can be performed when there is concern for crystal disease or infection 2
• Landmark-based aspiration: Acceptable when effusion is large and easily palpable 2

Synovial Fluid Analysis Required

The aspirated fluid must be analyzed for:

• Cell count with differential: Essential to distinguish septic arthritis (typically >50,000 WBC/μL with >75% PMNs) from other causes 3
• Gram stain and culture: To identify bacterial pathogens 1
• Crystal analysis: To diagnose gout (monosodium urate crystals) or CPPD (calcium pyrophosphate crystals) 1

Critical Pitfalls to Avoid

• Never administer intra-articular corticosteroids before excluding infection: This can worsen septic arthritis and lead to joint destruction 4
• Do not delay aspiration when infection is suspected: Septic arthritis requires urgent diagnosis and treatment to prevent permanent joint damage 1
• Ensure adequate imaging before aspiration: Plain radiographs (AP, lateral, sunrise/Merchant, and tunnel views) should be obtained first to exclude fracture, tumor, or other bony pathology 1, 4

Special Considerations

Pediatric Patients

In children with painful swollen knee, the likelihood of septic arthritis is >90% when both age <5 years and CRP >2.0 mg/dL are present, making aspiration critical in this population 3

Lyme Arthritis

For persistent or recurrent joint swelling after initial antibiotic treatment in Lyme arthritis, intra-articular corticosteroid injections are not recommended during active infection or while awaiting response to antibiotics 5. Aspiration should be performed to assess synovial fluid PCR status before considering symptomatic treatment 5

Osteoarthritis with Effusion

Intra-articular corticosteroid injection is indicated for flare of knee pain, especially if accompanied by effusion, but only after infection has been excluded by synovial fluid analysis 1. This provides 1-2 weeks of symptom reduction in approximately two-thirds of patients 2