What is the appropriate approach to diagnosing and managing a knee effusion?

Knee Effusion: Diagnostic and Management Approach

Initial Diagnostic Strategy

For any patient presenting with knee effusion, obtain plain radiographs first (AP, lateral, sunrise/Merchant, and tunnel views), followed by joint aspiration with synovial fluid analysis to establish the diagnosis—this is the most appropriate initial procedure rated 9/9 by the American College of Radiology. 1, 2

When to Obtain Radiographs

• Obtain radiographs immediately if the patient has focal tenderness, inability to bear weight, or visible effusion after trauma 1
• For patients ≥55 years old (Ottawa rules) or ≥50 years old (Pittsburgh rules), radiographs are indicated even with ability to walk 1
• In children ≥5 years old with knee effusion after any trauma (including overuse injury), radiographs are mandatory to rule out fracture 3
• The lateral view at 25-30 degrees flexion allows visualization of joint effusion and lipohemarthrosis (indicating intra-articular fracture) 1

Joint Aspiration: The Cornerstone of Diagnosis

Perform arthrocentesis after reviewing radiographs, as this provides sensitivity, specificity, and accuracy approaching 100% for diagnosing infection and establishing the etiology of effusion. 1, 2

Critical Timing Considerations

• Withhold antibiotics for at least 2 weeks before aspiration to increase organism recovery rates—false-negative aspirations commonly occur with preaspiration antibiotic treatment 1, 2
• If clinical suspicion for septic arthritis is high, perform urgent arthrocentesis before initiating antibiotics 2, 4
• Weekly repeat aspirations may be necessary if initial aspiration is negative but clinical suspicion remains high; in 16% of cases, more than three aspirations are needed for positive culture 1

Synovial Fluid Analysis Protocol

Complete analysis must include: 2

• Total cell count and differential leukocyte count
• Aerobic AND anaerobic bacterial cultures
• Crystal analysis (if clinically indicated)
• Consider advanced testing: leukocyte esterase, alpha-defensin, CRP, and nucleic acid amplification 1

The alpha-defensin test demonstrates 97% sensitivity and 96% specificity for periprosthetic joint infection, and when combined with synovial fluid CRP, achieves 97% sensitivity and 100% specificity 1

Aspiration Technique Optimization

• Use ultrasound or fluoroscopic guidance to improve accuracy, especially for small effusions or targeting specific compartments 1, 5
• Long needles (22 gauge/3.5-inch spinal needle) achieve 90.9% success rate versus 41.2% with shorter 18 gauge/1.5-inch needles 6
• A "dry tap" does NOT exclude infection 1

Adjunctive Laboratory Testing

Obtain ESR and CRP in all patients with suspected infection—the combination provides optimal sensitivity and specificity for detecting infection, with only one infected knee having negative results on both tests in validation studies. 2

• CRP >10 mg/L warrants repeat joint aspiration or biopsy 1
• Blood cultures should be obtained if fever is present or acute symptom onset suggests bloodstream infection 2

Pediatric-Specific Considerations

In children with knee effusion, immediately assess for Kocher criteria (fever, elevated ESR, elevated WBC, inability to bear weight) to identify septic arthritis, which requires urgent intervention. 3

• Elevated CRP is highly predictive of septic arthritis in pediatric knee effusions 3
• Do NOT apply transient synovitis of the hip guidelines to knee effusions—these have different clinical characteristics and management 3

Role of Advanced Imaging

Ultrasound

• Ultrasound is as accurate as MRI for diagnosing popliteal cysts and detecting cyst rupture 5
• Use ultrasound to confirm suspected effusions, quantify degree of effusion, and guide aspiration procedures 5
• Ultrasound can assess patellar polyethylene component thickness and detect postsurgical pseudoaneurysms 1

MRI

• Consider MRI when concomitant pathology is suspected (meniscal injury, ACL tear, articular cartilage injury) or to characterize known lesions 1
• MRI is NOT routinely indicated as initial imaging for acute trauma 1

Avoid These Imaging Pitfalls

• Do NOT routinely use bone scans, leukocyte scans, CT, or PET scans to diagnose prosthetic joint infection—these should not replace aspiration 2
• Radiographs alone cannot distinguish infection from aseptic loosening or particle disease 1

Therapeutic Aspiration Considerations

Aspiration provides only temporary clinical improvement lasting approximately one week due to early re-accumulation of effusion. 7

• Aspiration is most beneficial in post-traumatic effusions for immediate symptom relief 7
• Following aspiration of persistent effusions, corticosteroid injection may reduce synovitis in approximately two-thirds of patients 5
• Aspiration should be performed with consideration in traumatic effusions—while it aids diagnosis in non-traumatic cases, the benefit is temporary 7

Common Pitfalls to Avoid

• Never rely solely on imaging without fluid analysis—radiographic findings cannot distinguish between infection, loosening, or other pathologies 2
• Never assume Gram stain is sufficient—it has poor sensitivity and specificity; always obtain cultures 1
• Never skip radiographs before aspiration—underlying structural abnormalities must be assessed first 2
• Avoid aspirating through infected skin or cellulitis to prevent iatrogenic septic arthritis 4
• In prosthetic knees, orthopedic surgery should assess for aspiration 4