What is the appropriate evaluation and management for a patient presenting with knee effusion?

Evaluation and Management of Knee Effusion

Begin with plain radiographs (AP and lateral views) as the mandatory first imaging study for any patient presenting with knee effusion, followed by immediate arthrocentesis when infection is suspected or when the etiology of an atraumatic effusion is unclear. 1, 2

Immediate Clinical Assessment

Determine trauma history first to distinguish traumatic from atraumatic causes, as this fundamentally alters your diagnostic and management pathway. 1

Red Flags Requiring Urgent Evaluation

• Inability to bear weight for 4 consecutive steps mandates immediate radiographic imaging. 1
• Inability to flex the knee to 90 degrees due to large effusion requires radiographs without delay. 1
• Fever, systemic symptoms, or recent bacteremia suggest septic arthritis—an orthopedic emergency requiring urgent arthrocentesis and orthopedic consultation. 1, 2
• Absent distal pulses, pallor, paresthesia, or paralysis indicate possible acute limb ischemia requiring urgent CTA of the lower extremity. 1

Key Physical Examination Findings

• Focal bony tenderness at the patella, tibial plateau, or malleoli raises suspicion for fracture. 1
• Self-noticed swelling combined with positive ballottement test has a likelihood ratio of 3.6 for confirming effusion and is associated with internal derangement. 3
• Perform Lachman test at 20-30 degrees flexion to assess for ACL injury in traumatic cases. 4

Initial Imaging Protocol

Plain radiographs are non-negotiable as the first imaging study, regardless of how obvious the effusion appears clinically. 1, 2

What Radiographs Reveal

• Fractures, tumors, osteomyelitis, hardware complications, and chronic bone changes that alter management. 1
• Joint effusion, soft-tissue swelling, periosteal reaction, or gas in soft tissues suggesting necrotizing infection. 1
• Early osteomyelitis (< 14 days) may show only soft-tissue swelling, but films guide subsequent advanced imaging. 1

Arthrocentesis: When and Why

Perform joint aspiration immediately after radiographs when infection is suspected—do not delay for other studies. 1, 2

Indications for Arthrocentesis

• Suspected septic arthritis (fever, acute onset, systemic symptoms)—arthrocentesis is the reference standard for diagnosis. 1
• Atraumatic effusion of unknown etiology to rule out infection or crystal disease. 1, 2
• Post-traumatic effusion to identify lipohemarthrosis, which indicates occult intra-articular fracture despite negative radiographs. 2
• Crystal arthropathy precipitated by minor trauma must be excluded. 2

Synovial Fluid Analysis Priorities

• Cell count with differential to assess for infection (>50,000 WBC/mm³ suggests septic arthritis). 2
• Gram stain and culture if infection is suspected. 2
• Crystal analysis to exclude gout or pseudogout. 2
• Gross appearance for lipohemarthrosis (fat globules floating on blood indicate occult fracture). 2

Technical Considerations

Use image guidance (ultrasound, CT, or fluoroscopy) to ensure accurate needle placement and avoid contaminating adjacent tissues. 1

Advanced Imaging Based on Initial Findings

When Radiographs Show Fracture or Hardware Issues

CT without IV contrast is indicated to delineate fracture patterns, assess hardware loosening, evaluate for non-union, and detect osteolysis. 1

When Radiographs Are Normal or Show Only Effusion

MRI without IV contrast is the next-line study for suspected infection, internal derangement, or persistent pain. 1, 5

• MRI has 97% diagnostic accuracy for ACL tears and accurately depicts effusion extent, synovitis, popliteal cyst rupture, meniscal tears, ligament injuries, and occult fractures. 4, 5
• A negative MRI effectively excludes osteomyelitis. 1
• Add IV contrast to MRI when characterizing soft-tissue abscesses or delineating infection extent. 1

When MRI Is Contraindicated or Unavailable

CT with IV contrast can evaluate soft-tissue compartments, differentiate cellulitis from myositis or abscess, detect gas and foreign bodies, and guide procedures. 1

Management Algorithm by Etiology

Traumatic Effusion

• If radiographs are negative and patient can bear weight: conservative management with close follow-up. 1
• If symptoms persist or mechanical symptoms develop at 5-7 days: obtain MRI without contrast. 1
• If lipohemarthrosis is present on arthrocentesis: consider CT to characterize occult fracture. 2

Atraumatic Effusion

• Perform arthrocentesis to establish diagnosis and rule out infection or crystal disease. 1
• Obtain blood cultures if fever is present or acute symptom onset. 1
• Check ESR and CRP as the combination provides best sensitivity and specificity for prosthetic joint infection. 1

Prosthetic Joint Effusion

• Plain radiographs plus ESR/CRP testing, followed by arthrocentesis with synovial fluid analysis unless surgery is immediately planned. 1
• Do not routinely use bone scans, PET, or MRI for initial diagnosis of prosthetic joint infection. 1

Role of Ultrasound

Bedside ultrasound can detect joint effusion, soft-tissue abscess, periosteal thickening, and guide aspiration, but it must not replace plain radiographs as the initial imaging modality. 1

• Ultrasound limitations include inability to visualize bone lesions, susceptibility to metal artifact, and operator dependence. 1
• Power Doppler ultrasound can demonstrate increased synovial blood flow associated with knee pain. 5

Therapeutic Aspiration Considerations

Aspiration provides only temporary clinical improvement lasting approximately one week due to early re-accumulation of effusion. 6

• Aspiration is most beneficial for immediate symptom relief and establishing diagnosis in non-traumatic cases. 6
• In traumatic effusions, aspiration should be performed with consideration as it does not alter long-term outcomes. 6
• Corticosteroid injections may result in 1-2 week reduction in synovitis on MRI in two-thirds of patients. 5

Critical Pitfalls to Avoid

• Never skip radiographs, even when effusion is clinically obvious—fractures, tumors, and chronic changes must be excluded. 1, 2
• Never delay arthrocentesis in suspected infection—septic arthritis is an orthopedic emergency. 1, 2
• Do not obtain CT angiography routinely in knee trauma with normal vital signs and no vascular compromise. 2
• Remember that meniscal tears are often incidental in patients over 70 years, and asymptomatic tears are common in patients 45-55 years. 5