What is a knee effusion?

What is a Knee Effusion?

A knee effusion is an abnormal accumulation of fluid within the knee joint space, which can result from traumatic injury, overuse, infection, inflammatory arthritis, crystal deposition, or systemic disease. 1

Clinical Definition and Pathophysiology

A knee effusion represents excess synovial fluid or other fluid types (blood, pus, lipid-laden fluid) that accumulates in the intra-articular space of the knee joint. 1, 2 The fluid accumulation causes visible swelling and can be detected through physical examination or imaging studies. 3

Common Etiologies

Traumatic Causes

• Hemarthrosis from ligamentous injuries, meniscal tears, or fractures is the most common cause of acute traumatic knee effusions, particularly in pediatric patients 2
• Acute trauma typically produces rapid fluid accumulation within hours of injury 1
• Post-traumatic effusions can rarely contain lipid-rich chylous fluid from fat pad necrosis, appearing milky white on aspiration 4

Atraumatic Causes

• Inflammatory arthritis including rheumatoid arthritis and juvenile idiopathic arthritis 1, 2
• Infectious arthritis (septic joint) requiring urgent diagnosis and treatment 2
• Crystal deposition diseases such as gout or pseudogout 1
• Overuse syndromes and degenerative osteoarthritis 1
• Tumors and other space-occupying lesions 1
• Paralysis-related effusions in spinal cord injury patients, which are typically benign and noninflammatory 5

Diagnostic Approach

Imaging Detection

• Radiographs can delineate effusions but have limited sensitivity and specificity 6
• Ultrasound is highly sensitive for detecting even small effusions (as little as 1 mL in the hip) and can guide aspiration procedures 6, 3
• MRI accurately depicts the extent of effusion, presence of synovitis, and associated pathology like bone marrow lesions 6
• Ultrasound is as accurate as MRI in diagnosing popliteal cysts and detecting cyst rupture 6, 3

Joint Aspiration

• Arthrocentesis is essential for effusions of unknown origin to establish diagnosis through synovial fluid analysis 7, 1
• Fluid analysis can distinguish between inflammatory, infectious, hemorrhagic, and crystal-induced effusions 1, 2
• Ultrasound or fluoroscopic guidance improves aspiration accuracy, especially for small or loculated effusions 3, 8

Clinical Significance

Diagnostic Value

The presence of an effusion indicates intra-articular pathology but is nonspecific—the underlying cause must be determined through history, physical examination, imaging, and often synovial fluid analysis. 1, 2

Management Implications

• Aspiration provides only temporary symptom relief (lasting approximately one week) due to rapid re-accumulation, particularly in traumatic effusions 7
• Corticosteroid injection following aspiration may reduce synovitis in approximately two-thirds of patients, though benefits are typically short-lived (1-4 weeks) 3, 8
• The absence of effusion is a strong negative predictor of response to corticosteroid injection 8

Important Clinical Pitfalls

• Not all knee swelling represents true joint effusion—periarticular soft tissue edema, popliteal cysts, and bursitis can mimic intra-articular effusion 6
• In paralyzed patients, benign noninflammatory effusions are common and should not automatically trigger aggressive workup 5
• Aspiration should be performed with consideration in traumatic cases as it provides only temporary benefit and carries infection risk 7
• Always consider septic arthritis in the differential, particularly in atraumatic effusions with systemic symptoms, as this requires urgent intervention 2