What is knee effusion, also known as water on the knee?

What is "Water on the Knee"?

"Water on the knee" is the common term for knee effusion—an abnormal accumulation of fluid within the knee joint cavity that causes visible swelling and can result from trauma, arthritis, infection, or various other joint disorders. 1

Definition and Clinical Presentation

Knee effusion represents excess fluid collection in the intra-articular space of the knee joint. 2 This fluid accumulation manifests as:

• Visible joint swelling that distends the suprapatellar pouch and parapatellar regions 1
• Palpable fluid detectable on physical examination, though small effusions may be missed clinically 1
• Restricted range of motion and pain, particularly with weight-bearing 3
• Joint stiffness that worsens with activity 2

Common Causes

The differential diagnosis requires distinguishing between traumatic and atraumatic etiologies:

Traumatic Causes

• Hemarthrosis (blood in the joint) from ligamentous injuries, meniscal tears, or fractures—the most common cause in acute presentations 4, 2
• Ligamentous injuries including ACL, PCL, or collateral ligament tears 2
• Meniscal tears and cartilage injuries 2
• Fractures including occult fractures not visible on initial radiographs 1

Atraumatic Causes

• Osteoarthritis—routinely presents with mild to moderate effusions on examination 5
• Rheumatoid arthritis—contains approximately 100% more extractable fluid than osteoarthritic knees 6
• Infection (septic arthritis)—requires urgent diagnosis and treatment 4, 2
• Crystal deposition diseases (gout, pseudogout) 2
• Inflammatory arthropathies and postinfectious conditions 4

Diagnostic Approach

Physical Examination

Compare the affected knee with the unaffected knee systematically to detect subtle effusions. 2 Key examination findings include:

• Ballottement test to detect moderate to large effusions 2
• Bulge sign for small effusions 2
• Assessment for warmth, erythema, and tenderness to distinguish inflammatory from mechanical causes 4

Imaging Evaluation

Radiographs are the initial imaging modality and can demonstrate effusion as soft-tissue swelling, though they are limited for depicting internal joint structures. 1

Ultrasound excels at detecting even small effusions that may be missed clinically, particularly in the suprapatellar pouch, and can guide arthrocentesis. 1 Small amounts of fluid are readily visualized with longitudinal and transverse scans in the suprapatellar region. 1

MRI is more sensitive than clinical examination for detecting effusions and should be the next examination when radiographs show effusion but pain persists, as it accurately depicts the extent of fluid and presence of synovitis. 5

Clinical Significance by Etiology

In Osteoarthritis

• Effusion presence indicates active disease and predicts better response to intra-articular corticosteroid injections 5
• Changes in effusion severity correlate with risk of frequent knee pain (p=0.045 for improving effusions) 5
• Large effusions in OA associate with higher grades of mechanical destruction but not increased neutrophil counts 6

In Rheumatoid Arthritis

• Large effusions associate with increased synovial fluid neutrophil counts (p=0.04) but not arthritis grade 6
• Mechanical compression during arthrocentesis decreases failed diagnostic attempts from 56.7% to 26.9% and increases fluid yield by 108% 6

Management Considerations

Arthrocentesis (joint aspiration) provides only temporary improvement lasting approximately one week due to early re-accumulation, but is valuable for:

• Establishing diagnosis in atraumatic effusions of unknown origin 3
• Immediate symptomatic relief, particularly in post-traumatic cases 3
• Distinguishing infectious from non-infectious causes through synovial fluid analysis 4, 2

Critical Pitfall

Purulent arthritis or tuberculous arthritis should be the presumptive diagnosis in cases with purulent-appearing fluid until proven otherwise, even when considering rare entities like chylous effusion. 7 Arthroscopic irrigation and debridement may be necessary to confirm diagnosis in traumatic cases with atypical presentations. 7