Knee Effusion Clinical Tests

Knee effusion refers to the accumulation of fluid within the knee joint due to irritation of the synovial membrane. The fluid may be:

• Serous (synovial fluid)

• Hemorrhagic (hemarthrosis, usually post-trauma)

• Purulent (infection/septic arthritis)

The purpose of clinical tests is to confirm the presence of intra-articular effusion and differentiate it from soft tissue swelling or bony enlargement.

Clinical Relevance

Knee effusion typically presents as diffuse, symmetrical swelling, obscuring the normal contour of the knee. In subtle cases, effusion may only be detected by palpating the suprapatellar bursa.

Common causes include:

• Osteoarthritis – chronic joint inflammation with fluid accumulation

• Traumatic dislocation or ligament injury – acute hemarthrosis

• Infection (septic arthritis) – serious condition with purulent effusion

• Inflammatory arthritis – e.g., rheumatoid arthritis

• Overuse injuries – repetitive strain

• Tumors – rare cause of effusion

On palpation:

• Synovial thickening feels firm

• Clear effusion fluctuates fluidly

• Hemarthrosis feels more gelatinous

Clinical Tests for Knee Effusion

1. Patellar Tap Test (Ballottement Patellae)

• Purpose: Detects moderate effusion by determining if the patella floats on fluid.

• Procedure:

  • Patient lies supine with extended knee

  • Examiner pushes fluid from suprapatellar pouch into the joint

  • Gentle downward pressure applied to patella

• Interpretation:

  • Positive: Patella sinks then rebounds (“ballottement”)

  • Negative: Patella rests firmly against femur

2. Bulge / Wipe / Stroke Test

• Purpose: Detects small effusions by observing fluid redistribution.

• Procedure:

  • Patient supine with extended knee

  • Examiner strokes fluid upward along medial side, then downward along lateral side

  • Observe for fluid wave returning medially

• Interpretation:

  • Positive: Visible fluid wave on medial knee

  • Negative: No visible fluid shift

Highly sensitive for subtle effusions not detectable by the patellar tap test.

3. Fluid Wave Test

• Purpose: Detects small effusions (<30–50 ml).

• Procedure:

  • Examiner compresses parapatellar gutters simultaneously

  • Patient rises while pressure is maintained

  • Release of pressure → observe fluid return

• Interpretation:

  • Positive: Fluid visibly refills parapatellar recesses

  • Negative: No fluid return

4. Sweep Test

• Purpose: Detects effusion by systematically moving joint fluid.

• Procedure:

  • Examiner “sweeps” fluid across suprapatellar and medial recesses

• Interpretation:

  • Positive: Observable shifting of joint fluid

Evidence and Reliability

• Bulge sign – most reliable for detecting effusion (Rₐ = 0.97)

• Balloon test – extremely high reliability (PABAK = 0.99)

• Patellar tap test – best for moderate effusion

• Stroke / Bulge / Fluid wave tests – most sensitive for small effusions

Conclusion

Clinical tests for knee effusion are effective in distinguishing between small, moderate, and large joint effusions:

• Patellar Tap Test – best for moderate effusions

• Bulge & Fluid Wave Tests – most sensitive for small effusions

• Balloon Test – demonstrates highest reliability

For optimal diagnostic accuracy, these tests should be combined with a thorough clinical examination and, when needed, imaging studies to confirm diagnosis and guide management.