The patellar tap test, also known as the ballottement test, assesses for fluid accumulation (effusion) in the knee joint. Here's how to perform it:
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Position the Patient: Have the patient lie supine (on their back) with their knee extended and relaxed.
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Milk the Effusion (If Necessary): If visible swelling isn't immediately apparent, gently milk any fluid from the suprapatellar pouch (above the patella) downwards towards the patella. This helps to concentrate the fluid beneath the patella, making the test more reliable.
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Apply Pressure:
- Place one hand (or fingers) a few inches above the patella to compress any fluid.
- With the other hand, use one or two fingers to firmly but gently tap or press downwards directly onto the patella.
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Observe for Floating: Feel for a distinct "tap" or "click" as the patella moves downwards through the fluid and impacts the femur (thigh bone). You should also feel the patella rebound slightly back up when you release the pressure. This "floating" sensation indicates fluid accumulation.
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Interpretation:
- Positive Test: If the patella feels like it's floating and you feel a tap against the femur, it suggests a significant effusion.
- Negative Test: If you don't feel the patella moving through fluid and impacting the femur, it suggests minimal or no effusion.
Important Considerations:
- Apply gentle pressure. Excessive force isn't necessary and can cause discomfort.
- Compare with the unaffected knee if possible, to assess for subtle differences.
- A small amount of fluid may be normal. The significance of the test result should be interpreted in the context of the patient's other symptoms and clinical findings.
- This test is subjective and relies on the examiner's experience.
