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Special Test

Patellar Tap Test

Patellar Tap Test (or as it called Patellar Ballottement Test) is a useful technique for detecting the effusion of the knee joint.

How do you do a Patellar Tap Test?

  • With the patient in supine position,
  • Using one hand, the clinician grasps the patient’s thigh at the anterior aspect about 10 cm above the patella, placing the fingers medial and the thumb lateral. The patient’s knee is extended.
  • With the other hand, the clinician grasps the patient’s lower leg about 5 cm distal to the patella, placing the fingers medial and the thumb lateral.
  • The proximal hand exerts compression against the anterior, lateral, and medial aspects of the thigh and, while maintaining this pressure, slides distally.
  • The distal hand exerts compression in a similar way and slides proximally.
  • Using the index finger of the distal hand, the clinician now taps the patella against the femur.
See Also: Patellar Instability
Patellar Tap Test -  Patellar Ballottement Test
Patellar Tap Test

What is a positive Patellar Tap Test mean?

A Patellar Tap Test positive is indicative of a significant synovial effusion or hemarthrosis in the knee joint.

  • In the normal knee joint with minimal free fluid, the patella moves directly into the femoral condyle and there is no tapping sensation underneath the clinician’s fingertips.
  • In the knee with excess fluid, the patella is “floating” ; thus, ballottement causes the patella to tap directly against the femoral condyle. This sensation is transmitted to the clinician’s fingertips.

Sometimes, this test can produce false-positive results. When this is the case, the uninvolved side usually tests positive as well.

Patellar Tap Test Accuracy

In a study by Marlous Kastelein 1, the ballottement test was used to assess knee effusion caused by trauma; when compared to MRI effusion the test had the following accuracy:

  • Sensitivity: 83%
  • Specificity: 49%

Knee Effusion

The amount of swelling present may provide the clinician with valuable information regarding the internal damage that may have resulted. Diffuse swelling indicates fluid in the joint or synovial swelling, or both. An effusion can be detected by noticing the loss of the peripatellar groove and by palpation of the fluid. A perceptible bulge on the medial aspect suggests a small effusion; this sign may not be present with larger effusions.

When assessing swelling, the examiner must determine the type and amount of swelling that is present.

The examiner must differentiate between swelling and synovial thickening. Normally the knee contains 2 mL of synovial fluid.

A bloody effusion (hemarthrosis) is usually traumatic, caused by a ligament tear (usually of the anterior cruciate ligament), osteochondral fracture, or peripheral meniscus tear. A hemarthrosis usually develops very quickly, within 1 to 2 hours. On palpation it has a “doughy” feeling and is relatively hard to the touch. The overlying skin feels warm. Presence of marrow fat (oil droplets) in hemarthrosis indicates intra-articular fracture

Nontraumatic synovial effusion caused by chronic joint disease normally takes several hours or even days to develop. The examiner feels a fluctuant joint effusion; the overlying skin is mildly warm to the touch.

Synovial effusion usually develops from overuse of the knee and disappears after a few days of inactivity.

In the case of a joint infection, the development of the effusion is delayed. The overlying skin is very hot and red. Palpation demonstrates a large joint effusion that is taut and tender. Movement is clearly limited by pain.

Tense effusion must be aspirated to relieve pain, to prevent quadriceps inhibition and for a diagnosis.


  1. Kastelein M, Luijsterburg PA, Wagemakers HP, Bansraj SC, Berger MY, Koes BW, Bierma-Zeinstra SM. Diagnostic value of history taking and physical examination to assess effusion of the knee in traumatic knee patients in general practice. Arch Phys Med Rehabil. 2009 Jan;90(1):82-6. doi: 10.1016/j.apmr.2008.06.027. PMID: 19154833.
  2. Maricar N, Callaghan MJ, Parkes MJ, Felson DT, O’Neill TW. Clinical assessment of effusion in knee osteoarthritis-A systematic review. Semin Arthritis Rheum. 2016 Apr;45(5):556-63. doi: 10.1016/j.semarthrit.2015.10.004. Epub 2015 Oct 22. PMID: 26581486; PMCID: PMC4823277.
  3. Winkel D, Matthijs O, Phelps V: Examination of the Knee. Gaithersburg, MD: Aspen, 1997.
  4. Dutton’s Orthopaedic Examination, Evaluation, And Intervention 3rd Edition.
  5. Clinical Tests for the Musculoskeletal System 3rd Edition.
  6. Clinical Assessment and Examination in Orthopedics, 2nd Edition Book
  7. Ronald McRae – Clinical Orthopaedic Examination 6th Edition Book
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