Patellar Tap Test

 Patellar Tap Test

What is the 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.

See Also: Patellar Instability

How do you do a Patellar Tap Test?

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.

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

Other tests for knee effusion include:

Brush Test (Stroke, Wipe test):

Brush Test is used for assessing minimal effusion.

The patient lies supine. The examiner uses one hand to “smooth out” the medial side of the knee from distal to proximal. With the other hand the examiner presses the superior recess moving distally from a proximal and lateral position.

Even with a mild joint effusion, a wave of fluid may be felt in the medial, distal part of the joint.

Brush Test knee
Brush Test

Dancing Patella Test:

The patient is supine or standing. With one hand, the examiner smoothes the suprapatellar pouch from proximal to distal while pressing the patella against the femur with the other hand or moving it medially and laterally with slight pressure.

Resilient resistance (a dancing patella) is abnormal and suggests effusion in the knee.

Dancing Patella Test
Dancing Patella Test

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:

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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