Effusion tests of the Knee

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For these tests, several names are used such as; ballottement patellae, patellar tap, effusion test, fluid wave test, fluid displacement test, bulge, wipe and stroke test. At the time of writing, these tests were not documented in RCTs or other articles therefore the sources of the information in this article are academic textbooks.

Purpose:
Accumulation of fluid due an irritation of the synovial membrane is known as effusion; it could be serous, blood, or pus.
If there is swelling in the knee it should be evaluated to know whether it is a soft tissue swelling or a bony swelling of a joint effusion.

Perform the patellar tap test or fluid displacement test to determine the presence of fluid in the knee joint. The patellar test is best for identifying moderate-sized effusions. The fluid displacement test is better for smaller effusions.

Clinical aspect:

Knee effusion is usually diffuse, symmetric and associated with a loss of knee contour, although the joint effusion may be minimal and may be distinguished by pushing the suprapatellar pouch.
Any condition in or around the knee can result in an effusion.
When we palpate the level of the medial and lateral condyles about 2 cm posterior to the medial and lateral edges of the patella the synovium is superficial and is only covered by skin and subcutaneous fat. If there is a synovial thickening, a dense structure is felt. Swelling can be intraarticular or extraarticular.
If there is effusion associated with patellofemoral pathology, there is the possibility of moderate-to-severe patellofemoral arthrosis, severe plical inflammation, chrondrol loose bodies and osteochrondral damage.
The intraarticular swelling is often due to a synovitis and it feels soft and does not elicit a fluid wave. A clear effusion flows like a liquid, whereas blood feels like a jelly mass.
Extraartrictular swelling is not movable and feels like a thick, soft tissue

An effusion can be associated with the following disorders;
-Osteoarthritis
- Trauma
- Infection
- Arthritic disorders
- Overuse

- Tumour (less common)

Techniques:

- Patellar tap test

Description of the test :
The patient is lying in supine with the leg extended. The physiotherapist putts pressure on the proximal side of the knee in an effort to squeeze the fluid out of the suprapatellar pouch. The fluid can be moved under the patella while maintaining the pressure on the suprapatellar pouch, he uses his other hand to press up on the medial and lateral recesses forcing the fluid under the patella . Tapping down the patella with the index to create an upward and downward movement and a palpable ‘click’ as the patella hits the underlying femur.

If the test is negative the femur and the patella are already in contact.
A positive test is when the patella can be felt to move down through the fluid and rebounds on the patella. The test can be false positive, therefore we must always test both sides to compare.

http://fysiovaardig.boom.nl/?id=bju240407.30072008138686

-Bulge, wipe or stroke test / Fluid displacement test

Description of the test
The patient in supine, with the knee in an extended position. The physiotherapist strokes upwards with the edge of the hand on the medial side of the knee to milk the fluid ca. 10 cm proximal of the patella into the lateral compartment, and continues pushing the fluid downwards on the lateral side.

A positive test if the physiotherapist sees a fluid moving towards the medial side.
http://fysiovaardig.boom.nl/?id=bju240407.30072008138484

- Fluid wave test:
Is used when the effusion is less than 30-50 cc. The patient is in supine, the physiotherapist presses his fingers in both parapatellar gutters. Because there’s a pressure from below upward, the gutters are emptied. The patient is asked to stand while the physiotherapist keeps his fingers in the parapatellar gutters, if the physiotherapist releases his fingers, and the fluid comes back in the parapatellar gutters, it is a positive sign.

Reliability of the tests:
Effusion were assessed by the bulge sign, balloon test, and patellar tap. Of these tests the bulge sig was the most reliable ( Rc 0,97), however the assessment of effusion by balloon test also achieved a post standardization PABAK of 0,99.