Glossary
Effusion and the bulge test
Quick answer
An effusion is extra fluid inside the knee joint, often called water on the knee. The bulge test detects small amounts by stroking fluid out of one side of the knee and watching it ripple back. Effusion signals that something inside the joint is irritated or injured, explains Dr. Pablo Gelber, knee surgeon at Aman Hospital in Doha.
How swelling is checked
With your leg straight and relaxed, the surgeon strokes the inner side of the knee upwards to push fluid away, then strokes down the outer side. If there is extra fluid, a small bulge reappears on the inner side within a second or two. This is the bulge or sweep test, and it is painless.
For larger effusions, the surgeon presses fluid down from above the kneecap and taps the kneecap, which bounces against the bone. This is the patellar tap.
What it tells the surgeon
Timing is a strong clue. Swelling within an hour or two of an injury usually means bleeding inside the joint, often from an ACL tear, a kneecap dislocation or a fracture. Swelling that builds overnight is more typical of a meniscus tear. Recurrent swelling after activity is common with arthritis or cartilage damage.
A hot, red, very painful swollen knee, especially with fever, may be an infection or gout and needs urgent assessment the same day. In these cases fluid is often drawn off with a needle and tested.
Frequently asked questions
Should the fluid always be drained?
No. Many effusions settle with rest, ice and treating the cause. Drainage is used to relieve a tense knee or to test the fluid when infection or crystals are suspected.
Why does my knee swell after football?
Repeated swelling after sport suggests an internal problem such as a meniscus tear, cartilage wear or instability. It is worth an examination rather than simply resting each time.
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