A knee effusion is excessive joint fluid inside the knee joint, also known as 'water on the knee'.

Page updated June 2023 by Dr Sheila Strover (Clinical Editor)

knee effusion

An acute knee effusion is one that comes on suddenly. A chronic effusion is one which is there all the time. It is an indication of an underlying problem in the knee, such as a torn meniscus, torn anterior cruciate ligament, gout or arthritis.

effusion of knee contained within joint capsule

Side view of the knee, showing a swollen capsule, tense with a joint effusion. The joint capsule is watertight, and since the fluid of an effusion is secreted by the cells of the inner lining, the swelling can become quite tense.

What is water on the knee?

Patients often describe an effusion as 'water on the knee', but the fluid is not actually water and it is not 'on the knee' but contained within the capsule of the knee. The fluid of an effusion is secreted by the cells of the 'synovium' which form the inner lining of the capsule of the knee joint, and will differ depending upon the trigger. The fluid is usually viscous and slightly yellow, and contains white blood cells and other cells commonly seen in inflammations. In certain conditions there may be crystals within the fluid.

Back to top


Will fluid on the knee go away on its own?

This is an active system, and fluid can also be resorbed back into the blood. If there is something in the joint triggering inflammation or irritation, then the rate of secretion will exceed the rate at which the fluid is resorbed, and that determines how persistent is the effusion. If the effusion persists, is painful and limits movement, then it can be drained (aspirated).

Back to top


Pros and Cons of draining a swollen knee

Draining and effusion with a wide-bore needle and syringe is usually an easy matter, and the patient may gain instant relief of discomfort. But there is always the risk of introducing infection. The aspirated fluid can be sent to the laboratory for analysis.

Back to top


Water on the knee

Enclosing the articulating part of the knee joint is a water-tight capsule made of a strong fibrous material lined on the inside with a thin layer of secretory cells.

A simple explanation of the term 'water on the knee'.


Dr Sheila StroverĀ (Editor)
BSc (Hons), MB BCh, MBA

See biography...

Relevant material -

eBook - Common Knee Symptoms

- by Dr Sheila Strover (Clinical Editor)

eBook that gives easy-to-read explanations of common knee symptoms.