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.

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?
• Will fluid on the knee go away on its own?
• Pros and Cons of draining a swollen knee
• Forum discussions

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.

Peer-reviewed papers

• Quote from peer-reviewed paper:

  Fluid in the knee can be appreciated clinically by "...either visual inspection, or dynamic testing involving (1) movement of fluid across the knee with the presence of effusion denoted by reappearance of fluid distension (bulge sign) and (2) pressure over the patella with the presence of effusion determined by palpable ballottement or tapping of the patella against the femoral condyle (patellar tap), or palpable distension of the underlying joint line by fluid fluctuation (balloon sign)."

  Citation: 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.

End of paper

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Will fluid on the knee go away on its own?

The fluid inside the knee capsule 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).

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Pros and Cons of draining a swollen knee

Draining and effusion with a wide-bore needle and syringe ('aspiration' or 'arthrocentesis') 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.

Peer-reviewed papers

• Quote from peer-reviewed paper:

  "Knee joint aspiration is often an outpatient procedure. ....To avoid complications, the clinician should be familiar with the anatomy. The risk of complications can be minimized by using an appropriate technique."

  Citation: Akbarnia H, Saber AY, Zahn E. Knee Arthrocentesis. [Updated 2022 Aug 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470229/

End of paper Quick links

• Haemarthrosis

• Aspiration

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Forum discussions

• Advice on MRI results

  Patient wanting clarification of MRI report.

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