Fluid on the knee — a knee effusion, or “water on the knee” — is a painful build-up of fluid in and around the knee joint. We usually treat it with simple measures such as ice, medication, and a compression wrap. But sometimes we need to drain the fluid, a procedure called a knee aspiration. So, how is knee drainage performed, and when is it appropriate?
Causes of fluid in the knee
The knee is a synovial joint, lined by synovium that produces the synovial fluid, which lubricates and protects the cartilage. When the joint is diseased, it produces excess fluid. Common causes include osteoarthritis, inflammatory arthritis, infection, and traumatic injuries such as meniscal or cruciate ligament tears. Rarer causes include tumours such as pigmented villonodular synovitis (PVNS).
Why do we perform a knee aspiration?
We drain fluid from the knee for two reasons:
- Diagnosis — to determine the cause of the effusion by analysing the fluid.
- Symptom relief — to reduce swelling when simple measures such as ice and ibuprofen haven’t worked. An effusion larger than about 50 ml often causes pain and restricts movement, and draining it eases the symptoms.
Can you diagnose knee problems from the fluid?
Yes. Not all effusions are the same — fluid from a meniscal tear differs from fluid caused by inflammatory arthritis, such as gout. We send the drained fluid to a laboratory to identify the cells present, and microbiologists look for bacteria or crystals that point to the cause. We combine this with X-rays, MRI, and blood tests to confirm the diagnosis.
How is a knee aspiration performed?
We usually perform a knee aspiration in a clean clinic room, targeting the top of the joint just above the kneecap. First, we clean the skin with an antiseptic. Next, we pass a needle through the skin into the joint — using ultrasound to guide it, which improves accuracy and helps us drain the fluid effectively. Finally, we apply a bandage to the puncture site. The whole procedure takes a few minutes.
Where the fluid is due to inflammation or injury, we sometimes inject cortisone, hyaluronic acid, or PRP at the same time. We also advise regular icing and a compression bandage afterwards to help prevent the effusion from returning.
Risks of knee drainage
The procedure is low-risk, especially in a clean room under ultrasound guidance. There’s a small chance the effusion returns or that infection is introduced, and superficial bruising or swelling at the entry site may occur.
Does draining fluid from the knee hurt?
Usually not. You may feel slight discomfort as the needle enters the joint, but removing the fluid itself shouldn’t be painful. Ultrasound improves needle accuracy and reduces discomfort. If we send the fluid for analysis, we’ll discuss the results with you a few days later.
What to do after a knee aspiration
Aftercare matters. Leave the bandage in place until your doctor advises you to remove it and keep the entry site clean. You can return to normal activities, but hold off on sports or longer walks until you’re cleared. If the knee is sore, ice it and take ibuprofen or another NSAID every 4–6 hours until the pain settles.
Frequently asked questions about knee aspiration.n
How can I reduce fluid on the knee without draining it?
Simple measures often help: ice packs for 10 minutes every 4–6 hours, a compression bandage, NSAIDs such as ibuprofen or diclofenac, and a knee brace for walking or sport. Aspiration is reserved for larger or persistent effusions or when the cause needs to be investigated.
Can a knee effusion come back after aspiration?
Yes — fluid can return within days or weeks. The key is identifying and treating the underlying cause, which is what stops it from recurring.
How much fluid is drained from a knee?
It varies with the cause and severity, from a few millilitres to well over 50 ml. Larger effusions (above about 50 ml) are the ones most likely to cause pain and restricted movement, and to benefit from draining.
Is a knee aspiration better under ultrasound?
Yes. Ultrasound accurately guides the needle into the joint, improves the completeness of fluid drainage, and reduces discomfort compared with a blind procedure.
Will I need an injection at the same time?
Sometimes. If the effusion is due to inflammation or arthritis, we may inject cortisone, hyaluronic acid, or PRP after draining — but not always. It depends on the cause.
Final word from Sport Doctor London about knee aspiration
A knee aspiration is an effective way to both diagnose the cause of a knee effusion and relieve the symptoms. See an experienced doctor who performs it under ultrasound, which improves both accuracy and comfort.
To discuss a swollen knee with Dr Masci in London, contact the team here or call +44 (0) 203 488 0350.
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