“Knee joint aspiration” is a technique doctors use to remove fluid from the knee. It’s a fairly common procedure, and very quick to perform. And there are a few different reasons why you might have one. Here’s what you need to know.
What is knee aspiration?
In simple terms, it involves inserting a needle into the knee to withdraw some of the fluid that has collected underneath the joint lining. This fluid is a mixture of blood cells and lubricating chemicals. Its purpose is to protect the joint, and the free movement of your joint, as you go about your day-to-day tasks.
Why would I need an aspiration?
There are two main reasons:
1. Diagnosis – because we want to take a sample that will tell us more about an underlying condition
2. Therapy / treatment – because your knee is swollen and we want to relieve pressure there
How do you use it for diagnosing?
For diagnosis, the first thing we want to rule out is an infection. This often leaves the knee feeling hot, red and painful, and you might be feeling ill too; it’s very important to treat an infected knee quickly, because infections can be dangerous. After that, we’ll be looking for evidence of a condition that’s causing you pain or mobility problems. This could be chronic (an ongoing problem) or acute (a sudden flare-up or injury). The most common examples are inflammatory conditions like wear-and-tear arthritis, rheumatoid arthritis or gout.
What do you do with the fluid once you’ve removed it?
Once removed (see below), we send it to the lab(s) for analysis. The microbiology experts can use it to test for infections – one common example is a Gram stain, which swiftly detects certain types of bacteria. This service is available 24/7, so we can get you treated very quickly if we need to. Another type of test looks for the presence of uric acid crystals, which is a marker of gout. We may also look to do blood tests (for “inflammatory markers”, for example), x-rays, or other scans, in addition to the aspiration.
How would you use it for therapy on my knee?
When a knee swells up, it’s usually because the body is trying to protect the joint by producing more of that lubricating fluid. But the volume of new fluid can be quite substantial – even as much as 180-200ml, which can cause the knee to swell up dramatically. Aside from mobility problems, this puts pressure on the joint – which can be painful. Here the aspiration aims to drain off some of the fluid (perhaps as much as 120-130ml) to relieve pressure within the joint. While this often relieves the pain, it’s important to remember that the procedure itself isn’t curing the problem. We still need to know what’s causing the swelling so we can treat that underlying condition separately.
How do you perform the aspiration?
You can have an aspiration in clinic, in A&E or, occasionally, in theatre. The target area is usually just above and to the side of the kneecap. We’ll swab the area with antiseptic. Then we’ll follow that with a local anaesthetic if you’re having one (a pinprick with a needle). We then insert the aspiration needle, through the skin and down through the joint lining. The fluid is drawn off into a syringe – or several syringes, if you’re having a pressure-relieving procedure. It takes about 30 seconds to draw fluid for a diagnostic sample and 2-3 minutes for the therapeutic treatment. Once complete, we’ll put a small bandage over the injection site.
Does it hurt?
You may feel a little discomfort, or mild pain, when the needle is first applied. There may also be a feeling of pressure on the joint. Withdrawing the fluid usually doesn’t cause discomfort. But, as mentioned, we can numb the area using a local anaesthetic if we need to.
What happens afterwards?
You can usually go straight home after the aspiration. You may want to take some simple painkillers, if it feels uncomfortable. You’ll also want to keep the site of the injection clean and dry until the bandage is removed (we’ll let you know when you can do that). If we’re running tests on the fluid, you’ll hear back from us once we have the results.
Does swelling-related pain disappear completely after an aspiration?
If your aspiration was aimed at relieving fluid build-up (rather than diagnosis), we’d normally hope to see a reduction in pain almost immediately. This doesn’t always happen, though, so it may be necessary to find alternative ways to manage your problem – things like steroid injections, a regular icing regime, or help from a rheumatology expert. It’s also worth noting that the fluid may build up again over the coming weeks and months. Again, the key thing is to identify the underlying problem in your knee, so we can arrange the best possible ongoing treatment for you. In the short term, though, an aspiration can be very useful in reducing the unpleasant and painful effects of excessive fluid on the knee.
Interested in finding out more?
Read our Starter Guide to Swollen Knees
Find out more about how your knee works in the Beginner’s Guide to the Knee
Learn more about wear-and-tear arthritis in The 60-Second Guide to Osteoarthritis
Are you getting persistent swelling in your knee? If so, it’s important to have it properly investigated. You can do that here in the clinic by booking an appointment with us. We’ll make sure you’re seen, examined and treated as soon as possible.