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Knee aspiration: what it is, why it is done and what to expect
First published February 2018 · Updated October 2026
Knee aspiration (arthrocentesis) is a short in-office procedure in which a doctor uses a sterile needle and syringe to drain extra fluid from a swollen knee, after numbing the skin. Removing the fluid can ease pressure and pain and make it easier to bend the knee. The fluid can also be sent to a lab to look for causes such as infection or gout. A corticosteroid is sometimes injected at the same visit. Most people can go back to light activity soon after, avoiding strenuous activity for a few days.
A knee can swell after an injury, with arthritis and other joint conditions, or with an illness that affects the joints. The swelling, often called water on the knee (knee effusion), is extra fluid in or around the joint. Common causes are an acute injury, arthritis, other joint disorders and infection. When the knee is tight with fluid, or the cause is unclear, a physician may recommend knee aspiration, also called arthrocentesis, to drain it.
In this video, Ashley Marcus, MD, drains a large amount of fluid from the knee of a snowboarder who had completely torn his ACL and was doing physical therapy while he waited for surgery. She explains that the fluid in an inflamed knee "is full of inflammatory substances," which is part of why the knee hurts, and that the fluid also keeps him from bending it. Draining it should ease his pain and help his therapy, because a knee that is not moved can lose motion and the muscles can weaken before surgery (see pre- and post-surgical rehab). The same applies to arthritis, she says: removing the fluid "can lead to relief even within an hour of the procedure."
What is knee aspiration?
Knee aspiration is a procedure in which a sterile needle and syringe are used to drain excess fluid from the knee joint. It is usually done under local anesthetic. Draining enough fluid can ease pain and swelling and bring some relief. The fluid can also be sent for synovial fluid analysis, a lab test that helps find the cause of the swelling. Our physicians can use ultrasound imaging to see the fluid and guide the needle.
When is knee aspiration done?
Knee aspiration is done for two reasons: to find out what is causing the swelling (diagnosis) and to treat it.
To help find the cause
- To evaluate a swollen knee (joint effusion)
- To look for signs of a fracture inside the joint
- To check unexplained swelling and pain in a single joint
- To check for a possible joint infection (septic arthritis)
- To help identify the type of arthritis or other joint condition
- To look for crystals, as in gout
- To study the fluid itself in a lab
To treat the knee
- Relieving a tense, swollen knee. When the swelling is large or painful, draining the fluid relieves pressure and pain and makes it easier to move the knee.
- Draining infected fluid. When a joint infection is known, draining the fluid can bring relief, improve movement and help protect the joint.
- Injecting medication. Aspiration is sometimes followed by an injection into the joint at the same visit.
How is knee aspiration done?
- The physician examines the swollen knee and chooses the best spot for the needle.
- The skin is cleaned with an antiseptic, and a local anesthetic numbs the area.
- The needle is placed into the joint space, and the fluid is drawn out into the syringe.
- If needed, medication is injected through the same needle before it is removed.
- A small bandage goes on, and the knee may be wrapped for compression.
Recovery after knee aspiration
In most cases, removing the extra fluid eases swelling and pain and makes it easier to move the knee. Medication and compression can help keep the fluid from coming back. How long full relief takes, from a few weeks to a few months, depends on what caused the swelling, and some injuries, such as a torn ligament, may still need a repair. Follow your doctor's advice about rest and activity, and avoid strenuous activity for a few days after the procedure.
Call your doctor if you notice:
- Fever
- Redness, swelling, bleeding or other drainage where the needle went in
- Pain around the site that gets worse instead of better
Your doctor may give you other instructions, depending on your situation.
What is knee aspiration and injection?
Sometimes medication is injected into the joint right after the fluid is drained. The most common reason is a corticosteroid (cortisone) injection for knee osteoarthritis, done as an office procedure. A steroid injection can bring real relief, especially during a flare-up of a long-term joint condition that causes a large amount of swelling. Some people have a short flare of soreness after the injection, which usually settles. Doctors limit how often steroid is injected into the same joint. Read more on our joint injections page, or watch our clinicians drain and inject swollen knees in how to relieve knee pain and swelling.
When to see a sports medicine doctor for knee pain
See a sports medicine physician about your knee pain if you notice any of these warning signs:
- Pain deep in the knee that lasts more than a day or two. The inside of the knee is not covered by muscle, so pain deep in the joint rarely comes from a muscle. It can come from an injury to the meniscus, the cartilage cushion between the thigh bone and shin bone that absorbs shock and helps the knee bend and straighten smoothly, or from damage to the cartilage lining the joint.
- Swelling that lasts more than a day or two. Swelling inside a joint often causes pain and stiffness, and the knee may click as tendons move over the swollen area. Sometimes the knee feels swollen even when it looks normal.
- Loss of motion. If you cannot fully bend or straighten the knee for more than a day, it can mean significant swelling inside the joint or another injury.
- Instability. A knee that feels like it will give out or collapse may have a ligament injury.
Related care at Precision Care Medical & Sports Performance
Watch and read more
Where to get a knee aspiration near you
Knee evaluations and joint injections are offered at our Brooklyn, Staten Island and Midtown Manhattan offices. Call ahead to confirm which office is best for a knee aspiration. You can request an appointment online or call the office closest to you. An online request is not a confirmed booking until the office contacts you.
If you are looking for knee aspiration near me, our Brooklyn office serves patients from Coney Island, Brighton Beach and Bay Ridge, our Staten Island office serves patients from New Dorp, Grant City, Todt Hill and Great Kills, and our Midtown Manhattan office serves Midtown East. A sports medicine doctor examines the knee first; call ahead to confirm which office is best for the procedure, or request an appointment.
150 E 55th St, Suite 401, New York, NY 10022
(212) 840-0135
Questions patients ask
What is knee aspiration?
Knee aspiration, also called arthrocentesis, is a procedure in which a doctor uses a sterile needle and syringe to drain extra fluid from a swollen knee. It is done in the office, usually after numbing the skin with a local anesthetic.
Why would you need fluid drained from your knee?
To relieve a tense, painful knee that is hard to bend, and to find the cause of the swelling. The fluid can be tested in a lab for signs of infection, gout and other joint conditions.
Is knee aspiration painful?
The skin is numbed with a local anesthetic first, so most people feel pressure more than pain. Many people feel relief once the pressure of the fluid is gone. Some soreness for a day or two afterward is common.
What is the recovery after knee aspiration?
Most people can go back to light activity soon after the procedure and should avoid strenuous activity for a few days. Call your doctor if you develop a fever, redness, swelling or drainage at the needle site, or pain that gets worse.
Can the fluid come back after knee aspiration?
Yes. If the underlying cause, such as arthritis or an injury, is still there, the knee can fill with fluid again. Treating the cause, with medication, compression, physical therapy or an injection when appropriate, helps keep it from coming back.
Who performs knee aspiration at Precision Care Medical & Sports Performance?
Knee aspiration is done in the office by one of our physicians, who also treat sports injuries and knee arthritis. The skin is usually numbed first with a local anesthetic, and ultrasound can be used to see the fluid and guide the needle.
Is knee aspiration a non-surgical treatment?
Yes. It is a needle procedure done in the office, not surgery, and it is one part of non-surgical orthopedic care for a swollen knee. Draining fluid can ease pressure and pain, and the fluid can be sent to a lab to help find the cause of the swelling.
Educational only, not medical advice. These videos and this article explain common treatments in general terms. They are not a diagnosis or a treatment plan for you. Results vary from person to person. See a qualified clinician about your own symptoms, and call 911 or go to an emergency room for an emergency.
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