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What Is Knee Arthrocentesis?

By Jacquelyn Gilchrist
Updated Feb 10, 2024
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Knee arthrocentesis is a medical procedure that involves drawing excess fluid out of the knee joint with a needle. Also called knee joint aspiration, this is usually a straightforward procedure that can be performed on an outpatient basis, and does not require general anesthesia. A doctor may recommend knee arthrocentesis for patients who suffer from knee pain. Drawing out synovial fluid can relieve pressure on the joint, and it also serves as a diagnostic tool to determine the underlying cause of the knee condition.

There are a wide range of possible underlying causes for knee pain, such as arthritis and water on the knee, which generally refers to a buildup of fluid in the joint. Removing the fluid, or aspirating it, reduces the pressure on the joint, and alleviates pain and inflammation. The doctor can also send a sample of the fluid to the laboratory for testing to determine the underlying medical cause.

To begin a knee arthrocentesis, the patient must first remove clothing over the area and lie on the exam table. The doctor may ask the patient to either extend the knee straight outward or bend it at a 90-degree angle. He will sterilize the skin and apply a local anesthetic to numb the area. A needle will be inserted into the knee joint and the doctor will draw fluid into the syringe. In some cases, several syringes will be needed to draw additional fluid out of the joint.

After removing the syringe from the needle, the doctor may use the same, still inserted needle to inject a corticosteroid drug into the joint. This injection is used to alleviate pain and reduce the inflammation. If the procedure is used for diagnostic purposes, the laboratory will run tests on the fluid to check for any underlying medical problems. Patients may resume normal activities following a knee arthrocentesis; however, they must be careful not to exert a great deal of pressure on the joint. They should contact their doctors if they experience a fever, increased swelling, or increased pain.

Knee arthrocentesis is generally considered to be safe for most people, but patients should disclose their other medical conditions as a safety precaution. Those with an infection may need to wait until the infection clears to undergo the procedure. Patients with a skin condition on the knee may also be unable to have a fluid aspiration. It is also important for patients to disclose their medications and any supplements they take, such as blood thinners, aspirin, and steroid drugs.

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