Arthritis, infection, and joint injuries are associated with inflammation and swelling. This swelling occurs when excess fluid collects inside a joint capsule. Your physician may remove the excess fluid using a needle and syringe. This procedure is called joint aspiration or arthrocentesis.

Joint aspiration involves taking fluid out of a joint, sometimes for diagnostic purposes. See What is ar Arthrocentesis (Joint Aspiration)?

Joint aspiration is generally considered a quick, safe procedure. It typically involves a local anesthetic, so you’ll feel minimal pain and discomfort. It may be done in your doctor’s office or a hospital setting. Medical imaging, such as ultrasound, may be used to ensure that the needle enters the joint capsule, minimizing the potential risk of damage to other tissues.

See The Joint Aspiration Procedure

This page describes a joint’s anatomy and three reasons why your doctor may recommend an aspiration.

Your joint contains fluid

Before having a joint aspiration, it’s helpful to know a bit about the area it will target. A joint capsule is made up of flexible soft tissues that envelope a joint—the area where the two bones meet.

  • Normally, a joint capsule contains just a drop or two of joint fluid (synovial fluid).
  • A joint that is inflamed by arthritis may contain several times the normal amount of fluid.
  • The joint capsule may also contain blood after an acute injury or pus during an infection (septic arthritis).

Depending on the joint and your anatomy, a joint may contain excess fluid but not appear swollen.

See How Do Synovial Joints Work?


Joint aspiration may be recommended because:

1. You need an accurate diagnosis

The fluid inside the affected joint can be examined, providing important clues to help confirm or rule out a diagnosis. For example, microscopic crystals in joint fluid indicate gout or pseudogout.

A doctor can send the aspirated fluid to a lab. The lab may be asked to:

  • Examine the fluid under a microscope
  • Run a chemical analysis
  • Perform a culture and gram stain

A doctor may also learn things just by looking at the aspirated fluid—for example, pink or red fluid indicates the presence of blood and suggests an injury, and opaque, yellowish fluid suggests septic arthritis.

See Diagnosis through Synovial Fluid Analysis

2. Your swollen joint is stiff and painful

Arthritis and other joint problems can lead to swelling that causes or worsens pain and stiffness. You may notice that it’s difficult to bend or completely straighten the swollen joint. Removing some of the joint fluid can relieve pressure and ease your symptoms.

It’s common for swelling to return after a joint aspiration, 1 Chiodo CP, Logan C, Blauwet CA. Aspiration and Injection Techniques of the Lower Extremity. J Am Acad Orthop Surg. 2018 Aug 01;26(15):e313-e320. As cited in Seidman AJ, Limaiem F. Synovial Fluid Analysis. In: StatPearls. Treasure Island (FL): StatPearls Publishing; January 22, 2019. Accessed October 14, 2020. and a joint may be aspirated more than once.

See Understanding Joint Pain


3. You’re getting an injection

A doctor may recommend injecting an arthritic joint with cortisone, hyaluronic acid, or another therapeutic liquid into a joint capsule as part of treatment. A joint aspiration may be performed before the injection. Aspirating excess fluid makes room for the therapeutic liquid and helps ensure that it is highly concentrated in the joint.

Joint aspirations can be done on almost any joint, most often the knee joint. Many people recover right away, while others may need to rest the affected joint for a couple of hours.

See Therapeutic Injections for Knee Arthritis

For most people, joint aspiration is a safe and uneventful procedure. Contact your doctor if you experience moderate to extreme re-swelling, discoloration or rash around the injection area, fever, bleeding or discharge from the needle’s entry point, or pain that is not adequately controlled with over-the-counter pain medication and ice packs.

Learn more:

Arthrocentesis Recovery and Potential Risks

When to Consider Therapeutic Injections for Knee Arthritis Pain

Gabriella Ode, MD, is an orthopedic surgeon specializing in knee, shoulder, and elbow surgery. After completing residency training at Atrium Health/OrthoCarolina in Charlotte, North Carolina, she completed a fellowship in sports medicine and shoulder surgery at Hospital for Special Surgery in New York City.