When a joint causes pain, swells, is red, or has limited range of motion, a doctor may recommend using a needle and syringe to remove fluid from the joint. This procedure is called arthrocentesis, or joint aspiration, and it may be done to obtain fluid for diagnostic lab testing, to alleviate pressure and relieve joint pain, or both.
The most commonly aspirated joint is the knee. However, arthrocentesis can be performed on other joints, such as the hip, elbow, wrist, ankle, shoulder and the knuckle of the big toe (the first metatarsophalangeal joint).
Synovial Fluid (Joint Fluid)
The aspirated fluid is called synovial fluid, or joint fluid. It is normally slippery and viscous and can be found in joints and bursae.
Synovial Fluid in Joints
A small amount of synovial fluid is found in most joints. The fluid, along with cartilage, facilitates smooth joint movement and provides cushioning between the joint's bones.
Synovial Fluid in Bursae
Thin, slippery sacs in the joints called bursae contain synovial fluid, and they provide cushioning and reduce friction between a joint's bones and surrounding soft tissue (e.g. muscle and ligaments). When a bursa is inflamed, it is called bursitis, and the bursa may swell with excess fluid.
Depending on the circumstances, a doctor performing arthrocentesis will remove fluid from the joint or from a joint's bursa.
Synovial fluid's composition can change over time and reflects the relative health of joints and bursae.
To aid in determining a diagnosis, a physician may examine the aspirated synovial fluid and send it to a lab for further analysis.
When is Arthrocentesis Performed?
Arthrocentesis is typically done for one of three reasons:
- Additional information is needed to make a diagnosis
- Relieving pressure in the joint will help alleviate pain and discomfort
- Excess fluid needs to be removed before a therapeutic injection is given
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Arthrocentesis for Diagnosis
A doctor may recommend arthrocentesis to confirm or rule out a diagnosis if one or more of the following is suspected:
- An infection in the joint
- A joint disorder such as gout, pseudogout, or rheumatoid arthritis
- Bursitis (either septic or non-septic bursitis)
- A traumatic injury has caused bleeding into the joint space
- Excess fluid is causing pressure and pain in the joint
The synovial fluid that is collected during arthrocentesis may undergo analysis. Information from that analysis can aid in diagnosis and setting a course for treatment.
Arthrocentesis to Reduce Pain
If a diagnosis has already been established, a doctor may aspirate a painfully swollen joint solely to improve patient comfort.
For example, if a patient has a swollen knee from osteoarthritis, and there are no signs of infection or other problems, a doctor might perform arthrocentesis and dispose of the fluid rather than sending it to a lab for analysis.
Arthrocentesis Prior to an Injection
In these cases, a diagnosis has already been made and the excess fluid may or may not be causing the patient discomfort.
Contraindications for Arthrocentesis
A doctor may decide not perform a joint aspiration if the patient has:
- A known source of infection at another site
- A skin infection or wound at the point of injection
- Certain bleeding disorders
A doctor may decide against an in-office arthrocentesis if the joint is particularly difficult to access with a needle (e.g. arthrocentesis of the hip may be difficult in obese patients).
In these cases, the doctor may recommend arthrocentesis in a hospital setting, where real-time X-rays or ultrasound may be used to assist needle placement.
Who Performs Arthrocentesis?
Orthopedic surgeons are doctors who specialize in musculoskeletal problems and can perform arthrocentesis. If the affected joint is an artificial joint (e.g. knee replacement) an orthopedic surgeon should be consulted.
Trained rheumatologists, family doctors, internal medicine doctors, and emergency medicine doctors may also perform the procedure.