Fluid accumulation in the knee joint, commonly known as “water on the knee” or knee effusion, signals an underlying issue causing the joint space to overfill. This excess fluid is medically addressed through joint aspiration, or arthrocentesis, which involves using a needle and syringe to withdraw the fluid. Only a qualified healthcare provider, such as an orthopedic surgeon or rheumatologist, should perform this procedure. Attempting to drain a swollen knee at home is extremely dangerous due to the high risk of introducing infection directly into the joint.
The Need for Drainage
Doctors recommend knee fluid drainage for two primary reasons: immediate relief and definitive diagnosis. Therapeutically, removing excess fluid alleviates intense pressure and pain caused by a large effusion. This is often the quickest way to reduce swelling, significantly improving a patient’s comfort and range of motion.
The diagnostic purpose is equally important, as analyzing the fluid determines the root problem. The fluid’s color, clarity, and composition indicate the source of swelling. Common causes requiring this step include acute injury causing bleeding (hemarthrosis), infectious arthritis, or crystal-induced arthritis, such as gout.
In cases where septic arthritis, a joint infection, is suspected, immediate aspiration is a medical necessity to identify the bacteria and begin appropriate treatment, preventing rapid and permanent joint destruction. The fluid analysis guides the subsequent long-term treatment plan. Even if a large effusion is not painful, a doctor may still aspirate the joint for a sample if the cause remains unclear following physical examination and imaging studies.
Understanding the Procedure
The process of arthrocentesis is performed in an outpatient clinic or doctor’s office, taking only a few minutes to complete. The patient is usually positioned on an examination table with the knee extended or slightly flexed, often with a towel placed underneath for support and to open the joint space slightly. This positioning helps ensure safe access to the joint capsule.
The healthcare professional first thoroughly cleanses the skin around the insertion site with an antiseptic solution like iodine or chlorhexidine to maintain a sterile field. A local anesthetic, such as lidocaine, is then injected into the skin and deeper tissues along the planned needle path to numb the area. This step minimizes patient discomfort during the subsequent aspiration.
A larger, sterile needle, an 18- or 20-gauge, attached to a syringe, is then inserted into the joint space. The needle is commonly inserted using a parapatellar approach, either just lateral or medial to the kneecap (patella), aimed toward the intercondylar notch of the femur. For more challenging aspirations, the physician may use ultrasound guidance to visualize the needle tip entering the fluid pocket accurately.
Once the needle is correctly positioned within the joint, the physician gently pulls back on the syringe plunger to withdraw the accumulated fluid. They may “milk” or gently compress the area around the patella to push the remaining fluid toward the needle tip, maximizing the amount of fluid removed. After all the fluid has been aspirated, the needle is withdrawn, and a sterile dressing is applied to the puncture site.
What Happens After the Fluid is Removed
Immediately after the procedure, the needle site is covered with a small bandage, and the physician may apply pressure to control any minor bleeding. Patients are often advised to rest the knee and limit strenuous activity for the next 24 hours to prevent re-swelling. Applying ice to the joint can also help reduce residual swelling and soreness, which is common for a day or two as the local anesthetic wears off.
The aspirated fluid is quickly transferred into specialized containers and sent to a laboratory for synovial fluid analysis. The lab performs a number of tests, including a white blood cell (WBC) count, which helps differentiate inflammatory conditions from non-inflammatory ones, and a Gram stain and culture to check for the presence of bacteria indicating an infection.
The lab also examines the fluid under a microscope for specific crystals, which indicate gout (monosodium urate crystals) or pseudogout (calcium pyrophosphate crystals). This analysis provides the necessary information to confirm the diagnosis and initiate a targeted, long-term treatment plan. Complications from arthrocentesis are rare but include localized bruising, temporary pain, or infection if sterile technique is compromised.