Knee Aspiration: Why and How Fluid Is Drained From a Swollen Knee

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A knee aspiration is a short office procedure that removes fluid from inside the knee joint with a needle and syringe. We use it to take pressure off of a swollen knee and, in the same few minutes, to find out what is causing the swelling. After more than 20 years and over 700 joint replacements a year, I can tell you the fluid often answers the question faster than a scan.

Here is what I tell my own patients: the fluid is just a message and diagnosing it correctly changes the plan.

What is a knee aspiration?

Knee aspiration is the removal of joint fluid through a hollow needle placed into the knee joint. The medical term is arthrocentesis: arthro means joint and centesis means to puncture with a needle in order to draw fluid.

Every healthy knee holds a small amount of synovial fluid, the slippery liquid that lubricates cartilage, typically only a teaspoon or two.

When a knee is injured or inflamed, the joint lining makes far more. A badly swollen knee can fill a 60 milliliter syringe, sometimes twice. Patients feel that as tightness, fullness, and a knee that will not fully straighten.

Why is a knee aspiration done?

A knee aspiration is done for two main reasons: to find out what is inside the joint, and to relieve the pressure that fluid creates. Most of the time it does both in a single visit.

What the fluid tells us

Sending the sample to the laboratory answers questions a simple knee X-ray cannot. In my office, the four findings that change management most often are:

• Infection. A high white blood cell count and a positive culture point to septic arthritis, which is an emergency and needs urgent surgical washout.
• Crystals. Uric acid crystals confirm gout; calcium pyrophosphate crystals confirm pseudogout. Both are treated with medication, not surgery.
• Blood. Blood in the joint (hemarthrosis) after a twisting injury suggests a torn ligament, a cartilage tear, or a fracture.
• Inflammation. The cell count helps separate ordinary wear-and-tear arthritis from rheumatoid and other inflammatory types.

If you want the fuller picture of why knees fill with fluid in the first place, our guide to the most common causes of knee swelling covers each one in turn.

How draining the fluid helps you feel better

Pressure inside the joint capsule is a real source of pain. The capsule is a tough sleeve that does not stretch quickly, so a fast build-up of fluid hurts more than the size of the swelling suggests.

A full knee also shuts the quadriceps down. The muscle senses the pressure and stops firing well, which is why a swollen knee feels unsteady on stairs. Taking the fluid out restores motion and lets physical therapy work.

How is a knee aspiration performed?

A knee aspiration takes about 10 minutes from start to finish, and no preparation is needed. Here is the sequence:

1. You lie back with the knee either straight or resting over a small towel roll.
2. I feel for the landmarks around the kneecap and mark the entry point, usually a soft spot just above and to the outer side of the patella.
3. The skin is cleaned with an antiseptic solution and given time to dry.
4. A small amount of local anesthetic numbs the skin and the tissue underneath.
5. A larger needle on a syringe passes into the joint, and the fluid is drawn back slowly.
6. If a cortisone injection is planned, the medication goes in through that same needle before it comes out.
7. A simple adhesive bandage goes over the site. You walk out.

Ultrasound guidance helps when the swelling is small or hard to feel. For a large, obvious effusion, landmarks are reliable and imaging adds little.

Does a knee aspiration hurt?

Most patients describe a pinch, then pressure, then relief. The numbing medicine stings for a few seconds; for most people that is the worst of it.

The relief surprises them. As the syringe fills, the tightness eases, and many patients bend the knee further before they get off the table.

What does the color of the knee fluid mean?

Color and thickness give an immediate answer while the laboratory work is pending. What I look for in the syringe:

• Clear pale yellow, thick and slippery. Normal or mildly irritated joint fluid, typical of osteoarthritis.
• Cloudy or thin yellow. Active inflammation, often gout or an inflammatory arthritis.
• Thick, opaque, or pus-like. Possible infection. This one goes to the laboratory immediately.
• Bright red blood. Bleeding inside the joint, usually from a recent injury such as a ligament tear or fracture.
• Blood with shiny fat droplets floating in it. Strongly suggests a fracture involving the joint surface.

Bloody fluid in a young athlete sends me looking for structural damage, often with an MRI. Straw-colored fluid after years of aching points back to knee arthritis, which is where a standing knee X-ray and its 0 to 4 grading tells the rest of the story.

How long does relief last after a knee aspiration?

Relief typically starts within a day and can last anywhere from a few days to several months, depending entirely on what caused the swelling. Aspiration treats the fluid, not the reason the fluid formed.

A randomized study of 167 patients with sudden knee effusion found real improvement in pain, motion, and swelling after aspiration, but the advantage had largely faded by the end of the first week because fluid re-accumulated. That matches what I see in clinic.

When the cause is gout and the gout gets treated, the fluid usually stays gone. When the cause is worn cartilage that is still worn, the knee refills.

This is why I rarely stop at the aspiration. Pairing it with a cortisone injection, therapy, and the rest of a nonsurgical joint pain treatment plan buys far more time than draining alone.

What are the risks of a knee aspiration?

Serious complications are uncommon. The risks worth knowing are these:

• Infection. The most important risk and a rare one when sterile technique is used. This is the reason we never rush the skin prep.
• Bruising or bleeding at the site. Common, minor, and self-limited.
• Soreness for a day or two. Expected, and ice handles it.
• A post-injection flare. If cortisone is given, roughly 1 in 50 people get a temporary flare of pain and swelling several hours later that settles within a few days.
• A dry tap. Sometimes little or no fluid comes back, usually because the swelling sits outside the joint rather than inside it.

Blood thinners are a common worry and rarely a barrier. Routine anticoagulation is not by itself a reason to avoid aspiration, though I always want the full medication list first.

When should you call after a knee aspiration?

Call the office the same day if you notice any of the following after a knee aspiration:

• Fever of 100.4 degrees Fahrenheit or higher, or chills
• Pain that is worse 24 hours later rather than better
• Spreading redness or heat around the knee
• Drainage or bleeding from the needle site

These are uncommon, and they need to be seen quickly rather than watched over a weekend.

Can fluid be drained from a knee replacement?

Yes, and it should be done by an orthopedic surgeon rather than in an urgent care setting. A swollen knee replacement raises the question of infection around the implant, and fluid analysis is the most useful test we have for answering it.

The technique is the same, but the standard for sterility is higher. Bacteria introduced into an artificial joint cause a far bigger problem than in a natural one. If your knee replacement swells, feels warm, or starts hurting after a period of doing well, call rather than wait.

What if the fluid keeps coming back?

Repeated swelling means the underlying diagnosis is still active. Draining the same knee over and over is not a treatment plan.

Depending on the cause, the next steps usually include a cortisone injection, a course of hyaluronic acid gel injections, physical therapy, weight management, or medication for gout or inflammatory arthritis.

If the swelling follows a twist or a catching sensation, we look at the cartilage cushions, and our guide to how a meniscus tear is diagnosed explains that pathway. If the X-ray shows bone rubbing on bone and the knee refills every few weeks despite good care, joint replacement enters the discussion.

Talk with a knee specialist in Franklin, Tennessee

A knee that swells repeatedly is telling you something, and it is worth listening to early. Our office sees patients from Franklin, Nashville, Columbia, Spring Hill, and across Middle Tennessee.

You can reach the Bone and Joint Institute of Tennessee at 3000 Edward Curd Lane, Franklin, TN 37067, or call (615) 791-2630. To book directly, use the online scheduling page.

This article is for education only and does not replace a medical evaluation. Treatment decisions should be made with a qualified orthopaedic provider who has examined you. Dr. Cory Calendine, MD is a board-certified orthopedic surgeon at the Bone and Joint Institute of Tennessee in Franklin, Tennessee.

References

1. Paschos NK, Giotis D, Abuhemoud K, Georgoulis AD. Effectiveness of aspiration in knee joint effusion management: a prospective randomized controlled study. Knee Surg Sports Traumatol Arthrosc. 2014;22(1):226-232. PubMed
2. Villa-Forte A. How to do knee arthrocentesis. Merck Manual Professional Edition. Reviewed June 2025. View source
3. Tantillo TJ, Katsigiorgis G. Arthrocentesis. In: StatPearls. Treasure Island, FL: StatPearls Publishing; updated 2023. View source
4. Yui JC, Preskill C, Greenlund LS. Arthrocentesis and joint injection in patients receiving direct oral anticoagulants. Mayo Clin Proc. 2017;92(8):1223-1226. DOI
5. Tarar MY, Malik RA, Charalambous CP. Bleeding complications in patients on warfarin undergoing joint injection or aspiration: systematic review and meta-analysis. Rheumatol Int. 2023;43(2):245-251. DOI
6. Cleveland Clinic. Arthrocentesis (joint aspiration). Reviewed December 2024. View source