Arthrocentesis of the knee

Procedure

A needle is passed into the swollen knee to draw off joint fluid, both to find the cause of the swelling and to ease pain.

Updated Nov 13, 2026

What it is

Knee arthrocentesis is the sterile aspiration of synovial fluid from the knee joint. The knee is the joint most often aspirated because it is large, superficial and easy to reach, and its suprapatellar pouch holds most of an effusion. The main reason to do it is diagnostic: an acutely hot, swollen joint is septic arthritis until proven otherwise, and only the fluid can confirm infection, crystals (gout, calcium pyrophosphate disease) or blood. It is also therapeutic, relieving a tense effusion or haemarthrosis, and it is the route for intra-articular injections once infection has been excluded. The usual landmark approach is superolateral, with the knee extended and relaxed: the needle enters about 1 cm above and 1 cm lateral to the superolateral corner of the patella and is aimed under the patella toward the intercondylar notch; a medial approach is an alternative. Ultrasound shows the effusion, avoids vessels and improves success, especially with small effusions or obese patients. The sample goes for cell count, Gram stain, culture and crystal microscopy; how to read the results is covered on the synovial fluid page.

Indications

Diagnostic: suspected septic arthritis, suspected gout or calcium pyrophosphate disease, unexplained effusion or monoarthritis.
Diagnostic: suspected haemarthrosis after trauma, where fat droplets in the blood suggest an intra-articular fracture.
Therapeutic: relief of a large, tense effusion or haemarthrosis that limits movement and causes pain.
Therapeutic: injection of corticosteroid or other agents into a joint once infection has been excluded.
Follow-up: serial aspiration of a septic joint to monitor drainage and response to treatment.

Contraindications and precautions

Absolute: cellulitis, an infected wound, a burn or a skin lesion over the entry site; choose another approach or refer.
Relative: coagulopathy or anticoagulation; aspiration with a fine needle is usually safe, but severe bleeding disorders need correction or a specialist.
Relative: prosthetic joint; aseptic risk is high and orthopaedics should be involved before puncture.
Relative: bacteraemia, because a needle could seed an uninfected joint.
Precautions: an uncooperative patient, a very small effusion (use ultrasound), and allergy to antiseptic or local anaesthetic.

Equipment

Sterile gloves, antiseptic solution (chlorhexidine in alcohol or povidone-iodine), sterile drape and gauze.
Local anaesthetic with a fine needle for the skin, and an 18–20 G needle on a large syringe for aspiration.
A spare syringe and a haemostat to change syringes without moving the needle in a large effusion.
Tubes: a sterile tube for Gram stain and culture, an anticoagulated tube for cell count, a tube for crystals, and blood-culture bottles if available.
Ultrasound with a linear probe and sterile cover when available; a small adhesive dressing.

Step by step (and why)

Explain, consent and check for allergies and anticoagulants — the patient must know the risks and keep the leg still.
Lay the patient supine with the knee extended and quadriceps relaxed, a towel under the knee — a relaxed muscle opens the space under the patella.
Mark the site 1 cm above and 1 cm lateral to the superolateral patella, or confirm fluid by ultrasound — this avoids cartilage and vessels.
Clean widely with antiseptic, let it dry and use a no-touch technique — skin flora is the main source of iatrogenic infection.
Infiltrate local anaesthetic down toward the capsule — the capsule is the painful layer; anaesthetic can be drawn back to confirm the track.
Advance the needle under the patella toward the notch, aspirating as you go — a flash of fluid shows you are in the joint.
Have an assistant press on the medial suprapatellar pouch — it pushes fluid toward the needle and improves the yield.
If the flow stops, rotate the bevel or withdraw slightly; do not probe blindly — synovium or fat may block the needle and blind probing hurts cartilage.
Withdraw, apply pressure and a dressing, and fill the tubes at once — delays lower cell counts and culture yield.

Knee approaches for arthrocentesis (choose the side where the effusion bulges most).

Approach	Knee position	Entry point and direction	When to use
Superolateral	Extended, relaxed	1 cm above and lateral to the superolateral patella, toward the notch under the patella	First choice for moderate and large effusions
Medial parapatellar	Extended, relaxed	Midpoint of the medial patella, 1 cm medial, under the patella	Effusion bulging medially; lateral skin not usable
Anterior (infrapatellar)	Flexed about 90°, seated	Beside the patellar tendon, below the patella, toward the notch	Patient cannot extend the knee; smaller effusions

Complications

Dry tap from a small effusion, a blocked needle or a wrong angle; ultrasound guidance reduces it.
Iatrogenic septic arthritis, rare with strict aseptic technique.
Bleeding or haemarthrosis, mostly in patients with a bleeding disorder.
Damage to articular cartilage from deep or repeated passes; vasovagal reaction during the procedure.
After a steroid injection: a short-lived painful flare, skin atrophy or depigmentation at the site, and a transient rise in blood glucose in people with diabetes.

Next step

Send the tubes at once for cell count with differential, Gram stain, culture and crystal microscopy, labelled with the joint and the time.
Turbid fluid, fever or a high clinical suspicion: take blood cultures, start empirical antibiotics after the sample and call orthopaedics the same day for washout.
Fat droplets in bloody fluid after trauma point to a fracture: obtain imaging and refer to orthopaedics.
Interpret the cell count and crystals with the synovial fluid page; a positive crystal result does not exclude coexisting infection.
Ask the patient to return with fever, increasing pain, redness or swelling of the knee in the following days.

Test yourself

Think of the answer, then tap the question to see it.

What is the main contraindication to knee arthrocentesis?

Infection of the skin or soft tissue over the entry site, such as cellulitis, because the needle can carry bacteria into the joint; choose another approach or refer.

Where does the needle enter in the superolateral approach to the knee?

About 1 cm above and 1 cm lateral to the superolateral corner of the patella, with the knee extended, aimed under the patella toward the intercondylar notch.

Which tests are requested on synovial fluid from a hot, swollen knee?

Cell count with differential, Gram stain, culture (ideally also in blood-culture bottles) and crystal microscopy under polarised light.

Why is a steroid never injected into a joint that might be infected?

Corticosteroid suppresses local defences and can hide and worsen septic arthritis, which destroys cartilage within days; infection must be excluded first.

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Arthrocentesis (StatPearls (NCBI Bookshelf), 2023); Ultrasound-Guided Arthrocentesis (StatPearls (NCBI Bookshelf), 2025).

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