[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [Procedures](https://durgical.com/en/clinical-skills/areas/procedures/) › Arthrocentesis of the knee

# 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

## Key points

- A single hot, swollen joint is septic until proven otherwise: aspirate before starting antibiotics whenever this does not delay treatment.
- Cellulitis or another skin infection over the puncture site is the main contraindication: going through it can seed the joint.
- Anticoagulation and a prosthetic joint are relative contraindications: weigh the risk; prosthetic joints are aspirated by, or with, orthopaedics.
- Send fluid for cell count, Gram stain, culture and crystal microscopy; inoculating blood-culture bottles raises the culture yield.
- Never inject a steroid into a joint that might be infected.

On this page

- The basics Key points
- What it is
- Indications
- Equipment
- Step by step (and why)
- Knee approaches for arthrocentesis (choose the side where the effusion bulges most).
- Next step
- Elsewhere in DurgiPedia

- Clinical Complications

- Safety Contraindications and precautions

- Study Test yourself

- Connections Connections

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-skills/arthrocentesis-of-the-knee/)

## Elsewhere in DurgiPedia

- [Knee effusion: patellar tap and bulge sign](https://durgical.com/en/clinical-exam/knee-effusion-patellar-tap-and-bulge-sign/)
- [Joint examination: inspection, palpation and range of motion](https://durgical.com/en/clinical-exam/joint-examination-inspection-palpation-and-range-of-motion/)
- [Infectious arthritis](https://durgical.com/en/general-pathology/infectious-arthritis/)
- [Gout](https://durgical.com/en/general-pathology/gout/)
- [Pseudogout (CPPD disease)](https://durgical.com/en/general-pathology/pseudogout/)
- [Synovial joints: capsule, synovial membrane and articular cartilage](https://durgical.com/en/histology/synovial-joints-capsule-synovial-membrane-and-articular-cartilage/)
- [Clean and sterile technique: medical and surgical asepsis](https://durgical.com/en/nursing/clean-and-sterile-technique-medical-and-surgical-asepsis/)

## Connections

## Related 4

- [Synovial fluid: septic, crystal or inflammatory?](https://durgical.com/en/clinical-skills/synovial-fluid-septic-crystal-or-inflammatory/)
- [Infiltrating local anaesthetic and digital blocks](https://durgical.com/en/clinical-skills/infiltrating-local-anaesthetic-and-digital-blocks/)
- [Ultrasound-guided vascular access](https://durgical.com/en/clinical-skills/ultrasound-guided-vascular-access/)
- [CRP, ESR and procalcitonin](https://durgical.com/en/clinical-skills/crp-esr-and-procalcitonin/)

## Related conditions 8

- [Septic Arthritis of the Knee](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/)
- [Gout of the Knee](https://durgical.com/en/conditions/gout-of-the-knee/)
- [Pseudogout (CPPD) of the Knee](https://durgical.com/en/conditions/pseudogout-of-the-knee/)
- [Knee Osteoarthritis](https://durgical.com/en/conditions/knee-osteoarthritis/)
- [Gout](https://durgical.com/en/conditions/gout/)
- [Rheumatoid arthritis](https://durgical.com/en/conditions/rheumatoid-arthritis/)
- [Prepatellar Bursitis](https://durgical.com/en/conditions/prepatellar-bursitis/)
- [Haemophilia A and B](https://durgical.com/en/conditions/haemophilia-a-and-b/)

## In Pharmacology 3

- [Local anesthetics](https://durgical.com/en/pharmacology/classes/local-anesthetics/)
- [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/)
- [Chlorhexidine](https://durgical.com/en/pharmacology/chlorhexidine/)

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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](https://www.ncbi.nlm.nih.gov/books/NBK557805/) (StatPearls (NCBI Bookshelf), 2023); [Ultrasound-Guided Arthrocentesis](https://www.ncbi.nlm.nih.gov/books/NBK573084/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/clinical-skills/arthrocentesis-of-the-knee/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/arthrocentesis-of-the-knee/