How to perform arthrocentesis of the knee

Does your patient need a knee arthrocentesis? Read about how to ensure a quick and painless knee tap here!
Last update19th Nov 2020

Anatomy of the knee

The knee joint is located behind the patella between the tibia and the femur. The joint space will bulge and feel squishy when it accumulates fluid. A knee tap will help determine what kind of fluid is present.

Figure 1. Anatomy of a knee joint.

Ideal positioning for arthrocentesis of the knee

For a knee arthrocentesis, or knee tap, you need to properly position your patient:

  • Patient is laying down
  • Hip is in a bit of external rotation
  • Affected knee is bent at 15°
  • Rolled towel placed under the knee to help with positioning
Figure 2. Proper positioning of a knee for arthrocentesis. The patient is laying down with hip in a bit of external rotation, affected knee bent at 15°, and a rolled towel placed under knee.

Once the patient is positioned, you will stand on the side of the unaffected leg to perform the arthrocentesis procedure on the affected knee.

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How to perform arthrocentesis of the knee

The arthrocentesis procedure requires seven simple steps to ensure a safe, clean, and painless knee tap:

  1. Direct the 18 gauge (G) needle 1 cm below the medial portion of the patella.
  2. Keep the needle parallel to the bed.
  3. Aspirate as soon as your needle enters the skin.
  4. Advance the needle until you get fluid into your syringe.
  5. Extract the joint fluid.
  6. Take out the needle out and apply a bandage.
Figure 3. Performing a knee arthrocentesis. Using an 18 gauge (G) needle, keep the needle parallel with the bed and push the needle 1 cm below the patella. Aspirate and advance the needle until fluid fills the syringe. Extract the joint fluid, take out the needle, and apply a bandage.

The freeze-spray tip

Injecting lidocaine in the skin is painful for patients, and an unnecessary additional step. You can use ethyl chloride spray instead, which freezes the skin so that the nerves do not feel the pain of the needle.

You’ll need an assistant to help you with this—it can even be a member of the patient’s family! Get your assistant to spray the marked spot, and as they are spraying, stick your needle directly into the joint.

Congratulations, you now know the basic procedure for a knee arthrocentesis!

That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

Recommended reading

  • Roberts, J. 2019. “Arthrocentesis”. In: Roberts and Hedges’ Clinical Procedures in Emergency Medicine and Acute Care. 7th edition. Philadelphia: Elsevier.  

About the author

Siamak Moayedi, MD
Professor and Director of Medical Student Education, University of Maryland and Course Director, Essential and Critical Procedures, Emergency Medicine.
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ACCME accredited, UEMS accredited, Comenius EduMedia Siegel 2017, BMA Highly recommended