Tap a Knee

After a traumatic injury, blood may collect in the sac behind the knee cap, causing swelling, pain, and immobility. This collection of blood should be removed.

Prepare the skin with an antiseptic solution.

Feel the knee cap (patella). The point of insertion of the needle will be just underneath the knee cap at the top (anterior) lateral (to the side) margin.

Inject a small amount of local anesthetic at the injection site, or place a topical anesthetic agent in that area.

Use a large bore needle (like an 18 gauge) attached to a syringe.

Insert the needle, directing the tip to a point just below the center of the knee cap.

You will feel a "pop" as you enter the sac. Aspirate as much blood as you can. If you fill one syringe, grasp the needle hub with a hemostat, untwist the syringe, empty it, re-attach it to the needle, and aspirate more. 40-50 cc of blood is not unusual. As the blood is aspirated, the patient will feel some relief.

When no more blood can be aspirated, grasp the needle hub with a hemostat (for stability) and unscrew the syringe. Replace it with a 5cc syringe filled with 1% lidocaine. Inject the lidocaine back into the sac. It should flow easily.

The lidocaine will relieve pain, and make the later examination much more comfortable. Move the knee (flexing and extending) to distribute the lidocaine evenly throughout the sac. 

Watch a video demonstrating this technique.

Following a twisting injury, the right knee is markedly swollen and the patient has significant pain. Prep the skin with antiseptic solution. Feel the top (anterior), lateral edge of the knee cap. Using local anesthetic for the skin, insert the large bore needle just underneath the knee cap at the anterior, lateral edge. The needle will enter the fluid or blood filled sac just beneath the knee cap. Aspirate (several times if necessary) until all blood or fluid is removed. Then inject 5 cc of lidocaine back into the sac to provide pain relief and make the followup examination easier and more comfortable for the patient.

Contributed by Scott D. Flinn, MD

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