Tapping the knee

Arthrocentesis can be a very painful procedure for patients—even with a lidocaine injection. So how can we minimize patient discomfort without compromising technique? In this video, we'll cover the anatomy of the knee, the ideal positioning of a patient's leg to avoid discomfort, and the needle approach for knee arthrocentesis. We'll also cover a unique freeze spray technique that numbs the area and eliminates the need for a painful lidocaine injection.

In this course, you will learn a variety of procedures used in emergency medicine. These procedures will help you to diagnose and treat common infections, injuries, and other problems, such as joint infections, dislocations, soft tissue abscesses, and priapism. You’ll also cover the ultimate airway rescue procedure—the cricothyrotomy. This masterclass is a companion to the Emergency Procedures Masterclass (Part 1).

In this lesson, we'll cover the anatomy of the knee, ideal positioning of the patient's leg, and the needle approach for the knee arthrocentesis. At the end of the lesson, I'll also review my "freezy" spray technique, which makes the procedure faster by eliminating the lidocaine step. Let's start with the anatomy. The knee joint is located between the tibia and the femur and is behind the patella.

The joint space will bulge and feel squishy when it accumulates fluid. Now let's look at how to properly position the patient. Have your patient lay down with their hip in slight external rotation and their knee in 15 degrees of flexion. A towel roll under the knee will help you with this. You will stand on the side of the unaffected leg.

I like to use this medial knee approach because it's more comfortable for the patient to externally rotate their hip rather than internal rotation. Now for the procedure - direct your 18 gauge needle one centimeter below the medial portion of the patella and try to keep it parallel to the bed.

Here's an arthrocentesis I performed on a cadaver, direct your 18 gauge needle one centimeter below the medial portion of the patella and try to keep it parallel to the bed and aspirate as soon as your needle enters the skin. Advance the needle until you get synovial fluid into your syringe. Next, take the needle out and apply a bandage. Now for the "freezy" spray pearl I promised.

I personally think that injecting lidocaine in the skin is painful for the patient and an unnecessary additional step. So I like to use ethyl chloride spray instead. The spray freezes the skin and the nerves don't feel the pain of the needle. You'll need someone else to help you with this. It can be the patient's family or literally anyone standing around.

As the spray is happening, stick your 18 gauge needle directly into the joint. As soon as your needle bevel is inside the skin start aspirating, you'll see the synovial fluid enter your syringe immediately. In this video, I'm using a large 30cc syringe and I'm not using a sterile field or sterile gloves because I've already marked my landmark and only the sterile needle is touching the skin.

Sometimes you'll need multiple syringes if there's a huge effusion - and for that scenario, you'll need to have a sterile glove to hold the needle in place while you switch syringes. Congratulations, you have successfully completed a knee arthrocentesis.