An arthrocentesis can be approached in several ways, but how do you know which way to go when there are no obvious landmarks? Use ultrasound to help you! By the end of this lesson, you'll know how to use ultrasound to find the biggest pocket of effusion and what things to avoid when performing ultrasound guidance for arthrocentesis. Get ready to perform your own procedural ultrasounds!
Take our Procedural Ultrasound Masterclass and start using ultrasound to improve the safety of your procedures. Your instructor, Dr Sara Damewood–the Emergency Ultrasound Section Chief and Clinical Ultrasound Fellowship Director at the University of Wisconsin–will guide you through the essentials of procedural ultrasound.
[00:00:00] There are many approaches to doing an ultrasound-guided arthrocentesis. The parapatellar, medial or lateral approach is generally preferred, to avoid other structures. You can also look suprapatellar, from the medial or lateral approach, and also infrapatellar, from the medial and lateral approach. How do you decide where to go when there's no landmarks? You can use ultrasound to find the biggest pocket of fluid. Here, is an example of an effusion from the medial approach.
[00:00:30] The fluid appears hypoechoic and without any edges. It's just filling the space here. Here, is a smaller effusion via a medial approach. If you saw this pocket of fluid, you might look in other places to try and a larger area to drain. Here, is the lateral view on the same patient. There's really not much fluid to drain here either. Let's look at another place. Here, is fluid in the suprapatellar space. See the hypoechoic regions.
[00:01:00] This would be a good area to perform an arthrocentesis. Once you found your pocket, another important consideration is to look for structures, in the path of your needle. What do you see here? It's a superficial vessel. There is one main artifact to know of when scanning the knee, anisotropy. This is the large quadriceps tendon, with a concept of anisotropy. You can see,
[00:01:30] that as the operator scans through the tendon, it tends to look more dark, when the probe is off access. And then the fibers appear bright again, when the probe is directly on access. Anisotropy translates to mean, directionally dependent and as a result of the fibrillar nature of the structure such as tendons and nerves. This quality leads to the ultrasound image depending on the angle between the structure of interest and the ultrasound beam, also known as the angle of insonation. For example, when the angle of insonation is perpendicular to
[00:02:00] a tendon, the tendon will appear bright and normal, like the diagram on the left. However, as the angle varies, the tendon can take on a more hypoechoic appearance, like the diagram on the right. Great. Now, you understand how to find the biggest pocket of fluid before doing an arthrocentesis and the concept of anisotropy.