Using POCUS to examine musculoskeletal pathologies can help to shape your differential diagnosis, inform your decision about performing an arthrocentesis, or simply provide more specificity about a patient. In this lesson, from our POCUS Masterclass, you'll learn an approach for recognizing an effusion in relatively superficial joints, such as the knee or elbow.
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[00:00:00] Evaluation for joint effusion is not always straightforward. Physical examination can be unreliable, especially in patients who are obese. With ultrasound, you can directly visualize intra-articular fluid. This can help shape your differential diagnosis, inform the decision to perform an arthrocentesis or just provide more specificity for the patient. In this lesson, we'll learn an approach to recognizing an effusion, for relatively superficial joints such as the knee or elbow. We’ll typically
[00:00:30] use a high-frequency linear transducer. For obese patients, there may be some incidences where we need a low-frequency transducer. All joints have some synovial fluid. The essential idea of ultrasound, to evaluate for an effusion, is that we're going to be looking for fluid that is growing in size. Swelling beyond the boundary of where we would normally expect the joint capsule to be, indicating an effusion. We'll look at some specific examples next. Let's start with the hip. Here's an example of a normal hip. Here's the femoral head and notice that there's a small,
[00:01:00] tiny amount of fluid in the joint space there. This is a normal amount of synovial fluid. If we draw an imaginary line in our minds between the two bones articulating to create the joint space, we can see that this fluid doesn't rise up above the joint capsule. Let's see this in a clip. The operator fans through to ensure that there's no fluid and we can see that this is normal, without any joint fluid evident. Contrast this with a hip effusion seen on the contralateral affected side of the same patient. Here's the femoral head and this is the joint capsule space.
[00:01:30] We draw our imaginary line and can see that here the anechoic fluid rises up above that line, indicating that a joint effusion is present. Let's consider the knee next. Here's the basic anatomy. There's the patella and we can see the tibia there. The patellar tendon connects them with its fibular pattern. In our mind, we can draw an imaginary line between the patella and tibia and ask ourselves whether we see fluid rising above it. In this case, we don't. This is a normal knee. Contrast that with a patient with a knee effusion. Here's the tibia,
[00:02:00] there's the joint space. There's a large volume of anechoic fluid distending the patellar tendon, seen with its fibular pattern here further upwards. And we can see that there's a large effusion, amenable to arthrocentesis, if needed. Effusions don't always have to appear anechoic with simple fluid. This is an example of a patient with a hemorrhagic knee effusion, a hemarthrosis. Let's review the anatomy. Here's the patella and there's the tibia. This is the joint space. If we draw an imaginary line and imagine connecting the patella with the
[00:02:30] tibia, we can see that there's material here that rises above that line but has external echoes, that's because this is consistent with blood. It's a complex effusion. Let's look at it in a clip. Notice how the operator attempts to compress the fluid and you can see that there's some actual movement, as they depress the blood back into the joint space. This further confirms that we're looking at an effusion. Let's move into the upper extremity and look at the wrist. Here we have an example of a normal wrist. There's the radius and there we see the lunate.
[00:03:00] We draw our imaginary line, connecting between the two and see there's no material rising above it. Contrast that to a patient with a wrist effusion. There's the radius and there's the lunate. We draw our imaginary line. What do you think? Is it a simple effusion or complex? It looks a little more complex, right? We can see that there's some internal echoes in this material, part of that is rising up above the joint space here. This is consistent with an effusion. Next, we'll discuss the elbow. Here's a normal elbow seen in short-axis view.
[00:03:30] There's the hyperechoic appearance in the olecranon and just superficial to it, the fat pad. If we draw an imaginary line, connecting the condyles of the olecranon, we can see that there's no material, no fat rising up above it. There's no effusion here. Here's the same elbow seen in long-axis view. There we have the humerus, hyperechoic appearance of a dome and there's the olecranon. There's the triceps, just superficial to the bone. And we can see the olecranon fossa fat pad there as well. Draw our imaginary line to connect the bones and we can see that the
[00:04:00] fat pad doesn't rise above. This is normal. Now, let's take a look at an effusion in short-axis. If we draw our imaginary line, we can see how it rises up above. This is consistent with an elbow effusion. This is also the origin of the sail sign seen on x-ray. You can see again that this material appears complex because this is a hemorrhagic effusion. There was an intra-articular fracture. Here's the same elbow, now seen in long-axis. We can see that we have complex material with internal echoes
[00:04:30] arising above that imaginary line. We've confirmed in a second plane, that we've got an effusion. We can also use ultrasound to evaluate small joints in the fingers or toes. Here's an example of a patient with a metatarsophalangeal effusion. At the top, we see an affected digit with the effusion and below is a normal example. Notice how we have a small area of fluid rising above the joint space here. In the normal digit, there's a small amount of synovial fluid but nothing rising above the imaginary line
[00:05:00] connecting the two bones of the joint. Of course, it makes sense that in a digit, an effusion might not necessarily be big but you can certainly see it with ultrasound. Here's a clip of the same patient. You may notice that there's some internal hyperechoic material inside the effusion. This is actually a gouty tophi. The patient underwent arthrocentesis, confirming that they had crystal disease. Now, we know what to look for when using ultrasound to evaluate for effusions. We don't have to worry about equivocal or difficult physical examinations anymore because now we can see
[00:05:30] the fluid if it's there.