Hi everyone, I'm Shelby Johnson from Mayo Clinic in Minneapolis, and this is an ultrasound consult for Baker's cyst. For the posterior knee, I generally use a linear mid-frequency or curvilinear probe depending on the patient's body habitus. The patient will be position prone with their knee just in slight flexion. And as for other regions, I like to have this home base. So for the posterior knee, my home base is this view on the right with the femoral condyles, the medial and lateral gastrocnemius muscles, and the popliteal neurovascular structures in view. And I've outlined those structures here. And then throughout this consult, I've placed little red boxes that are indicative of the positioning of my probe to help with orientation. So to evaluate for Baker's cyst, from that home base, I move the transducer slightly medial to visualize the semimembranosus tendon and the medial gastrocnemius muscle tendon. So by definition, a Baker's cyst is a distention of the semimembranosus medial gastrocnemius bursa. And so fluid at that interval is a very reliable indicator of a Baker's cyst. So in the still image on the left, I've outlined the semitendinosus tendon superficially, the semimembranosus tendon, and the medial gastrocnemius muscle and tendon. And you can see that in this view, at least, the semimembranosus tendon is more hypoechoic in comparison to the medial gastrocnemius tendon. And that's due to anisotropy. So it's very easy to get fooled at this spot and think that the hypoechoic semimembranosus tendon is actually bursal fluid. So by tilting the transducers, cephalad and caudad, like I'm doing in this video on the right, you can kind of see those two structures come into view. And so for this individual, there's no fluid between those tendons and thus no Baker's cyst. Now these next examples are ultrasound evaluations of Baker's cysts. So Baker's cysts can present with a variety of characteristics. They are typically well-defined anechoic or hypoechoic fluid collections with posterior acoustic enhancement. And they're located at, again, that interval between the semimembranosus tendon and the medial head of the gastrocnemius. So if there's a fluid collection or mass present at a different location, then by definition, it's not a Baker's cyst. And you need to think about other things on your differential, like extra articular ganglion cysts, soft tissue masses, popliteal aneurysms, et cetera. So this video on the left, we are at the posterior medial knee with lateral to our right. And you can see that as I tilt that transducer probe, you can visualize again that anisotropy of the semimembranosus and the tendon of the medial head of the gastroc. But as opposed to the last image, there's a very well-defined, largely anechoic fluid collection at that interval. And that's consistent with Baker's cyst. And as I tilt that probe, you can see some synovium within that cyst. And so you may also see loose bodies from osteoarthritis, hemorrhagic debris, or even hyper-coacceptations or proliferative synovial linings. In the video on the right, we are in a transverse view over the posterior medial knee. And so you can see the three different components of the cyst as I translate from proximal to distal. So the base is this deeper portion, which is just deep to the medial head of the gastrocnemius, but superficial to the posterior capsule of the knee. The body, which is the more superficial aspect of the bursa. And then, hard to see in this video, but there's a neck that connects those two components. And those components create almost a crescent shape in this transverse view. As with most structures, you should see the cyst in both short and long axis. And so this is a sagittal view of the Baker's cyst. In the example on the left, where we have distal to the right, you can see that large body of the Baker's cyst, just superficial to the medial gastroc. And in the example on the right, you can see the body, again, superficial, as well as that deeper base component, which, if large enough, which it is here, can track distally to lie between the medial gastroc superficially and the soleus deep. So in reference to our case, this was a 61-year-old with ongoing knee pain due to osteoarthritis and known Baker's cyst. The transverse image on the left is a great example of that crescentic shape and the three components of the Baker's cyst. The base, as noted by the asterisk, the neck that sits between the semimembranosus tendon and the tendon of the medial head of the gastroc, and then the body more superficially. And then in the longitudinal image on the right, you can see the distal extent of that Baker's cyst with hemorrhagic debris that has filled that space. In this individual, her pain was complicated by rupture of the cyst, which is the most common complication of a Baker's cyst. You can actually mimic acute thrombophobitis or DDT. So it should be in your differential for acute caffeine, especially in someone with a known history of Baker's cyst. With rupture, you may see subcutaneous fluid superficial to the medial gastrocnemius, and you may see partial or complete emptying of a Baker's cyst at its typical location between the semimembranosus and the medial gastroc.

3rd Edition, CASE 31

3rd Edition

Knee

Baker's cyst

ultrasound diagnosis

posterior knee examination

fluid collection

differential diagnosis