Hi, this is Sagar Wagle, one of the musculoskeletal radiology fellows at Stanford University. We have a case of a 61 year old female with posterior knee pain and sweating and there is a clinical concern for Baker's cyst. Before we look at the ultrasound images, let's look at anatomy of posterior knee and Baker's cyst. Here we have distal femur, proximal tibia, fibula. Medially, we have the semimembranosus muscle and semitendinosus tendon on top. We have medial and lateral head of gastrocnemius and biceps femoris. There are multiple types of cysts and masses that can occur in posterior knee and for a cyst to be a Baker's cyst, it has to originate between the semimembranosus and the medial head of gastrocnemius muscle. Let's look at MRI of a normal knee. This is semimembranosus muscle. This is semitendinosus tendon. Sometimes this look is called cherry on top. So the semitendinosus on top of semimembranosus. The medial head of gastrocnemius would originate somewhere around here as I scroll down. So as I scroll down, this is the medial head of gastrocnemius and as I keep going down, if there were to be a Baker's cyst, it would originate between the medial head of gastrocnemius and the semimembranosus. Let's look at an abnormal case. We have a 45-year-old man with posterior knee pain for one week. This is semimembranosus. This is semitendinosus, again cherry on top. The medial head of gastrocnemius would originate somewhere in this region as I scroll down. So here we see the medial head of gastrocnemius originating and we have a cyst in between the semimembranosus and medial head of gastrocnemius. So this is a Baker's cyst. This person also has degenerative changes and joint diffusion that we will not focus on. There's some some debris and septation in the cyst and as we scroll down, we see this edema around the cyst. This indicates that the cyst has leaked or ruptured and sometimes this can cause pain. So here we can look at the Baker's cyst in our case versus normal interface between the medial head of gastrocnemius and the semimembranosus. I have one other case of a 39-year-old female with posterior knee mass for one month and again here we can see a cyst between the medial head of gastrocnemius and semimembranosus. As you go down, this is semimembranosus. This is medial head of gastrocnemius. This is semimembranosus. This is medial head of gastrocnemius and we can see the cyst in between and we also see edema around the cyst that indicates that this cyst has leaked or ruptured. One more time, semimembranosus, medial head of gastrocnemius and some edema around. So let's go back to our original case. Here Mg indicates medial head of gastrocnemius, Sm indicates semimembranosus and there's a cyst in between these two which is a bigger cyst. Here we can see a needle going to the cyst. So this cyst was treated by aspiration. Thank you for watching the video.

3rd Edition, CASE 31

3rd Edition

Knee

Baker's cysts

posterior knee pain

MRI imaging

cyst aspiration

semimembranosus