A baker cyst is a common finding on ultrasound of the leg. This is a cyst that is located on the back of the knee. This cyst lies between the gastrocnemius muscle and the tendon of the semimembranosis tendon and communicates with the knee joint. This can be an asymptomatic finding or one that causes pain. Additionally, these cysts can sometimes leak or rupture and produce pain. Your doctor may feel a mass behind the knee and order an ultrasound to take a look.
This is very common finding on ultrasound done to look for deep vein thrombosis or clot of the leg. The technologists usually look at the back of the knee to look for these cysts as they can mimic the symptoms you would have with clot. It is common that these cysts cause no problems and are not responsible for your symptoms.
On ultrasound, the cyst will be filled with fluid and have the typical appearance of cyst. Often, these cysts will have debris or other complex appearance. There is usually no internal blood flow, which the technologist will look for. In the cases of leak or rupture, we will see fluid going down the leg, sometimes all the way to the calf.
It is important to confirm its cystic nature, as some rare tumors can have a similar appearance in the back of the knee. In cases that are not clear, an MRI of the knee may be ordered. The cysts can be associated with arthritis or after trauma. These cysts are more common in older adults as well
I often see these cysts left alone and not treated. In some cases, a specially trained radiologist will remove the fluid from the cyst to treat a cyst which is causing pain. This is done under ultrasound guidance so that the needle is placed directly into the cyst. When cysts are especially large, a surgery may be done to remove it. In some cases, the cyst can compress the blood vessels and nerves causing additional trouble.