A popliteal cyst, also known as a Baker cyst, is a fluid-filled sac that develops in the popliteal fossa (the space behind the knee). It typically forms when excess joint fluid from the knee escapes through a one-way valve mechanism into a bursa between the medial head of the gastrocnemius and the semimembranosus tendon. On ultrasound, a typical popliteal cyst appears as an anechoic or hypoechoic, well-defined collection with a characteristic comma or crescent shape, often with a neck communicating with the knee joint. Baker cysts are frequently associated with underlying knee pathology such as arthritis or meniscal tears. Ultrasound is valuable for distinguishing a Baker cyst from other causes of posterior knee swelling, such as popliteal artery aneurysm or deep vein thrombosis, and for detecting complications like cyst rupture.
Sonography Term