Baker's cyst Disease

A Baker's cyst, also known as a popliteal cyst, is a benign swelling of the semimembranosus or more rarely some other synovial bursa found behind the knee joint. It is named after the surgeon who first described it, William Morrant Baker (1838–1896). It is not a "true" cyst, as an open communication with the synovial sac is often maintained.