High Quality Content by WIKIPEDIA articles! A Baker`s cyst, also known as a popliteal cyst, is a benign swelling of the semimembranous 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). This is not a "true" cyst, as an open communication with the synovial sac is often maintained.