Hi! Welcome to HCM! A Popliteal cyst is a fluid-filled swelling that develops behind the knee in the popliteal fossa.Primary politeal cyst develops just behind a healthy knee joint in younger people and children while a secondary cyst can develop if there is an underlying problem within the knee, such as arthritis or a tear in the meniscal cartilage. sometimes too much synovial fluid is produced within affected knee joint. the pressure inside the knee increases, stretching the joint capsule which bulges out into the back of the knee, forming the popliteal cyst that is filled with synovial fluid which can rupture as in your case.Treatment is mostly done with arthroscopic surgery.You must consult your orthopedic surgeon. Hope this helps!