I had a large sac of fluid behind my knee drained, but it came back. What do I do now?

It sounds like you may have had a Baker's cyst also known as a popliteal cyst. A cyst like this can be persistent and cause painful knee swelling and loss of motion. Some doctors advise having the fluid removed again. But if you have to do this
more than three times, then removing the cyst is an option.

Doctors can use a special tool called an arthroscope to go inside the knee joint. The fluid is removed and the wall of the cyst is cut out. The doctor also takes out the valve that allows fluid to move from the joint into the cyst.

A recent study from South Korea reports this treatment method is 100 percent successful. In a group of 14 patients with recurrent cysts, none of the cysts came back after removal, even after a year.