I've had the fluid removed from a cyst behind my knee twice now. Why does it keep coming back?

Doctors think there are several reasons for this. First, the cysts have thick walls with twisted, deep roots. The body can't dissolve or absorb this tissue. There's also a valve between the cyst and the joint. This opening allows fluid to move from the joint into the cyst.

Often other damage in the knee adds to the problem. A new study by two doctors in South Korea report better results for cyst removal using arthroscopy. A special tool with a tiny TV camera is inserted into the cyst. The fluid is taken out. Then the cyst wall is removed with a motorized shaver. Any other damage in the joint is repaired at the same time. They've had 100 percent success in treating cysts this way.