Swelling from joint. Baker's cysts are a result of a weakness in the capsule of the knee joint. If there is cause for increased fluid pressure in the knee (arthritis or meniscus tear) fluid may leak out of the capsule and form a cyst behind the knee. The swelling can come and go. If persistent, addressing the cause (i.e. A meniscus tear) often results in disappearance of the cyst. Surgery rarely done for cyst alone.
Mass behind the knee. A baker's cyst forms in the area on the back of the knee joint. They are usually noticed as a smooth mass in that area. They are sometimes very painful and often require surgery.