-- Benign tumor on the back of the knee joint. Tumor consists of accumulated fluid that protrudes between two groups of muscles behind the knee. May result from injury or from diseases, such as arthritis or gout. See Arthritis; gout. Baker's cyst can be surgically removed if it becomes painful, unsightly or presses on blood vessels or nerves. If caused by disease, it usually disappears after successful treatment of the underlying disease.