A Baker cyst is a buildup of joint
fluid behind the knee. It creates a tight bump behind the knee.
Joint fluid helps the knee move smoothly. A Baker cyst happens when excess fluid is pushed out to the back of the knee.
Sometimes the cause is not known. In adults, it may be due to underlying problems with the knee joint. In children, it may be due to problems with the small fluid-filled sac (bursa) between the bone and soft tissue of the knee.
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