behind knee definition
By Mayo Clinic Staff. A Baker's cyst is a fluid-filled cyst that causes a bulge and a feeling of tightness behind your knee. The pain can get worse when you fully flex or extend your knee or when you're active. A Baker's cyst, also called a popliteal (pop-luh-TEE-ul) cyst, is usually the result of a problem with your knee joint, such as arthritis or a cartilage tear.