BAKER'S CYST - A synovial cyst or pouch that occurs in the synovial lining of the knee. Presents in the popliteal fossa (behind the knee). Synovial fluid escapes from the knee joint and into the cyst in individuals who suffer from degenerative and other joint disease. Typically these cysts are not painful unless swelling is extensive. Aspiration of the cyst is therapeutic only temporarily since recurrence is common. Most disappear spontaneously after several years. Larger cysts can be removed surgically. Rupture of a Baker's cyst usually manifests as painless swelling of the calf.