Popliteal (Baker's) cyst, knee (photograph and arthrogram)

A synovial cyst is a soft, fluid-filled mass under the skin that may occur over any synovial joint. It is caused by herniation of synovial tissue, which then fills with fluid. In popliteal, or Baker's, cyst a fluctuant swelling is present in the popliteal area, occasionally extending well into the calf, as shown on the left. An arthrogram of a popliteal cyst in a patient with rheumatoid arthritis is shown on the right. If the cyst ruptures, acute swelling and pain identical to thrombophlebitis may occur. Ultrasound may be done to delineate the problem, and an arthrogram can be used to confirm the diagnosis.