Baker's cysts of knee. X-ray of the knees of a patient with cysts caused by rheumatoid arthritis. The cysts appear as dark areas just below the knee joints. Also called Baker's cysts, these swellings are caused by increased pressure within the knee joint due to a build up of fluid. The fluid causes the synovial membrane that covers the joint to balloon out backwards behind the knee. Most Baker's cysts are painless and many disappear without treatment. If the cyst ruptures, however, the fluid is released into the calf muscles, causing pain and inflammation. Surgery is sometimes needed to remove troublesome cysts.