A hard fluid filled lump behind the knee. A Baker’s cyst occurs as a result of increased pressure in the knee joint due to build up of fluid. Such a build up is a feature of disorders such as rheumatoid arthritis. The cyst is formed by a backward ballooning-out of the synovial membrane covering the knee joint.
Many Baker’s cyst are painless, and some disappear spontaneously, sometimes after many months. Occasionally, a cyst may rupture, causing fluid to seep down between the layers of the calf muscles. This can result in pain and swelling in the calf that may mimic a deep vein thrombosis.
Diagnosis of a Baker’s cyst is confirmed by ultrasound scanning. Treatment is rarely needed but in a few cases surgery may be performed.