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Causes
By Mayo Clinic staff
A door hinge needs oil to swing smoothly, to reduce the friction between its moving parts and to minimize wear and tear. Similarly, the cartilage and tendons in your knees rely on a lubricating fluid called synovial (si-NO-vee-ul) fluid. This fluid helps your legs swing smoothly and reduces friction between the moving parts of your knees.
Synovial fluid circulates throughout your knee and passes in and out of various tissue pouches (bursae) throughout your knee. A valve-like system exists between your knee joint and the bursa on the back of your knee (popliteal bursa). This regulates the amount of synovial fluid going in and out of the bursa.
But sometimes the knee produces too much synovial fluid. Most commonly this is caused by an inflammation of the knee joint, such as occurs with various types of arthritis. A knee injury, especially a cartilage tear, also is a common cause of Baker's cyst.
When the popliteal bursa fills with fluid and expands, the result is a bulge called a Baker's cyst. In texture, it's similar to a balloon filled with water.
