Also known as Baker's Cysts.
Three bones meet within the knee joint: the femur, or thighbone, the tibia, or shinbone, and the patella, or kneecap. Between the three bones lies a bursa, a fluid-filled sac that lubricates the joint. As a result of age and wear, the bursa may create extra fluid. The fluid may leak out of the wall of the bursa, forming a fluid-filled sac known as a Popliteal or Baker's cyst. The condition can be painful and often accompanies osteoarthritis, gout, or a meniscal injury.