A baker's cyst is a hernia of the synovial sac that encloses the knee joint. Since the knee joint is so large, and carries the weight of the body with every step, this is the joint where where pressure most commonly causes the joint fluid to form a hernia. The symptoms of a baker's cyst are directly related to where the hernia happens. Nearly all of these occur on the back of the knee because that is where the protecting tissues are the thinnest. This part of the knee has the greatest concentration of nerves and the pressure of the cyst on a nerve can cause extreme knee and leg pain.