Baker’s cyst
Baker’s cyst [8] is also called a popliteal cyst. It is a fluid-filled swelling that results in a painful lump at the back of the knee. This lump often causes stiffness and limits your ability to move your knees. The pain is most severe when you try to stretch or bend your knees.
The swelling is due to a buildup of synovial fluid behind the knee. Synovial fluid is a clear, viscous fluid secreted by the lining of the joints that is meant to act as a lubricant. Sometimes the knee produces this fluid in excess. Pressure from overuse of the knee can force this fluid through a one-way valve to the back of the knee. This results in the bulge known as Baker’s cyst.