Popliteal or baker's cysts
A cyst is a fluid filled sac in any location of the body. The most common around the knee is called a Baker’s cyst or popliteal cyst. There is a one way valve which goes from the knee to this sac in the back of the knee. Typically, damage within the knee causes swelling and the fluid is pumped from the knee to this fluid filled sac. This creates swelling and sometimes pain in the back of the knee. This may cause problems achieving full bending or full straightening of the knee.
Typically, draining or cutting out this cyst does not help unless the source of the fluid inside the knee is fixed at the same time. Usually it means that arthroscopy is necessary to fix or remove torn menisci or joint lining cartilage. Over time, the cyst usually disappears or reduces in size and does not need drainage or removal.
Occasionally these fluid filled sacs burst and cause swelling and fluid to travel down the back of the calf. This can be quite painful for up to three months and has a slightly higher risk of causing blood clots in the leg, but is usually self limiting.