Causes

Baker’s cysts are benign cystic tumors found on the back of the knee joint. The cyst forms with an accumulation of fluid between the 2 groups of muscles behind the knee. This cyst may form from an injury or inflammation of the membrane lining of the knee joint. You notice a bulge behind your knee, and you feel tightness back there, too. The pain gets worse when you fully extend your knee or when you're active.

What could be the cause?

A likely explanation is that you have a Baker's cyst, also called a popliteal cyst. A Baker's cyst is usually the result of a problem with your knee joint, such as arthritis or a cartilage tear. Both conditions can cause your knee to produce too much fluid, which can lead to a Baker's cyst. Treating the larger problem usually relieves the swelling and discomfort of a Baker's cyst.

These cysts occur most often in adults between 55 and 70, and in children between 4 and 7 years old. As many as one in five people with other knee problems develop a Baker's cyst.

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 fluid. This fluid helps your legs swing smoothly and reduces friction between the moving parts in 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 or a knee injury, especially a cartilage tear.

The popliteal bursa behind your knee is most likely to bulge from the excess synovial fluid. When the 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 jelly.