Popliteal bursitis, which represents the inflammation of the popliteal bursa, is caused by an excess of fluid coming from the knee joint, which protrudes into the back of the knee. When an abundance of fluid in the knee joint is compressed by the weight of the body, between the bones of the knee joint, it becomes trapped and ends up separating from the joint, eventually forming a sac filled with liquid.

The popliteal bursa, a small sac with fluid located between the tendons of the muscles located on the posterior part of the thigh (femoral and semimembranosus biceps) and one of the calf muscles, helps reduce the friction between the muscles, allowing smooth, painless movements of the knee.

Cysts are lined sacs that have fluid in them. They are formed around the joints due to a bulging in the capsule joint. A popliteal bursitis develops from the capsule into the knee joint. It ends up protruding into the tissues of the back of the knee. A popliteal bursitis can be felt as a soft swelling in the back of the knee.

Four bones come together to make up the knee joint: knee (kneecap), thigh (femur), thick bone at the front of the lower leg (tibia) and thin bone on the side of the lower part of the foot (fibula).

A popliteal bursitis is more commonly known as Baker's cyst or less commonly as a popliteal cyst. Popliteal refers to the muscle next to it (called popliteus) and a bursitis is an inflamed sac filled with fluid.