Knee bursitis is a condition in which one or more small fluid-filled sacs near the knee joint become inflamed. The sacs are called bursae. Bursae reduce friction and cushion pressure points between bones and tendons, muscles and skin near the joints. Any of the bursae in the knee can be affected by painful swelling, also called inflammation. But most often, knee bursitis happens over the kneecap or on the inner side of the knee below the joint. Knee bursitis causes pain and can limit your movement. Treatment often includes a mix of self-care techniques and medical treatments to ease pain and inflammation.