Bursitis of the Knee (Pes Anserine Bursitis)

This is a swelling of a fluid-filled sac called a "bursa." It's on the inner side of your knee, between the tibia and the tendons that attach to your hamstring muscle. You have similar sacs near other large joints throughout your body. They act as cushions between your bones and your soft tissues. Normally they have a small amount of fluid inside them. But sometimes they can swell. We call that "bursitis."