Bursae in the knee

A "bursa" is a small fluid-filled sac that sits near a bone. It cushions and protects nearby tissues when they rub on or slide over bones. These sacs, called "bursae," are found in many places throughout the body. Bursitis happens when a bursa gets irritated and swollen.

The bursae in the knee most likely to get bursitis are called the "prepatellar bursa" and "superficial infrapatellar bursa."