Bursae (plural) are usually closely related to tendons where there is a lot of frictional movement.
Bursae of the knee
In the knee there are several bursae (plural of bursa), the main ones being:
Occasionally bursae become inflamed and distended with fluid. They can also become infected. The condition of an inflamed bursa is called bursitis.
"Many different types of bursae are seen around the knee joint. Thorough knowledge of the anatomy of the region is of utmost importance for arriving at a correct diagnosis.