The bursa is a fluid-filled sac placed between two structures to help cushion friction during movement. Usually it's located between a tendon and the bone. The bursa cushions the tendon as it moves over the bone. Sometimes there's a bursa between two ligaments.

In bursitis the bursa gets inflamed. The patient has pain mostly on movement. The most common cause is repeated motion. Arthritis, infection, and injury can also cause bursitis. Bursitis occurs more often with aging as the bursa becomes thinner and dried out.

Problems with bursitis occur in the feet, hips, knees, shoulder, and elbows. There are several bursae around the knee joint. Kneeling is a common cause of one type of knee bursitis. This is sometimes called housemaid's knee. Another type of bursitis occurs most often in people who install carpet, but anyone can get bursitis of the knee.