Small fluid-filled sacs that protect muscles, tendons, and other soft tissues around joint areas from external stimuli or bone tissues are called bursae. Bursae primarily serve as cushions in areas where bones may be subject to friction or trauma. They are most commonly found around the knee, shoulder, and hip joints in the body.

When bursae are subjected to acute trauma or mild but frequent and prolonged pressure, they can swell by accumulating fluid, similar to what happens after an impact. This condition is known as bursitis. If bursitis appears quickly (less than three weeks) and responds to treatment, it is considered acute bursitis. Bursitis that does not respond to treatment or recurs frequently is defined as chronic bursitis.

While numerous bursae exist in many joints throughout the body, one of the most common types of bursitis is prepatellar bursitis. This condition occurs when the prepatellar bursa, located directly in front of the kneecap (patella) between the skin and the patella, becomes inflamed or fills with fluid. The prepatellar bursa plays a protective role by preventing excessive friction and strain on the kneecap when placing the knee on the ground.