Do not be scared. Bursae are small fluid-filled sacs that cushion the knee joints and allow the tendons and ligaments to slide easily over the joint. These sacs can swell and become inflamed with overuse or repeated pressure from kneeling. This is known as bursitis. Most cases of bursitis are not serious and can be treated by self-care. However, some instances may require antibiotic treatment or aspiration, which is a procedure that uses a needle to withdraw excess fluid.