Bursae cushion the movement between the bones, tendons and muscles near the joints. Bursitis is most often caused by repetitive movement and is known by several common names including weaver's bottom, clergyman's knee, and miner's elbow, depending on the affected individual's occupation and area of injury. There are over 150 bursae in the human body. Usually bursae are present from birth, but they may form in response to repeated pressure. Each sac contains a small amount of synovial fluid, a clear liquid that acts as a lubricant.
Inflammation causes pain on movement.
In the knee there are four bursae, and all can become inflamed with overuse. The most common cause of bursitis is repeated physical activity, but it can flare up for no known reason. It can also be caused by trauma, rheumatoid arthritis, gout, and acute or chronic infection. Pain and tenderness are common symptoms. If the affected joint is close to the skin, as with the shoulder, knee, elbow, or Achilles tendon, swelling and redness are seen and the area may feel warm to the touch.