Bursitis is a condition that affects the fluid-filled sacs, called bursae, that cushion the friction between bones, tendons, and muscles near the joints. There are around 160 bursae throughout the human body. In bursitis, the bursae become inflamed which leads to pain and stiffness in the area. Shoulders, elbows, and hips are the most common areas affected by bursitis but it can also occur near the knees, heels, and at the base of the big toe. Symptoms of bursitis include aches and stiffness near the affected joint, pain when palpating the area, swelling, and redness.
The most common cause of bursitis is repetitive movements or positions that put pressure on the bursae around a joint. Other causes include injury or trauma when direct pressure is applied to the bursae, inflammatory forms of arthritis, gout, and infection.
The bursae are lined by synovial tissue which produces fluid that lubricates and reduces friction. In the case of bursitis, the synovial lining becomes inflamed and thickened and produces excessive fluid which leads to the symptoms of localized pain and swelling. The inflammation is caused by synovial cells that multiply and increases collagen formation. Over time, the bursal lining can become replaced by fibrous tissue.