Housemaid's knee": clinical photograph showing a pre-patellar bursa, a swelling the size of a tennis-ball on a person's knee. Bursitis is the inflammation of a bursa, a small, fluid-filled sac of fibrous tissue which is lined with synovial membrane. Bursae exist to reduce friction over the moving parts in joints and where tendons pass over bones. Bursitis may result from injury, infection or rheumatoid synovitis and causes pain & may also restrict movement in an adjacent joint. Treatment of bursitis is through rest & the use of steroid injections."