The most common causes of hip pain are arthritis, bursitis, muscle strain, and nerve irritation. Arthritis happens because the hip is a "ball and socket" joint formed by the pelvic bone and the end of the femur bone. Both bones are covered with a smooth layer of protective cartilage; loss of this cartilage from wear and tear, inflammation, or injury causes arthritis. The hip joint and the large muscles that cover it are lubricated by five large bursa sacs and the fluid contained in the hip joint. Each sac produces lubricating fluid and functions to reduce pressure and friction around the joint. These bursa sacs can become irritated from injury, excessive pressure, and overuse. Inflammation of a bursa is called bursitis. Three major muscle groups help to move the hip through a wide range of movements. Overuse and irritation of these muscles can lead to muscle strain. Nerve irritation occurs when the major nerves controlling lower leg function cross the hip. Irritation of these nerves can cause pain through the hip and/or down the leg. Injury to the sciatic nerve frequently causes pain along the outer thigh or down the back of the leg. This is discussed separately.