The rotator cuff is a group of four muscles and their tendons that wrap around to form a "cuff" over the upper end of the arm at the shoulder. The rotator cuff helps guide the shoulder through many motions by allowing lift and rotation to the arm while stabilizing the ball of the shoulder within the socket. The space over the top of the rotator cuff is occupied by a bursa which is a fluid sac between the tendon and the acromion bone. Bursitis, a common condition, is inflammation of this fluid sac and occurs with repetitive overhead activity or overuse of the arm. The term impingement refers to pinching of the rotator cuff tendons and bursa against the acromion bone above it.