The "rotator cuff" is a group of 4 muscles that are responsible for keeping the shoulder joint stable. Unfortunately, injuries to the rotator cuff are very common, either from injury or with repeated overuse of the shoulder. Injuries to the rotator cuff can vary as a person ages. When left untreated, this injury can cause severe pain and a decrease in the ability to use the arm. ACC (New Zealand) have produced a useful brochure that outlines the facts about rotator cuff tears, treatment, self management and prevention.