The most flexible joint in the entire human body, our shoulder joint is formed by the union of the humerus, the scapula (or shoulder blade), and the clavicle (or collarbone). Commonly thought of as a single joint, the shoulder is actually made up of two separate joints - the glenohumeral and acromioclavicular joints. These two joints work together to allow the arm both to circumduct in a large circle and to rotate around its axis at the shoulder. Although both of these joints are held together by extensive ligament and muscle attachments, certain types of force can easily weaken the shoulder. The shoulder joint is vulnerable to dislocations from sudden jerks of the arm. To reinforce the shoulder, a group of four muscles called the rotator cuff extend from the scapula and surround the head of the humerus to both rotate the arm and prevent dislocation . Chronic or acute wear and tear of shoulder joint can lead to the painful tearing of the tendons of the rotator cuff or a torn labrum. Both of these conditions are very painful and may require surgery to remove or reattach the torn tissue
