The shoulder is a ball and socket joint, made up of three bones, the humerus (upper arm bone), the scapula (shoulder blade) and the clavicle (collar bone). The ball at the top end of the humerus (humeral head) fits into the socket (glenoid) of the scapula to form the shoulder joint. The humeral head and glenoid are covered with cartilage, which allows for smooth joint motion.
The shoulder has the greatest range of motion of any joint in the body, allowing you to position your hand anywhere in space. The coordinated activity of numerous muscles working together in set patterns is required to produce this motion.
The rotator cuff is a group of muscles closely wrapped around the top of your arm bone (humerus). These muscles originate from your shoulder blade and their tendons form a cuff which covers the joint. The rotator cuff muscles are important to normal shoulder function. Together they act to stabilize and rotate the shoulder joint.