Your shoulder is composed of three bones: your upper arm bone (humerus), your shoulder blade (scapula) and your collarbone (clavicle). The head of your upper arm bone fits into a rounded socket in your shoulder blade. A variety of muscles and tendons (rotator cuff) keep your arm bone centered in the socket. They cover the head of your upper arm bone and attach it to your shoulder blade. Most shoulder problems fall into four major categories: tendon inflammation or tendon tear, instability, arthritis, or fracture.