Your shoulder is made up 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. This socket is called the glenoid. A combination of muscles and tendons keeps your arm bone centered in your shoulder socket. These tissues are called the rotator cuff. 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 (bursitis or tendinitis) or tendon tear, Instability, Arthritis, or Fracture (broken bone). Other much less common causes of shoulder pain are, infection, and nerve-related problems.