Three bones make up your shoulder. They are the upper arm bone (humerus), the shoulder blade (scapula), and the collarbone (clavicle).
The head of the upper arm bone fits into a rounded socket in the shoulder blade. This socket is called the glenoid. A combination of muscles and tendons keeps the arm bone centered in the shoulder socket. These tissues are called the rotator cuff. They cover the head of the upper arm bone and attach it to the shoulder blade.
Most shoulder problems fall into four major categories:
Tendon inflammation (bursitis or tendinitis) or tendon tear
Fracture (broken bone)
Other much less common causes of shoulder pain are nerve-related problems, tumors and infection.