The shoulder is made up of three bones: the scapula (shoulder blade), the humerus (upper arm bone) and the clavicle (collarbone).

The tendons of four muscles form the rotator cuff. The muscles are called the supraspinatus, infraspinatus, teres minor, and subscapularis. The rotator cuff connects the humerus with the scapula (shoulder blade) and helps raise and rotate the arm. As the arm is raised, the rotator cuff also keeps the humerus tightly in the socket (glenoid) of the scapula.

The part of the scapula that makes up the roof of the shoulder is called the acromion. Between the acromion and the rotator cuff tendons there is a bursa. There are many bursae all over the body where tissues must move against one another. The bursa is a lubricated sac of tissue that protects the muscles and tendons as they move against one another. The bursa simply allows the moving parts to slide against one another without too much friction.

The humeral head of the shoulder (the ball portion) has several blood vessels that enter around the base of the articular cartilage (the smooth surface that covers the bone surface). Certain types of fractures of the shoulder can injure these blood vessels, leading to a condition where the ball no longer has any blood supply. This can lead to a collapse of the joint surface even if the fracture heals. Over time, this can result in arthritis of the joint. This condition is called aseptic necrosis of the humeral head. In some cases, when such fractures occur, your doctor may recommend a shoulder replacement rather than try to fix the broken bones. The risk of developing avascular necrosis is so high that it makes more sense to go ahead and replace the humeral head immediately. Otherwise, if the fracture was fixed and aseptic necrosis resulted, you would need a second operation to replace the shoulder.