The shoulder is one of the most flexible of the body’s joints. The shoulder joint is what allows people to move their arms and, as a result, move their hands to where they need to be used. To be so flexible, the shoulder must have a wide range of movement. Unfortunately, this ability to move so freely means the shoulder joint is one of the less stable joints in the body.
The shoulder is a ball and socket joint. However, unlike other ball and socket joints in the body where the ball is nearly surrounded by the bone “cup” part of the joint, the ball end of the upper arm bone (humerus) instead rests in a shallow cup located on the end of the shoulder blade (scapula). A cuff of cartilage (labrum) forms an extended cup in which the ball end of the arm bone rests.
The entire shoulder joint is held in place by a complex arrangement of muscles, tendons and ligaments.
Some of these muscles and tendons form the shoulder’s rotator cuff, a group of four tendons and four muscles that work together and form a “cuff” over the upper end of the arm. The four muscles are the subscapularis, the supraspinatus, the infraspinatus and the teres minor. The muscles start on the shoulder blade (scapula) and are connected to the upper part of the arm bone (humerus) by the tendons that are attached directly into the bone. The rotator cuff helps a person raise and rotate the arm and stabilize the shoulder within the joint.
In addition, the shoulder joint has a subacromial bursa, a fluid-filled cushioning sac between the deltoid muscle, the curved section of bone that forms the top of the shoulder (acromion) and the rotator cuff.
When one of the elements of this complex joint is injured, worn down by overuse or simply out of balance, the result usually is shoulder pain.