ANATOMY - Shoulder
The shoulder is a ball and socket joint. The ball is called the head of the humerus and the socket is called the glenoid (it's part of your shoulder blade, also known as the scapula). Sometimes, arthritis can form here. On top of this ball and socket joint is another bone known as the acromion. This is a frequent place for bone spurs to form. Right next door to the acromion is the acromioclavicular joint or "AC joint" for short. This is a common place for shoulder separations.
A group of four (4) muscles helps to move your shoulder joint; they are called the rotator cuff. These muscles work together to help get your arm up over your head, as well as rotate it in and out. That's why rotator cuff injuries usually result in weakness, especially in trying to raise the arm overhead. One of the four muscles is injured much more frequently than the others; it is known as the supraspinatus muscle.
In addition, these rotator cuff muscles function to help keep your shoulder "in socket", or "located" (when the shoulder comes out of socket, it's called "dis-located"). You have several ligaments in your shoulder that help to keep it in place. Finally, there's an "O-ring" around the socket, called the labrum, which also helps keep your shoulder in socket and causes pain and popping when it's torn.
