The shoulder comprises bones, ligaments, tendons, and muscles that connect the arm to the torso. The three bones that make up the shoulder joint include the clavicle (collarbone), scapula (shoulder blade), and humerus (long bone of the arm).
The shoulder has two joints that work together to allow arm movement. The acromioclavicular (AC) joint is a gliding joint formed between the clavicle and the acromion. The acromion is the projection of the scapula that forms the point of the shoulder. The AC joint gives us the ability to raise the arm above the head. The glenohumeral joint, or shoulder joint, is a ball-and-socket type joint. The "ball" is the top, rounded part of the humerus, and the "socket" is the bowl-shaped part of the scapula, called the glenoid, into which the ball fits. This joint allows the arm to move in a circular rotation as well as towards and away from the body. The labrum is a piece of cartilage that cushions the humerus head and the glenoid. This cartilage also helps to stabilize the joint.
The rotator cuff is a group of four muscles that pull the humerus into the scapula. The rotator cuff muscles stabilize the glenohumeral joint and help with rotation of the arm.
Two sac-like structures called bursa are also located in the shoulder. The bursa secrete a lubricating fluid, which helps reduce friction between the moving parts of the joint.
Together, all of these structures create one of the most flexible joints in the body.