The shoulder joint is actually an intricate complex of four articulations that, working together, allow for more motion than any other joint in the human body. The main joint is called the glenohumeral joint. This is a ball and saucer joint. The ball portion of the proximal humerus (arm bone) articulates against the saucer-shaped glenoid portion of the scapula (shoulder blade). This is the joint that is injured in a shoulder dislocation. The scapula, in turn, rests against the upper posterior rib cage, and there is actually a fair amount of movement between the scapula and the rib cage normally. The scapula also articulates with the clavicle (collar bone) through the acromion process of the scapula. This joint is referred to as the acromioclavicular joint and is the joint that is injured during a shoulder separation.
An intricate complex of ligaments and muscles provides stability to these joints and allows for smooth, painless motion. Through injury or degeneration, portions of the shoulder complex can be damaged or wear out. This can result in pain and stiffness, as well as weakness, instability, and loss of function of the shoulder.