The shoulder is the junction of three bones: the humerus (upper arm bone), the clavicle (collarbone), and scapula (shoulder blade). A group of four muscles and their respective tendons, known as the rotator cuff, connects the humerus to the shoulder socket, or glenoid, stabilizes the joint, and allows the upper arm to be lifted and rotated. In a dislocation, the humerus is forced out of the glenoid cavity by a strong blow or severe weakness of the rotator cuff. A dislocation can occur in one of three types: anterior (forward), posterior (backward), and inferior (downward). An anterior dislocation of the shoulder is by far the most common dislocation. Because of the great range of motion allowed by the shoulder joint, it is the most-frequently dislocated joint in the body. 
Image courtesy of Grant's Atlas of Anatomy
A partial dislocation of the shoulder is known as a Subluxation. A full dislocation is very painful, and, since the rotator cuff tendons are weakened when it occurs, the chance of recurrent dislocation, a condition known as instability, increases with each occurrence.
Causes and Risk Factors
Causes
Since shoulder dislocations most often occur in the anterior (frontward) direction, the most common way to dislocate it is to suffer a backwards blow while the shoulder is raised. The condition can also be caused by a stiff upward or backward blow to the shoulder.
Risk Factors
Wrestlers, basketball players and football players comprise the biggest risk group for shoulder dislocations. Twisting, lifting and hitting, as well as throwing sports, also pose a higher risk of dislocation. Those with arthritis, degeneration of the rotator cuff, or poor muscle tone are also at great risk.
Last updated: Aug-27-07
