What is dislocation of the shoulder? What causes a shoulder dislocation?

One of the most moveable joints in the human body is by far the shoulder joint, this joint enables the arm to move in a vast array of various directions. The flexibility and ability to move in a lot of directions has one drawback and that is that it leaves the shoulder joint vulnerable to being unstable and becoming dislocated easily.
 
The genohumeral joint is made up of the head of the humerus, which inserts into the glenoid fossa of the scapule (shoulder blade). The glenoid fossa of the scapula is a shallow groove that the arm bone sits in, and makes the shoulder joint. This fossa is quite shallow so the arm bone doesn't have a lot of support when it sits within this fossa. Because of this the shoulder joint needs many other adjacent structures like ligaments to keep it's stableness. Within the joint, the labrum (a fibrous ring of cartilage) extends from the glenoid fossa and gives a much deeper receptacle for the humeral head. The capsule tissue that envelopes the joint also helps maintain stability. The tendons from the rotator cuff muscles also offer a considerable amount of support for the shoulder joint.
 
shoulder dislocation
 
Shoulder dislocations take place due to the humerus bone being displaced from the glenoid fossa. The shoulder can be dislocated in a number of directions, and a dislocated shoulder is defined by the location where the humeral head ultimately ends up after it has been dislocated. 90 % or more of shoulder dislocations are anterior dislocations, which means that the humeral head has been moved to a position in front of the joint. Posterior dislocations are those where the humeral head has moved backwards toward the shoulder blade.