The shoulder joint is called a ball-and-socket joint. The ball is the rounded top of the bone in the upper arm (humerus), which fits into the socket — the cup-shaped outer part of the shoulder blade. When the top of the humerus moves out of its usual location in the shoulder joint, the shoulder is said to be dislocated. A related injury called a shoulder subluxation occurs when the top of the humerus is only partially displaced and not totally out of its socket.
In some cases, a shoulder is dislocated when the arm is pulled or twisted with extreme force in an outward, upward or backward direction. This extreme force literally pops the top of the humerus out of its socket. In other cases, a shoulder dislocation is the result of a fall on an outstretched arm, a direct forceful blow to the shoulder, a seizure or a severe electric shock. Seizures and shock can cause shoulder dislocations because they produce extreme, unbalanced muscle contractions that can wrench the humerus out of place.
Doctors classify shoulder dislocations into three types, depending on the direction of the dislocation:
Anterior dislocation — The top of the humerus is displaced forward, toward the front of the body. This is the most common type of shoulder dislocation, accounting for more than 95% of cases. In young people, the cause is typically sports-related. In older people, it usually is caused by a fall on an outstretched arm.
Posterior dislocation — The top of the humerus is displaced toward the back of the body. Posterior dislocations account for 2% to 4% of all shoulder dislocations and are the type most likely to be related to seizures and electric shock. Posterior dislocations also can happen because of a fall on an outstretched arm or a blow to the front of the shoulder.
Inferior dislocation — The top of the humerus is displaced downward. This type of shoulder dislocation is the rarest, occurring in only one out of every 200 cases. It can be caused by various types of trauma in which the arm is pushed violently downward.
Shoulder dislocations are the most common joint dislocation seen by emergency room doctors, accounting for more than 50% of all dislocations treated in hospitals. Young adult men and older women tend to be the groups with the highest rate of shoulder dislocations.
Almost all shoulder dislocations are related to trauma. Occasionally, the dislocation occurs after ordinarily harmless motions, such as raising an arm or rolling over in bed. In these mysterious cases, the real cause may be that the shoulder ligaments are abnormally loose. Loose ligaments are sometimes due to an inherited condition that can increase a person's risk of dislocation in other body joints as well.