What Is It?
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:
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.
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