Many young children and athletes have experienced a shoulder dislocation just from day to day activities. I have seen auto accidents dislocate this very important joint as well.
The shoulder moves in all directions because of its unique construction. The shoulder is a ball and socket joint composed of three bones: the collarbone (clavicle), shoulder blade (scapula) and the upper arm (humerus). When this joint experiences force such as from a tackle in football, falling or being involved in a serious auto accident the humerus can separate from the socket.
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When your shoulder dislocates your arm is not only useless, you go through a considerable amount of pain until it is reinserted into the joint. A person trained in first ad or doctors can easily pop the shoulder back into place—usually with little complications, but x-rays may be taken to make sure the shoulder there is no other damage.
With severe auto accidents there are often other complications that accompany the dislocated shoulder. Cartilage is present in the shoulder joint in order to keep the humerus connected in the joint. If the shoulder experiences enough force the cartilage may be ripped- making the joint unstable as there is nothing there to keep the bone in place. If the cartilage is torn surgery is necessary to repair and reattach the cartilage. Another possible complication is a Hill-Sachs lesion which occurs when the humerus hits the clavicle as it is pulled from the socket. This force will chip bone from the top of the humerus, flattening it which will lead to further shoulder instability, painful movement and an increased risk of dislocation in the future.