Shoulder dislocation is the most common form of joint dislocation and accounts for almost half of this type of injury. The most common form of dislocation is a forward dislocation of the humeral head, known as an anterior dislocation. Most of the time, this occurs when the arm is to the side, externally rotated and moved backward, and there is a forward force on the upper arm, forcing the joint to pop out into a fragile position. . A blow to the back of the arm, a fall onto the outstretched hand (FOOSH), strong external rotation, as well as abduction of the shoulder can cause a dislocation.
Posterior shoulder dislocation is rare and occurs when the arm rotates inward and along the body, and is most often caused by muscle spasms in the large back and chest muscles due to a seizure or electric shock. A downward dislocation of the joint can occur if the arm is moved outward and turned outward with great force, as the arm bone rises against the lower part of the shoulder blade and thus pushes the joint out of its place. Posterior dislocation is most commonly associated with side effects such as damage to the nerves and blood vessels or injury to the muscles of the shoulder rotator cuff.
A posterior shoulder dislocation can occur with the joint tilted to be unstable in all directions, and is often present in patients with joint hypermobility. Multidirectional instability is the medical term for this syndrome that runs in families, in young adults under the age of 30, and occurs on both shoulders.