Posterior Shoulder Dislocation Injuries

A shoulder dislocation is an injury that occurs when the ball comes out of the ball-and-socket shoulder joint. Shoulder dislocations typically occur as a result of either traumatic injuries (falls, motor vehicle collisions, etc.) or from athletic injuries. 

Most shoulder dislocations occur when the ball comes out of the front of the shoulder—a so-called anterior shoulder dislocation. About 95% of shoulder dislocations are anterior dislocations.

The shoulder can also dislocate out of the back of the shoulder, although this is a much more unusual injury. When the ball comes out of the back of the shoulder socket, the injury is called a posterior shoulder dislocation. Posterior dislocations are important to recognize, since their treatment is slightly different, and unfortunately, these injuries can be easily overlooked.

One of the reasons these injuries are missed is that the arm is held in a position that seems normal. Typically, after a posterior shoulder dislocation, the upper arm is held by the side, with the forearm held against the body. This is similar to the position you might hold your arm with a contusion type of injury, making the dislocation of the joint less obvious.

X-ray of shoulder dislocation
Science Photo Library / Getty Images

Causes 

Like anterior dislocations, posterior dislocations can also occur after major injuries, but they can also occur after seizures and electric shocks.

The muscles that internally rotate the shoulder are much stronger than the muscles that externally rotate the shoulder. Because of this strength imbalance, sudden, forceful contractions—such as those experienced during a seizure or shock—can push the ball out of the back of the shoulder.

Typically, after a traumatic injury such as a fall or sports injury, people are looking for the possibility of a shoulder dislocation. Yet when someone has a seizure or electric shock, most people aren't thinking about the possibility of a shoulder dislocation.

Therefore, these injuries are sometimes overlooked because attention is on other aspects of the patient's health. In these cases, shoulder pain may be attributed to a contusion as a result of the seizure or shock.

Treatment 

The most important initial step in the treatment of a posterior dislocation, once the injury is recognized, is to reposition the ball into the ball-and-socket shoulder joint. Repositioning the shoulder joint, called "reducing" the joint, is typically not complicated, but it is much more easily tolerated with anesthesia to relieve pain and discomfort.

Surgical treatment of the dislocation may be needed, especially when bone damage accompanies the dislocation. In the case of a posterior shoulder dislocation, it is common that when the shoulder comes out of joint, the ball forcefully strikes the edge of the socket. This can cause an impaction-type of fracture called a reverse Hill-Sachs defect. A Hill-Sachs defect is a common finding with an anterior dislocation. A similar injury, except on the opposite side of the ball and thus called a reverse Hill-Sachs, can occur with a posterior dislocation.

Other injuries can also occur in association with posterior shoulder dislocations. These include fractures of the proximal humerus, tears of the labrum, and rotator cuff tears. In addition to the treatment of the dislocation, it is important to ensure proper treatment of these associated injuries.

Prognosis

As discussed, posterior dislocations are less common than anterior dislocations. The prognosis seems to be largely related to the amount of bone and cartilage damage that occurs at the time of the dislocation. Concerns for patients who have dislocated their shoulder include the possibility of recurrent (repeat) dislocations.

Recurrent dislocations become especially problematic with larger bone defects, since there is less stability in the shoulder if the bone is damaged. 

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