Shoulder Dislocation

Anatomy

The shoulder is a ball-and-socket joint. The humeral head (ball) sits in the glenoid (socket) and allows the large range of motion that humans have at the shoulder. This range of motion allows extremes of motion, such as those that an elite pitcher uses to throw the ball.

There are a number of structures that help stabilize the glenohumeral joint. First, to help the humeral head fit more snugly in the glenoid, the glenoid is surrounded by a ring of cartilage called the labrum. Second, the glenohumeral ligaments add further stability to the joint. These ligaments are thickened areas of the shoulder capsule, which encompasses the entire joint. Both the labrum and the glenohumeral ligaments can be injured when the humeral head dislocates out of the glenoid.

In addition, important arteries and nerves travel in this area and can be injured from a shoulder dislocation.

Causes

As mentioned, more than 90% of traumatic shoulder dislocations are anterior, with the head of the humerus dislocating out of the front of the glenoid. This usually occurs as the result of a fall with the arm in the abducted (held away from the body) and externally rotated position or when the arm is hit from the front when held in this same position, as can happen in a football tackle.

Less frequently (24%), the dislocation is posterior, with the humeral head dislocating out of the back of the joint. This usually occurs when the arm is forward elevated and internally rotated. These dislocations are less likely to occur from sports (although they can occur with the pass block in football) and are more likely the result of a motor vehicle accident, seizure, or electric shock (Figure 1).

Symptoms

The patient with an acutely dislocated shoulder is usually in a good deal of distress. He or she holds the arm slightly away from the body and in internal rotation. The patient avoids moving the joint due to the pain, and on examination, it will be found that there is little ability to rotate the shoulder.

If the patient has had previous shoulder dislocations, he or she may present without an acute dislocation but with complaints of the shoulder “popping in and out,” especially when his or her arm is in the throwing position.

Diagnosis

In the acute setting, the history of the mechanism of the injury, as well as the physical exam, is usually enough to make the diagnosis. The examiner should always perform a good neurovascular exam (checking the pulses and nerve supply to the arm both in the central and peripheral distribution) as damage may have been done to important structures. The axillary nerve, which supplies the large deltoid muscle of the shoulder, is not uncommonly [Page 1296]damaged from a shoulder dislocation. Axillary nerve integrity can be assessed by testing the sensation of the skin over the deltoid muscle on the outside of the arm.

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