Dislocations of the shoulder are one of the most common injuries to the shoulder, especially in younger active athletes and those who participate in contact sports. There is also a small group of people that have increased flexibility and do not truly dislocate their shoulder, but their shoulder is looser than the average person.
The shoulder is a ball and socket joint, but the anatomy of the shoulder allows for an amazing amount of flexibility. The humeral head, or ball, sits on the glenoid, a very shallow socket. It resembles a golf ball on a tee. Because the socket is so shallow, the shoulder relies on the soft tissue around the shoulder for stability. The labrum, a thickened cartilage layer around the glenoid, and the ligaments and capsule, provide a majority of the stability to the shoulder. The muscles of the rotator cuff and scapula also provide stability.
The shoulder is most commonly dislocated when the arm is abducted and externally rotated, in a position as if you were throwing a baseball. When there is a strong force with the arm in this position, the humeral head can dislocate out the front of the glenoid. When this occurs, it can either come partially out, which is termed a shoulder subluxation, or come entirely out, which is termed a shoulder dislocation. Although some people are able to reduce their shoulder (put it back into joint), many others need to have their shoulder reduced in an emergency room.
When a shoulder dislocates, there is most often an injury to the front of the labrum, termed a “Bankart Lesion”. There is also an injury to the humeral head termed a “Hill-Sachs Lesion”. The Bankart lesion is what is repaired during surgery for a unstable shoulder.
The natural history of athletes with a shoulder dislocation depends on the age of the athlete. In younger athletes (less than 20 years old), there is a very high risk of re-dislocating the shoulder. However, as we get old, the risk decreases. In people over the age of 50, there is a chance that the shoulder dislocation occurs along with a rotator cuff tear.
The diagnosis of a shoulder dislocation is most commonly made by a patient’s history and physical exam. Patients usually describe how the injury occurred and how the shoulder was put back into joint. Typically, the physical exam demonstrates instability, although in the initial period the exam often only demonstrates tenderness around the shoulder.
Radiographs are performed usually at the time of dislocation as well as after the shoulder has been put back in. Radiographs help determine if there was any significant injury to the bone of the humeral head or glenoid.
An MRI is often obtained to evaluate the soft tissue injury around the shoulder. An MRI allows the surgeon to visualize the injury to the labrum (the “Bankart lesion”) and determine if there is any other injury to the shoulder such as a rotator cuff tear.