Causes / typical injury mechanism: Posterior shoulder instability, which can include subluxations and dislocations, is associated with posterior labral tears and encompasses a relatively rare spectrum of disease. Acute posterior shoulder dislocations are most commonly caused by trauma and require urgent attention and reduction. Placing an axial load on a humerus that is flexed, adducted, and internally rotated causes the humeral head to lever out of the posterior glenoid and dislocate or sublux. Acute posterior dislocation can also result from direct forces originating on the anterior shoulder, such as colliding with a dashboard during a motor vehicle collision. Other well-known causes of acute posterior shoulder dislocation include seizures, electrocution, and falling on an outstretched hand.

Posterior shoulder instability with subluxation is more common than posterior dislocation and generally more of a chronic problem caused by repetitive microtrauma. Repetitive posterior forces on the humeral head (axial load to a humerus that is flexed, adducted, and internally rotated, with similar force vectors as in acute posterior dislocations) can lead to progressive capsular stretching and posterior labral tears. Instability itself can also result from recurrent posterior shoulder dislocations.

Classic history and presentation: Chronic posterior shoulder instability is most commonly seen in football linemen, weight lifters (especially bench pressing), and overhead throwing athletes. It is important to note that posterior shoulder instability can often be associated with inferior shoulder instability due to the intimate relationship of the posteroinferior capsule and the posterior band of the inferior glenohumeral ligament (PB-IGHL). Posterior labral tears can sometimes present as deep shoulder pain without obvious instability symptomology.

Prevalence: Glenohumeral (shoulder) instability affects approximately 2% of the general population; however, posterior instability only accounts for 2%-12% of shoulder instability cases (anterior instability is the most common form of shoulder instability).

Risk factors: Risk factors include activities that result in posteriorly directed forces on the humeral head, including playing football (especially linemen), weight lifting (especially bench pressing), overhead throwing, swimming, performing gymnastics, and wrestling. Other risk factors include seizures (epilepsy), certain connective tissue disorders causing hypermobility such as Ehlers-Danlos syndrome, and excessive glenoid retroversion and erosion.

Pathophysiology: Both acute posterior shoulder dislocation and posterior shoulder instability are commonly associated with tears in the posterior glenoid labrum. The glenoid labrum is a circular band of fibrocartilaginous tissue that acts to deepen the glenoid cavity by 50% to allow greater humeral range of motion without dislocation. As the humeral head dislocates (or subluxates) out of the posterior glenoid, it weakens and tears the labrum as it glides over it.