Dislocations and subluxations occur when the normal arrangement of bones entering a joint is changed.  In a subluxation, an incomplete dislocation has occurred.  Although the relationship of joint structures is altered, contact between joint surfaces remain.

The shoulder accounts for 50% of all joint dislocations and 95% of shoulder dislocations are anterior, as in this case.  The mechanism of injury is typically due to external rotation and abduction of the arm.  40% of shoulder dislocations are recurrent and they commonly occur in young individuals. 

In anterior shoulder dislocations, the humerus will typically lie posterior and medial to the glenoid joint and sit beneath the coracoid process.  When one sees a dislocation of the glenohumeral joint, one should be concerned for an accompanying fracture.  25% of acute and 75% of recurrent fractures will have a "Hill-Sachs lesion", which is a compression fracture of the posterolateral aspect of the humeral head produced by the impaction of the humerus against the superior rim of the glenoid fossa.  A second type of fracture accompanying anterior shoulder dislocations is the "Bankart lesion."  This lesion occurs as the humerus fractures off a piece of the inferior glenoid rim. 

Posterior shoulder dislocations account for only 2-4% of shoulder dislocations.  These dislocations are notoriously difficult to diagnose by plain film.  An estimated 70% of all posterior shoulder dislocations are missed when plain films are used alone.  If one sees a patient with bilateral posterior shoulder dislocations, injury secondary to seizure activity is likely.

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