Snapshot A 27-year-old G1P1 woman with a newborn boy is evaluated on the labor and delivery floor for left shoulder pain. She had a seizure secondary to eclampsia 30 minutes prior to evaluation. On exam, her left arm is adducted and internally rotated. Her range of motion is limited due to pain. An axillary lateral radiograph demonstrates a posterior shoulder dislocation. Introduction Clinical definition complete posterior displacement of the humerus relative to the glenoid relevant definitions "dislocation" refers to complete displacement of the humeral head out of the glenoid "subluxation" refers to translation of the humeral head relative to the glenoid surface without frank dislocation Epidemiology Incidence rare injury comprises 2-4% of all shoulder dislocations majority of shoulder dislocations are anterior posterior dislocations are more much likely to be missed than anterior dislocations Risk factors seizure disorder occupational exposure to sources of high voltage electricity Etiology Tetanic muscle contractions pull humerus posteriorly out of glenoid seizures electrical shocks Pathoanatomy normal glenohumeral anatomy humeral head articulates with the glenoid and is held there by several static and dynamic stabilizers static stabilizers glenohumeral ligaments glenoid labrum negative intraarticular pressure dynamic stabilizers rotator cuff muscles tendon of the long head of the biceps brachialis Associated conditions reverse Bankart lesion posteroinferior labrum detachment following impaction of the humeral head against the posterior glenoid reverse Hill-Sachs lesion impaction fracture of the anteromedial humeral head Presentation Symptoms pain deformity limited range of motion Physical exam the arm is held adducted and internally rotated different from anterior dislocation in which the arm is held abducted and externally rotated prominent posterior shoulder and coracoid Imaging Radiographs indications radiographs are indicated in any patient in whom a shoulder dislocation is suspected anteroposterior (AP) and axillary lateral or scapular Y-views must be taken findings AP radiograph may be normal may show a "lightbulb sign" due to internal rotation of the arm the humeral head appears rounded like a lightbulb in internal rotation axillary lateral or scapular Y-view best views to demonstrate a dislocation Differential Anterior shoulder dislocation distinguishing factors the arm is abducted and externally rotated radiographs will demonstrate anterior dislocation Rotator cuff tear distinguishing factors the arm will not be held in adduction and internal rotation radiographs will demonstrate normal placement of the humeral head against the glenoid Treatment Nonoperative acute reduction followed by sling immobilization indication most posterior shoulder dislocations can be treated nonoperatively Complications Nerve injury axillary or suprascapular nerves Recurrence Prognosis Unfavorable recurrent dislocation