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Posterior Shoulder Dislocations are uncommon (strong supporting structures vs. anterior)
But commonly missed by physicians
Mechanism: Direct blow anterior shoulder/FOOSH with shoulder internally rotated and ADDucted)
May also see with seizure/electric shock (tetanic contraction)
Clinical findings subtle
Shoulder held in ADDuction and internal rotation. Patient unable to externally rotate arm from this position. If habitus allows, anterior shoulder depression/posterior fullness.
Radiology: Decreased overlap between humeral head and glenoid fossa. Proximal humerus fixed in internal rotation looks like a light bulb on a stick.
Y view will show subtle posterior displacement of humeral head (not as dramatic as is in anterior dislocations!)
http://cdn.lifeinthefastlane.com/wp-content/uploads/2009/06/posterior_shoulder_dislocation_005.jpg
http://eorif.com/Shoulderarm/Images/Shoulder-dislocationP1.jpg