Posterior Shoulder Dislocations

• A rare type of shoulder dislocation
• Accounts for 2-4% of all shoulder dislocations
• Classic mechanism of injury is a seizure or electrocution
• Reported to occur bilaterally in 15% of cases
• Often missed on the initial visit.
• Patient will complain of pain with movement of the shoulder and the arm is held in internal rotation.
• Can be missed on the AP, lateral and Y-views of the shoulder.
• Axillary or modified Axillary views are the best view to visualize a posterior shoulder dislocation. Shown below:

(A posterior shoulder dislocation will show the humeral head displayed superiorly in the image away from the clavicle which is the inferior most bone)

Some things to look for on the AP view that will suggest a posterior shoulder dislocation:

• Lightbulb sign – The head of the humerus in the same axis as the shaft producing a lightbulb shape
• The ‘rim sign’ – Widening of the glenohumeral space
• The vacant glenoid sign – the anterior glenoid fossa appears empty

Life in the Fast Lane as a great discussion of posterior shoulder dislocations at http://lifeinthefastlane.com/posterior-shoulder-dislocation/

 Best way to make the diagnosis --- suspect it and get an axillary view.