What are the two key findings on the AP view which should make you suspect a posterior dislocation?
1) Lack of congruency of the humeral head articular surface and glenoid, with no overlap of the two silhouettes. 2) Internal rotation of the humerus, leading to a rounded appearance (known as lightbulb sign).
Single AP view of the shoulder demonstrates lack of overlap between the glenoid and the humeral head. The shoulder is rotated such that the humeral head has a 'light bulb' appearance.