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 <br> 2) internal rotation of the humerus, leading to a rounded appearance of the humeral head (known as light bulb sign)
There is lack of overlap between the glenoid and the humeral head. The shoulder is rotated such that the humeral head has a 'light bulb' appearance.