A single AP film may appear normal in a posterior dislocation [2, 5, 6]. Subtle findings on the AP film include lightbulb sign (shape of the humeral head caused by internal rotation), lack of normal overlap between the glenoid and humeral head, and trough sign: an impaction fracture of the humeral head illustrated in Figure 2 [4-6]. Thus, it is essential to obtain additional views, such as an axillary, scapular Y, or Velpeau view [1-6].
Bedside ultrasound may also be used to quickly confirm the diagnosis (Figure 3).
Figure 2: Left: Note that the head of the humerus appears to be shaped like a lightbulb due to persistent internal rotation, there is an impaction fracture of the anteromedial humeral head (red line), and that the articular surface of the glenoid and the humeral head are not parallel (blue lines). Right: Compare this to the post-reduction AP radiograph with improved alignment. Author’s own image, illustrations by Dr Tabitha Ford.
Figure 3: Ultrasound image demonstrating the humeral head dislocated posteriorly relative to the glenoid. The dotted lines represent the normal position of the humeral head. Author’s own images with illustrations by Dr Victor Huang.