The AP x-ray of the shoulder shows a light bulb sign, suggestive of a possible posterior dislocation of the shoulder.

The Y view shows the head of the humerus in alignment with the glenoid cavity and this finding rules out a posterior dislocation.

The patient had her shoulder strapped in a sling with shoulder in extreme internal rotation which caused the light bulb appearance on the AP view.

Not all light bulb appearance of the shoulder is due to a posterior dislocation. If in doubt, repeat the x-ray with proper positioning of the affected shoulder or scrutinize the Y view carefully.

Reference: Accident & Emergency Radiology. A Survival Guide by Negel Raby et al. Second Edition.