The AP view of the left shoulder demonstrates a light bulb appearance of the humeral head. Y view obtained showed a posterior dislocation of the head of humerus.
A key finding on the AP view is symmetrical appearance of the head of the humerus referred to as ‘light bulb sign’. Another cause of symmetrical appearance on AP shoulder x-ray is extreme internal rotation of the shoulder in the absence of dislocation. So, a second view (either Y view or axillary view) should be obtained to confirm dislocation.
Reference: Grainger and Allison’s Diagnostic Radiology, 6th edition.