Lightbulb sign - refers to the abnormal AP radiograph appearance of the humeral head in posterior shoulder dislocation. When the humerus dislocates it also internally rotates such that the head contour projects like a lightbulb. Because the sign is subtle it is often missed by the untrained observer. Axillary view (bottom right) demonstrates the posterior dislocation much more obviously.
from our tumblr blog
http://radiologysigns.tumblr.com/
