Lightbulb sign - posterior shoulder dislocation
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Posterior dislocation of the right shoulder on frontal projection can be difficult to identify. Note the poor congruency of the glenohumeral line, not forming a nice lens, and the lightbulb shape of the rotated humeral head.
The humeral head is forced into internal rotation as it dislocates posteriorly, giving appearance of a lightbulb (see lightbulb sign).