The trough line sign is a sign of posterior shoulder dislocation on AP films.
In posterior dislocation, the anterior aspect of the humeral head becomes impacted against the posterior glenoid rim. With sufficient force, this causes a compression fracture on the anterior aspect of the humeral head. This compression fracture is analogous to the Hill-Sachs compression fracture seen with anterior shoulder dislocation of the glenohumeral joint.
Frontal radiographs reveal two nearly parallel lines in the superomedial aspect of the humeral head.