The trough line sign is a sign of posterior shoulder dislocation on AP shoulder radiograph.
Pathology
In a 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.
Radiographic features
Plain radiograph
Frontal radiographs reveal two nearly parallel lines in the superomedial aspect of the humeral head.
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