The trough line: a radiographic sign of posterior shoulder dislocation.

Over 50% of posterior dislocations of the shoulder are not recognized initially because of failure to appreciate the subtle clues available on the standard frontal projection. In 15 of 20 cases of posterior dislocation, two parallel lines of cortical bone were identified on the medial aspect of the humeral head. One line represents the medial cortex of the humeral head. The other was found to represent the margin of a troughlike impaction fracture. This is designated the "trough line". This line proved to be an important clue in the recognition of posterior dislocations of the shoulder.