• Posterior dislocation may be missed initially on frontal radiographs in 50% of cases.
• Signs on frontal radiographs are subtle, including the trough line sign and the loss of normal “half-moon overlap”.
• Axillary, scapular Y, or posterior oblique projections are needed for confirmation.
• The absence of external rotation on images in a standard shoulder series is a clue to posterior dislocation.
• Loss of half-moon overlap: On a normal true anteroposterior image, there is a half-moon overlap between the humeral head and the glenoid. In a posteriorly dislocated shoulder, there is lateral displacement of the humeral head with respect to the glenoid (losing the half-moon overlap)
• Trough line sign: 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 dislocation of the glenohumeral joint. Frontal radiographs reveal two nearly parallel lines in the superomedial aspect of the humeral head.

Shown below are the X ray findings of posterior shoulder dislocation.

Courtesy: RadsWiki, copyleft source.

Shown below are the X ray findings of anterior shoulder dislocation.

Courtesy: RadsWiki, copyleft source.

Shown below are the X ray findings of anterior shoulder dislocation post reduction.

Courtesy: RadsWiki, copyleft source.