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From the question on page 173.
Posterior shoulder dislocation.
Anteroposterior right shoulder
Several findings compatible with a posterior shoulder dislocation are present (fig 1). The humeral head is fixed in internal rotation, producing a “light-bulb” appearance and there is increased distance between the anterior glenoid rim and the medial aspect of the humeral head. The posterior humeral head is impacted onto the posterior rim of the glenoid (ellipse). A tiny fracture fragment is also seen overlying the axillary recess (arrow).
This view shows posterior dislocation of the humeral head relative to the glenoid. The humeral head should overlap the glenoid fossa (ellipse) on this view (fig 2).
The humeral head is seen impacted onto the posterior rim of the glenoid with an impaction fracture of the anteromedial humeral head (reverse Hill–Sachs or McLaughlin defect). A normal axillary radiograph of the right shoulder is also shown for comparison (fig 3). Orthogonal views more clearly delineate the posterior dislocation and are required for accurate diagnosis with plain radiographs.