AP radiograph reveals apparent increase in space between the anterior rim of the glenoid fossa and the medial aspect of the humeral head. This is termed the "positive rim sign." It may be present in posterior dislocation because the normal glenoid fossa faces as much anteriorly as it does laterally. Since the dislocated humeral head rests against the posterior glenoid rim, the space between the anterior rim and the humeral head appears increased.
Reference
Arndt JH, Sears AD: Posterior dislocation of the shoulder. AJR Am J Roentgenol 1965, 94:639-635.
