Half Moon Sign - Orthopaedia

Location	Shoulder
Diagnosis	Absent in posterior dislocation of the shoulder
Discussion	Normal left shoulder x-ray (A) and second abnormal left shoulder x-ray (B) with posterior dislocation; notice the lack of overlap of the humeral head with the glenoid. Normally the medial part of the head of the humerus overlaps the glenoid fossa to form a shadow shaped like a half-moon which reaches down to the inferior border of the fossa. This disappears in posterior dislocation.
Reference	Nobel W: Posterior traumatic dislocation of the shoulder. J Bone Joint Surg [Am] 1962, 44:523-538. Schild H, Muller HA, Klose K: The halfmoon sign. Australas Radiol 1982 Nov;26(3):273-7.