The lateral crescent sign was first described by Burkhardt et al. [1]. This sign represents the lateral compression and stretching of the fat of the inguinal canal and its contents by the hernia sac. The contents of the inguinal canal are the testicular vessels, vas deferens, and branches of the genitofemoral nerve (men) or the round ligament together with branches of the ilio inguinal nerve (women) [1]. This sign is useful for the diagnosis of early direct inguinal hernia [1]. Direct inguinal hernias (less common than the indirect type) are usually acquired and with an incidence increasing with age [2, 3]. They protrude through the Hesselbach triangle above the inguinal ligament and lie medial to the inferior epigastric vessels [1]. Direct inguinal hernias are more frequently observed in men and have a lesser rate of complications (such as strangulation) compared to indirect inguinal or femoral hernias [2, 3]. In the direct form of inguinal hernia, the contents of the inguinal canal are compressed and laterally deviated by the herniated omentum or viscera into a semi-circle of tissue (Fig. 1A, B) that resembles a crescent moon (Fig. 2). This sign is more commonly observed at early stages of direct inguinal hernia. With progression, the contents of the canal are further squeezed and the crescent-like appearance tends to disappear [1].

Fig. 1
figure 1

Axial contrast-enhanced CT image (A). And color-coded image (B). Showing a left direct inguinal hernia. The fat and the other inguinal canal contents (outlined in yellow) are flattened by the herniated fat and omentum (arrow) into a thin lateral “moon-like” crescent. The common femoral artery (red dot) and vein (blue dot) are seen coursing laterally and posteriorly to the hernia

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Fig. 2
figure 2

Source https://fr.wikipedia.org/wiki/%C3%89clipse#/media/File:Solar_Eclipse-4745.jpg

Moon crescent during the solar eclipse of 2012.

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