A hidden tunnel at the skull base: A descriptive anatomical study of the mastoid foramen in dry skulls

The mastoid foramen (MF) is a small but clinically significant anatomical structure located on the mastoid process of the temporal bone, showing considerable individual variation in its morphology and localization. This descriptive morphometric study aimed to provide data relevant to surgical planning by defining the frequency, morphometric characteristics, and relationship of the mastoid foramen with bony reference points. A total of 185 dry human skulls (370 hemiskulls) of unknown sex were examined. The diameter of the largest mastoid foramen, defined as the major mastoid foramen (MMF), and its distances to the asterion and mastoid process were measured using a digital caliper. The relationship of the MF with the occipitomastoid suture and the transverse plane passing through the zygomatic arch was evaluated. In addition, the number, localization, and quadrant-based distribution of accessory mastoid foramina (MFa) relative to the MMF were assessed. The most common finding was the presence of 2 mastoid foramina (37.8%), followed by a single foramen (34.3%). The MMF diameter ranged from 0.60 to 7.65 mm. The mean distance from the MMF to the asterion was 18.10 ± 5.34 mm, and to the mastoid process was 29.60 ± 5.38 mm. The MF was most frequently located anterior to the occipitomastoid suture and inferior to the zygomatic arch line. Accessory mastoid foramina were identified in approximately half of the hemiskulls and were most commonly clustered in the anterosuperior and posterosuperior quadrants. This anatomical study demonstrates significant variations about the mastoid foramen with respect to frequency, diameter, and spatial relationships to anatomical landmarks. A detailed evaluation of the clinical significance of these variations suggests a potential contribution to the prevention of surgical complications; however, confirmation of their applicability in clinical practice would benefit from further support by radiologic imaging and clinical studies.