The S1 dorsal foramen: Nuances of anatomy.

Background: Access to the dorsal S1 neural foramen is frequently performed, but can be challenging.

Purpose: To report previously undescribed nuances of dorsal S1 foraminal anatomy, including a bony flange that may obstruct access to the foramen.

Methods: Fluoroscopic - advanced imaging correlations of the S1 foraminal aperture were performed. The anatomy of the S1 foramen was examined in 27 cadaveric specimens (N ​= ​49 S1 foramina) and the foraminal and flange dimensions quantified. The S1 foramen was examined in 50 CT datasets (N ​= ​100 S1 foramina), and its dimensions quantified in PA and ipsilateral oblique orientations.

Results: The medial aspect of the S1 dorsal foramen is variably shielded by a bony flange, which extends from medial to lateral. The foraminal aperture is covered by a ligament extending from this flange to the lateral aspect of the foramen. On the cadaveric specimens, the mean vertical dimension of the foraminal aperture was 8.6 ​mm; the transverse dimension was 8.9 ​mm. The mean transverse dimension of the flange was 2.8 ​mm. The CT images demonstrated that the transverse dimension of the S1foraminal aperture increased from a mean of 6.23 ​mm in the PA orientation to 7.90 ​mm in the ipsilateral oblique.

Discussion: The bony flange at the S1 dorsal foramen shields its medial aspect; the flange on average covers one-third of the transverse dimension of the foramen, and in 14% covered one-half. The flange has little cortical bone at its margins and may be unseen at fluoroscopy. Ligamentous tissue extending laterally from the flange covers the remainder of the foramen. The transverse dimension of the foramen increased significantly from a posterior-anterior trajectory to an ipsilateral oblique trajectory, as the flange no longer obstructed access.

Conclusion: Access to the S1 dorsal foramen may be confounded by an unseen bony flange covering its medial aspect. An ipsilateral oblique trajectory targeting the lateral margin of the foramen will maximize successful access.