Retrospective lumbosacral CT analysis and prospective observational study of the ipsilateral tunnel view technique for fluoroscopy-guided selective S1 transforaminal epidural injection

Background The purpose of this study was to retrospectively observe the anatomic relationship between dorsal S1 foramen (DS1F) and ventral S1 foramen (VS1F) through computed tomography (CT) analysis and to prospectively determine the optimal angle of ipsilateral tunnel view technique for performing S1 transforaminal epidural steroid injection (S1-TFESI). Methods The axial lumbosacral CTs taken between in 208 consecutive patients and the following measurements were obtained on both sides: (1) the alpha-angle was defined as an angle between a sagittal line passing through the center of the sacrum and an imaginary line passing through the center of DS1F, (2) the largest diameter of DS1F and VS1F. The fluoroscopy was adjusted to show the largest L5/S1 intervertebral disc space, which was defined as the cephalad angle, and tilted to the ipsilateral oblique side until the entrance of DS1F had a well-defined, round shape, which defined as the beta-angle in 40 humans. Results CT measurements showed that the alpha-angle was 26.3 +/- 3.3 degrees (15-38 degrees) and the diameter of DS1F was 7.1 +/- 0.7 mm (4-10.9 mm), which was significantly smaller than the diameter of VS1F, 10.1 +/- 1.0 mm (7.2-13.8 mm). The beta-angle was 24 +/- 4.6 degrees, which was not much different from the alpha-angle and the cephalad angle was 23 +/- 4.6 degrees. The success rate of S1-TFESI was 100% and there were no procedure-related complications. Conclusions The entrance of DS1F is easily identified with an ipsilateral 25 degrees-tunnel view technique while performing S1-TFESI, and it is a clinically applicable approach.