Optimizing S1 nerve root block using three-dimensional computed tomography imaging: Identifying the ideal fluoroscopic angle and predicting S1 neural foramen position.

Background: S1 nerve root block (S1NRB) is frequently used to diagnose and treat lumbosacral and lower limb pain. However, the visibility of the S1 neural foramen can be obscured by factors including intestinal gas, increasing the procedure's technical difficulty. This study identified the optimal fluoroscopic angle for S1NRB and standardized the technique using three-dimensional (3D) computed tomography (CT) images.

Methods: We analyzed 3D CT images of 101 patients with lumbar degenerative diseases (lumbar disc herniation and lumbar spinal canal stenosis). The reference position angle (RPA) was defined as the angle where the superior endplate of the sacrum appears straight, whereas the tunnel view angle (TVA) was defined as the angle where the anterior and posterior S1 neural foramina coincide. We evaluated the relationship between the RPA and TVA, measured the position of the S1 neural foramen at the RPA using the S1 spinous process and sacroiliac joint as landmarks, and analyzed correlations with sex and body size.

Results: The mean RPA and TVA were 32.4 ± 6.4° and 34.0 ± 6.2°, respectively, with a mean difference of 1.5 ± 2.8° (r = 0.897, p < 0.0001). The horizontal distance from the S1 spinous process to the S1 neural foramen was 23.1 ± 2.1 mm, and to the sacroiliac joint was 52.0 ± 4.3 mm, with a ratio of 44.5 ± 3.4 %. The vertical distance from the S1 spinous process to the S1 neural foramen was -1.2 ± 1.7 mm caudally. In 84.2 % of the cases, the S1 neural foramen was located 0-4 mm caudal to the S1 spinous process and at 40-50 % of the horizontal distance between these landmarks. These parameters showed no significant differences based on sex or body size.

Conclusions: RPA, using the superior endplate of the sacrum as a reference, closely approximates the TVA and is a useful indicator of the optimal fluoroscopic angle for S1NRB. The S1 neural foramen position can be predicted using anatomical landmarks, irrespective of patient characteristics.