INTRODUCTION: S1 nerve root block (S1NRB) is widely used for diagnosing and treating lumbosacral and lower limb pain. However, visualization of the S1 neural foramen can be obscured by factors such as intestinal gas, increasing procedural difficulty. Additionally, fluoroscopy-guided procedures raise concerns about radiation exposure for both patients and medical staff. This study aimed to identify the optimal fluoroscopic angle for S1NRB and standardize the technique using three-dimensional computed tomography (3D-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, while the tunnel view angle (TVA) was defined as the angle where the anterior and posterior S1 neural foramina coincide, representing the optimal trajectory for needle insertion in S1NRB. We evaluated the relationship between RPA and TVA and measured the position of the S1 neural foramen at the RPA using the S1 spinous process and sacroiliac joint as landmarks. Correlations between these parameters and patient characteristics (sex, height, weight, BMI) were analyzed. Lumbar lordosis and sacral slope were also evaluated using lateral X-ray images.
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±2.1mm, and to the sacroiliac joint was 52.0±4.3mm, with a ratio of 44.5±3.4%. The vertical distance from the S1 spinous process to the S1 neural foramen was -1.2±1.7mm caudally. In 84.2% of cases, the S1 neural foramen was located 0-4mm 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. Furthermore, in 96% of patients, the S1 neural foramen was located within 35-55% horizontally and 0-6mm caudally of these reference points.
DISCUSSION: Our study demonstrates that RPA, using the superior endplate of the sacrum as a reference, strongly correlates with TVA and serves as a useful indicator for determining the optimal fluoroscopic angle for S1NRB. Additionally, the anatomical position of the S1 neural foramen can be predicted using anatomical landmarks, irrespective of patient characteristics. These findings have the potential to improve the efficiency, safety, and success rate of S1NRB procedures. The standardized, patient-specific approach proposed in this study enables more accurate localization of the S1 neural foramen and may contribute to reducing fluoroscopy time and radiation exposure.