Does the Position of the Polyaxial Screw Head in Patients With L5-S1 Stabilization Lead to an Increased Difficulty in L5 Transforaminal Nerve Injection? A Three-Dimensional Computerized Tomography Study

Background Challenges may be encountered if transforaminal nerve injection (TFNI) is required in patients who have undergone posterior transpedicular stabilization (PTS) surgery to the L5-S1 level. In this study, we investigated the contributory factors that lead to these challenges. Methods We selected 125 patients who underwent PTS surgery involving the L5-S1 segment, between 18 to 70 years of age to be included in the study. The demographic data of the patients, body mass indexes (BMI), postoperative spondylolisthesis grades, heights of the iliac crest, and the positions of the polyaxial screw head were assessed. The shortest trajectory of L5-TFNI, the distance of the needle entry point (NEP) to the midline, and optimum viewing angles (VA) were measured on the three-dimensional computed tomography (CT) sections. Results Pre-PTS surgery, in males compared to females, NEP was noted to be more medial (p=0.007), the needle trajectory was shorter (p=0.001), and the optimal VA was narrower (p=0.001). Increasing BMI and increasing height of the iliac crest caused the TFNI trajectory to become longer. Post-PTS surgery, angulation of polyaxial screw heads of more than 15 degrees laterally in both genders significantly caused a decrease in VA (p=0.001). Conclusions Using the reconstruction technique in 3D CT, we demonstrated that pedicle screw heads angled laterally, a higher iliac crest height, and an increased BMI make L5-TFNI difficult to be performed. Locking the stabilization system while targeting the most neutral position for polyaxial screw heads during surgery may facilitate the L5-TFNI.