Computed Tomography Study Of The Difference Between S1 And S2 Alar-iliac Screw Placement In Chinese Adults Population

Objective:To assess the radiographic parameters of S1 and S2 sacral alar iliac (SAI) screws in Chinese adult population and to compare the parameters of the S1-AI and S2-AI techniques.Methods:Three dimensional computed tomography (3DCT) reconstructions of sixty Chinese skeletally mature patients with normal pelvis were analyzed. Trajectories of S1AI and S2AI techniques with maximal length and width were obtained by manipulating and rotating the 3D pelvis. The length and angulation parameters were evaluated and compared.Statistical analysis:The data were expressed as mean ± standard deviations for variables. The differences of the data between S1-AI and S2-AI techniques were analyzed by paired-samples’t test. The measurements between genders were also performed using independent-samples’t test, respectively. Statistical significance was set at p< 0.05. All analyses were carried out using the SPSS (Statistical Package for the Social Sciences, version 17).Results:Based on the ideal trajectory, the ideal trajectory parameters of SI AI and S2AI technique were shown as follows:The S1AI trajectories were more caudal than S2AI trajectories with average sagittal angulation 26° more for males and 24° more for females. The sagittal angulation was 57.25 ± 4.37° (SI) vs.31.02 ± 7.43°(S2) (p<0.05) for male and 58.61 ± 5.11°(S1) vs.34.16 ± 6.02°(S2) (p<0.05) for males. The lateral angulation of SI AI trajectories were average 2° less than that of S2AI trajectories. The lateral angulation was 38.14 ± 3.10 (S1) vs.40.25 ± 2.84°(S2) (p<0.05) for males and 37.24 ± 2.96°(S1) vs.39.25 ± 2.64°(S2) (p<0.05) for females. The maximal length of trajectories were 119.46 ± 3.77mm (S1) vs.120.04 ± 6.52mm (S2) (p=0.551) for males, 108.21 ± 5.24mm (S1) vs.109.47 ± 4.63mm (S2) (p=0.434) for females. Before piercing the sacroiliac joint, the trajectories processed 43.12 ± 4.63mm (S1) vs.35.75 ± 5.03mm (S2) (p<0.05) for males,39.73 ± 5.85mm (S1) vs.32.16 ± 5.28mm (S2) (p<0.05) for females in the sacrum. Bilateral iliac width along the optimal trajectory was mm 22.05 ± 4.91 (S1) vs. mm 21.49 ± 3.22 (S2) (p=0.672) for male and 16.93 ± 2.12mm (S1) vs.15.35 ± 2.71mm (S2) (p=0.366) for female. The starting point of S1AI technique was deeper and more lateral from the midline than that of S2AI technique.Conclusion:The application of S1AI and S2AI screws into the sacrum and ilium of Chinese adult population is feasible along with the angle and length of ideal trajectory. Differences between S1AI and S2AI ideal trajectories and between genders should be noticed in clinical operation. Preoperative CT scans and reconstruction should be performed to evaluate proper trajectory parameters and to choose appropriate screws.