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Objective : The aim of this study was to measure radiographic parameters for optimal placement of posterior second sacral alar screw(S2AS)for instrumentation and fusion. Methods: Three dimensional computed tomography(3D CT)reconstructions of 60 normal adult pelvises were generated to map the S2 alar screw.The entry point was typically chosen lateral and superior to the S2 dorsal foramen.Ideal S2AS trajectories were explored by rotating and cutting the 3D pelvis,ensuring that the trajectories were of maximum length and width.After an optimal trajectory and an entry point were identified,ralated linear anatomic parameters as well as sagittal and transverse angles were determined. Results: CT scans from sixty patients were reviewed.The ideal S2 alar screw trajectories could be identified in each pelvis.According to this morphometric study,the trajectories for females were more lateral in the transverse plane(F: 33.73 ± 5.99 vs.M: 29.82± 4.11,p = 0.000),but the caudal angulation in the sagittal plane showed no difference between genders(F: 48.13 ± 4.85 vs.M: 48.22± 2.58,p = 0.199).The maximal lengths of trajectory were significantly longer than female(F: 40.82 ± 4.29 vs.M: 43.42± 4.02,p = 0.001).The distance from entry point to the S2 midline(F: 23.36 ± 2.98 vs.M: 23.10± 2.74,p = 0.613),and skin in the transverse plane showed no difference between genders(F: 34.92 ± 8.89 vs.M: 35.38± 9.88,p = 0.568).All the left and right screw trajectory parameters in females were shown no differences: sagittal angle were L 49.17 ± 4.77 vs.R 49.10 ± 5.01(p = 0.958)and laterally transverse angle were L 33.30 ± 6.14 vs.R 34.17 ± 5.92(p = 0.580).The maximal lengths of trajectory were L 41.02 ± 3.99 vs.R 40.62 ± 4.62 mm(p = 0.717).The entry point was L 23.39 ± 3.17 vs.R 23.33 ± 2.83 mm(p = 0.939)lateral to the second sacral midline,and L 34.52 ± 8.78 vs.R 35.33 ± 9.13 mm(p = 0.728)underneath the skin.All the left and right screw trajectory parameters in males were shown no differences: sagittal angle were L 48.33 ± 2.42 vs.R 48.10 ± 2.75(p = 0.729)and laterally transverse angle were L 29.56 ± 3.53 vs.R 30.06 ± 4.66(p = 0.642).The maximal lengths of trajectory were L 43.27 ± 3.45 vs.R 43.56 ± 4.57 mm(p = 0.783).The entry point was L 23.36 ± 2.59 vs.R 22.83 ± 2.90 mm(p = 0.459)lateral to the second sacral midline,and L 35.38 ± 9.49 vs.R 36.38 ± 9.39 mm(p = 0.688)underneath the skin. Conclusion: S2 alar screw can serve as an alternative sacral fixation point to provide additional biomechanical stability of lumbosacral constructs.It is possible to determine a trajectory with ideal entry and maximum length through the S2 alar by 3D CT.Individual trajectory in each case with CT imaging should be indentified before surgery.