Posterior second sacral alar iliac screw insertion: anatomic study in a Chinese population

To provide radiographic parameters for optimal placement of posterior second sacral alar iliac (S2AI) screw for instrumentation and fusion of scoliosis to the second sacral level in a Chinese population.

S2AI screw trajectories were mapped on three-dimensional computed tomography (3DCT) reconstructions of 60 normal adult pelvises. 1 mm inferior and 1 mm lateral to the S1 dorsal foramen were chosen as the entry point, and ideal S2AI screw trajectories were explored by rotating and cutting the 3D pelvis, ensuring that the trajectories were of maximum length and width. The directions and depth of these determined trajectories were then measured.

The ideal S2AI screw trajectories could be found in each pelvis. The left and right screw trajectory parameters for males were shown as follows: angulation was L 29.15 ± 8.60° vs. R 29.96 ± 8.28° (p = 0.286) caudally in the sagittal plane and L 36.49 ± 3.14° vs. R 37.16 ± 3.14° (p = 0.165) laterally in the transverse plane. The maximal and intrasacral lengths of trajectory were L 121.25 ± 8.33 vs. R 120.63 ± 7.54 mm (p = 0.460) and L 26.20 ± 3.31 vs. R 26.92 ± 4.76 mm (p = 0.268). The entry point was L 28.87 ± 3.33 vs. R 29.79 ± 3.55 mm (p = 0.186) lateral to the second sacral midline, and L 44.14 ± 11.87 vs. R 43.89 ± 12.53 mm (p = 0.687) underneath the skin. The trajectories for females were more caudal (L: 34.50 ± 6.56° vs. 29.15 ± 8.60°, p = 0.009; R: 35.72 ± 7.53° vs. 29.96 ± 8.28°, p = 0.007) in the sagittal plane, but the lateral angulation in the transverse plane showed no difference between genders (p > 0.05). The female iliac medullar cavities were obviously narrower than those of males (L: 14.76 ± 2.46 vs. 16.98 ± 3.52, p = 0.006; R: 14.94 ± 2.60 vs. 17.00 ± 2.81, p = 0.005). Although the average maximal length of trajectories for females were about 5 mm shorter than those of males, intrasacral length were equal to those of males. Furthermore, both the distance from entry point to the S2 midline and skin in the transverse plane showed no difference between genders.

The feasibility to insert S2AI screws to the sacrum and ilium in an Asian population along with the ideal entry angle and length of trajectory were identified for clinical practice.