Fig. 18

The percutaneous S2AI screw insertion technique. A Entry point determination. Left: Localization of the S1 foramen using a standard probe. The white arrows refer to the S1 upper endplate, the SI joint, and the acetabulum. Middle: Localization of the S2 foramen. Right: The entry point (white star) is 10 mm lateral to the midpoint between S1 and S2 foramina. B Trajectory verification. Left: The probe is advanced keeping its tip in the center of the (teardrop) identified on the oblique obturator view. Middle: In the anteroposterior view, the probe is directed laterally in the transverse plane and caudally in the sagittal plane. Right: The screw is advanced in the direction of the AIIS (white star) (From Yilmaz E, Abdul-Jabbar A, Tawfik T, et al. S2 alar-iliac screw insertion: Technical note with pictorial guide. World Neurosurg 2018;113:296–301, Fig. 2, p. 297; Fig. 4, p. 298.) [55]