![]() ![]() Sagittal, coronal, and axial views showing measurement methodology for the patient with Castellvi type IIIa transitional anatomy shown in Fig. Used in combination, the PSIS and sacral laminar slope provide anatomical landmarks on both the ilium and the sacrum, which allow a safe screw trajectory above the sciatic notch, while adjusting for variability in the sagittal screw angle in relation to pelvic tilt ( Fig. A line perpendicular to the sacral laminar slope determines the sagittal screw trajectory. The PSIS is used to determine the cephalocaudal location of the starting point along the lateral border of the dorsal sacral foramina. There are two important local anatomical landmarks that are critical to reliable placement of freehand S2AI screws: 1) the posterior superior iliac spine (PSIS), and 2) the sacral laminar slope. Figure is available in color online only. White arrows show approximate starting point, 15 mm cephalad to the caudal PSIS, and trajectory of the S2AI screws. Triangles indicate the most caudal aspect of the PSIS. Dotted line highlights asymmetrical position of sciatic notches (*) bilaterally. Posterior view of right-sided Castellvi type IIIa lumbosacral junction, associated with L5 contribution to the SI joint and elevation of right hemipelvis. In this special setting, using the “one size fits all” starting point caudal to the S1 dorsal foramen can result in unreliable screw trajectories, especially when placing the screws without fluoroscopy. However, freehand placement of S2AI screws can be challenging, especially in the setting of transitional lumbosacral anatomy, where unilateral lumbarization/sacralization of the transitional vertebra introduces asymmetry in the dorsal sacral landmarks and occasionally elevates the hemipelvis ( Fig. We recently published our series of patients treated with freehand S2AI screws along with a description of the technique, demonstrating safe and reliable placement of 100 consecutive S2AI screws with no neurovascular complications. There have been few published studies regarding freehand placement of S2AI screws. This method improves intraoperative efficiency by allowing a seamless transition from freehand pelvic instrumentation to freehand thoracolumbar screw placement, which decreases radiation exposure and decreases surgeon fatigue by obviating the need to wear lead protective gear. 6, 7, 11, 13 The senior author prefers to place S2AI screws using the freehand technique guided by local anatomical landmarks only, without intraoperative fluoroscopy. ![]() 4 Multiple techniques have been described to optimize screw placement, including fluoroscopic guidance, robotic assistance, surgical navigation, and manual palpation of the greater trochanter. S2AI screws provide stable pelvic fixation with screw heads that are in line with S1 screws, with much less prominence, and require much less tissue dissection compared to traditional pelvic screws. T he S2-alar-iliac (S2AI) screw for spinopelvic fixation has enjoyed a tremendous increase in popularity over the past decade. ![]()
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