The three-dimensional printed template guided technique for S2AI screw placement and a comparison with freehand technique

Background Sacropelvic fixation continues to present challenges when involved in the adult spinal deformity correction. The S2 alar iliac (S2AI) fixation is commonly used in sacropelvic fixation. Several techniques, including intraoperative navigation and freehand technique, were used for S2AI screws placement. The aim of this study is to analyze the anatomic parameters for S2AI screw trajectory in Asian population and introduce a novel technique described as three-dimensional printed template guided technique (TGT). Meanwhile, the accuracy and safety of this technique were compared with conventional freehand technique. Methods The S2AI trajectory parameters were measured in100 Asian adult volunteers. Parameters were compared between different genders. Forty-eight adult patients who underwent S2AI screw placement were reviewed, 28 patients received freehand technique and 20 patients received TGT technique. Postoperative CT was used to assess the accuracy of screw trajectory and cortex violation related complications were recorded. Results The cephalocaudal angles (CA), maximal length of screw pathway (ML), narrowest width of pathway within the iliar teardrop (NW), distance from the center of teardrop to sciatic notch(SD) and distance of the start point distal to S1 dorsal foramen(DD) showed significant gender-related difference (p<0.05). All 48 patients were placed S2AI screws bilaterally (40 screws in TGT vs 56 screws in freehand). One screw penetrated iliac cortex in TGT group but 10 screws penetrated iliac cortex in freehand group (3% vs 17.9%) (p<0.05). Conclusion Approximately 30-35° of cephalocaudal angle and 39°mediolateral angle are appropriate for S2AI screw placement in Asian patients. Either freehand or TGT technique is safe for S2AI screw placement. TGT technique is more accurate compared with conventional freehand technique. Trial registration This is a retrospective study. Key words Sacropelvic fixation, Second sacral alar iliac (S2AI) screw, three-dimensional printed template, freehand technique