[Entry point and screw path study of second sacral alar-iliac screw trajectory in spinal pelvic fixation under O-arm navigation].

<b>Objective:</b> To explore the feasibility of using the entry point and screw path parameters of sacroiliac (S₂AI) screws inserted under O-arm 3D computer navigation as a reference for freehand screw insertion in patients with degenerative spinal deformities. <b>Methods:</b> A retrospective analysis was conducted on the clinical data of 66 patients with degenerative spinal deformities who received S₂AI screw fixation assisted by the O-arm 3D computer navigation system at Nanjing Drum Tower Hospital Affiliated with Nanjing University Medical School from January 2017 to April 2022. The patients included 6 males and 60 females, with a mean age of (64.3±5.9) years. Preoperatively, the entry point was set as the intersection of 1 mm from the outer and lower edges of the S₁ foramen, adjusted intraoperatively by navigation, and verified postoperatively by full spinal CT scans and 3D reconstructions. Using the S₁ screw entry point as the origin, the position of the bilateral S₂AI screw entry points, tailward angulation (SA), outward angulation (TA), and vertical distance from the entry point to the skin (SD) were measured on postoperative CT 3D images. The accuracy of S₂AI screw placement and any intraoperative and postoperative complications were recorded. Differences in entry point coordinates and screw path parameters between the left and right sides were compared. The intraclass correlation coefficient (ICC) was used to assess intra-observer and inter-observer agreement. <b>Results:</b> The coordinates and screw path parameters for the S₂AI screws placed using the O-arm navigation demonstrated good intra-observer and inter-observer consistency (ICC>0.75). The left entry point was located (8.08±1.39) mm laterally and (24.47±2.20) mm caudally from the S₁ entry point, while the right entry point was (8.09±1.41) mm laterally and (24.40±2.54) mm caudally, with no significant difference between the left and right sides (both <i>P</i>>0.05). The left TA was 46.33°±3.44°, SA was 39.14°±6.12°, and SD was (60.38±13.37) mm; the right TA was 46.37°±3.41°, SA was 39.59°±5.89°, and SD was (60.30±12.24) mm, with no significant differences between the left and right parameters (all <i>P</i>>0.05). There was no significant neurovascular complications intraoperatively or two weeks postoperatively, with a screw insertion accuracy of 97.7% (129/132). <b>Conclusions:</b> In the fixation of the pelvis in degenerative spinal deformities, the use of O-arm navigation-assisted S₂AI screw insertion has a high accuracy rate. It is recommended that for freehand screw insertion, the entry point should be located 24 mm caudally and 8 mm laterally from the outer and lower edges of the superior articular process of S₁, with an TA of approximately 46° and a SA of approximately 39°.