A distal start point facilitates safer insertion of S2-alar-iliac screws fixation

This study aimed to show that a more distal insertion of S2-alar-iliac screws (S2AI) towards the S2 foramen facilitates a more natural trajectory and results in fewer breaches of the iliac wing due to the anteroposterior relationship of the sacrum and pelvis. Thirty-six consecutive patients who had received S2AI and postoperative computed tomography (CT) imaging were retrospectively reviewed. A freehand technique was used to insert all screws. Screws were stratified into two groups (Zone A and Zone B) based on the insertion point within the sacrum. Zone A was defined as screw insertion proximal to the second sacral foramen. Zone B was defined as screw insertion at or distal to the level of the second sacral foramen. CT imaging was reviewed to identify passage of the screw through either the fibrous or bony sacroiliac joint, and to check for instrumentation breach of the iliac wing. Sixty-two S2AI in 36 patients were identified. Twenty-six screws (42%) were inserted into Zone A and 36 (58%) into Zone B. There were 11 instrumentation breaches of the iliac wing in Zone A (42%) and 0 in Zone B (p < 0.0001). In Zone A, 24 screws (92%) passed through the upper, fibrous portion of the sacroiliac joint. In Zone B, no screws passed through this region (p < 0.0001). In conclusion, insertion of S2AI at or distal to the second sacral foramen facilitates improved screw placement due to the anteroposterior relationship of the sacrum and pelvis at this level.