Abstract
BACKGROUND: A classic posterior starting point of sacral-alar-iliac screw (SAIS) for spinopelvic fixation may not be optimal for sacroiliac joint fixation. We aimed to compare the safety and biomechanical performance of SAIS/reverse SAIS (RSAIS) implanted via the classic (lateral S1-S2 dorsal foramina border) versus modified (medial S1-S2 dorsal foramina border) posterior starting point for sacroiliac joint fixation. METHODS: Eighty normal pelvic CT scans were used to measure the outer widths (OWs) of the SAIS/RSAIS posterior starting point of the classic and modified methods. 7.3 mm SAIS were implanted using the two methods on each side of 20 3D-printed models to examine sacral-side screw trajectory penetration. Fourteen Tile C1 injury models were divided into two groups and fixed with S2AIS implanted using either the classic (penetrating) or modified (non-penetrating) method to compare the stiffness and maximum loads of the final fixations. Six patients received percutaneous SAIS/RSAIS fixation using the modified method. Screw positions (Smith criteria), pain (visual analogue scale, VAS) and function (Majeed score) were assessed. RESULTS: The modified method gave larger OWs for both S1AIS (7.80 vs. 5.11 mm) and S2AIS (5.85 vs. 3.04 mm) and reduced sacral-side screw trajectory penetration (S1AIS: 2.5% vs. 25%; S2AIS: 60% vs. 95%) than the classic method (all p < 0.05). Modified method fixation exhibited 13.8% higher stiffness and 26.2% greater maximum load than the classic method (p < 0.05). All SAIS/RSAIS positions were rated "excellent". During a postoperative follow-up of 7-20 months, no screw loosening or fixation failure occurred. Mean VAS increased from 5.6 preoperatively to 1.6 postoperatively, and mean Majeed from 59.7 to 81. CONCLUSION: The medial border of S1-S2 dorsal foramina provides a safer and more stable posterior starting point for SAIS/RSAIS in sacroiliac joint fixation, particularly for S2AIS/RS2AIS, outperforming the lateral border method.