Anatomic safeguards and the low risk of vascular injury during percutaneous tricortical S1 pedicle screw fixation – a cadaver study

Percutaneous tricortical S1 pedicle screw fixation (PTSPSF) offers superior fixation compared to bicortical or monocortical methods but poses potential risks to major intrapelvic vessels. Strangely, clinical complications are rare. This study aims to elucidate the reasons for this discrepancy. In 17 fresh human cadavers, 34 pedicle screws were placed under fluoroscopic guidance, ensuring the distal two threads to exceed the anterior cortical promontory wall. 12 specimens were turned supine and anatomically dissected. The relation between the sacral promontory tip, screw tips and intrapelvic vessels was carefully examined. In 5 specimen 3D fluoroscopy was conducted to measure the distance between the screw tips and the surrounding blood vessels. No intraluminal screw placements were observed. In 2 cases (8.3%), screw tips penetrated the anterior longitudinal ligament and abutted the right common iliac vein (CIV), but without causing impression or injury of the vessel wall. In 2 other cases the ligament remained intact and the screw trajectory pointed dorsally to the CIV. Again the wall of this vessels remained intact. In 84%, the screws were surrounded by osteophytes and did not touch or point to major vessels. Osteophytes at the sacral promontory are typically not visible during lateral fluoroscopy-guided surgery, yet they are commonly present in older individuals. As demonstrated in our study, screw trajectory plays a critical role in avoiding vascular injury, with osteophytes potentially offering an added layer of protection to the right common iliac vein (CIV) and other major vessels. In a post-mortem setting, our findings suggest that even when screw tips penetrate the anterior longitudinal ligament and extend two threads beyond the bone, there is no direct injury to the walls of major blood vessels.