Preoperative 3D-Planned S1 Corridors Transferred into 2D Fluoroscopy Allow for Safe Intraoperative Large-Diameter Implant Placement: Description of a Novel Sacroiliac Fixation Technique and Proof of Concept in 137 Implantations

Background and Objectives: Percutaneous iliosacral screw fixation is a standard treatment for posterior pelvic ring instability and sacral insufficiency fractures. However, conventional transsacral S1 screw fixation is associated with notable complication rates, most commonly implant loosening; dysmorphic sacral anatomy increases the risk of iatrogenic L5 or S1 nerve root injury. This study presents a modified S1 trajectory to engage the high-density bone of the anterior and cranial S1 vertebral body (promontory) by transferring preoperative 3D planning to intraoperative 2D fluoroscopy. Materials and Methods: This retrospective study analyzed implant placements for posterior pelvic ring instability, including high-velocity trauma and fragility fractures of the pelvis (FFPs). Preoperative computed tomography (CT) multiplanar reconstruction defined a modified corridor from a posterior-caudal iliac entry point directed cranially and ventrally into the S1 promontory. The 3D trajectory was transferred intraoperatively using standard 2D fluoroscopy (lateral, anteroposterior, inlet, and outlet views) with the patient prone. In cases of reduced bone quality or intended sacroiliac fusion, 3D-printed titanium implants (triangular or cylindrical threaded, 10.0–13.5 mm outer diameter) were selected over 7.5 mm cannulated screws. Results: Overall, 137 implants were placed in 71 patients: 13 cannulated screws in high-velocity pelvic ring trauma, 72 triangular titanium sacroiliac fusion implants (iFuse Implant System®, SI-Bone), and 52 threaded titanium fusion implants (iFuse TORQ®, SI-Bone) in patients with FFP. The modified trajectory consistently engaged the anterior and cranial S1 vertebral body. Postoperative 3D CT confirmed accurate placement of all implants. No iatrogenic nerve injuries or revisions for implant malposition occurred. Mean follow-up was 12 ± 9 months. Conclusions: Preoperative 3D CT planning combined with standard 2D fluoroscopy guided a modified S1 trajectory toward the cranial S1 vertebral body. Accurate and safe implant placement was achieved in the prone position without navigation systems, providing a practical alternative when standard transverse trajectories are limited by narrow bony corridors or sacral or pelvic dysmorphy.