Safety and Early Clinical Outcomes of Sacroiliac Fusion for Sacral Fractures via a Modified S1 Corridor: An Exploratory Comparison of Triangular and Threaded 3D-Printed Titanium Implants

Background and Objectives: Fragility fractures of the sacrum are increasingly recognized in elderly osteoporotic patients and can cause persistent pain, immobility, and progressive pelvic ring instability. We previously described a modified S1 corridor targeting the higher-density cranial–ventral S1 vertebral body for safe percutaneous implant placement. This study reports the clinical outcomes of osteoporotic and pathologic sacral fractures treated through this corridor and presents an exploratory comparison of two 3D-printed titanium sacroiliac fusion implants: the triangular iFuse™ Implant System and the threaded iFuse TORQ™ (SI-BONE, Inc., Santa Clara, CA, USA). Materials and Methods: This retrospective single-center study analyzed 58 patients with fragility or pathologic sacral fractures (FFP IIIb–V; OF 2–5) treated via the modified S1 corridor between January 2021 and March 2026. Of these, 27 patients received triangular iFuse™ implants, and 31 patients received threaded iFuse TORQ™ implants. The primary outcome was procedural safety (implant revision, neurological injury, cortical breach). Secondary outcomes included the Oswestry Disability Index (ODI), Visual Analog Scale (VAS) for pain, operative time, and need for additional fixation. Group comparisons used non-parametric tests; multivariable linear regression adjusted for age, fracture severity (OF classification), and additional fixation. Results: At a mean follow-up of 17.8 ± 9.1 months (iFuse) and 4.1 ± 2.1 months (TORQ), no implant revisions, clinically significant cortical breaches, or neurological injuries were observed in either group. Patient-reported outcome data were available for 48 of 58 patients (20 iFuse, 28 TORQ). Patients in the iFuse™ group were older (80.9 ± 8.5 vs. 76.0 ± 9.8 years; p = 0.050), and OF classification differed between groups, with a higher proportion of OF 4 fractures in the TORQ™ group (p = 0.003). After adjustment for age, OF classification, and additional fixation, TORQ™ implantation was associated with a 15.6-point lower postoperative ODI (p < 0.001), a 13.5-point greater ODI improvement (p < 0.001), a 0.94-point lower postoperative VAS (p = 0.006), and a 1.36-point greater VAS reduction (p < 0.001). Findings remained significant in a sensitivity analysis restricted to patients without additional fixation. Conclusions: Sacroiliac fusion via a modified density-oriented S1 corridor was safe for both large implant types, with no revisions in the analyzed cohort. In this retrospective, non-randomized cohort, threaded iFuse TORQ™ implants were associated with better patient-reported outcomes after adjustment for baseline imbalances. Because the groups were not randomized and follow-up duration differed substantially, these comparative findings are hypothesis-generating and require confirmation in prospective, controlled studies.