S2-Alar-Iliac Pelvic Fixation With Standard vs Porous Fusion/Fixation Implants: An Inverse Probability Weighted 1-Year Preliminary Comparative Analysis.

BACKGROUND: Recently, porous fusion/fixation implants (PFFIs) with a 3D-printed porous, fenestrated, bone self-harvesting design were developed for S2-alar-iliac (S2AI) spinopelvic fixation. Prior studies have demonstrated the safety and feasibility of using PFFI for S2AI spinopelvic fixation, but their impact on outcomes remains unknown. This study compared 1-year outcomes of PFFI vs standard threaded screws. METHODS: This is a single-center retrospective cohort study evaluating consecutive patients who underwent S2AI spinopelvic fixation between 2019 and 2024. Patients who underwent S2AI fixation for long-segment fusion, high-grade L5-S1 spondylolisthesis, L5-S1 pseudarthrosis, and lumbosacral junction fractures were included. Patients with iliac fixation, prior spinopelvic fixation, injection-proven sacroiliac joint dysfunction, or lack of 1-year clinical and radiographic follow-up were excluded. Baseline demographic, clinical, and radiographic variables were collected. The primary outcome was distal failure. Patients were stratified by S2AI implant design. Inverse probability weighting was then performed to adjust for confounding and cohort imbalance, followed by comparative analysis of 1-year outcomes. RESULTS: Ninety patients were included. Median age (interquartile range [IQR]) was 67 (61-74) years, including 59 (65.6%) women. Median (IQR) number of levels fused was 6.5 (4.0-9.0). There were 33 (36.7%) and 57 (63.3%) patients with PFFI vs standard S2AI implants, respectively. Overall distal failure and re-operation rates were 12.2% and 5.6%, respectively. Injection-proven sacroiliac joint dysfunction following S2AI fixation occurred in 3.3%. PFFI implants were significantly larger in diameter and length, but there were no other significant differences in baseline demographics, spinopelvic parameters, fusion levels, osteotomies, L5-S1 interbody technique, and bone morphogenetic protein-2 use between cohorts. After IPW, there were no significant differences in distal mechanical implant failure, L5-S1 pseudarthrosis, and sacroiliac joint dysfunction at 1 year. The PFFI cohort had a significantly lower rate of pelvic screw loosening (P = 0.011). CONCLUSIONS: In this preliminary short-term (1 year) analysis, PFFI and standard threaded implants demonstrated comparable rates of distal failure and sacroiliac joint dysfunction. Studies evaluating longer-term (≥2 years) clinical and radiographic outcomes are required. CLINICAL RELEVANCE: PFFIs used for S2AI spinopelvic fixation demonstrated similar short-term 1-year outcomes compared with standard threaded implants, with a lower rate of pelvic screw loosening.