Purpose <p>Mechanical failure after minimally invasive posterior pelvic fixation remains a major challenge in elderly patients with sacral fragility fractures (SFF), particularly in the presence of severe osteoporosis. Multiple bilateral fixation strategies are currently used, yet their relative mechanical reliability remains poorly understood. This study compared four commonly applied bilateral constructs, with a specific focus on the biomechanical behavior of the transsacral bar (TSB).</p> Methods <p>A retrospective cohort study was conducted including 164 patients aged ≥ 60 years who underwent minimally invasive fixation for Fragility Fractures of the Pelvis (FFP) Types II–IV between 2018 and 2025. Patients were treated using one of four standardized constructs: (1) two long transsacral screws (2LS), (2) a hybrid configuration combining one long with two short screws (1L2S), (3) four short iliosacral screws placed in a multiplanar arrangement (4&#xa0;S), or (4) a TSB construct applied either as a dual-bar configuration or combined with antirotational screws. The primary endpoint was radiographic loosening at 3 and 12 months. Secondary outcomes included operative time, pain trajectories, early mobility, and complications. Multivariable logistic regression was used to identify independent predictors of loosening.</p> Results <p>Implant loosening occurred in 17% of patients after 2LS, 6% after 1L2S, and 4% after 4&#xa0;S fixation, whereas no loosening was observed following TSB fixation (<i>p</i> = 0.013). Operative duration differed among techniques, with the TSB group averaging 70.6&#xa0;min. Pain improved in most patients by 3 months and continued to decline at 12 months. Mobility recovery was favorable across all groups. Regression analysis identified fixation technique as the only independent predictor of loosening (<i>p</i> = 0.0055). No mechanical failures or revisions occurred in the TSB cohort.</p> Conclusions <p>Despite all constructs being bilateral, their mechanical behavior differed substantially. The TSB construct demonstrated the highest stability, with no cases of loosening, suggesting a distinct biomechanical advantage in osteoporotic sacral fixation. This advantage likely relates to transiliac cortical anchorage combined with locking-nut–mediated preload, which may reduce implant micromotion. These findings support the TSB as a robust fixation option in elderly patients with compromised bone quality. Prospective studies are warranted to further delineate its clinical role across varying fracture patterns.</p> Level of Evidence <p>Level III, retrospective cohort study.</p>