Biomechanical advantages of double-trajectory screws technique combined with multiple-rod constructs in lumbar pedicle subtraction osteotomy: a finite element analysis study.

INTRODUCTION: Although multi-rods constructs (Multi-RCs) have been demonstrated to reduce the risk of rod fracture (RF) following pedicle subtraction osteotomy (PSO), they offer limited improvement in terms of screw-related complications. The double-trajectory technique (combining traditional trajectory (TT) and cortical bone trajectory (CBT) screws) has been demonstrated to enhance screw fixation. Consequently, the employment of the double-trajectory technique in conjunction with Multi-RCs has the capacity to augment the fixation strength of both rods and screws. Thus, the study aimed to evaluate the biomechanical performance of a novel configuration combining double-trajectory screws with multi-rod in lumbar PSO. METHODS: A validated L1-5 nonlinear FE model was modified to simulate a 25° PSO at L3. Three configurations were analyzed under both normal and osteoporotic conditions: (a) 2R-TT (one rod with TT screws on each side); (b) 4R-TT (one rod with TT screws on each side, supplemented by a lateral satellite rod); and (c) 4R-TT/CBT (one rod with TT screws and one rod with CBT screws on each side). Loads of 400 N and 7.5 Nm in various directions were applied, with L5 fixed. Outcomes measured included range of motion (ROM) and maximum von Mises stress on rods and screws. RESULTS: The 4R-TT/CBT configuration had the smallest ROM in all directions, with reductions of 17.3%-26.4% compared to 2R-TT and 10.9%-16.8% compared to 4R-TT. It also showed the lowest von Mises stress on rods, particularly in flexion, where reductions ranged from 20.5% to 21.4% compared to 2R-TT and 20.9%-21.4% compared to 4R-TT. Considering the maximum von Mises stress on the screws, it exhibited the lowest and demonstrated a substantial reduction of 8.9%-22.8% and 19.5%-29.8% compared to the 2R-TT and 4R-TT configurations, respectively. CONCLUSION: The 4R-TT/CBT configuration provided superior biomechanical performance in lumbar PSO, enhancing construct stiffness, and substantially reducing stresses on rods and screws compared to the 2R-TT and 4R-TT configurations. However, this was a FE study with inherent simplifications. Therefore, the findings should be further confirmed by cadaveric or clinical studies in the future.