Cage-Orientation-Dependent Biomechanics of Endoscopic-Compatible Bidirectional Cortical Bone Trajectory-Modified Translaminar Facet Screw Fixation versus Bilateral Percutaneous Pedicle Screw Fixation in Osteoporotic TLIF: A Finite Element Study

BACKGROUND: Bilateral percutaneous pedicle screw (BPPS) fixation is constrained by cancellous bone dependence in osteoporosis and limited compatibility with endoscopic workflows. The newly proposed bidirectional cortical bone trajectory-modified translaminar facet screw (BCBT-MTFS) fixation enables endoscopic bilateral cortical fixation. Its uncertain biomechanics motivated a comparison with BPPS across cage insertion angles. Methods: Geometry-specific L1-S1 finite element models from three osteoporotic spines simulated L4-5 TLIF with BPPS or BCBT-MTFS at cage insertion angles of 30°, 45°, 60°, and 90° under a 400 N follower preload and 7.5 N·m pure moments. Multilevel range of motion (ROM), adjacent-disc and implant stresses were designated as primary outcomes, alongside additional measures of implant-bone interface loading. The Angle Sensitivity Index (ASI), Composite Mechanical Response (CMR), Mechanical Coupling Index (MCI), and Integrated Advantage Score (IAS) were calculated from the primary outcomes to integrate multidimensional biomechanical responses and evaluate overall construct performance. Results: Relative to BPPS, BCBT-MTFS reduces multilevel ROM (12.7-46.2%) and rod stress (24.5-60.3%) in flexion, selectively lowers cranial disc and rod loading under axial rotation and lateral bending, and reduces estimated screw-bone shear force across all tested conditions. Trade-offs include higher cranial disc stress in flexion and, in extension, weaker restraint (24.5-49.0% greater fused-segment ROM), higher implant loading and cage-endplate pressure. ASI identifies cage and posterior instrumentation stresses as the most cage-insertion-angle-sensitive metrics in BCBT-MTFS, versus disc stresses in BPPS. With increasing cage obliquity, BCBT-MTFS shows smoother CMR and declining MCI, whereas BPPS shows CMR amplification near 60° and stronger high-obliquity coupling. IAS favors BCBT-MTFS for cranial disc, cage, and rod stresses, and BPPS for screw stress and fused-segment control. Conclusions: In osteoporotic TLIF, cage orientation shapes construct biomechanics through fixation-specific load sharing rather than stiffness alone, supporting joint selection of fixation and cage position based on anticipated failure modes, local mechanical reserve, and endoscopic requirements.