Biomechanical evaluation of a fully cortical-threaded screw for modified cortical bone trajectory fixation: Combined experimental and finite element study

Although fully cortical-threaded screws are introduced as a design adaptation tailored to the cortical-dominant load path of modified cortical bone trajectory (MCBT) fixation, the biomechanical consequences of this design across the screw-bone interface and fusion construct remain insufficiently defined. Therefore, this study compared fully cortical-threaded MCBT screws with clinically used dual-threaded screws through an integrated experimental and finite element (FE) framework, spanning in vitro ovine vertebral biomechanical test and specimen-specific L4 vertebra and L1-S1 fusion models. At the screw level, compared with the control group, fully cortical-threaded MCBT screws increased maximum pull-out strength by 111.2% and multidirectional stiffness by 75%-89% in L4 vertebral FE analysis, with biomechanical testing showing corresponding increases of 39.1% in insertion torque and 41.9% in maximum pull-out strength. At the fusion-construct level, fully cortical-threaded MCBT fixation limited fused-segment motion and decreased stress across the cage, instrumentation, and vertebral bone, indicating a more coordinated load-transfer pattern rather than a simple increase in interface-level strength. These effects were consistent across fusion strategies, but procedure-specific mechanics remained evident, with PLIF producing more symmetric load sharing and TLIF retaining intrinsic asymmetry owing to unilateral facet joint resection. Overall, the fully cortical-threaded screw design for MCBT promoted continuous and stable bone-screw load transfer, translating interface-level gains into coordinated load distribution, greater fusion-construct stability, and lower deformation-driven stress concentration. These findings indicate that aligning screw architecture with the cortical-dominant load path is a mechanically rational design strategy within MCBT fixation, particularly in biomechanically demanding settings.