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.

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