Effect of Surgical Tightening Torque on the Pull-Out Strength of Screws in Vertebral Body Tethering

Background/Objectives: Screw loosening and vertebral fractures remain common after vertebral body tethering (VBT). Because tightening torque sets screw preload, its biomechanical effect warrants explicit modeling. In this paper, a Finite Element (FE) model, supported by ex vivo porcine vertebral tests, was developed and validated that incorporates torque-induced pre-tension to quantify vertebral stress, aiming toward customizable VBT planning. Methods: An FE model with pre-tension and axial extraction failure was parameterized using ex vivo tests on five porcine vertebrae. A laterally inserted surgical screw in each specimen was tightened to (Formula presented.) Nm. Axial extraction produced failure loads of (Formula presented.) kN. This is also considered in the FE model to validate the failure scenario. Results: Torque alone generated peak von Mises stresses of (Formula presented.) MPa (cortical bone 1) and (Formula presented.) MPa (trabecular), lower than prior reports. With added axial load, peaks rose to (Formula presented.) MPa and (Formula presented.) MPa, exceeding typical ranges. However, predicted failure agreed with experiments, showing (Formula presented.) mm displacement and a conical displacement distribution around the washer. Conclusions: Modeling torque-induced pre-tension is essential to reproduce realistic stress states and anchor failure in VBT. The framework enables patient-specific assessment (bone geometry/density) to recommend safe tightening torques, potentially reducing screw loosening and early fractures.