Experimental and micro-finite element analysis of the toggling behaviour of a carbon fibre-reinforced PEEK pedicle screw

Primary stability of pedicle screws is critical for successful spinal fixation, particularly in osteoporotic patients, where screw loosening occurs in over 50 % of cases. This study combined experimental and numerical approaches to evaluate the primary stability of carbon fibre-reinforced polyether-ether-ketone (CF/PEEK) screws under toggling loading conditions. Fifteen human cadaveric vertebrae with low bone mineral density were tested using a multidirectional loading setup with custom-designed, 3D-printed guides for accurate screw placement. Toggling tests were conducted under displacement-controlled linear loading-unloading ramps with increasing displacements (1, 2, and 4 mm). To complement the experiments, a sample-specific, explicit non-linear micro-finite element (μFE) model was developed to simulate the toggling tests. Experimental and numerical peak forces and tangential stiffnesses were compared at different displacement levels. Experimentally, no significant correlation was found between mean bone volume fraction in the vicinity of the screw and peak forces. Similarly, volumetric bone mineral density did not show significant correlation with the experimental biomechanical variables. However, μFE-derived peak forces showed significant correlation with experimental measurements at all displacement levels (R² = 0.67, R² = 0.60, R² = 0.59 at 1 mm, 2 mm and 4 mm displacement, respectively), with the model slightly overestimating the experimental values at higher displacements (e.g., 4 mm). Across all data points, μFE and experimental peak forces strongly correlated (R² = 0.92), following a power-law relationship. This study demonstrates that non-linear μFE models can reliably predict the primary stability of pedicle screws, offering potential for optimizing screw designs and reducing clinical failure rates associated with loosening.