Estimation of pedicle screw fixation strength from probe indentation force and screw insertion torque: A biomechanical study on bone surrogates of various densities

Posterior pedicle screw fixation is commonly used for patients with spinal disorders. However, failure of fixation is reported in many cases and surgeons have only little information. The objective of this study was to assess the correlation between the probe indentation force, screw insertion torque and the pullout force using bone surrogates of different densities. The indentation force and insertion torque were measured using a custom made test bench during screw insertion into polyurethane foam blocks. The two variables were significantly correlated to pullout force and to density. A high correlation was also found between indentation force and the peak insertion torque. The proposed methods for measuring indentation force and screw insertion torque were reproducible. This study suggests that the peak screw insertion torque and the indentation force can predict the screw fixation strength in synthetic bone models. Additional tests should be performed on animal and human specimens to confirm and to translate these findings to clinical applications.