Biomechanical investigation of the factors related to pedicle screw fixation strength

Spinal pathologies or injuries can severely compromise the quality of life for the patients. The surgical intervention is often performed using internal fixation devices. Fixation with pedicle screws is a well-established method providing spinal stability and deformity correction. However, reported rates of fixation failure because of screw loosening have become a major concern, especially with the appearance of new and more powerful surgical techniques. Numerous experimental studies have been devoted to pedicle screw fixation strength evaluation. The evaluation methods are commonly including preoperative measurements of bone mineral density, screw insertional torque measurement and pullout tests. Bone mineral density measurement gives only to some extent an estimation of the pedicle screw fixation strength. Several studies indicated that the screw insertional torque measurement can provide predictive information on the fixation strength. However, the latter was not confirmed by other studies. This controversy illustrates the need for improving the understanding of factors related to pedicle screw fixation strength. In addition, there is a need for better understanding the mechanisms of pedicle screw loosening leading to failure and their effects on the fixation strength. \n \nThe main objective of this doctoral thesis was to improve the understanding on the mechanisms of pedicle screw loosening and the factors related to pedicle screw fixation strength. This objective is related to two hypotheses: 1) the indentation force measured while performing the pilot hole and the torque observed during screw insertion are related to the screw pullout force and stiffness; 2) cyclic bending load (toggling) on pedicle screw in craniocaudal (CC) and mediolateral (ML) directions loosens the screw and affects the pullout force and stiffness. \n \nThree specific objectives were defined to verify the hypotheses using two experimental protocols. The first specific objective was to develop and validate tools measuring the indentation force while performing the pilot hole and the insertional torque during pedicle screw insertion. The second objective was to compare the screw loosening mechanisms through toggling in different modes and evaluate their effects on pedicle screw pullout force and stiffness. Finally, the third objective was to establish the relationships between the indentation force, the insertional torque and the screw pullout force and stiffness. \n \nThe first protocol was performed on synthetic bone surrogates mainly to explore the first specific objective. Furthermore, to account for the effect of various bone densities and toggling modes on pullout force and stiffness, pedicle screw were pulled out with and without toggling from synthetic bone surrogates of three different densities. With five repetitions, a total of 36 trials have been completed. Finally, potential relationships between the indentation force and the insertional torque with the pullout force and stiffness were explored. The second protocol was performed on porcine vertebrae to investigate the second and the third specific objectives. As the second specific objective, three toggling modes (CC, ML and no toggling (NT)) were performed on porcine lumbar vertebrae ranging from L1 to L3. The screws were then submitted to axial pullout test. A complete design of experiment with two factors and three levels (32 = 9 trials) was used to investigate on the main effect of toggling mode and vertebral level on screw pullout force and stiffness, as well as their quadratic interactions. With five repetitions, a total of 54 trials were performed on 27 isolated vertebrae, using both pedicles. Finally, potential relationships were investigated between the indentation force while performing a pilot hole, the insertional torque during screw insertion, and the pullout force and stiffness with and without toggling. \n \nThe results of the first protocol suggest that screw toggling significantly affects the pullout force (P = 0.01) and stiffness (P < 0.0001). A higher pullout force and stiffness was demonstrated for higher density without toggling. The effect of density was higher than the effect of toggling on pullout force. The indentation force while performing the pilot hole was significantly correlated to pullout force and stiffness (r = 0.99, P < 0.0001 and r = 0.92, P < 0.0001 respectively). Strong correlations were also shown between the insertional torque during screw insertion and the pedicle screw pullout force and stiffness (r = 0.98, P < 0.0001 and r = 0.91, P < 0.0001 respectively). \n \nThe study on porcine vertebrae showed that screw toggling significantly affects the pullout force (P = 0.0004) and stiffness (P < 0.0001). The lowest pullout force and stiffness were illustrated for CC toggling while the highest ones were shown without toggling (NT). The effect of vertebral level on pullout force was higher than the effect of toggling. Lower pullout force and higher stiffness were observed at anatomically lower vertebra level (L3). The effect of toggling was more important on the stiffness than the pullout force at all vertebral levels. Significant differences in pullout force were shown between CC toggling and no toggling (P = 0.03) and between CC and ML toggling (P = 0.02). For the stiffness, significant différences were shown between all toggling modes (CC and NT: P < 0.0001, ML and NT: P = 0.0002, CC and ML: P = 0.0003). The indentation force, the insertional torque and the BMD had the highest important correlations with pullout force with and without toggling (0.71 < r < 0.88). For the stiffness after CC toggling and with no toggling, the indentation force, the insertional torque and the BMD had the most important negative correlations (-0.60 < r < -0.85). Multiple regression analysis showed that BMD and insertional torque improve the estimation of pullout force after CC or ML toggling (R2 > 0.85, P < 0.0001). For pullout force without toggling BMD and pedicle area were the main contributing factors to the regression model. For the stiffness with and without toggling, the indentation force was the single best factor with highest contribution to the regression model. \n \nIn conclusion, pedicle screw toggling significantly affects the pedicle screw pullout force and stiffness. Screw toggling, in particular CC toggling, should be considered in the biomechanical evaluation of pedicle screw fixation strength. Furthermore, the contribution of toggling was more important on the stiffness than the pullout force. The effect of vertebral level should be considered in determining the fixation strength. The developed instruments and methods for indentation force measurement during pilot hole creation and insertional torque measurement during screw insertion were reproducible, and provide valuable data to estimate pedicle screw pullout force and stiffness. The relationship between the pilot hole indentation force and screw insertional torque, and the screw pullout force and stiffness are a affected by the toggling mode. Indentation force and insertional torque measurements, together with BMD measurement, are recommended for a better estimation of pedicle screw fixation strength after CC toggling.

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