Contribution to FE modeling for intraoperative pedicle screw strength prediction

Abstract

Although the use of pedicle screws is considered safe, mechanical issues still often occur. Commonly reported issues are screw loosening, screw bending and screw fracture. The aim of this study was to develop a Finite Element (FE) model for the study of pedicle screw biomechanics and for the prediction of the intraoperative pullout strength. The model includes both a parameterized screw model and a patient-specific vertebra model. Pullout experiments were performed on 30 human cadaveric vertebrae from ten donors. The experimental force-displacement data served to evaluate the FE model performance. μCT images were taken before and after screw insertion, allowing the creation of an accurate 3D-model and a precise representation of the mechanical properties of the bone. The experimental results revealed a significant positive correlation between bone mineral density (BMD) and pullout strength (Spearman ρ = 0.59, p < 0.001) as well as between BMD and pullout stiffness (Spearman ρ = 0.59, p < 0.001). A high positive correlation was also found between the pullout strength and stiffness (Spearman ρ = 0.84, p < 0.0001). The FE model was able to reproduce the linear part of the experimental force-displacement curve. Moreover, a high positive correlation was found between numerical and experimental pullout stiffness (Pearson ρ = 0.96, p < 0.005) and strength (Pearson ρ = 0.90, p < 0.05). Once fully validated, this model opens the way for a detailed study of pedicle screw biomechanics and for future adjustments of the screw design.

Ultra–high-molecular-weight polyethylene sublaminar tape as semirigid fixation or pedicle screw augmentation to prevent failure in long-segment spine surgery: an ex vivo biomechanical study

“…FE modeling on spinal fixation techniques is primarily focused on PS fixation. [27][28][29][30] In our study, FE modeling provided insight into the applied bending moment and the load distribution within the vertebrae as a result of load transferred through PS and ST fixations. If no bending moment can be transferred (as in classic pullout strength tests), an axial force of 333.3 N would have resulted in our setup (Equation 1).…”

Can a Priori Unknown Values of Biomechanical Parameters Be Determined with Sufficient Accuracy in Mbs Using Sensitivity Analysis? Analyzing the Interaction Characteristics between Cervical Vertebra and Pedicle Screw

“…The main objective of the numerical model validation was the verification of the correctness of the model responses with respect to the available real data or to the outcome of existing validated models. Generally, the behavior of screw-vertebra simulation models is validated against experimental force-displacement measurements carried out from pullout tests [32,33]. In order to ensure that such a real scenario can be validly reproduced using multibody simulation and that an entered non-linear stiffness can be correctly processed in the current model, the experimental test of Pezowicz et all [25] was modeled.…”

“…The focus of the following studies has been the analysis of mechanical properties of the screws-vertebra structure and the effects of traditional and cortical bone trajectory pedicle systems. Abbeele et all [14] predicted the intraoperative pullout strength through a patient-specific FE model of a standard cylindrical pedicle screw inserted in the lumbar vertebra. The impact of different screw sizes on pedicle screw fixation in osteoporotic vertebrae was quantitatively determined using FE simulation by Keitaro et all [15].…”

“…In the current research there is no existing direct way to measure biomechanical properties such as stiffness and damping of the screw inserted in a human spine in vivo. In the most cases, those parameters are inferred using pullout experiments on cadaver or animal vertebrae [25][26][27]. A few approaches that apply optimization methods are proposed in the literature to identify the stiffness and damping values for the various spinal flexible structures [28,29].…”

Patient-Specific Finite Element Models of Posterior Pedicle Screw Fixation: Effect of Screw’s Size and Geometry

“…Many studies assessed the optimal fixation to treat a burst fracture by simulating with FE models a system composed of three vertebrae and two intervertebral discs implanted with different configurations of rod and screws (e.g., monolateral vs. bilateral, short segment vs. long segment) (Li et al, 2014;Elmasry et al, 2017;Su et al, 2018;Wang et al, 2019). Other studies focused on the vertebra-screws interactions and proposed FE models validated with experimental measures: FE models were found to be good predictors of pull-out strength and stiffness obtained by experimental tests better than apparent density estimated from CT images (Abbeele et al, 2018;Chevalier et al, 2018;Widmer et al, 2020). The screw size and other insertion-related parameters have been tested with linear FE models (Qi et al, 2011;Newcomb et al, 2017), with non-linear FE models (material non-linearities, contact mechanics) (Chen et al, 2003;Bianco et al, 2017Bianco et al, , 2019Molinari et al, 2021), or assuming the bone as heterogeneous material with elastic properties driven by the local bone mineral density (BMD) (Matsukawa et al, 2016(Matsukawa et al, , 2020Biswas et al, 2019;Molinari et al, 2021).…”