A Finite Element Analysis And Clinical Study Of Posterior Dynamic Fixation Techniques In Lumbar Spine

BackgroundWith the modernization process, there having an unprecedented development in transportation industry and construction business, accompany with the change of people’s live concept, lumbar diseases have been increasing and becoming more complex each year, which makes the spinal surgery face new challenges clinically and urgently needs to be explored in depth.Lumbar is more mobile than the thoracic spine and direct contact with it. The vertebral body of lumbar is a large, box-shaped bone that serves the weight-bearing function of the vertebrae. With its complicated geometry and kinematics, it remains one of the most fragile parts of human musculoskeletal system. The understanding of cervical anatomy, biomechanics and injury mechanisms is the key to treat cervical disorders.Lumbar spine biomechanics research is an important section of human biomechanics study. Many researchers pointed out that the biomechanical factors play an important role in exploring the etiology, pathogenesis, treatment and prevention of the lumbar spinal disease. In the past, studies on lumbar spine mainly used traditional experimental means of biomechanical tests. But the disadvantage of this method is the complexity of experimental means, and when carried out a variety of operating conditions, the experiment is often costly, time-consuming and low efficiency. Moreover, because it can not directly test in humans, it is difficult to accurately reflect the more intrinsic biomechanical situation after application different load. In order to solve this problem, in recent years, with the improvement of digital technology, more and more researchers use numerical analysis methods, namely, based on the theory of traditional mechanical analysis, numerical analysis methods such as finite element method (finite element method, FEM) and so on, for linear and nonlinear stress and deformation analysis. It can be simulated using a variety of lumbar spinal disorders by FEM, making biomechanical studies on bones’ complex geometric structures, boundary conditions and material nonuniformity of the lumbar issues have possible solutions.FEM has powerful capabilities of modeling and can simulate complex geometric structures, material parameters and different force in the dynamic or static state, which has increasingly been applied to the human body biomechanics. Because of the complexity of lumbar vertebras, zygapophyseal joints, ligaments and other structures of lumbar spine, too much work in finite element modeling as well as the different characters of detailed parameters of the various organizations, it is more difficult to simulate than other large joints by FEM. Existing finite element models of lumbar spine, most of them have not constructed the multiple levels of lumbar spine perfectly. The models can’t be used for the research related to multi-level lumbar spine analysis.Based on previous studies, we have taken a long time to explore the effective methods of modeling, and then established finite element models of normal human lumbar spine with fine anatomical structures, through simulating lumbar posterior decompression and interbody fusion in L4～L5with three different pedicle screw system (USS, ISOBAR TTL and DYNESYS), to explore their biomechanics effect on lumbar spine. And treat and follow up lumbar disease with Dynesys and Isobar TTL system in order to evaluate the clinical and radiological effect.Objective1. According to spiral CT scan images of a normal male volunteers’lumbar spine with neutral position, to establish finite element model of normal human lumbar spine (L3～S1) with fine anatomical structures with Mimics11.1、Geomagic studio10.0、HyperMesh10.0and Abaqus6.8software. Moreover, its validity should be verified, so that it can reflect the mechanical characteristics of the normal human lumbar spine (L3-S1).2. According to the three-dimensional finite element model (FEM) of normal lumbar spine established, three finite element models were reconstructed by different fixation techniques including USS, Isobar TTL and Dynesys system. The same compressive preload combined with the same pure moment in flexion, extension, left-right lateral bending, and left-right axial rotation was applied to the models, so as to explore ROM of L3-S1, intradiskal pressure of adjacent segment and the stress distribution and maximum value of the fixation devices during various motion conditions.3. To treat and follow up lumbar disc degeneration disease with Dynesys and Isobar TTL system, and to evaluate the clinical and radiological effect.Methods1. The lumbar spine geometries were determined from CT images of a26year old healthy man. The finite element model (L3-S1) was constructed by the combination of software package Mimics11.1, Geomagic studio10.0, HyperMesh10.0and Abaqus6.8. Intersegmental ranges of motion was calculated after L3-S1being subjected to loads of moments10Nm and150N preload and S1was rigidly fixed while loads were applied at the L3for flexion, extension, lateral bending and axial rotation. For validation of the model, their predicted intersegmental range of motion was compared with the results by Yamamoto et al.2. Based on the intact finite element model, the models were generated by simulating decompression and interbody fusion in L4-L5with three different pedicle screw system (USS, ISOBAR TTL and DYNESYS). According to the applied different fixation system (USS, ISOBAR TTL and DYNESYS), The sizes and locations of screws and rods were confirmed in the intact L3～S1model using HyperMesh10.0to obtain the appropriate internal fixation systems. A compressive preload of150N combined with a pure moment of10Nm in flexion, extension, left-right lateral bending, and left-right axial rotation was applied to the models. ROM of L3～S1, intradiskal pressure of adjacent segment, the stress distribution and maximum value of the fixation devices during various motion conditions were explored.3. From2007to2012,50patients with lumbar disc degeneration disease were treated with Isobar TTL system, and the other50patients with Dynesys system. The clinical effete was evaluated by JOA. The ROM of lumbar segments and degeneration of adjacent segment was measured by X-ray film.Results1. The final intact model consisted of32,341elements and162,044nodes. With the same compressive preload combined with the same pure moment, the study summarize the comparison of the intersegmental responses between the intact model and previously published data under combined flexion-extension, left-right lateral bending, and left-right axial rotation. All the predicted responses were in good agreement with the published data by Yamamoto et al.2. Under flexion, extension, left-right lateral bending, and left-right axial rotation conditions, the Von Mises stress of the screws inserted by different fixation systems have no significant differences. All the screws have high stress concentration at the middle part of the screw. Maximal stress level of three fixation systems is all less than100MPa in different conditions. Compared to the reconstructed model by Isobar TTL and Dynesys system, the total lumbar ROM of both reconstructed models have no noticeable differences with the intact model, while the USS system decreases obviously, especially in flexion and extension condition. The intradiskal pressures of adjacent segments increase under the different conditions. The greatest increased was obtained in USS system, whereas the lowest increased was obtained in Dynesys system.3. According to the follow-up clinical observation on JOA and radiographic imaging, it showedDynesys system with the preoperative activity showed no significant difference, but Isobar TTL system compared with the preoperative activity has slightly decreased. But the two fixation system in the treatment of lumbar degenerative disease of the follow-up period were satisfactory, can effectively prevent adjacent segment degeneration, is the treatment of lumbar degenerative disease of an effective non-fusion, dynamic stabilization methodConclusions1. The intact FE model consists of four vertebrae (L3, L4, L5and S1), three intervertebral discs (L3-L4, L4-L5, and L5-SI), and includes all the important components of the lumbar spine such as cortical bone, cancellous bone, intervertebral discs, and ligaments. Each intervertebral disk consisted of disk annulus and disc nucleus.The anatomic detailed finite element model of the human lower lumbar spine realistically simulates the complex kinematics of the lumbar cervical spine (L3-S1) region which can simulate the natural condition and facililate the further biomechanical research.2. The pedicle-screw of three systems had high stress concentration at the middle part of the screw. According to the maximal stress level of each fixation system, the screw was hardly fracture on the basis of the yield strength of Titanium. Compared to the reconstructed model by Isobar TTL and Dynesys system, the total lumbar ROM of both reconstructed models have no noticeable differences with the intact model, while the USS system decreases obviously, especially in flexion and extension condition. The intradiskal pressures of adjacent segments increase under the different conditions. The greatest increased was obtained in USS system, whereas the lowest increased was obtained in Dynesys system. Consequently, it considers that these two kinds of dynamic fixation systems, Isobar TTL system and Dynesys system, could maintain the entire ROM of L3-S1. Besides, the Dynesys system avoids the accelerated adjacent segment degeneration in theory.3. Based on the clinical and radiological effect after follow up, the ROM of L3-S1in Dynesys group was closed to the pre-operation, while in Isobar TTL was decreased in a little range when compared to the pre-operation. The short-term clinical effect of dynamic fixation with Isobar TTL and Dynesys on lumbar disc degeneration disease was satisfying. Accordingly, dynamic fixation with Isobar TTL or Dynesys could be one of available treatment.