| BackgroundWith the modernization process, there having an unprecedented development in transportation industry and construction business, accompany with the change of people's live concept, cervical 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.Cervical spine is the base of skull's weight-bearing, and direct contact with the thoracic spine. 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.Cervical 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 cervical spinal disease. In the past, studies on cervical 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 cervical spinal disorders by FEM, making biomechanical studies on bones'complex geometric structures, boundary conditions and material nonuniformity of the cervical 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 cervical vertebras, zygapophyseal joints, ligaments and other structures of cervical 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 cervical spine, most of them have not constructed the multiple levels of cervical spine perfectly. The models can't be used for the research related to multi-level cervical 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 cervical spine with fine anatomical structures, through simulating the multi-level laminectomy, three-column injuries and internal fixation patterns in the models, to explore their biomechanics effect on cervical spine.Obiective 1. According to spiral CT scan images of a normal male volunteers' cervical spine with neutral position, to establish finite element model of normal human cervical spine (C3～C7) with fine anatomical structures with Mimics 11.1,Geomagic studio10.0,HyperMesh 10.0 and Abaqus6.8 software. Moreover, its validity should be verified, so that it can reflect the mechanical characteristics of the normal human cervical spine (C3～C7)2. According to the three-dimensional finite element model (FEM) of normal cervical spine established, three finite element models were reconstructed by different fixation techniques following C4-C6 level laminectomy. 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 intersegmental ranges of motion during various motion conditions and the stress distributions of the fixation devices.3. According to the three-dimensional finite element model (FEM) of normal cervical spine established, three finite element models were reconstructed by different fixation techniques following C4/5,C5/6 two-level three-column injuries (TCI). 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 total range of motions during various motion conditions and the stress distributions of the fixation devices.Methods1. The lower cervical spine geometries were determined from CT images of a 26 year old healthy man. The finite element model(C3～C7)was constructed by the combination of software package Mimics 11.1, Geomagic studio 10.0, HyperMesh 10.0 and Abaqus6.8. Intersegmental ranges of motion was calculated after C3～C7 being subjected to loads of moments 1.8 Nm and 74 N preload and C7 was rigidly fixed while loads were applied at the C4 for flexion, extension, lateral bending and axial rotation. For validation of the model, their predicted intersegmental ranges of motion was compared with the results by Finn et al.2. Based on the intact finite element model(FE/Intact),the model were generate by simulating C4-C6 level laminectomy. According to the applied different fixation techniques (PS, LS and TS techniques), The sizes and locations of screws and rods were confirmed in the intact C3-C7 model using HyperMesh 10.0 to obtain the appropriate internal fixation systems. A compressive preload of 74 N combined with a pure moment of 1.8 Nm in flexion, extension, left-right lateral bending, and left-right axial rotation was applied to the models. Intersegmental ranges of motion during various motion conditions and the stress distributions of the fixation devices were explored.3.Based on the intact finite element model(FE/Intact),the model were generate by simulating C4/5,C5/6 two-level three-column injuries. According to the applied different fixation techniques (PS, LS and TS technique), three finite element models were reconstructed by different fixation techniques following C4/5,C5/6 two-level three spinal columns injuries by the HyperMesh 10.0 software. A compressive preload of 74 N combined with a pure moment of 1.8 Nm in flexion, extension, left-right lateral bending, and left-right axial rotation was applied to the models. Total ranges of motion during various motion conditions and the stress distributions of the fixation devices were explored.Results1. The final intact model consisted of 130,748 elements and 30,820 nodes. 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 datas 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 Moroney, Panjabi, Finn and Ng et al.2. Compared to the normal model, the intersegmental motion of the C4-C5 and C5-C6 segments decreased with posterior cervical fixation. According the intersegmental motions data, three fixation techniques have the no noticeable differences in intersegmental biomechanical stability. Under flexion, extension, left-right lateral bending, and left-right axial rotation conditions, the screws inserted by techniques have the noticeable differences. Maximal stress level of TS was over 100 MPa in the conditions, while PS and LS were less than 100 MPa in the conditions. TS technique induces noticeable differences in the stress compared to the posterior cervical fixation technique, regarding the higher stress level on fixation devices.3. Compared to the reconstructed model by TS and PS technique, the total cervical biomechanical stability of the reconstructed model by LS techniques have the noticeable differences. Under flexion, extension, left-right lateral bending, and left-right axial rotation conditions, the screws inserted by techniques have the noticeable differences. Maximal stress level of TS was over 9800 MPa in the conditions. Maximal stress level of PS was over 1000 MPa and less than 1100 MPa. Maximal stress level of LS was less than 200 MPa in the conditions.Conclusions1. The intact FE model consists of five vertebrae (C3, C4, C5, C6, and C7), four intervertebral discs (C3-C4, C4-C5, C5-C6, and C6-C7), and includes all the important components of the cervical 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 cervical spine realistically simulates the complex kinematics of the lower cervical spine(C3-C7) region which can simulate the natural condition and facililate the further biomechanical research.2. Compared to the intersegmental motions data, the use of posterior cervical fixation offers immediate stability of the cervical spine following laminectomy, thus allowing early rehabilitation. The screws inserted by TS technique had high stress concentration at the middle part of the screw. Screw inserted by PS and LS techniques had high stress concentration at the actual cap-rod-screw interface. TS technique induces noticeable differences in the stress compared to the posterior cervical fixation technique, regarding a higher stress level on fixation devices. Choosing fixation devices containing zirconium alloys or suitably prolonging the need for external bracing are necessary for reducing the higher risk of fracture on fixation devices.3. Compared to the total motions data, the use of posterior cervical fixation offers immediate stability of the cervical spine following C4/5,C5/6 two-level three spinal columns injuries, thus allowing early rehabilitation. However, PS and TS techniques can provide the better stability than LS technique. The screws inserted by TS technique had high stress concentration at the middle part of the screw. Screw inserted by PS and LS techniques had high stress concentration at the actual cap-rod-screw interface. According to the yield strength of titanium alloy, TS technique induces noticeable differences in the stress compared to PS and TS techniques. The stress level of TS technique is close to 104 MPa level. Consequently, It considers that we should use the PS technique firstly to treat cervical TCI. If use TS technique to treat it, we should suitably prolonging the need for external bracing or add anterior cervical operation for reducing the higher risk of fracture on fixation devices.
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