| BackgroundWith the modernization process of21th century, there having an unprecedented development in transportation industry and construction business,Cervical spondylosis has become one of the common and frequently occurring illness in clinic and brought great harmful to suffers, their families and society 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.Surgical techniques can be broadly divided into anterior surgery, posterior surgery, or combined anterior and posterior surgical techniques.Anterior cervical decompression and fusion (ACDF) allows direct removal of the compressive abnormality that compresses the spinal cord and nerve root from the front, with stabilization obtained by anterior arthrodesis. However, The disadvantages include the need for graft healing, related to bone grafting, such as dislodgement, fracture, and nonunion of the grafted bone and recurrence of myelopathy due to adjacent segment degeneration. Particularly, as more levels are incoporated into the construct, cervical motion is adversely affected, adjacent segment degeneration seen more obvious. Posterior surgery, such as laminectomy, laminectomy and fusion, and laminoplasty. Prior to the advent of anterior cervical spine surgery, laminectomy was the most common approach to decompression for multilevel myelopathy. Whereas, there were significant problems associated with postlaminectomy, including kyphosis secondary to iatrogenic destabilization of the cervical spine, and lead to delayed neurological deterioration. Restenosis of spinal canal due to scar tissue formation, and will lead to spinal cord injury due to mild trauma.With the update treatment theory for cervical disc degeneration gradually, the method develops from rigid internal fixations to elasticity internal fixation, adopts the spinal motion preservation instead of the intervertebral fusion. Therefore, the development and application of cervical artificial disc replacement(C-ADR), indeed, might be the operation of the revolution.Based on the satisfactory clinical outcome and further study, a novel hybrid surgical technique combing ACDF and C-ADR was introduced to treat cervical multilevel degenerative disc disease.Hybrid surgical technique, as a promising alternative for treating multilevel degenerative disc disease, preserves the ROM of treating cervical segment, avoids the excessive stress on adjacent segment in order to obviates the progression of ASD, and save the high medical cost and of multilevel C-ADR for patients. Encouraging outcomes of the hybrid technique have been published by several literatures which were mainly for double-level However, few study focus on the application of the new surgical method to treat three-level cervical disorders. In particularly, the detailed biomechanical analysis for hybrid technique is absent.An FE cervical model including the entire subaxial cervical spine (C2-C7) had been built to simulate the physiologic condition possibly. Then double-level ACDF and single-level C-ADR were inserted in three ways. Our purpose of the FE study is to observe biomechanical changes including ROM, facet joint force, ligament tension force and the distribution of Von Mises stresses on the prosthesis in the subaxial cervical spine and investigate which construction has a better biomechanical pattern. Objective1. According to spiral CT scan images of a normal male volunteers’ cervical spine with neutral position, to build finite element model of normal human subaxial cervical spine (C2-C7) with fine anatomical structures with Mimics11.1、Geomagic studio10.0、HyperMesh10.0and Abaqus6.8software. As the range of motion were in agreement with previous experiments., the current subaxial cervical FE model was able to reflect the mechanical characteristics of the normal human cervical spine (C2-C7)2. According to the three-dimensional finite element model (FEM) of normal cervical spine established, three finite element models were reconstructed by different hybrid surgery of C3-C6level including two level ACDF and one level C-ADR simulation. 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. Biomechanical respond to the operational simulation models was assessed by the ROM, facet joint force and ligament tension force of the intact subaxial cervical spine and the distribution of stress on the Mobi-C artificial prosthesis.Methods1. The cervical spine geometries were determined from CT images of a26year old healthy man. The finite element model(C2-C7)was reconstructed by the combination of software package Mimics11.1, Geomagic studio10.0, HyperMesh10.0and Abaqus6.8. For validation of the FE cervical model, a pure moment of1.8Nm was imposed in flexion-extension, left-right lateral bending and left-right axial rotation loading condition with74N axial compression force superior to C2and C7was rigidly fixed to simulate the boundary condition in all degrees of freedom constrained.. To assess the validity of FE model, ROM was compared against the previous results in the vitro reports of the published literature.2. Based on the intact finite element model(FE/Intact),the model were generate by simulating C3-C6level hybrid sugery. Coronal CT images of Mobi-C were scan to reconstruct the three-dimension model of prosthesis. Material properties of the CoCr alloy and UHMWPE was obtained from previous literature. In the current study, three alternations of hybrid surgery were simulated as following:M1:C3/4ACDF, C4/5ACDF, C5/6C-ADR; M2:C3/4ACDF, C4/5C-ADR, C5/6ACDF; M3:C3/4C-ADR, C4/5ACDF, C5/6ACDF. To develop the surgical models, anterior longitudinal ligament (ALL), annulus and nucleus pulposus of the insertion areas were removed consistent with the standard surgical technique. ABAQUS v6.9.1was used to reconstruct the hybrid surgery simulation model. For ACDF, a strut bone graft which assumed to be cancellous bone property was rigidly fused at operative level. For C-AD, the endplate of prosthesis replaced the vertebral endplates which were accurately removed to fit the prosthesis maximizing. Biomechanical respond to the operational simulation models was assessed by the ROM, facet joint force and ligament tension force of the intact subaxial cervical spine and the distribution of stress on the Mobi-C artificial prosthesis.Results1. The final intact model consisted of130,429elements and30,181nodes. 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 different loading conditions: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 and Finn. Therefore, the current subaxial cervical FE model was able to investigate the biomechanical changes in hybrid technique surgery based on these findings.2. Biomechanical comparison between the hybrid technique models and intact model under the different loading conditions demonstrated:(1) The ROM of C-ADR segments were increased while the segments adjacent to the operative level (C2/3、C6/7) were no significant different. For the total ROM of treatment groups, M2was closed to the intact group.(2)The peak stress concentrated around the bilateral parts of the inlay core, the maximum stress was30.44Mpa in M2; The highest stress forced on the middle and posterior region of the endplate with the maximum stress being80.11Mpa in M2.(3)The facet joint force of operative levels was increased to some extent, whereas, facet contact force adjacent to the treatment levels (C2/3, C6/7) had scarcely difference among three hybrid groups compared with the intact group.Conclusions1. The intact FE model consists of five vertebrae (C2, C3, C4, C5, C6, and C7), four intervertebral discs (C2-C3, 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 (C2-C7) region which can simulate the natural condition and facililate the further biomechanical research.2. Hybrid surgical technique is a safe and reliable application of multi-level cervical DDD, especially maintaining the entire subaxial cervical ROM to prevent the ASD theoretically. However, the novel technique increases the ROM, facet joint force and ligament tension beyond the normal level, which may produce new segment degeneration in the C-ADR segment. Balance the advantage of the hybrid technique against risk, M2reconstruction is a more suitable for patient. On the other hand, due to the limitations of the finite element method, further testing in the cadaveric model is necessary as well as long-term clinical follow-up. |
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