| BackgroundSpondylolysis is a common disease and has a high risk of developing spondylolisthesis, as well as traction on the spinal cord and nerve root, leading to spinal disorders or low back pain when the lumbar spine is subjected to high external forces. It occurs in4-8%of the general population, which is higher in young athletes, approximately31%. Surgical treatment of lumbar spondylolysis could be grouped into three categories:decompression, lumbar intrasegmental fusion and direct repair of isthmus. The most frequently used surgical procedure for symptomatic lumbar spondylolysis is fusion, which determines a loss of spinal movement and subsequently an overload of adjacent segment. Some of scholars proposed direct repair of spondylolysis by using intrasegmental fixation construct and bone-grafting of the pars defect such as Buck’technique, Scott’technique and Scott modified technique However, it is commonly found in traditional rigid spinal fusion—abnormally large motion at the adjacent level and subsequent degeneration. A dynamic stabilization system allows for a motion pattern similar to that of a healthy motion segment, a strong reduction in facet joint and intradiscal pressure during flexion and extension.It is necessary to search the alloy whose functional, qualities, biocompatibility, enhancing both the possibility and the execution of less invasive surgeries. So, In the year of1998, we had designed a "C" shape memory alloy intrasegmental fixator(MAIF), the biomechanical evaluation, finite element analysis and clinical application of the fixation had been completed, proving to well-designed and easily to handle, and could afford a new reliable alternative for fixation in treating spondylolysis. However, The MAIF indicated that some certain breakthroughs had been taken. With the advantages of alloy and anatomical biological feature, we attempted to develop new intrasegmental fixation devices. Then an aliform nitinal SMA intrasegmental fixation device (ASMAF) was designed to treat spondylolysis and provided better clinical results. Now, in order to prevent adjacent segment degeneration and spinal stenosis, a neotype aerofoil nitinal SMA fixation device (NASMAF) was designed.We were used to investigate the traditional techniques, lacking detailed the internal structural response to external loading. Thus, for more accurate assessment of biomechanical behavior of different fixation devices, finite element(FE) method was emploied in recent years, with the improvement of digital technology. It can be simulated using a variety of spinal disorders by FEM, making biomechanical studies on bones’complex geometric structures, boundary conditions and material nonuniformity of the spinal 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 and fixation designs. Based on previous studies, we have taken a long time to explore the effective methods of modeling and created a three-dimensional nonlinear finite element model of the lumbosacral spine. The lumbar spondylolysis was simulated by fracturing the bilateral pars interarticularis of the posterior element in the L4vertebral body. Both of techniques repair surgeries were simulated in the model, to explore their biomechanics effect on lumbosacral spine.Objective1. According to spiral CT scan images of a normal male volunteers’ lumbosacral spine with backlying position, to establish finite element model of normal human lumbosacral vertebrae with Mimics10.01、Geomagic studio12.0、 HyperMesh10.0and Abaqus10.1software. Moreover, its validity had been verified, so that it can reflect the mechanical characteristics of the normal human L1～S spine.2. According to the three-dimensional finite element model (FEM) of normal lumbosacral spine established, two finite element models were reconstructed by different fixation techniques following L4level with the bilateral pars defect. The same compressive preload combined with the same pure moment in flexion, extension, left lateral bending, and left axial rotation was applied to the models, so as to explore von Mises stress in the disc annulus during various motion conditions and the stress distributions of the fixation devices.Methods1. The lumbosacral spine geometries were determined from CT images of a27years old healthy man. According to spiral CT scan images of0.625mm thickness, to establish finite element model of normal human lumbosacral vertebrae with Mimics10.01、Geomagic studio12.0、HyperMesh10.0and Abaqus10.1software. Simulations were performed on the FE model by imposing500N of axial preload as corresponding weight of upper torso of a normal adult.7.5Nm of bending moment was then applied on L1to simulate flexion, extension, left lateral bending and left axial rotation. S1vertebra was constrained all degree of freedom at the inferior surfaces. The developed FE model was validated in previous studies, during which the range of motion (ROM) of this intact model was compared with that of the cadaveric specimen in vitro test by White et al.2. Based on the intact finite element model, the model were generated by simulating L4level spondylolysis. Both ASMAF’technique and NASMAF’ technique repair surgeries were simulated. A compressive preload of500N combined with a pure moment of7.5Nm in flexion, extension, left lateral bending, and left axial rotation was applied to the models. Von Mises stress in the disc annulus during various motion conditions and the stress distributions of the fixation devices were explored.Results1. The final intact model consisted of31,431nodes and161,404elements. With the same compressive preload combined with the same pure moment, the study summarize the comparison of the intrasegental 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 White, Yamamoto et al.2. For the FEM of spondylolysis, maximum stresses of disc at the surgical level increased remarkably in flexion, extension, left lateral bending, and left axial rotation, respectively, compared to the intact model. After direct repair by ASMAF’ technique, the increased disc stress due to spondylolysis tended to be unchanged. But for the NASMAF model, the disc stress was decreased mostly to the intact level, Which have the noticeable differences. Maximal stress level of NASMAF was over100Mpa in the conditions, while ASMAF was less than100Mpa in the conditions.Conclusions1. The intact FE model consists of five vertebrae (LI, L2, L3, L4and L5) and sacrum, five intervertebral discs (L1/2, L2/3, L3/4, L4/5, L5/S1), and includes all the important components of the lumbosacral spine such as cortical bone, cancellous bone, intervertebral discs, and ligaments (anterior longitudinal ligaments, posterior longitudinal ligaments, transverse ligament, ligamentum flavum, interspinous ligament, supraspinous ligament, capsular ligament). The anatomic detailed finite element model of the human lumbosacral spine realistically simulates the complex kinematics which can simulate the natural condition and facililate the further biomechanical research.2. Compared to the normal model, the intrasegmental stresses of the L4～L5decreased with NASMAF, which had more advantages than ASMAF. Under flexion, extension, left lateral bending, and left axial rotation conditions, the ASMAF had high stress concentration at the middle part and curved part, while the NASMAF had high stress concentration at the junction of the swings and the U-shape base. Maximal stress level of NASMAF was over100Mpa in the conditions. In summary, loading on the motion segment in presence of isthmic spondylolysis caused stress concentration over the SMA and the annular region of the intervertebral disc. The NASMAF may decrease subsequent disc failure, potentially reducing the risk of adjacent segment degeneration or subsequent spondylolisthesis. |
PDF Full Text Request