Biomechanical study of lumbar spine with dynamic stabilization device using finite element method
Introduction
Currently, spine procedures are aimed primarily at arthrodesis (fusion), which is a versatile and effective tool in the management of a wide variety of spinal instabilities, deformities and painful conditions. Pedicular fixation (fusion) is the gold standard in terms of increasing biomechanical rigidity and clinical fusion rates because pedicle screws are the strongest component of spinal implants. Conversely, several reports have established that spinal fusion with pedicle fixation accelerates the degeneration of adjacent motion segments because the relative immobility of fused spinal segments transfers stress to adjacent segments [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].
Nonfusion technologies have been developed with the goal of reducing the incidence of arthrodesis-related morbidity. Implant types include total disc replacements, prosthetic nuclear implants and posterior dynamic stabilization devices. Although implants offer some theoretical advantages over fusion, new potential problems such as mechanical failure, device migration, same level degeneration and implant subsidence are associated with new technologies. Furthermore, the efficacy of non-fusion implants in the prevention of adjacent level degeneration was not yet proved. The popularity of non-fusion implants is based more on the lack of satisfaction with conventional spinal fusion rather than their proved superiority.
This study determined whether the fused segment led to an acceleration of the degeneration of the adjacent motion segments through the measurement of the ROM and the disc pressure. Verification of increased stress adjacent to the level of fusion substantiated the superiority of the dynamic stabilization device compared to fusion. Comparing the ROM and the disc pressure of the dynamically stabilized spine with the intact spine, the usefulness of the dynamic stabilization device was established. The proper degree of stiffness of the dynamic stabilization device was determined and applied to its design.
Section snippets
Fusion of the lumbar spine
Fusion of the lumbar spine has been widely used to manage spinal instability because the surgery is a convenient approach to stabilize the spine. Spinal fixation devices can be used to form a rigid construct with the spine to replace bone, restore alignment, maintain position and prevent motion in the treatment of fractures, degenerative disease and congenital deformities. Most fixation devices are used primarily to promote fusion by bone grafting. Spinal bony fusion is indispensable for
Intact model
A finite element (FE) model of lumbar spine (L2–L5) was created for the present study. Fig. 2 shows the total procedure of the reconstruction of the lumbar spine used in this study. L2–L5 vertebral bodies were reconstructed from computed tomographic (CT) images of the lumbar spine of a male subject using the commercial 3D reconstruction program, AMIRA 3.1.1 (Mercury Computer System, Berlin, Germany) [38]. The 3D data were edited to produce smooth surfaces using the commercial 3D reverse
Validation of the intact model
The kinematics data of the present FE model were compared with the in vitro experimental data obtained using cadavers in the study of Yamamoto et al. [37]. The same loading conditions, 10 N m, were used for validation of flexion, extension, and lateral bending, respectively. The validation of rotation was excluded because there was insufficient information of the axis of rotation in the study performed by Yamamoto et al. The results of the validation are listed in Table 6. The data obtained in
Conclusion
A three-dimensional nonlinear FE analysis was performed to investigate the advantages of dynamic stabilization and its influence on the adjacent intervertebral motion segments. The ROM and the disc pressure of the dynamically stabilized spine were similar to those of the intact spine. The pressure on the L3–L4 segment decreased slightly and the pressure on the adjacent level decreased in comparison with the fused spine. The dynamic stabilization device having a stiffness of 10–15 N/mm made the
Acknowledgements
This research was supported by the Laboratory of Excellency Research Programme of the Ministry of Education and Human Resources Development (MOE), the Ministry of Commerce (MOCIE), Industry and Energy, and the Ministry of Labour (MOLAB) in Korea. This research is also partially supported by Spine Laboratory of Stanford University.
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