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/lp/de-gruyter/modeling-of-lumbar-spine-equipped-with-fixator-MTQrWXbl6n
- Publisher
- de Gruyter
- Copyright
- Copyright © 2015 by the
- ISSN
- 1895-9091
- eISSN
- 1896-530X
- DOI
- 10.1515/bams-2015-0032
- Publisher site
- See Article on Publisher Site
Abstract
Anatomical model of fragment of lumbar spine and rod fixator with screw attachment were made. Numerical analyses (use of finite element method) were executed. Results show that stresses in the analyzed structure depend on the size of the fixator used. Authors suggest further research. Keywords: distribution of stresses; finite element method; modeling. in vertebrae and annulus fibrosis in intervertebral disc) and more elastic filling (cancellous bone and nucleus pulpous) (Figure 1) [36]. Introduction Spine is one of the most significant human organs, which is responsible for movement, stabilization and shock amortization. Its composite structure makes spine a defense tool for spinal column with large possibilities of movement [1]. That is why contusions and destruction of this organ are so important to be treated with good planning system. Modern medical surgery techniques are commonly based on accurate computer planning. This publication focuses on the most frequently operated area of spine, which is the lumbar section. The purpose of this study was to do a biomechanical analysis of lumbar spine with the use of screw-attachment spinal fixator. Figure 1:Lumbar spine model consisting of four vertebrae made in Femap 11.0. Materials and methods The spine model was constructed with four lumbar vertebrae (Figures 14) approximated to the same size, which is the average size of vertebrae of young women [2], and four intervertebral disks. Each module was created as a composite of tougher peripheral material (cortical bone *Corresponding author: Tomasz Madej, Department of Machine Design and Technology, AGH University of Science and Technology, Kraków, Malopolskie, Poland, E-mail: tmadej@agh.edu.pl Justyna Gerke: Comarch, Kraków, Malopolskie, Poland Anna M. Ryniewicz: Department of Machine Design and Technology, AGH University of Science and Technology, Kraków, Malopolskie, Poland Figure 2:Lumbar spine model equipped with fixator (size of section, 4 mm). 258Gerke et al.: Modeling of lumbar spine Figure 3:Visualization of stresses of the anatomical model of fragment of lumbar spine. Table 1:Comparison of basic mechanics parameters of modeled materials [7]. Structure Young modulus, MPa 12,000 140 0.1 4.2 200,000 Poisson ratio 0.3 0.2 0.45 0.45 0.3 Cortical bone Cancellous bone Nucleus pulposus Annulus fibrosis Fixator elements made of Ti6Al4V Figure 4:Visualization of stresses in section of anatomical lumbar vertebrae. fixator in a real situation are grown together, in the areas where models were connected common nodes method is used. For better comparison there were made three models: one anatomical structure of lumbar spine and two models equipped with fixator with different sizes of section side (4 and 6 mm). Each model was charged with a force of 300 N from above. In the analyzed case, the modeled implant was attached to bone structure of the lumbar spine with the use of transpedicular screws. The fixator model consists of two rods with square section differing by size of section side (4 and 6 mm) and eight spinal screws ø4. Mechanical parameters used in modeling are presented in Table 1 and the whole model of four lumbar vertebrae with spinal fixator in Figure 2. In calculation of stresses in the modeled structures, the authors used finite element model in Femap 11.0 program. A total of 140,000 thetraedric elements had size of side 2.4 mm. Taking into account the fact that bone and Results The anatomical model of lumbar spine shows concentration of stresses in front reaching 3000 kPa in the transition area, dropping toward the back side of the structure. Stress level in bone structure was much higher than this, occurring in the intervertebral discs (where stress level was close to zero) (Figures 3 and 4). Similar analyses were made for model equipped with fixator with square section side of 4 mm showing lower stresses (reaching about 1135 kPa) in structure of fragment of lumbar spine. Moreover, contrast of stress Gerke et al.: Modeling of lumbar spine259 level between different materials showed the stress to be milder in this model than in the case of anatomical spine model. In the transverse section of vertebral structure authors noticed stress concentration in the screw area and stress penetration to the fixator structure. In fixator rod stress concentration was located in the attachment area (Figures 5 and 6). Figure 5 is a visualization of stresses in the model of fragment of lumbar spine equipped with fixator (section size 4 mm, screw ø4). Shown from top are above and back views. Similar analyses were made for model equipped with fixator with side square section side of 6 mm showing drastic stresses that fail in the whole analyzed structure (stresses reached 92 kPa) with simultaneous growth in fixator. Similarly to the previous case the highest stress levels in fixator structure were reached in the attachment area (Figures 7 and 8). Discussion Rapid disappearance of stresses into intervertebral discs structure points to its unique amortization function which was noticed in the literature [8, 9]. Stress levels in the front area of the lumbar spine were higher than in the back side; in the case of instrumentation of fixator (4 mm of section side) it was about 10 times lower. Authors confirmed the legitimacy of lumbar spine fixating for lowering stress level which can help slow down scoliosis behavior or make faster the healing process of the destroyed part of spine. Equipping the fixator with too large rod section side (6 mm) made the disappearance of stresses in the back section of lumbar structure almost complete. This situation, according to Wolfs law, can cause osteolyses process to begin and leads to screw slack. Moreover, excessive stress differences between front and back sides of Figure 5:Visualization of stresses in model of fragment of lumbar spine equipped with fixator (section size, 4 mm; screw ø4). From top: above and back view. 260Gerke et al.: Modeling of lumbar spine Figure 6:Visualization of stresses in section of lumbar vertebrae equipped with fixator (section size, 4 mm; screw ø4). From left: section on screw level and above. Figure 7:Visualization of stresses in model of fragment of lumbar spine equipped with fixator (section size, 6 mm; screw ø4). From top: above and back view. Gerke et al.: Modeling of lumbar spine261 Figure 8:Visualization of stresses in section of lumbar vertebrae equipped with fixator (section size, 6 mm; screw ø4). From left: section on screw level and above. vertebral structure can make uneven wear of bone structure and leads to loss of functionality. Authors of publications [7, 1018] show similar stress distribution, except for stress concentration in rod structure. According to [7], the highest stresses were noticed in the middle of the screw attachment (in the current publication authors located concentration of stresses in the rod in the screws attachment area) which could be caused by rods linker application in [7, 15]. Financial support: None declared. Employment or leadership: None declared. Honorarium: None declared. Competing interests: The funding organization(s) played no role in the study design; in the collection, analysis, and interpretation of data; in the writing of the report; or in the decision to submit the report for publication. Conclusions Authors confirmed the legitimacy of using a fixator in the cases of stopping scoliosis process in lumbar spine structures. High stress dependence on implant size makes planning process more necessary before operation treatment. In this publication authors focused on the case of fixator attached to spine structure by transpedicular screws. This is an indication of continuing research in this subject with the use of different types of attachments like hooks and anchors in different spine sections. More reliable results can be gained by taking a spine model from an MRI or CT scan. The current publication can be the basis for further research. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Journal
Bio-Algorithms and Med-Systems – de Gruyter
Published: Dec 1, 2015