Modeling and simulation of the cervical spine : mechanical stress in injuries

Abstract

Zsfassung in dt. SpracheInjuries to the human neck are most of time very dangerous and in numerous cases they lead to severe damages to the human body. Due to the fact that the number of serious injuries to the human neck has risen in our modern society, the research in this field is gaining more and more importance. Reasons are among others high-speed transportation as well as increasing leisure-time activities. Hence, main objective is the individual safety and, thus, the identification of the most relevant improvement potentials. A very critical part of the human neck is the upper cervical spine. The spinal cord is the main nerve fiber that runs through it. This nerve fiber controls nearly every essential function. Therefore, its protection has highest priority. In the human body this protection function is performed, amongst others, by the vertebrae of the backbone. The vertebral column, especially the upper cervical spine, is because of its high range of movability a common place for dangerous injuries. To be more precise, fractures occurring in the vertebrae of the upper cervical spine are responsible for many life-threatening injuries. Due to this fact, the goal of this thesis was to get a deeper inside into the upper cervical spine and investigate fractures in the vertebrae of this part of the human body. At the division of Neuronic Engineering, KTH, a Finite Element Model of the human cervical spine has been developed and used extensively since 1996. One main question was if it is possible to use the vertebrae in the existing level of detail in order to predict vertebral fractures. To investigate this question, the second vertebrae, also called axis or shorter C2, is picked out, as this axis is the most common place for fractures in the upper cervical spine. A new finite element model for the C2 vertebrae was established and compared to the behavior with the already existing one from the KTH neck model. Based on computer tomography data of the vertebrae, a three dimensional model of the axis was created and a Finite Element meshing is done with the help of the software ICEM CDF. After that, material properties and boundary conditions were added with the help of the software LS-DYNA. In order to compare the models, two test case sets were defined. Firstly, the vertebrae was directly loaded with various forces at different places. Secondly, a whole fall scenario using the neck model was simulated to apply forces to the vertebrae. The results from the test case set 1 indicate that with the new axis model it is possible to simulate a similar behavior as with the existing KTH model. Further, with a mesh convergence analysis it could be shown that the new C2 Finite Element model has a slightly better convergence rate. Regarding the results of test case 2, it was concluded that both models could deliver the expected and reasonable Von Mises stress distributions. In the context of fracture modeling it can be said that with both models typical fracture patterns can be obtained. All results are in good agreement with experimental data and literature data of already accomplished models. To summarize, the basis for further investigations in the area of fracture prediction in the human vertebrae was elaborated with this thesis. It was shown that most important elements for the creation and application of an appropriate model are a proper geometry definition, a good choice of the finite element mesh and the selection of realistic test cases.8