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A level-set based medical image segmentation tool for the creation of spinal finite element models
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Journal of Biomechanics 2006, Vol. 39 (Suppl 1)
behavior with excellent agreement to in-vitro data, whereas the validated model diverged strongly. Discussion: To investigate the biomechanical performance of implants and the load distribution in the different spinal structures by a numerical analysis requires not only a good agreement of the intact segment also of the defect stage, which is initiated prior to implant insertion. Therefore, the described calibration method may be suggested to obtain clinical relevant data, but it is more extensive. Acknowledgement: This work was supported by the German Research Council (Deutsche Forschungsgemeinschaft), project: W1-1352/6-1 6468 We-Th, no. 21 (P68) Effect of laminectomy on stability of lumbar spine - FEM study P. Tich~ 1, Z. Hor~k 1, J. Koukalov~ 1, J. Michalec 2. 1Laboratory of Biomechanics, CTU in Prague, Fac. of Mechanical Eng., Prague, Czech Republic, 2Dept. of Mechanics C TU in Prague, Fac. of Mechanical Eng., Prague, Czech Republic The paper aims at the presentation of the lumbar spine finite element model used in a study of segmental kinematics including the tissue loading. Lumbar spine FE-model consisting of the three lumbar vertebras, two disc (including annulus and nucleus) and vast variety of ligaments. The primary model objective is its verification by experimental test data obtained from cadaver spine segments. A confirmed model was used for a study of various range of laminectomy effect. On this model we simulate a various loading states witch can spine segment arrive at under physiologic conditions (compression, flexion, extension and lateral bending). FE-model could predict the laminectomy impact on the operation approach. References [1] Petr~_ela M., Tich£ P., Vilimek M. An experimental assessment of lumbar vertebras mobility. In: EORS - 13th Annual Meeting of European Orthopaedic Research Society, Helsinki, Helsinki Fair Center, 2003, p. 46. [2] Vilimek M., Tich£ P. A Skiagraphy method using for lumbar vertebras movement. Gait & Posture 2003; 18(Suppl. 2): 80-123. [3] Vilimek M., Tich£ P. A Skiagraphy method using for lumbar vertebras movement delimitation. In: ESMAC 2003 - European Society for Movement Analysis in Adults and Children - 12th Annual Meeting. 2003; Marseille: The French Group of Movement Analysis in Adults and Children, p. 84. [4] Hor&k Z., Jirkov& L., Tich~ P., Sedl&~ek R. Experimental measurements of the lumbar spine stiffness. In: 22nd Danubia-Adria Symposium on Experimental Methods in Solid Mechanics. 2005; Vol. 1, pp. 94-95. [5] Hor&k Z., Jirkov& L., Tich£ E, Sedl&~ek R. Experimental measurements of the lumbar spine stiffness and Range of Mori6n. In: 4th YSESM; 2005; Vol. 1, pp. 141-142. 5107 We-Th, no. 22 (P68) A level-set based medical image segmentation tool for the creation of spinal finite element models A. Jones 1, D. Magee 2, A. Bulpitt 2, R. Wilcox 1. 1School of Mechanical
Engineering, University of Leeds, Leeds, UK, 2School of Computing, University of Leeds, Leeds, UK The increased use of finite element modelling as a research tool in the field of spinal biomechanics has sparked an interest in creating tools to simplify and speed up the model building process. In order to compare the finite element models with experimental data it is necessary to develop specimenspecific models. The current greatest challenge of creating such models is the identification of the unique geometry and features of each spinal specimen from medical images, such as those created by computed tomography. If the specimen is dry or the bone is osteoporotic then conventional thresholding and morphological operations offered by image processing packages will not be sufficient to extract a solid vertebral geometry. Currently it is typical for computational models of spinal segments to be created in a labour intensive manner, where individual grid points are selected by hand. During this project an image segmentation tool, based upon the level set method, was developed to facilitate the process of vertebral architecture extraction. The flexibility of the level set method is combined with multi-resolution techniques to tackle the segmentation of vertebrae in healthy or diseased states and at any orientation. The ability of the level set curve evolution method to capture non-spherical topologies allows for the natural segmentation of the posterior elements and spinal canal, as well as the vertebral bodies, in a spinal section. The use of multiple resolutions, meanwhile, allows the production of results in a practical time frame. This multi-resolution level set method is intended to form a part of an image processing package in a user-led environment. In combination with more straight-forward tools this technique is effective in reducing the time taken to generate finite element model architectures. Examples of completed finite element models are shown along with image segmentation results at various stages.
Poster Presentations 5101 We-Th, no. 23 (P68) Optimisation of FE model generation methods for reliable vertebrae representation A. Jones, R. Wilcox. School of Mechanical Engineering, University of Leeds,
Leeds, UK Introduction: Finite element modelling is becoming widely used in the assessment of new spinal treatments such as vertebroplasty. Interest in the quality of specimen-specific models has therefore increased. In order to undertake large scale modelling projects, methods of rapid model generation must be developed. The aim of this study was to investigate the relative contribution of various model attributes to the reliability of the results, and hence determine how the mesh generation process can be optimised. Methods: A set of porcine lumbar vertebral bodies were imaged using computed tomography and tested in the laboratory. The images were used to build a series of FE models and three comparative studies were undertaken: First, a series of manually-generated, generic and specimen-specific models were used to determine the relative effects of model geometry and material property values by comparison with the experimental results. Next, changes were made to the applied boundary conditions and the effect on the simulated results assessed. Finally, a new series of models of different mesh densities were generated using semi-automated methods and the simulated results compared with the manually-generated models. Results: The results from the first study showed that high levels of agreement between the experimental and FE results were only possible with models combining specimen-specific architecture and element-specific material properties (concordance >80%). In addition, small changes in the positions of the applied boundary conditions were found to have a substantial effects on the model results. The final study indicated that meshes generated by manual methods (103 hexahedral elements) could achieve similar levels of accuracy to automatically generated meshes of higher mesh density (105 tetrahedral elements). Conclusions: This study shows that in addition to the well established importance of attributes relating to material properties, those relating to the architecture of the specific vertebra and in particular the location of the boundary conditions, are crucial to the model's accuracy. The results of the meshing study highlight the need for methods which combine the accuracy of manual node placement with the efficiency of automatic meshing. 4009 We-Th, no. 24 (P68) Estimation of forces in lumbar spine and associated guy wires J.A. Hodgdon 1, C.S. Putcha 2. 1Navy Health Research Center, San Diego,
CA, USA, 2Department of Civil and Environmental Engineering, California State University, Fullerton, USA This paper deals with calculation of forces in the lumbar spine and the connected guy wires. McGill (2002) suggests this model of the lumbar spine connected to guy wires. In this model the lumbar spine is assumed to have the flexibility of bending in any direction with a large amount of muscle coactivation. The spine is likened to a flexible rod that buckles under compressive loading. It is also assumed that the lumbar spine has guy wires connected to it. These guy wires constitute complex interaction of stiffening structures along the spine and those forming the torso wall. These guy wires and the lumbar spine are assumed to connect at the base to the pelvis. In actuality, each of the guy wires can be considered like a combination of tendon and muscle. The load is assumed to be acting on the free joint downwards. For the purpose of this analysis, equivalent truss model with the guy wires treated like tendons (with equivalent modulus of elasticity) is considered. Thus the model is treated like a truss model with four members (lumbar spine, two guy wires and pelvis). The main intent of this research is to calculate the forces in the lumber spine and the connected guy wires and show mathematically that the force developed in the lumber spine is much less than the model without the connecting guy wires and hence more resistant to buckling. It is evident from the model that due to the load P acting at the free joint, there will be forces that will be developed in each of the four members of the truss. The forces in the lumbar spine and the associated guy wires are considered using the principles of mechanics and using the basic concepts of matrix analysis of structures. The actual physical structure is modeled as a equivalent truss with four members. 6544 We-Th, no. 25 (P68) Compact and cancellous bone behave as a "bimaterial beam model" of constant strength N. Sesic 1,2, M. Opalic 2, V. Nikolic 3. 1General Hospital Karlovac, Dpt. of
Surgery, A, Karlovac, Croatia, 2Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia, 3 Dep. of Anatomy, Faculty of Medicine, Osijek, Croatia A mechanical model of bone should describe stress and strain distributions according to minimum/maximum laws. At the macrolevel the shape of the bone
Journal of Biomechanics 2006, Vol. 39 (Suppl 1)
behavior with excellent agreement to in-vitro data, whereas the validated model diverged strongly. Discussion: To investigate the biomechanical performance of implants and the load distribution in the different spinal structures by a numerical analysis requires not only a good agreement of the intact segment also of the defect stage, which is initiated prior to implant insertion. Therefore, the described calibration method may be suggested to obtain clinical relevant data, but it is more extensive. Acknowledgement: This work was supported by the German Research Council (Deutsche Forschungsgemeinschaft), project: W1-1352/6-1 6468 We-Th, no. 21 (P68) Effect of laminectomy on stability of lumbar spine - FEM study P. Tich~ 1, Z. Hor~k 1, J. Koukalov~ 1, J. Michalec 2. 1Laboratory of Biomechanics, CTU in Prague, Fac. of Mechanical Eng., Prague, Czech Republic, 2Dept. of Mechanics C TU in Prague, Fac. of Mechanical Eng., Prague, Czech Republic The paper aims at the presentation of the lumbar spine finite element model used in a study of segmental kinematics including the tissue loading. Lumbar spine FE-model consisting of the three lumbar vertebras, two disc (including annulus and nucleus) and vast variety of ligaments. The primary model objective is its verification by experimental test data obtained from cadaver spine segments. A confirmed model was used for a study of various range of laminectomy effect. On this model we simulate a various loading states witch can spine segment arrive at under physiologic conditions (compression, flexion, extension and lateral bending). FE-model could predict the laminectomy impact on the operation approach. References [1] Petr~_ela M., Tich£ P., Vilimek M. An experimental assessment of lumbar vertebras mobility. In: EORS - 13th Annual Meeting of European Orthopaedic Research Society, Helsinki, Helsinki Fair Center, 2003, p. 46. [2] Vilimek M., Tich£ P. A Skiagraphy method using for lumbar vertebras movement. Gait & Posture 2003; 18(Suppl. 2): 80-123. [3] Vilimek M., Tich£ P. A Skiagraphy method using for lumbar vertebras movement delimitation. In: ESMAC 2003 - European Society for Movement Analysis in Adults and Children - 12th Annual Meeting. 2003; Marseille: The French Group of Movement Analysis in Adults and Children, p. 84. [4] Hor&k Z., Jirkov& L., Tich~ P., Sedl&~ek R. Experimental measurements of the lumbar spine stiffness. In: 22nd Danubia-Adria Symposium on Experimental Methods in Solid Mechanics. 2005; Vol. 1, pp. 94-95. [5] Hor&k Z., Jirkov& L., Tich£ E, Sedl&~ek R. Experimental measurements of the lumbar spine stiffness and Range of Mori6n. In: 4th YSESM; 2005; Vol. 1, pp. 141-142. 5107 We-Th, no. 22 (P68) A level-set based medical image segmentation tool for the creation of spinal finite element models A. Jones 1, D. Magee 2, A. Bulpitt 2, R. Wilcox 1. 1School of Mechanical
Engineering, University of Leeds, Leeds, UK, 2School of Computing, University of Leeds, Leeds, UK The increased use of finite element modelling as a research tool in the field of spinal biomechanics has sparked an interest in creating tools to simplify and speed up the model building process. In order to compare the finite element models with experimental data it is necessary to develop specimenspecific models. The current greatest challenge of creating such models is the identification of the unique geometry and features of each spinal specimen from medical images, such as those created by computed tomography. If the specimen is dry or the bone is osteoporotic then conventional thresholding and morphological operations offered by image processing packages will not be sufficient to extract a solid vertebral geometry. Currently it is typical for computational models of spinal segments to be created in a labour intensive manner, where individual grid points are selected by hand. During this project an image segmentation tool, based upon the level set method, was developed to facilitate the process of vertebral architecture extraction. The flexibility of the level set method is combined with multi-resolution techniques to tackle the segmentation of vertebrae in healthy or diseased states and at any orientation. The ability of the level set curve evolution method to capture non-spherical topologies allows for the natural segmentation of the posterior elements and spinal canal, as well as the vertebral bodies, in a spinal section. The use of multiple resolutions, meanwhile, allows the production of results in a practical time frame. This multi-resolution level set method is intended to form a part of an image processing package in a user-led environment. In combination with more straight-forward tools this technique is effective in reducing the time taken to generate finite element model architectures. Examples of completed finite element models are shown along with image segmentation results at various stages.
Poster Presentations 5101 We-Th, no. 23 (P68) Optimisation of FE model generation methods for reliable vertebrae representation A. Jones, R. Wilcox. School of Mechanical Engineering, University of Leeds,
Leeds, UK Introduction: Finite element modelling is becoming widely used in the assessment of new spinal treatments such as vertebroplasty. Interest in the quality of specimen-specific models has therefore increased. In order to undertake large scale modelling projects, methods of rapid model generation must be developed. The aim of this study was to investigate the relative contribution of various model attributes to the reliability of the results, and hence determine how the mesh generation process can be optimised. Methods: A set of porcine lumbar vertebral bodies were imaged using computed tomography and tested in the laboratory. The images were used to build a series of FE models and three comparative studies were undertaken: First, a series of manually-generated, generic and specimen-specific models were used to determine the relative effects of model geometry and material property values by comparison with the experimental results. Next, changes were made to the applied boundary conditions and the effect on the simulated results assessed. Finally, a new series of models of different mesh densities were generated using semi-automated methods and the simulated results compared with the manually-generated models. Results: The results from the first study showed that high levels of agreement between the experimental and FE results were only possible with models combining specimen-specific architecture and element-specific material properties (concordance >80%). In addition, small changes in the positions of the applied boundary conditions were found to have a substantial effects on the model results. The final study indicated that meshes generated by manual methods (103 hexahedral elements) could achieve similar levels of accuracy to automatically generated meshes of higher mesh density (105 tetrahedral elements). Conclusions: This study shows that in addition to the well established importance of attributes relating to material properties, those relating to the architecture of the specific vertebra and in particular the location of the boundary conditions, are crucial to the model's accuracy. The results of the meshing study highlight the need for methods which combine the accuracy of manual node placement with the efficiency of automatic meshing. 4009 We-Th, no. 24 (P68) Estimation of forces in lumbar spine and associated guy wires J.A. Hodgdon 1, C.S. Putcha 2. 1Navy Health Research Center, San Diego,
CA, USA, 2Department of Civil and Environmental Engineering, California State University, Fullerton, USA This paper deals with calculation of forces in the lumbar spine and the connected guy wires. McGill (2002) suggests this model of the lumbar spine connected to guy wires. In this model the lumbar spine is assumed to have the flexibility of bending in any direction with a large amount of muscle coactivation. The spine is likened to a flexible rod that buckles under compressive loading. It is also assumed that the lumbar spine has guy wires connected to it. These guy wires constitute complex interaction of stiffening structures along the spine and those forming the torso wall. These guy wires and the lumbar spine are assumed to connect at the base to the pelvis. In actuality, each of the guy wires can be considered like a combination of tendon and muscle. The load is assumed to be acting on the free joint downwards. For the purpose of this analysis, equivalent truss model with the guy wires treated like tendons (with equivalent modulus of elasticity) is considered. Thus the model is treated like a truss model with four members (lumbar spine, two guy wires and pelvis). The main intent of this research is to calculate the forces in the lumber spine and the connected guy wires and show mathematically that the force developed in the lumber spine is much less than the model without the connecting guy wires and hence more resistant to buckling. It is evident from the model that due to the load P acting at the free joint, there will be forces that will be developed in each of the four members of the truss. The forces in the lumbar spine and the associated guy wires are considered using the principles of mechanics and using the basic concepts of matrix analysis of structures. The actual physical structure is modeled as a equivalent truss with four members. 6544 We-Th, no. 25 (P68) Compact and cancellous bone behave as a "bimaterial beam model" of constant strength N. Sesic 1,2, M. Opalic 2, V. Nikolic 3. 1General Hospital Karlovac, Dpt. of
Surgery, A, Karlovac, Croatia, 2Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia, 3 Dep. of Anatomy, Faculty of Medicine, Osijek, Croatia A mechanical model of bone should describe stress and strain distributions according to minimum/maximum laws. At the macrolevel the shape of the bone