Sigal Ian A, Hardisty Michael R, Whyne Cari M
Orthopaedic Biomechanics Laboratory, Sunnybrook Health Sciences Centre, Toronto, ON, Canada.
J Biomech. 2008;41(7):1381-9. doi: 10.1016/j.jbiomech.2008.02.019. Epub 2008 Apr 7.
Despite recent advances in software for meshing specimen-specific geometries, considerable effort is still often required to produce and analyze specimen-specific models suitable for biomechanical analysis through finite element modeling. We hypothesize that it is possible to obtain accurate models by adapting a pre-existing geometry to represent a target specimen using morphing techniques. Here we present two algorithms for morphing, automated wrapping (AW) and manual landmarks (ML), and demonstrate their use to prepare specimen-specific models of caudal rat vertebrae. We evaluate the algorithms by measuring the distance between target and morphed geometries and by comparing response to axial loading simulated with finite element (FE) methods. First a traditional reconstruction process based on microCT was used to obtain two natural specimen-specific FE models. Next, the two morphing algorithms were used to compute mappings from the surface of one model, the source, to the other, the target, and to use this mapping to morph the source mesh to produce a target mesh. The microCT images were then used to assign element-specific material properties. In AW the mappings were obtained by wrapping the source and target surfaces with an auxiliary triangulated surface. In ML, landmarks were manually placed on corresponding locations on the surfaces of both source and target. Both morphing algorithms were successful in reproducing the shape of the target vertebra with a median distance between natural and morphed models of 18.8 and 32.2 microm, respectively, for AW and ML. Whereas AW-morphing produced a surface more closely resembling that of the target, ML guaranteed correspondence of the landmark locations between source and target. Morphing preserved the quality of the mesh producing models suitable for FE simulation. Moreover, there were only minor differences between natural and morphed models in predictions of deformation, strain and stress. We therefore conclude that it is possible to use mesh-morphing techniques to produce accurate specimen-specific FE models of caudal rat vertebrae. Mesh morphing techniques provide advantages over conventional specimen-specific finite element modeling by reducing the effort required to generate multiple target specimen models, facilitating intermodel comparisons through correspondence of nodes and maintenance of connectivity, and lends itself to parametric evaluation of "artificial" geometries with a focus on optimizing reconstruction.
尽管用于对特定标本几何形状进行网格化的软件最近取得了进展，但通常仍需要付出相当大的努力，才能通过有限元建模生成并分析适用于生物力学分析的特定标本模型。我们假设，通过使用变形技术使预先存在的几何形状适应以表示目标标本，有可能获得精确的模型。在此，我们介绍两种变形算法，自动包裹（AW）和手动地标（ML），并展示它们在制备大鼠尾椎特定标本模型中的应用。我们通过测量目标几何形状与变形几何形状之间的距离，以及比较有限元（FE）方法模拟的轴向载荷响应来评估这些算法。首先，使用基于显微CT的传统重建过程来获得两个自然的特定标本有限元模型。接下来，使用这两种变形算法来计算从一个模型（源模型）的表面到另一个模型（目标模型）的映射，并使用此映射对源网格进行变形以生成目标网格。然后使用显微CT图像来指定特定单元的材料属性。在AW算法中，通过用辅助三角化表面包裹源表面和目标表面来获得映射。在ML算法中，手动在源模型和目标模型表面的相应位置放置地标。两种变形算法都成功地再现了目标椎骨的形状，对于AW和ML算法，自然模型与变形模型之间的中位距离分别为18.8微米和32.2微米。虽然AW变形产生的表面与目标表面更相似，但ML算法保证了源模型和目标模型之间地标位置的对应。变形保留了网格质量，生成了适用于有限元模拟的模型。此外，在变形、应变和应力预测方面，自然模型和变形模型之间只有微小差异。因此，我们得出结论，使用网格变形技术可以生成精确的大鼠尾椎特定标本有限元模型。网格变形技术相对于传统的特定标本有限元建模具有优势，它减少了生成多个目标标本模型所需的工作量，通过节点对应和连通性维护促进了模型间比较，并且有助于对“人工”几何形状进行参数评估，重点在于优化重建。
