Short communication Finite element model development of a child pelvis with optimization-based material identification Jong-Eun Kim a,Ã , Zuoping Li b , Yasushi Ito a , Christina D. Huber c , Alan M. Shih a , Alan W. Eberhardt b , King H. Yang c , Albert I. King c , Bharat K. Soni a a Department of Mechanical Engineering, University of Alabama at Birmingham, Hoehn 330B, 1075 13th St. S., Birmingham, AL 35294, USA b Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294, USA c Department of Biomedical Engineering, Wayne State University, Detroit, MI 48202, USA article info Article history: Accepted 5 June 2009 Keywords: Pediatric pelvis Finite element Hexahedral mesh Material identification abstract A finite element (FE) model of a 10-years-old child pelvis was developed and validated against experimental data from lateral impacts of pediatric pelves. The pelvic bone geometry was reconstructed from a set of computed tomography images, and a hexahedral mesh was generated using a new octree- based hexahedral meshing technique. Lateral impacts to the greater trochanter and iliac wing of the seated pelvis were simulated. Sensitivity analysis was conducted to identify material parameters that substantially affected the model response. An optimization-based material identification method was developed to obtain the most favorable material property set by minimizing differences in biomechanical responses between experimental and simulation results. This study represents a pilot effort in the development and validation of age-dependent musculoskeletal FE models for children, which may ultimately serve to evaluate injury mechanisms and means of protection for the pediatric population. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Many experimental side impact studies have been conducted to investigate the biomechanical properties, structural responses, and injury tolerances of the adult pelvis (Viano et al., 1989; Beason et al., 2003; Etheridge et al., 2005; Yoganandan and Pintar, 2005). Finite element (FE) models of the adult pelvis have been successfully used to evaluate the pelvic structural responses under quasi-static or impact load scenarios that complement experimental studies (Dawson et al., 1999; Garc´ ıa et al., 2000; Majumder et al., 2004; Anderson et al., 2005; Song et al., 2006; Li et al., 2007). None of these studies, however, involved pediatric pelves, and relatively little information is available on the material properties, structural responses, and injury tolerance of the pediatric pelvis. Research efforts related to pediatric injury have been limited due to the difficulties in obtaining pediatric cadavers or medical image datasets. One experimental study reported lateral impact re- sponses of pediatric pelves using 12 whole child cadavers with ages ranging from 2 to 12-years-old (Ouyang et al., 2003). The objective of this study was to develop a FE model of a 10- years-old (10YO) child pelvis from medical image data. A simulation- and optimization-based material identification meth- od was used in combination with the data from the experimental study (Ouyang et al., 2003) to find the material properties of the pediatric pelvic bone and its associated soft tissues. 2. Materials and methods 2.1. Geometry and hexahedral mesh generation Axial computed tomography (CT) scans of a 10YO female abdomen and pelvis were acquired from a radiology database within the Children’s Hospital of Michigan, with approval by the Wayne State University Human Investigation Committee. No bony abnormalities were identified by a board certified radiologist. Using this CT data set (resolution of 512  512 pixels, and a slice thickness of 7.5mm), image segmentation and 3D reconstruction were performed in MIMICS 10.01 (Materialise Inc., Leuven, Belgium). A 3D volume was generated based on linear interpolation between 2D slices, and triangulated surfaces were rendered. The final surfaces (left and right coxal bones and sacrum/coccyx) were exported (Fig. 1a) to create a FE mesh. An automatic octree-based hexahedral mesh generation method was devel- oped with a new set of refinement templates to create geometry-adapted meshes (Ito et al., 2009). An octree core mesh was generated inside the triangulated surface model using an octree (Fig. 1b). A shape smoothing method was applied to the boundary surface of the octree core mesh to eliminate hump artifacts. A buffer layer was then added on the boundary surface (Fig. 1c). Node smoothing and boundary projection methods were applied to the mesh to recover the boundary surface and to improve the mesh quality (Fig. 1d). The final model had 181,505 eight-node hexahedra with minimum Jacobian of 0.11. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2009.06.010 Ã Corresponding author. Tel.: +1205 975 5889; fax: +1 205 975 7244. E-mail address: jkim@uab.edu (J.-E. Kim). Journal of Biomechanics 42 (2009) 2191–2195