# Generation of Computational 3D Models of Human Bones Based on STL Data and CAD Software Packages
## 1 Introduction  
Biomedical engineering combines the design and problem-solving skills of engineering with medical and biological sciences to advance health care treatment. In recent years, Computer-Aided Design (CAD) has been increasingly applied and integrated in the medical technology, including among other fields computer-aided surgery, design of orthopedic devices and implants, design of tissue scaffolds, reverse engineering (RE), and 3D reconstruction. Computer tomography (CT) medical imaging is the most used tool for viewing the internal structure of the human body but is limited by its 2D image presentation. Three-dimensional (3D) solid models of medical images bring more information in the diagnosis and treatment, and the reconstructed 3D solid models can be easily converted to Rapid Prototyping physical models or in Virtual Reality Modeling Language (VRML) format for visualization. Usually, a 3D bio-CAD model is reconstructed through either segmentation or volumetric representation. There are several methods that can be applied to reconstruct a 3D solid model for bio-medical imaging from its 2D CT image [1; 2; 3; 4; 5; 6; 7; 8]. Usually, the model is reconstructed through either segmentation or volumetric representation. One method involves a swept blend from the contours of each layer in point data, the second possibility is via voxel to stack and construct the model by using the marching-cube algorithm. Contour detection in each layer is also used to construct the mixed layers in the triangular stereolithography (STL) model. This format approximates 3D surfaces of a solid model with oriented triangles (facets) of different size and shape (aspect ratio) in order to get a smooth representation suitable for industrial processing of 3D parts using STL machines. However, such a representation is not suitable for the computational analysis using finite element analysis (FEA) because of inappropriate size and large aspect ratio of elements. The FEA mesh quality is critical to the FEA results; a good mesh with high quality can lead to results with high precision. The generation of a FEA mesh from a CAD model mainly depends on the geometric shape of the CAD model: the simpler the geometric shape of a CAD model, the less time the FEA mesh generation costs, and the better the quality of the generated mesh. To shorten the mesh generation time and improve the mesh quality, the CAD mesh model is required to be simplified, which can be solved by suppressing the detailed features on the mesh without any changes to the rest before FEA mesh generation [9; 10; 11]. In case of human bones, the STL geometry obtained after CT scanning is very complex, with a high level of detail. Applying a geometry simplification should not alter the local topology, stress concentrators, or other important geometrical details. A hybrid method that combines significant feature points for functional structure and the ones for anatomical appearance as presented in [12] for the reconstruction of an individual three-dimensional model of fractured long bone based on feature points could be a solution. In this paper, three methods for the conversion of 3D models of STL type into 3D solid models using different CAD systems are presented. For transformation, CATIA V5 (Computer Aided Three-dimensional Interactive Applications, Dassault Systemes, Velizy-Villacoublay, France) and PTC Creo Parametric 2.0 (Parametric Technology Corporation, Boston, MA, USA) were used. The quality of the conversion methods is tested by comparing the results and simulation time of finite element analysis of a zygomatic bone model, which is obtained by the two methods of conversion (using the specific functions of the CATIA software and geometric modeling using sketches in Creo Parametric software). The paper underlines the methodology to use standard functionalities available in the majority of commercial or even freeware CAD software. The paper is organized as follows. Section 2 presents the working conditions and investigated bone geometries.