# Generation of Computational 3D Models of Human Bones Based on STL Data and CAD Software Packages
## 4 Results  
The models of the zygomatic bone obtained using the two methods of conversion (using the specific functions of the CATIA software and geometric modeling using sketches in Creo Parametric software) were subjected to FEA. Before performing the analysis, it is important to get information regarding the model accuracy with respect to the original state; a smoother model will lead to an easier mesh generation and slightly different results under the same boundary conditions. A visual comparison of the two solid models with the base STL model is presented in Figure 16. By comparing the volumes of the 3D models, we can estimate the geometry changes introduced by the conversion techniques presented above. Thus, the CATIA model (Figure 16a) has a volume of 16.35 cm\({}^{3}\), while the Creo model (Figure 16b) has a volume of 16.66 cm\({}^{3}\), which is comparative with the initial volume of 16.41 cm\({}^{3}\) of the STL model. The small difference reflects the accuracy of the generated models.  
The FE analyses performed in this study were static and aimed to study the quality of the obtained models and to determine the time needed for performing the simulations.  
Figure 17 shows the load conditions (force uniformly distributed) and the kinematic constraints (locks on the connection areas) of two solid models obtained by conversion. The obtained model using CATIA software is shown in Figure 17a, while Figure 17b presents the parameterized model obtained by modeling using sketches in Creo Parametric software.  
Although identical load values and constraints were applied on both models, in Figure 17, a difference of density between the force and constraints symbols can be observed due to the different number of facets that form the models. The applied load corresponds to an impact force resulting in a trauma.  
Table 1 presents data referring to the mesh size of the two 3D solid models.  
Figure 18 shows the results of the two FE analyses, respectively the equivalent von Mises stress distribution in volume (left side) and in plane (right side) located at the middle of the model that includes all the elements of the zygomatic bone. A difference in the stress distribution caused by the geometric elements that form the solid models can be observed.  
Figure 15: The complete parametric solid model resulting from the union of components A and B.  
Figure 16: Comparison between generated and initial STL models (in yellow): (**a**) CATIA model; (**b**) Creo model.  
Figure 17: Loading conditions and kinematic constraints: (**a**) for the CATIA model; (**b**) for the Creo model.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CAD Software** & **Tetrahedral Elements** & **Edges** & **Nodes** & **Facets** \\ \hline CATIA model & 254,886 & 41,826 & 8739 & 58,574 \\ \hline Creo Parametric model & 11,324 & 16,148 & 2933 & 24,540 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Data referring to the mesh size.  
Regarding the time needed to perform the FEA, it can be seen in Table 2 that the analysis of the Creo Parametric model took only 73 s, this time being four times shorter than the time required for solving the CATIA model.  
It can be seen in Tables 1 and 2 that there is a clear difference between the time requirements for the models, the parameterized model having the advantage from all points of view. The explanation consists of the fact that it can be discretized with tetrahedral elements (higher order), resulting in fewer elements: 11,324 against 254,886 (see Table 1), i.e., 22.5 times less.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CAD Software** & **Total Time** & **CPU Time** & **RAM Usage** & **Disk Space** \\  & **[s]** & **[s]** & **[kB]** & **Usage [kB]** \\ \hline CATIA model & 295.21 & 330.13 & 1,281,335 & 1,477,632 \\ \hline Creo Parametric model & 72.82 & 129.86 & 878,993 & 967,680 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Computation time of the FE analyses.  
Figure 18: [PERSON] stress distribution: (**a**) for the CATIA model; (**b**) for the Creo model.