# Generation of Computational 3D Models of Human Bones Based on STL Data and CAD Software Packages
## 3 Conversion Methods
### Conversion Methods Using Geometric Modeling in Creo Parametric
Use of the \"shell\" function would not be possible if the model was not parameterized, for example, a geometry directly imported as an STL file. The parameterized models are also necessary to further design the hip prosthesis. Figure 9 presents the designed assembly femoral bone-hip prosthesis, and the correction of the neck-shaft angle (130\({}^{\circ}\)) and the arch shape of the femoral stem to facilitate the implantation can be noticed. For the fabrication of the implant, the Ti6 Al7 Nb alloy was the best choice because of its mechanical and biocompatibility properties; the hip prosthesis was realized by Selective Laser Sintering technology, from pure titanium, in the Rapid Prototyping Laboratory of the Technical University of Cluj-Napoca [18]. Figure 8: Tracing the parameterized 3D model: (**a**) use of the Swept Blend function; (**b**) parametrized solid model. The conversion of the 3D model (STL type) of the zygomatic bone, as shown in Figure 1b, is now presented. By using the Shrinkwrap function, a complex solid is obtained where a division into two parts is required (part A and part B--see Figure 10a). After that, a sketch in the section plane was created (see Figure 10b). The sketch is required for the reunion of the two components after the conversion into parametric solids. In order to obtain component A (Figure 11a), the Shrinkwrap solid was sectioned with a series of parallel planes, and these sections were sketches with Bezier curves (see Figure 11b). Since the initial model of the zygomatic bone contains both straight contours and flat and curved surfaces, after modeling them using the Blend function (Figure 12), it is necessary to adjust them via a series of operations using the Extrude Cut (Figure 12b) and Blend Cut functions (Figure 12c). These additional operations are intended to make the approximation of the initial model as accurate as possible. Figure 12c presents the final parameterized model obtained after applying all the modeling operations. Figure 10: The conversion of the 3D model (STL type) of the zygomatic bone: (**a**) 3D model components; (**b**) the bonding section. Figure 9: Designed assembly of a femoral bone with a hip replacement. Figure 11: The A component tracing: (**a**) Shrinkwrap model; (**b**) sketches necessary for modeling. Figure 12: The achievement of the A component—results of the following functions: (**a**) Blend—parameterized model; (**b**) Extrude Cut—obtaining straight edges and flat surfaces; (**c**) Blend Cut—obtaining curved surfaces. The 3D model of component B had a more complex shape, as can be seen in Figure 13a, and it required a new approach for the modeling methodology. The initial model was the basis for tracing external sketches (Figure 13b and detail B in Figure 14a). After obtaining the sketches, operations for removing the excess material from a predefined parallelepiped model were applied. For the upper part of the model (detail A in Figure 14), the same steps were applied as for modeling component A, with the difference that the sketches were arranged on the inclined plane normal to the axis of the zygomatic bone apopysis. Figure 14b shows the final parametrized model of the B component after applying the modeling functions: Blend--applied to the top surface, Blend Cut and Extrude Cut--applied to the exterior sketches that define the outer contour of the component. Figure 14: Tracing of the upper and lower part of the B component: (**a**) details of sketches; (**b**) the final parameterized model of the B component. Figure 5:0 The B component tracing: (**a**) Shrinkwrap model; (**b**) sketches necessary for modeling. Once the two parts, marked with yellow and green (A and B), were obtained by Boolean union operation (Merge) into the bonding area (see Figure 10b), the parameterized solid model was obtained, as shown in Figure 5:1  
No problems were encountered when the parametric solid models resulting from this conversion method (geometric modeling) were analyzed using FEA.