## 2 Materials and Methods  
Before experiment was performed, two appropriate Sarafix configurations were chosen based on their applicability in the orthodontic practices. Testing configurations were established with the respect to the location of a tibia bone and other specifics of importance. The tibia was located eccentrically, which resulted in a very complex stress state and susceptibility to external effects, i.e., traumas and fractures. At the same time, the position of the tibia bone facilitated the application of the fixation device. Two representative configurations of the Sarafix external fixation device is shown on Figure 1 (configurations B and C). It is important to remark that chosen configurations are examined with a simulated open fracture with bone marrow defect with the interfragment clearance of 50 mm. This value is obtained from the orthodontic practices data, and it corresponds to length of an opened fracture with greater defect of a bone marrow [25]. B configuration represents the unilateral external fixation device, which is composed of 4 half-pins fastened into upper and lower bone segments (Figure 1a). All of the half-pins are placed in the anterior posterior plane (AP plane) parallel to each other. Configuration B is mainly used for the tibia and upper extremities. C configuration represents the biplanar external fixation device also composed from 4 half-pins fastened into upper and lower bone segments (Figure 2b). Unlike B configuration, this configuration has an additional half-pin on the upper and lower segment, respectively, fitted into a plane that forms a 45-degree angle with the AP plane. This way, half-pins are located in two planes, and, along with other components, they form a so-called triangular, i.e., delta, construction used for biplanar fixation of the bone segments. During development of the Sarafix device with composite frame, special focus was directed to frame geometry, i.e., dimensions, which need to correspond to dimensions of the frame built from stainless steel. The material chosen for the composite frame is a recommended composite material with carbon fibers used for these types of constructions. Mechanical properties of the composite material used for frame construction are shown in Table 1.  
Before the experiment was conducted, it was necessary to manufacture the models of the human tibia bone that would be used for the Sarafix device application. Often used materials for bone models are PVC materials, wood, aluminum, copper, etc. This research considers human bone model made out of beech tree with mechanical properties similar to the human bone (Table 1) [26, 27]. The final form of the Sarafix device with the composite frame applied to the bone models is shown on Figure 2. \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Property**} & \multirow{2}{*}{**Mark**} & **Std-CF–UD** & **Beech Tree** \\ \cline{3-4}  & & & **Value** \\ \hline Longitudinal extensive strength & \(\chi_{\text{t}}\) & 1500 MPa & 135 MPa \\ Transversal extensive strength & \(\chi_{\text{t}}\) & 50 MPa & 2.8 MPa \\ Longitudinal pressure strength & \(\chi_{\text{c}}\) & 1200 MPa & 60 MPa \\ Transversal pressure strength & \(\chi_{\text{c}}\) & 250 MPa & 13 MPa \\ Longitudinal bending strength & \(\chi_{\text{S}}\) & 1200 MPa & 148 MPa \\ Shear strength & \(\text{S}\) & 70 MPa & 8.5 MPa \\ Longitudinal modulus of elasticity & \(\text{E}_{\text{L}},\text{E}_{\text{11}}\) & 135 GPa & 15.4 GPa \\ Transversal modulus of elasticity & \(\text{E}_{\text{T}},\text{E}_{\text{22}}\) & 10 GPa & 1.12 GPa \\ Poisson’s coefficient & \(\text{v}_{\text{12}}\) & 0.3 & 0.66 \\ Sliding modulus & \(\text{G}_{\text{12}}\) & 5 GPa & 0.45 GPa \\ Density & \(\rho\) & 1600 kg/m\({}^{3}\) & 740 kg/m\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of the composite material Std–CF–UD and beech tree [1]. Figure 1: Sarafix fixation device with steel frame; (**a**) B50 configuration, (**b**) C50 configuration.