## 2 Development of the External Fixation Model  
Before the test itself, two appropriate configurations of the Sarafix fixator that have the greatest application in orthopedic practice were selected. When forming the test configurations, the position of the tibia in the lower leg and other specifics were taken into account. Namely, the tibia in the lower leg is located eccentrically and close to the skin, which has the effect of making this bone extremely exposed to external influences, i.e., injuries and fractures, but at the same time, its specific position facilitates the application of fixators. Two representative configurations of the Sarafix fixator were selected for application on the lower leg and upper extremities under designations B and C (Figure 1). It is important to note that for the test configuration of the fixator, an open bone fracture with a bone structure defect in the value of the interfragmentary gap of 50 mm was simulated. This value was obtained based on data from orthopedic practice and corresponds to the length of an open fracture with a larger bone structure defect [17; 18]. Configuration B is a unilateral external fixator consisting of four half-pins placed in the upper and lower segment of the bone (Figure 1a). All half-pins are located in the AP plane parallel to each other. Version B is mainly used on the lower leg and upper extremities. Configuration C is a unilateral biplanar external fixator consisting of four half-pins each placed in the upper and lower segments of the bone (Figure 1b). Unlike version B, it has one half-pin each in the upper and lower segments placed in a plane that overlaps with an angle of 45\({}^{\circ}\) in relation to the AP plane. In this way, the half-pins are arranged in two planes and, together with the other components, form one so-called triangular or delta construction, which achieves biplanar stiffening of bone segments. During the development of the device for external fixation (Sarafix with a frame made of composite material) care was taken to ensure that it geometrically and dimensionally corresponds to the original device (frame made of stainless steel) in order to be able to compare the obtained results. Additionally, when choosing the composite material, it was taken into account that it should be the material that is most often used today in the manufacture of fixators, which is a composite material reinforced with carbon fibers. The mechanical characteristics of the composite material used in the manufacture of the fixator frame are shown in Table 1. \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Property**} & \multirow{2}{*}{**Mark**} & \multicolumn{2}{c}{**Std-CF-UD**} & \multicolumn{2}{c}{**Beech Wood**} \\ \cline{3-4}  & & \multicolumn{2}{c}{**Value**} \\ \hline Longitudinal tensile strength & \(\chi_{\text{t}}\) & 1500 MPa & 135 MPa \\ Transversal tensile strength & \(\chi_{\text{t}}\) & 50 MPa & 2.8 MPa \\ Longitudinal compressive strength & \(\chi_{\text{c}}\) & 1200 MPa & 60 MPa \\ Transversal compressive strength & \(\chi_{\text{c}}\) & 250 MPa & 13 MPa \\ Longitudinal flexural strength & \(\chi_{\text{S}}\) & 1200 MPa & 148 MPa \\ Ultimate in-plane shear strength & S & 70 MPa & 8.5 MPa \\ Longitudinal modulus of elasticity & E\({}_{\text{L}}\), E\({}_{11}\) & 135 GPa & 15.4 GPa \\ Transversal modulus of elasticity & E\({}_{\text{T}}\), E\({}_{22}\) & 10 GPa & 1.12 GPa \\ Poisson’s ratio & \(\
u_{12}\) & 0.3 & 0.66 \\ Shear modulus & G\({}_{12}\) & 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 wood [1; 19]. Figure 1: Sarafix fixation device with steel frame; (**a**) B50 configuration, (**b**) C50 configuration. Before performing the tests, it was necessary to create human bone models on which the Sarafix external fixation system versions would be placed. Different materials are used to make human bone models.