## 4 Experimental Testing of Fixators  
Experimental research was conducted in vitro, and necessary data were obtained with the material testing machine with usage of appropriate measuring equipment and side-tools used for supporting of the model (Figure 6). The goal of the experimental setup was to verify the structural analysis results. In real conditions, a fixation device is loaded via bone segments. Considering that fact, the experimental load of the fixation device was applied throughout the models of the bone segments [24; 31; 34]. Figure 6 shows the experimental setup for the Sarafix fixation device. Supporting of the ends for the upper and lower bone models was carried out with spherical joints mounted on the force transducer and lower side-tools. One of the side-tools was fixated to the base of the testing machine. Its upper part contained the spherical joint, which \"grabbed\" the lower bone model segment. The upper mobile side-tool also contained a spherical joint, which was connected with the force transducer. During the examination, deflection of the proximal bone segment model was monitored with the deflection transducer, while load was controlled with the force transducer (U2A by the HBM manufacturer--Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany), and with the usage of the material testing machine (Zwick GmbH & Co., model 143501, Ulm, Germany). Deflections of the bone segments at the fracture zone were monitored with the digital high-speed CMOS camera (PCO AG, type: pco. 1200 s, Kelheim, Germany) (Figure 6). Stress analysis was performed with the strain gauges of type 3/120 LY11 (HBM) and a data acquisition electronic device system QuantumX MX840B (HBM), used for signal receiving from the strain gauges. Strain gauges were connected into the Wheatstone's quarter-bridge configuration and with the QuantumX system via two measuring channels. Quarter-bridge configurations were composed from active and compensation strain gauges, as shown in Figure 7. Active strain gauges were applied on the opposite sides of the frame, i.e., MM- and MM+ control zones (Figure 7). This way their longitudinal axis matched the direction of the maximal principal strain. Compensation gauges are used to compensate the temperature effects, and they are the same type as the active gauges. Compensation gauges were mounted on a plate that was connected to the frame in an area near the control zones. The plate and fixation device frame were made out of the same material. After their application, all of the gauges were protected from the mechanical defects using the sealing tape. The fixation device frame, due to axial pressure load, was exposed to the eccentric pressure. This caused the specific principal strain condition, which is formed by bending strains and axial pressure strains (Figure 8). Using the superposition method, these strains can be defined as:  
\[\epsilon_{1}=-\epsilon_{\mathrm{p}}+\epsilon_{\mathrm{s}}=-\frac{\mathrm{F}}{ \mathrm{AE}}+\frac{\mathrm{M}}{\mathrm{EZ}};\;\epsilon_{3}=-\epsilon_{\mathrm{p }}-\epsilon_{\mathrm{s}}=-\frac{\mathrm{F}}{\mathrm{AE}}-\frac{\mathrm{M}}{ \mathrm{EZ}} \tag{5}\]  
Figure 7: Strain gauges arrangement; (**a**) mounting, (**b**) experiment. where \(\epsilon_{\rm p}\) is the strain component caused by the axial compressive force, \(\epsilon_{\rm s}\) the strain component caused by the bending moment, F the axial compressive force, A the area cross-section of the fixator connecting rod, E modulus of elasticity, M bending moment and Z section modulus of the fixator connecting rod. In this case, the bending strains are significantly greater than the pressure strains. Therefore, this form of a load is manifested with an unequal distribution of the main strains, i.e., the neutral axis does not match with the frame symmetry axis (Figure 8).