## 4 Experimental Section  
The experiment aimed to obtain information about the dependency between the tightening of the headless screw and to determine the normal force acting on the headless, self-tapping cannulated screw (same as in the calculation, i.e., Ti: 4.0/1.4 \(\times\) 30/7 mm by MEDIN a.s.; see Figure 5 and Table 1). The acquired results can then be compared with analytical, numerical, and stochastic calculations. The knowledge of axial forces acting during osteosynthesis is important when designing headless screws and their threads. In real-life fractures, bending stress is also present, but compression and tension are the dominant stresses on the bone fragments and screw during screw introduction (insertion). Moreover, this type of information is missing in literatures. The experiment was performed on a headless screw introduced into porcine leg bones purchased from a conventional butcher shop. The porcine bones are commonly used as a readily available substitute for human bones, including metatarals [22]. The use of porcine bones was in accordance with ethical standards and did not require approval from an ethics committee. Tightening the screw inside the bone fragments elicited a compressive force measured by a holed strain gauge transducer. First, the porcine bone fragments were pre-drilled, then a tensometric sensor (LC 8150-375-500, Tensometric Mestechnik GmbH, Wuppertal, Germany) was placed between the bone fragments, and a headless screw was inserted. In accordance with the operating procedures, the headless screw was tightened to a tightening torque of 2.5 Nm. This value was experimentally verified by previous measurements to  
Figure 10: Two-dimensional distribution of the reliability function values (horizontal axis represents calculated stress and vertical axis is yield stress, steps: 1,000,000)—Anthill software. ensure that the screw threads were not stripped while driving the screw into the examined bone fragments. The force was measured in quarter-turn increments, i.e., 90 deg, until the headless screw was tightened to 3.5 turns; this also included the moment of the thread shear (i.e., the maximum moment) in the bone. (Thread shear will cause the osteosynthesis to fail but will not destroy the implant.) The principle of the measurement is shown schematically in Figure 11.  
Figure 12 shows the real measurement of the compressive axial force acting on the headless screw. It depicts two fragments of a cleaned pork bone, joined with a screw, and a holed compression force sensor inserted between them. The investigated screw was made of titanium alloy Ti6 Al4V, the properties of which are specified by ISO 5832-3 (material for surgical implants). This alloy, together with the stainless steel 316L, are two of the most commonly used materials for the manufacture of implants [23]. The screw material was considered biocompatible, isotropic, and homogeneous, and its basic mechanical properties are shown in Table 3. The screws were manufactured by conventional machining methods. The measurements took place at the laboratory at the VSB--Technical University of Ostrava, Faculty of Mechanical Engineering, Department of Applied Mechanics (Ostrava, Czech Republic). In all, 10 measurements were performed. Figure 13 shows the dependence of the axial compressive force F on the number of turns n (in quarter-turns) of the headless bolt. Table 4 shows the maximum force F, number of turns for each measurement, and the values of arithmetic mean, median, and standard deviation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Modulus of Elasticity \(E\) [Pa]** & **Yield Limit [MPa]** & **Strength Limit [MPa]** \\ \hline Ti6 Al4V & \(1.06\times 10^{11}\) & 920 & 1000 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Basic information on the material of the applied screws. Figure 11: Schema of the experiment. After 2-2.5 turns, the curves reached their maximum, at which point the Herbert screw thread in the bone failed (bone damage), and the compressive force started to decline.