## 3 Results
### Comparison of FEM and Experimental Results  
The numerical results were only obtained for the axial distances of 70 mm and 90 mm, because these are the two distances more often used in clinical situations. The results are presented in Table 5, where the ratio CL/TL is also defined, as well as the percentage of the axial deformation in the callus. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Configuration** & **50 [mm]** & **CL/TL** & **70 [mm]** & **CL/TL** & **90 [mm]** & **CL/TL** \\ \hline AB & 30.66 \(\pm\) 4.97 & 15\% & 52.44 \(\pm\) 2.62 & 26\% & 56.85 \(\pm\) 1.20 & 28\% \\ AC & 40.84 \(\pm\) 5.69 & 20\% & 53.14 \(\pm\) 0.32 & 27\% & 56.10 \(\pm\) 0.66 & 28\% \\ BC & 36.93 \(\pm\) 5.65 & 18\% & 54.79 \(\pm\) 1.23 & 27\% & 57.78 \(\pm\) 0.90 & 29\% \\ ABC & 39.24 \(\pm\) 5.01 & 20\% & 52.54 \(\pm\) 2.75 & 26\% & 57.39 \(\pm\) 0.28 & 29\% \\ ACAB & - & - & 51.96 \(\pm\) 3.28 & 26\% & - & - \\ BCAB & - & - & 54.14 \(\pm\) 0.80 & 27\% & - & - \\ ABCAB & - & - & 50.73 \(\pm\) 3.33 & 25\% & - & - \\ ABCAC & - & - & 54.93 \(\pm\) 1.22 & 27\% & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Data obtained for a compression load of 200 N and a callus stiffness of 220 Nmm\({}^{-1}\). Figure 6: Experimental loads on the several pin configurations and distance between axes of 50 mm, linear regressions of the data: (**a**) in the range [0–100] N and (**b**) for the data in the range [100–200] N.  
In order to verify the agreement between the numerical and experimental results, the AC configuration was also analysed for a distance between axes of 50 mm and the results are presented in Figure 7. The letter N is used to identify the numerical results. The value of the cell load in the AC50-N data and the tensile/compression load of 200 N is 30.07 N. In Figure 7, is possible to see that for the axial distances of 50 mm and 70 mm, the numerical model seems to have a stiffer behaviour than the experimental model, whereas, for the axial distance of 90 mm, it is more flexible. The predicted callus force correlated well with the experimental values, when the axial distance of 70 mm was used, and the results are reported in Figure 8.  
The maximum error was of 9.18 N in the ABCAC pin configuration, which represent 16.7% of the force measured. In the symmetrical configurations the highest error was 14.5% for the ABC pin alignment. The lowest value of the predicted forces is for the ABC configuration, whereas, in the experimental tests, it was for the ABCAB configuration. This behaviour could be related to the locking stiffness of the various pin configurations that are presented in Figure 9. From the results presented in Figure 9a, for the symmetric pin configurations, it is possible to see that both numerical and experimental results show similar locking stiffness trends and the highest error was 21% for the BC pin alignment. Nevertheless, in the case of nonsymmetric pin configurations, Figure 9b, the numerical results are almost not sensitive to changes of pin configuration, whereas for the experimental results the BCAB pin alignment leads to the lowest locking stiffness. Figure 10 shows several numerical results for both AB and BC pin configurations, i.e., for the configurations with the highest and the lowest locking stiffness. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & \multicolumn{3}{c}{**70 [mm]**} & \multicolumn{3}{c}{**90 [mm]**} \\ \cline{2-7}  & **Force [N]** & **Def. [\%]** & **CL/TL** & **Force [N]** & **Def. [\%]** & **CL/TL** \\ \hline AB & 47.4 & 1.20 & 24\% & 68.31 & 1.73 & 34\% \\ AC & 46.61 & 1.18 & 23\% & 66.77 & 1.69 & 33\% \\ BC & 48.04 & 1.22 & 24\% & 68.75 & 1.74 & 34\% \\ ABC & 44.91 & 1.14 & 22\% & 63.9 & 1.62 & 32\% \\ ACAB & 47.12 & 1.20 & 24\% & 65.75 & 1.67 & 33\% \\ BCAB & 47.48 & 1.20 & 24\% & 70.67 & 1.79 & 35\% \\ ABCAB & 46.25 & 1.17 & 23\% & 67.94 & 1.72 & 34\% \\ ABCAC & 45.75 & 1.16 & 23\% & 66.97 & 1.70 & 33\% \\ \hline \hline \end{tabular}
\end{table}