## 4 Discussion  
The experimental evaluation performed in the previous section showed that the proposed system has accurate alignment performance for the distal screwing guidance while minimizing use of X-ray images. One of the important assumptions of this study was that guidance using the conventional mechanical jig has inherent limitation because the position of the jig changes during the assembly process and/or because the IMN itself deforms within the bone cavity during the insertion process. Thus, we divided the guidance process for distal screwing into two steps, a parallelization step and a fine-positioning step, for accurate and easy manipulation. A parallel guidance system with a laser pointer (PGSLP) was developed for the parallelization step, and a mechanical fine-adjustment device (FAD) was developed for the fine-positioning step. Previously, several mechanical jig systems had been proposed for guidance [6; 7; 8], but most of them did not mention how to align the drilling direction of the IMN hole efficiently to reduce the X-ray exposure. Different from these, the proposed system using the PGSLP and the FAD is novel, time-saving, and cost-effective. Although we could not completely eliminate the necessity of X-ray imaging, we believe that shooting 3-10 X-ray images are acceptable for this operation. The experiments showed that the parallelization process could be done in 72 s at maximum and 41.1 \(\pm\) 19.0 s on average, and no X-ray image was needed at all. The tilted angle error was 2.18\({}^{\circ}\) at maximum and 1.24 \(\pm\) 0.715\({}^{\circ}\) on average. During the experiments, it was possible to move the C-arm to make the offset of the reflected image of the laser pointer less than 5 mm, which is expected to produce sub-degree tilted angle error. However, the final results were somewhat larger than expected. It is believed that it is due to the calibration error of attaching the OTS markers. Thus, the calibration accuracy should be increased in order to improve the overall performance evaluation in future work. The C-arm used in this study can be remotely operated; however, in this experiment, manual operation was used in order to observe the feasibility of the proposed system using the conventional C-arm and also because manual operation was easier than remote operation. To remotely operate, additional mechanisms, such as a camera to visualize the position \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Subject** & **Index** & \begin{tabular}{c} **Parallelization** \\ **Time (s)** \\ \end{tabular} & \begin{tabular}{c} **Fine Positioning** \\ **Time (s)** \\ \end{tabular} & \begin{tabular}{c} **Total Time** \\ **(s)** \\ \end{tabular} & \
\begin{tabular}{c} **No. of X-ray** \\ **Images (EA)** \\ \end{tabular} &
\begin{tabular}{c} **X-ray Exposure** \\ **(cGy-cm2)** \\ \end{tabular} \\ \hline S1 & T1 & 72 & 145 & 217 & 6 & 1.14 \\ S1 & T2 & 16 & 336 & 352 & 10 & 2.71 \\ S1 & T3 & 52 & 342 & 394 & 10 & 3.67 \\ S2 & T4 & 51 & 310 & 361 & 7 & 2.94 \\ S2 & T5 & 27 & 205 & 232 & 6 & 2.58 \\ S2 & T6 & 31 & 238 & 269 & 9 & 2.66 \\ S3 & T7 & 27 & 140 & 167 & 6 & 1.22 \\ S3 & T8 & 63 & 81 & 144 & 3 & 1.00 \\ S3 & T9 & 31 & 64 & 95 & 3 & 1.31 \\ \hline Average \(\pm\) STD & 41.1 \(\pm\) 19.0 & 207 \(\pm\) 107 & 248 \(\pm\) 105 & 6.67 \(\pm\) 2.65 & 2.14 \(\pm\) 0.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measured parameters during experiments. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Index** & **T1** & **T2** & **T3** & **T4** & **T5** & **T6** & **T7** & **T8** & **T9** & **Average \(\pm\) STD** \\ \hline Tilted angle error (\({}^{\circ}\)) & 0.950 & 1.20 & 1.01 & 0.280 & 0.200 & 2.15 & 2.18 & 1.64 & 1.51 & 1.24 \(\pm\) 0.715 \\ Translational Error (mm) & 0.150 & 0.390 & 0.390 & 0.240 & 0.400 & 0.470 & 0.380 & 0.560 & 0.420 & 0.378 \(\pm\) 0.120 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tilted angle error in the parallelization step and translational error in the fine-positioning step.