# Desktop 3D Printing: Key for Surgical Navigation in Acral Tumors?
## 3 Results  
The error results obtained by comparing the 3D models from CT images before and after the sterilization processes are displayed in Figure 6. The mean error between 3D models is 0.13 \(\pm\) 0.14 mm for case 1 (Figure 6a) and 0.17 \(\pm\) 0.15 mm for case 2 (Figure 6b). For case 1, the surface finger area of the pinky, ring finger and middle finger and the middle surface area of the posterior part of the hand presented a higher error than the rest of the holder. The maximum error (2.30 mm) was located in some small specific points of the pinkie and ring finger. In case 2 (foot mold), some areas of the arch that tied the toes up and the surface that is in contact with the top part of the sole of the foot presented a higher error than the rest of the mold. The maximum error (2.77 mm) was located on small parts on the arch and back of the mold. No strong deformation is detected in any of the holders. The performance evaluation results of the IGS system on the two patient-specific 3D printed phantoms using both, the commercial pointer and the electric scalpel, are presented in Table 1. Each repetition corresponds to removing and placing back again the 3D printed limb on the mold. In case 1, the total root-mean-squared error (RMSE) across four repetitions was \(1.88\pm 0.18\) mm (NDI pointer) and \(1.57\pm 0.11\) mm (electric scalpel). In case 2, the RMSE was \(2.12\pm 0.18\) mm (NDI pointer) and \(1.95\pm 0.43\) mm (electric scalpel). In both cases, the electric scalpel obtained a lower RMSE than the commercial pointer. Sixty percent of the points taken with both tools presented an error below 1.97 mm. The IGS workflow was feasible in both clinical cases during surgery. For case 1, the hand was fixed to the mold by using elastic bands on the fingers (Figure 7b). The position of the hand was corrected several times because it was slipping on the mold. The NDI pointer was used as the reference pointer. For case 2, the foot was put into a bandage to avoid the slipping encountered during surgery for case 1. In this surgical procedure, the electric scalpel was chosen as a reference pointer. After the patient's limb was positioned into the mold, we observed that the navigation information was not correct on the screen. We presumed that this problem was caused by incorrect placement of the RB, so we registered the RB with the mold. Once this new position of the RB was established, the navigation continued with no further problems. Registration required less than 1 min. Video recordings of both clinical cases can be found in Video S1 (surgery of case 1) and Video S2 (surgery of case 2). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Case** & **TRE Metrics (mm)** & **NDI Pointer** & **Scalpel** & **All Reference Pointers** \\ \hline \multirow{4}{*}{**Case 1**} & **Mean** & \(1.80\pm 0.57\) & \(1.48\pm 0.53\) & \(1.64\pm 0.57\) \\  & **RMSE** & \(1.88\pm 0.18\) & \(1.57\pm 0.11\) & \(1.72\pm 0.22\) \\  & **Min** & \(0.87\pm 0.21\) & \(0.84\pm 0.12\) & \(0.85\pm 0.17\) \\  & **Max** & \(2.60\pm 0.37\) & \(2.42\pm 0.27\) & \(2.51\pm 0.33\) \\ \hline \multirow{4}{*}{**Case 2**} & **Mean** & \(2.06\pm 0.54\) & \(1.84\pm 0.77\) & \(1.95\pm 0.68\) \\  & **RMSE** & \(2.12\pm 0.18\) & \(1.95\pm 0.43\) & \(2.04\pm 0.34\) \\ \cline{1-1}  & **Min** & \(1.33\pm 0.23\) & \(0.70\pm 0.13\) & \(1.02\pm 0.37\) \\ \cline{1-1}  & **Max** & \(2.94\pm 0.31\) & \(2.80\pm 0.41\) & \(2.87\pm 0.37\) \\ \hline \multirow{4}{*}{**All Cases**} & **Mean** & \(1.92\pm 0.57\) & \(1.65\pm 0.68\) & \(1.78\pm 0.64\) \\  & **RMSE** & \(2.00\pm 0.22\) & \(1.76\pm 0.37\) & \(1.88\pm 0.32\) \\ \cline{1-1}  & **Min** & \(1.10\pm 0.32\) & \(0.77\pm 0.15\) & \(0.94\pm 0.30\) \\ \cline{1-1}  & **Max** & \(2.77\pm 0.38\) & \(2.61\pm 0.40\) & \(2.68\pm 0.40\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Target Registration Error (TRE) (mm) obtained using each reference pointer (NDI pointer and scalpel) for cases 1 and 2 as well as the combined results for both cases and reference pointers. (RMSE: root-mean-square error).