# Desktop 3D Printing: Key for Surgical Navigation in Acral Tumors?
## 4 Discussion  
In this study, we have presented a novel framework for acral tumor resection combining desktop 3D printing and surgical navigation. We used 3D printing technology to create a patient-specific mold that maintained the same position of the distal extremity during IGS as in the preoperative images. Furthermore, 3D printing allowed us to design a customized tracking attachment to navigate specific surgical tools. The feasibility of the suggested workflow has been evaluated on two patients with a malignant tumor in one extremity. Patient-to-image registration is a crucial step in surgical navigation. Still, the anatomical locations of our clinical cases include a large number of joints with complex movements, hindering an accurate alignment between the patient and the images. Our research proposes a solution to this problem, but several sources of error could limit the final navigation accuracy: the 3D-printed mold, the navigation system performance, and the integration of this solution in the surgical procedure. The proposed models are 3D-printed in PLA and have to be sterilized before surgery. This process, based on EtO at 37 \({}^{\circ}\)C, may still deform the objects decreasing navigation accuracy. To evaluate this error, we compared the virtual 3D models obtained from CT images acquired before and after the sterilization process. The results showed that the sterilization process produced a similar error in both cases below 0.2 mm. Higher errors were found in localized points, far from the target, which will not influence the results close to the tumor during resection. We believe that these errors are caused by the limited resolution of the CT scan, which may cause small inconsistencies during the segmentation step. Nevertheless, PLA has a low glass transition temperature, limiting its use in many hospitals where low temperature sterilization is not available. For this reason, other alternative materials could be used, such as stereolithography resins. Some of these resins withstand the high temperatures from more common sterilization techniques (e.g., autoclave), and have proved lower shape deformation after sterilization [30]. Simulation of the IGS system with patient-specific 3D-printed phantoms before surgery is not only valuable to facilitate preoperative planning, but it also allows the evaluation of the system accuracy before the intervention. The TRE of the whole system in these phantoms was below 1.9 mm. Our error values are higher than those reported in [23], where the authors placed an implant on a phantom using a Polaris Vicra Tracking System (Northern Digital Inc., ON, Canada). However, this difference is expected, since our framework includes an additional source of error arising from the positioning of the limb on the mold. Moreover, we achieved similar errors to [31], in which a multi-camera optical tracking system was also used. The errors were similar in both clinical cases, showing that the proposed methodology may be adapted to different extremity locations without altering the accuracy of the system. Commercial IGS systems do not only lack flexibility when adapting to specific requirements of tumor surgeries, but they are also limited by the tools that can be tracked during navigation. In this study, we were able to track not only a commercial pointer but also a surgical tool (scalpel), with a lower TRE during the simulations. This might be due to differences in the tip shape and how it fits in the conical holes while performing the experiment. The scalpel has a more rounded tip that might be positioned in a more consistent way when the location of the conical holes was recorded. These results encourage us to consider creating more tracking attachments for other surgical instruments, such as different scalpel models or drills, commonly used in tumor resection surgeries. The proposed workflow offered two different options for the image-to-patient registration during surgery.