# Desktop 3D Printing: Key for Surgical Navigation in Acral Tumors?
## 1 Introduction  
Surgical navigation allows relating the pose of specific instruments to the patient's imaging studies in real-time by means of a tracking system and patient-to-image registration [1]. This technology has shown potential benefits in orthopedic oncology surgeries on pelvic, sacral, spinal, and bone tumors [2; 3; 4; 5]. Nevertheless, some of these authors have not found strong evidence supporting a better surgical outcome for navigated operations compared to conventional surgery [6]. Commercial navigation systems have been designed explicitly for orthopedic procedures, such as OrthoPilot (B. Braun, Melsungen, Germany), ORTHOsoft (Zimmer Biomet, IN, USA), or NAV3i (Stryker Corporation, MI, USA). However, there are two limitations of these solutions in orthopedic tumor surgery: the lack of flexibility, necessary to adapt the system to the requirements of every case,and the high cost of these devices. Very particular procedures, such as the resection of acral tumors (those located in distal extremities such us hands or feet), have no clear benefit from navigation because it is complicated to adapt commercial systems to the necessities of those surgeries [7,8]. Nevertheless, due to the low amount of surrounding tissue, sometimes it is hard for the surgeon to ensure enough resection margin. Surgical navigation could provide the surgeon with the tumor's location, leading to a more precise surgical margin outcome than conventional surgery. However, these locations introduce a significant challenge during surgical navigation due to their large number of joints with complex movements (e.g., hand exhibits 27 degrees of freedom). This causes preoperative images not to correspond to the actual limb position during surgery, so the rigid transformation commonly applied for image-to-world registration will compromise navigation accuracy. To solve these problems, we propose to adapt the image-guided surgery (IGS) workflow using two enabling technologies: open-source software and three-dimensional (3D) printing. Software solutions such as 3D Slicer [9] can be an alternative in surgical navigation, offering extra flexibility that enables the development of innovative solutions not available in commercial systems [10, 11, 12, 13]. 3D printing, also known as rapid prototyping or additive manufacturing, allows anyone to easily convert 3D models into physical objects using a layering technique. During the last decade, the applications of 3D printing have increased exponentially in the medical area [14], including preoperative planning, bioprinting, and physician training. 3D printing already has an increasing impact in surgical practice, where the ability to interact with the patient's anatomy is crucial: anatomical models, surgical instruments, and implants/prostheses are the main areas identified in the recent literature [15, 16, 17, 18]. Orthopedic surgery is an area of particular interest [19] since 3D printed models facilitate the pre-contouring of plates, and cutting guides can be designed and printed adapted to each case [20,21]. Several companies offer support for manufacturing these tools. However, there is also a trend to implement desktop 3D printing inside the hospital [22], keeping the interaction with the printing process in-house and improving the involvement of the clinical staff in the workflow. 3D printing may expand the possibilities of other techniques such as surgical navigation, where patient-specific implants have enabled IGS in complex tumor resections [23]. In this study, we suggest combining desktop 3D printing and open-source software to navigate acral tumor resection surgeries. To do so, we propose a surgical workflow that includes patient-specific 3D-printed molds to ensure that the distal extremity position depicted in preoperative images resembles the one found during surgery. We also designed a specific 3D-printed attachment to track a surgical tool. An open-source navigation software has been adapted for guiding the resection of these tumors.