# Desktop 3D Printing: Key for Surgical Navigation in Acral Tumors?
## 2 Materials and Methods
### Navigation Software and Hardware  
A specific software application named _AcralTumorNavigation_ was developed as a module in the 3D Slicer platform to facilitate the surgical navigation of acral tumors [9; 25]. OptiTrack multi-camera optical tracking system (NaturalPoint Inc., OR, USA) provided real-time tool tracking during the accuracy experiments and surgery. This system is accurate and very robust for IGS navigation in large clinical scenarios, thanks to the use of multiple cameras. Even if the surgical team occludes the line-of-sight between some cameras and the tracked tools, the system will provide correct tracking data [26]. Eight near-infrared cameras (model OptiTrack Flex 13) were distributed around the operating room (OR) to cover the working area (Figure 4a). The tracking system transfers the positioning data to the _AcralTumorNavigation_ module using _PLUS-toolkit_[27] and the _OpenIGTLink_ communication protocol [28]. Two different tools were used as reference pointer during the surgical procedure: a passive 4-marker probe (Northern Digital Inc. [NDI], ON, Canada) (Figure 3b) and an electric scalpel with a specific optical marker configuration attached to a customized 3D-printed cap (Figure 3e). A pivoting procedure [29] enabled to obtain the relationship between the RB attached to the reference pointer and its tip. Both pointer tools were sterilized before surgery (the probe and the customized cap using a protocol based on EtO at 37 \({}^{\circ}\)C and the scalpel in autoclave).  
The limb mold ensures that the patient is in the same position as in the preoperative image, granting an automatic registration between the IGS system and the patient. This holder included an RB that allowed the tracking device to compensate for any movement of the patient's limb during surgical navigation. For this purpose, the RB needs to be registered to the mold. We proposed two alternative approaches to solve this registration problem. First, the RB and the mold could be 3D printed together, so no external registration would be necessary as the relative position is already known from the 3D model design. Second, if the RB and mold could not be printed together or if the RB was displaced right before surgery, registration could be calculated applying a fiducial-based rigid registration algorithm [29] by recording points on the conical holes made on the surface of the mold. This method could be employed intraoperatively.  
An external screen available in the OR (Figure 4b) displayed the _AcralTumorNavigation_ software during the intraoperative navigation. First, virtual 3D models and preoperative images of the patient were loaded in the navigation software. Then, the surgeon selected the reference pointer (commercial probe or an electric scalpel) to be used during navigation and performed pivot calibration. If any  
Figure 4: Operating room where surgeries of case 1 and 2 were performed, showing the distribution of the eight optical tracking system cameras in blue (**a**) and two surgical monitors in pink (**b**).  
misregistration between the limb mold and its corresponding RB was detected, the application allowed the physician to calculate an intraoperative registration by recording points on the mold's conical holes using the reference pointer previously selected. Once the patient's extremity was placed on the limb holder, the program displayed the real-time position of the reference pointer with respect to the medical images and virtual 3D models of the patient. Moreover, _AcralTumorNavigation_ could calculate the real-time distance of the reference pointer tip to any 3D model (e.g., tumor), giving visual feedback to the surgeon. It also enabled real-time visualization of the patient's volume-rendered image, or virtual 3D models, depending on surgeons' interest. Furthermore, the software could store point coordinates from the tip of the reference pointer, which were later used for surgical validation of the tumor resection margin.