# Desktop 3D Printing: Key for Surgical Navigation in Acral Tumors?
## 2 Materials and Methods
### Computer-Aided Design and Manufacturing  
We designed and manufactured several tools and models in this study: a mold to ensure the position of the limb for each patient, a customized cap to enable navigation of the surgical scalpel, and phantoms obtained from the limb of each patient to evaluate the navigation error in a controlled scenario. This section describes the methodology followed to create each object.  
Each mold was designed in Meshmixer (Autodesk Inc., San Rafael, CA, USA) software by extruding the surface of the limb's 3D mesh. This holder allows the limb to be fixed during the surgical procedure in the same position as in the preoperative CT image. The design of the mold considered  
Figure 1: Representation of the clinical cases. Case 1: patient with a soft-tissue sarcoma (green) located on the right hand. (**a**) Real image and (**c**) virtual image. Case 2: patient with a soft-tissue sarcoma (green) in the right foot. (**b**) Real image and (**d**) virtual image.  
the surgical approach and working area, to facilitate easy management during surgery, and presented a smooth surface on the limb side to limit any harm to the patient. It also included an attachment for retro-reflective optical markers (rigid body, RB) to enable navigation. The position of these markers was determined to avoid possible occlusions during navigation. Furthermore, we included several conical holes (\(\O\) 4 mm \(\times\) 3 mm depth) on the mold surface for point-based registration and error measurement. We also designed a customized cap with optical markers to track the electric scalpel during surgery.  
3D models (molds and the customized cap for the surgical tool) were manufactured with a desktop 3D printer (Witbox-2, BQ, Madrid, Spain) using polylactic acid (PLA) (Figure 2). The RB attachment included in the mold design of case 2 was 3D printed separately since the whole design did not fit in the printer bed (dimensions 297 mm \(\times\) 210 mm). 3D printed tools were sterilized to maintain asepsis of the surgical field as they may be in contact with the patient. The sterilization technique was based on ethylene oxide (EtO) at 37 \({}^{\circ}\)C to avoid deformation (the glass transition temperature of PLA is around 55-65 \({}^{\circ}\)C). It involved extensive degassing to remove the residual EtO [24]. The spherical markers used for navigation were attached during surgery, since they are supplied in individual bags in sterile condition.  
A patient-specific phantom was manufactured for each patient case to evaluate the performance of the IGS system before surgery (Figure 3a,d). Each phantom included a 3D printed copy of the mold made for surgery and the affected extremity limb with several conical holes (\(\O\) 4 mm \(\times\) 3 mm depth) on the model surface used to evaluate the accuracy of the navigation. Original 3D models of the mold and the limb from both cases are available in the supplementary material as 3D Model S1 (case 1 mold), 3D Model S2 (case 1 limb), 3D Model S3 (case 2 mold), and 3D Model S4 (case 2 limb).  
Figure 3: Image-guided surgery simulation on patient-specific 3D printed phantoms based on case 1 (left) and case 2 (right). The user is recording points from the conical holes made on the surface of the 3D printed phantoms (**a**,**d**) with the commercial pointer (**b**) in case 1, and with the electric scalpel (**e**) in case 2. The corresponding virtual scenes are shown in (**c**,**f**).  
Figure 2: Three-dimensionally (3D) printed models with a rigid body (RB) attached to them: (**a**) patient-specific mold for case 1; (**b**) patient-specific mold for case 2; (**c**) electric scalpel customizable cap.