# Hybrid Spine Simulator Prototype for X-ray Free Pedicle Screws Fixation Training
## 2 Materials and Methods
### Hardware Components  
The hardware components of the simulator include (Figure 1a): the spine phantom, the surgical instruments, the camera and markers for tracking, a calibration cube, as well as a desktop computer. The spine phantom contains patient-specific vertebral synthetic models, generated by processing CT datasets, and manufactured with a fused deposition modeling (FDM) 3D printer (Dimension Elite 3D Printer Stratasys Ltd., Minneapolis, MN, USA and Rehovot, Israel). The CT datasets are processed using a semi-automatic tool, the \"EndoCAS Segmentation Pipeline\" integrated in the ITK-SNAP 1.5 open-source software [22], to generate the 3D virtual meshes of the patient vertebrae. Then, following the image segmentation, these 3D meshes are refined into printable 3D models, via the open-source software MeshLab [23] by performing a few optimization stages (e.g., removal of non-manifold edges and vertices, holes filling, and finally mesh filtering). Acrylonitrile butadiene styrene (ABS) is used to manufacture vertebral models via 3D printing: this material is commonly employed for 3D printing in the orthopedic domain to replicate the bone mechanical behavior [24]. Mockups of intersomatic disks are manufactured with a soft room-temperature-vulcanizing silicone (RTV silicone), modeled to replicate the size and to maintain a mobility similar to the in-vivo human spine [25]. The spine mannequin is embedded in a soft synthetic polyurethane foam to simulate paravertebral soft tissues, and a skin-like covering made of RTV silicone is added to allow an accurate simulation of palpation and surgical incision (Figure 1b). Vertebrae tracking is necessary to update the Virtual X-Ray Visualization and the AR Visualization accordingly to any vertebral displacement during the simulated surgical intervention. The selected tracking strategy is based on the using of low-cost cameras to reduce the total cost of the simulator. Cameras and markers' positioning are chosen considering both technical requirements and the consistency with a real surgical scenario. ArUco markers [24], square (2 cm \(\times\) 2 cm) fiducial markers composed of a binary matrix (white and black) and a black border, are used for real-time tracking of anatomical models and instrumentation. Ad-hoc supports are designed to apply these planar fiducial markers on each vertebra, allowing us to determine their real-time pose during the entire surgical simulation. Given that the intervention is commonly performed with a posterior access, vertebral supports are designed to emerge from the bottom side of the phantom (Figure 2a), so that they are visible by a lateral camera (Cam\({}_{\text{Lat}}\)) which is used to track in real-time the pose of vertebrae. An additional top camera (Cam\({}_{\text{Top}}\)), opportunely calibrated with Cam\({}_{\text{Lat}}\), is placed above the phantom to acquire posterior images and then to offer AR views of the simulated torso, to help the trainee in the identification of pedicles and implantation of screws (e.g., vertebra models or virtual targets showing the ideal trajectory for screw insertion). At the same time, this second camera (Cam\({}_{\text{Top}}\)) offers an additional point of view to track the surgical instruments (e.g., pedicle access tool equipped with a trackable fiducial marker), when they are not visible for the Cam\({}_{\text{Lat}}\) camera. The calibration between the reference  
Figure 1: (**a, left**) Overview of the final design of the simulator hardware components: a spine phantom, equipped with markers for tracking, surgical tools with markers, two cameras for tracking, and a calibration cube. (**b, right**) The final appearance of the 3D printed spine embedded in the soft synthetic foam and covered with a skin-like silicone layer. systems of Cam\({}_{\text{Lat}}\) and Cam\({}_{\text{Top}}\) is performed as described in Section 2.2.2 thanks to the calibration cube.