# Hybrid Spine Simulator Prototype for X-ray Free Pedicle Screws Fixation Training
## 4 Discussion and Conclusions  
A recent literature review on orthopedic surgery simulation highlights the need for \"high-tech\" multimodal simulators allowing the trainees to develop visuospatial awareness of the anatomy and a \"sense of touch\" for surgical procedures. Starting from these considerations, in a previous study we have presented a hybrid simulator for open orthopedic surgery using the Microsoft HoloLens [14]. Although HDMs represent an ideal solution to develop an immersive training environment for open surgery, other non-wearable AR-enabling devices deserve consideration to develop hybrid simulators for training of image-guided surgical procedures such as spinal fluoroscopic guided interventions. For this reason, in this work we present and technically test a prototype of an innovative hybrid spine simulator for pedicle screws fixation, based on the use of a traditional stand-up display, that is consistent with the equipment of a real surgical scenario and thus appears the best technological choice in terms of realism. The proposed strategy to build the spine simulator takes advantage of:  
* patient-specific modeling to improve the realism of the simulated surgical pathology;
* rapid prototyping for the manufacturing of synthetic vertebral models;
* AR to enrich the simulated surgical scenario and help the learner to carry out the procedure;
* VR functionalities for simulating X-ray images of the anatomy to train for the uniplanar fluoroscopic targeting of pedicles without any exposure to harmful radiations. Fiducial markers are used to track in real-time the position of each vertebra (which can move relative to adjacent vertebrae to simulate the mobility of the human spine) and surgical instrumentation. Two calibrated cameras, arranged in an orthogonal configuration, are proposed to track the vertebral models (lateral camera) and to track the instrumentation and produce an AR view of the simulated torso (top camera). A simulated virtual C-arm is used to generate synthetic fluoroscopic projections, using a simple approach based on shader programming to achieve an \"X-ray effect\". Quantitative tests show that the proposed low-cost tracking approach is accurate enough for training in pedicle screws insertion in lumbar vertebrae. Future studies will improve the robustness of the simulator, involving clinicians to test the system and define the best positioning of the cameras (an ad-hoc support structure will be designed to hold in position the two cameras), fiducial markers, and lights so that the accuracy is maximized and, at the same time, the tracking set-up (camera, markers and lights) does not hinder the surgeon's movements and manipulation of the instruments. To this end, the proposed tracking strategy that is based on the optical tracking of the passive markers, is advantageous compared to other techniques using wired sensors (e.g., electromagnetic sensors). On the other hand, the selected tracking approach can fail due to marker occlusions: in our simulator, this is particularly true for the markers placed on the instruments. However, for our application, continuous instrument tracking is not necessary because, even in the real surgical workflow, fluoroscopic imaging is not continuous but intermittent to minimize the radiation dose to the patient. In the future, Face Validity, Content Validity, and Construct Validity tests will be performed for a complete assessment of the proposed simulator for training in pedicle screws insertion. Conceptualization, S.C., G.T. ; methodology, S.C., G.T., V.F. and P.D.P. ; software, S.C.; validation, S.C., V.M. ; formal analysis, S.C. and V.M. ; investigation, S.C.; resources, V.F. ; data curation, S.C. and V.M. ; writing--original draft preparation, S.C., G.T. and V.M. ; writing--review and editing, G.T., P.D.P. and V.F. ; visualization, V.F. ; project administration, S.C.; funding acquisition, V.F. All authors have read and agreed to the published version of the manuscript.