# Hybrid Spine Simulator Prototype for X-ray Free Pedicle Screws Fixation Training
## 1 Introduction
However, the availability of realistic haptic feedback is the bottleneck in developing high fidelity VR simulators, especially in open surgery, and conventional haptic interfaces are limited in the magnitude of the forces being rendered (so they do not enable a realistic simulation of the surgical instruments/bone interaction) [14], thus a hybrid approach based on 3D printing and augmented reality (AR) is promising to overcome current technological limits [14]. Hybrid simulators combine physical anatomical models with virtual reality elements by exploiting AR technologies to enrich the synthetic environment, for example: to visualize hidden anatomical structures, and/or additional information to guide the surgical tasks and to help the trainee [15; 16]. For these reasons, in a previous work, we presented a patient-specific hybrid simulator for orthopedic open surgery featuring wearable AR functionalities [14]. That simulator, which uses the Microsoft HoloLens (1 st gen) head-mounted display (HMD) [17] for an interactive and immersive simulation experience for hip arthroplasty training, received positive feedback from medical staff involved in the study to evaluate visual/audio perceptions, and gesture/voice interactions. Head-mounted displays are increasingly recognized as the most ergonomic solution for AR applications including manual tasks performed under direct vision like what happens in open surgery; however, literature studies [18] highlight some possible pitfalls to consider when using HMDs not specifically designed for the peripersonal space, to guide manual tasks: the perceptual issue related tovergence-accommodation conflict (VAC) and focal rivalry (FR) which hinder visual performance and cause visual fatigue [19], and the physical burdening of wearing an HMD for a prolonged period of time. Indeed, the HMD weight demands the forceful action of the neck extensors muscles to support and stabilize the neck and maintain the posture [20]. For these reasons the simulation environment should be carefully set up so that the virtual content appears in the optimal/comfort zone for most of the simulation, and so that the head tilt is sustainable. Furthermore, one should consider that the availability of an HMD during the simulation is a technological addon not coherent with the devices available in a traditional surgical room. Other non-wearable AR-enabling devices deserve consideration to develop hybrid simulators for the training of image-guided surgical procedures, such as spinal fluoroscopic-guided interventions, where the operator is constantly required to switch attention between the patient and a monitor showing real-time fluoroscopic images of the spinal anatomy. For these reasons, a traditional stand-up display appears the best technological choice in terms of realism, since it is consistent with the equipment of a real surgical scenario. In this work we describe and test an innovative hybrid spine simulator consisting of a torso mannequin with a patient-specific spine, and a PC with a traditional monitor to render the scene generated by a software module. Specifically, this latter provides:  
* A \"Virtual X-Ray Visualization\", simulating X-ray images of the anatomy to train for the uniplanar fluoroscopic targeting of pedicles without any exposure to harmful radiation. * An \"AR Visualization\", allowing the observation of the torso with overlaid virtual content to assist in the implantation of the screws at the proper anatomical targets (vertebral peduncles). Fluoroscopic images simulation has been previously proposed by [PERSON] et al. in [21] to improve the effectiveness of C-arm training for orthopedic and reconstructive surgery. Specifically, the system designed by [PERSON] et al. can generate digitally reconstructed radiographs based on the relative position of a real C-arm and a mannequin representing the patient: a virtual camera is used to simulate the X-ray detector, providing views of a CT volume (selected from a database containing surgical cases) from various perspectives.