# Hybrid Spine Simulator Prototype for X-ray Free Pedicle Screws Fixation Training
## 1 Introduction  
Simulation is becoming an essential part of surgical training as it allows for repetitive practice in a safe controlled environment whose complexity can be tailored to the trainee's expertise level and needs. Currently, there is a consensus among the orthopedic community that the acquisition of trainees' surgical skills should commence in a simulated training environment prior to progression to the surgical room [1] in order to improve surgical outcomes and patient safety. Pedic screws fixation, the gold standard among posterior instrumentation techniques to stabilize spine fusion, is a technically demanding procedure which requires long training to avoid catastrophic neurovascular complications due to screw misplacement. The risk of misplacement, which is exacerbated by the complexity of the anatomy (e.g., deformity of the spine together with dysplastic anatomy) can be very high, indeed literature studies report an error rate of 10-40% [2, 3]. This leads to the need of enriching traditional educational methods, largely based on the Halstedian model \"see one, do one, teach one\" [4], with surgical training sessions outside the operating theatre. According to the literature, cadavers are an effective medium for teaching surgical skill outside the operating room due to their realism [5]; however, their availability may be limited and their use has ethical, legal, and cost implications [6]. Since a few years ago, the use of three-dimensional (3D)-printed spine models has been emerging as a low-cost, easy-to-handle, and store alternative which allows the overcoming of ethical issues and/or legal constraints of training on cadavers [7]. Today, 3D printing is playing an emerging role in the domain of orthopedic education, allowing the development of high-fidelity anatomical reproductions which can be fruitfully used to learn and understand the pathological anatomy, but also to plan a surgical procedure, and to simulate surgical steps using real orthopedic instruments [8]. Furthermore, patient specific 3D-printed spine models, obtained starting from computed tomography (CT) images, overcome the intrinsic morphological constraints of cadavers. In fact, a user may wish to rehearse a particular surgical case or anatomical variation, but it is unlikely to find ex-vivo tissues that exhibit the desired pathology. Recent literature studies show that life-size 3D-printed spine models can be an excellent tool for training beginners in the use of free-hand pedicle screw instrumentation [9], enabling the training course administrator to select the surgical case from a digital library of anatomical models, and to tailor the simulation difficulty level to fit the needs of the learner [7]. 3D printing of patient-specific models offers great benefits for surgical training and rehearsal, and also overcomes limits of standard commercial mannequins which cover a very limited range of individual differences and pathologies. However, a recent review [10] on orthopedic simulation technologies suggests that _\"an ideal simulator should be multimodal, combining haptic, visual and audio technology to create an immersive training environment\"_: this highlights the need for further studies to develop \"high-tech\" simulators. Virtual reality (VR) has been well-established as a learning and training tool [11]. An interesting example of VR simulators available for spine surgery is the ImmersiveTouch Surgical Simulation Suite for Spine Surgery [12] that provides surgeons the ability to visualize 3D models of real patients, measure the exact dimensions of anatomical irregularities, etc. The effectiveness of this VR simulator was tested by [PERSON] et al. in [13], who demonstrated that it can effectively improve pedicle screw placement compared to traditional (oral/verbal/visual) teaching.