# Augmented Reality, Mixed Reality, and Hybrid Approach in Healthcare Simulation: A Systematic Review
The trainee can explore, repeat the surgical practice in a setting that \"fosters permission to fail\", recognize the mistakes and correct them, and try different approaches without jeopardizing patient safety. Thus, simulation-based medical training can be a platform for learning to mitigate ethical tensions and acquire/refine technical skills [12; 13; 14; 15]. The main simulation techniques in healthcare include virtual reality (VR) simulation, physical simulation, and hybrid (virtual-physical) simulation. VR simulation is based on the principles of immersion, interaction, and user involvement at different degrees: learners are immersed in a highly realistic virtual clinical environment (e.g., an operating room or an intensive care unit) and interact in real-time with the environment through virtual interactions/interfaces (e.g., virtual menus, point-and-click, etc.). Two interesting features of VR simulation are the objective evaluation of the trainee's performance, and the flexibility in selecting the virtual scenario. The latter allows the simulation to be adapted to the trainee's learning needs, enabling the learner to gain new cognitive skills, or develop and refine technical skills (e.g., instrumental navigation, clipping, slicing, etc.). However, two major limitations of VR-based simulation are the moderate sense of haptic feedback during interactions and the high costs making it prohibitively expensive for many institutions [16; 17; 18]. On the contrary, physical simulation is based on the reproduction of clinical scenarios with various degrees of realism. Low-fidelity scenarios include simple synthetic models (such as tubes or rings) made of plastic, rubber, and latex that are used to train basic clinical skills (such as knot-tying and suturing). High-fidelity scenarios include human organs or mannequins that can be used to perform the physical examination and to complete more complex procedures than just basic clinical tasks, and, unlike VR simulation, they provide the trainee realistic haptic feedback during all tool-tissue interactions [19; 20]. However, the main negative aspects of this type of simulation are the need to replace the model in case of a destructive task increasing training costs and the lack of the objective evaluation of trainee's performance. Hybrid simulators combine VR elements and physical models of the anatomy which can have an active or passive role in the simulation. Hybrid systems retain the natural haptic feedback of the physical simulation and the performance evaluation tools typical of VR simulation. We present a systematic review of simulation techniques based on augmented reality (AR), mixed reality (MR), and hybrid approaches in the context of healthcare, to investigate the challenges and trends in this discipline. Firstly, we define AR, MR, and hybrid approaches (Section 2.1), then we examine the simulation techniques in terms of the implementation of their virtual and physical components and the accuracy validity of the adopted techniques. Finally, we evaluate the clinical validity of the simulators and their frequency of use.