## 1 Introduction  
Currently, dentistry is benefiting from the development of modern digital transformation. Three-dimensional (3D) digital technology as well as computer-aided design and computer-aided manufacturing represent modern-day dentistry [1]. Nevertheless, innovations have been introduced in the dental field to improve dental education and clinical activity. Augmented Reality (AR) and Virtual Reality (VR) represent some of these innovations (Figure 1) and are part of the reality-virtuality continuum [2]. Commonly, traditional digital technologies in dentistry are structured into a three-step procedure which can be summarized as follows: (1) the digital image is acquired by a scanning device; (2) the operator can modify or change different dental aspect digitally, such as position or orientation of teeth; (3) the new information is transferred back to solid state or remain digital as a wax-up. With the introduction of AR and VR, these steps are simplified and implemented [3, 4]. Although AR and VR possess many common aspects, the outcomes and the users' experience are completely different. AR and VR systems require essential functionalities such as real and virtual data sources, tracking, registration techniques, visualization processing, perception locations, display types, and feedback mechanisms [5]. Instead of VR, AR generates an interaction between the real environment and virtual objects with the aim to combine virtual and real objects in a single real environment, to run interactively, and to register virtual and real objects reciprocally [6]. Monitor-based interfaces, monocular systems, head-mounted displays and other combined technologies are the instruments commonly used in AR systems. AR is mainly devoted to clinical practice in the medical field and improves the clinical procedure because it can allow clinical information to be visualized directly on the patient, overlapping the digital information and the real world [7]. In dentistry, the principal application of AR is related to overlapping digital information on the real world, basically \"improving the reality\" and live communication systems between patients and collaborators through the sharing of images, videos, and 3D models. Conversely, VR uses customized and advanced software and hardware to create a digital 3D reality in which all user's senses are stimulated with computer-generated sensations and feedbacks. VR, therefore, allows participants to experience simulated digital realities similar to those of physical reality, thus creating scenarios that are impossible to experience in the real world [8]. VR differs from AR in three basic systems: scene generator, a display device, and tracking and sensing. In VR, the display and scene generator is employed to simulate the real world; thus, the resolution of the monitor becomes vital for real-time interaction with the virtual world, allowing the user to be immersed in a digital-built reality [9]. Moreover, the user can use a specific device to interact with the virtual reality as a joystick and complex haptic systems (force feedback). VR applications are already in use in the dental field and mainly concern dental training: they allow students to simulate procedures in a virtual mouth, providing direct and immediate feedback. In addition to AR and VR, Mixed Reality (MR) represents an innovative tool for use in dentistry. Despite MR being often confused with AR, it is a combination of AR and VR. In MR, users can interact with both virtual and real objects in real-time, while simultaneously, these objects can interact with each other. This \"environment awareness\" implies that not only do virtual objects act in the real environment, but real objects can also modify the virtual elements, regardless of where the experience is taking place [2]. For instance, in pure MR, users would not be able to see a virtual instrument inside a drawer, unless they open it to look inside; in AR, the instrument would be overlaid, and it would be unnecessary to open the drawer.