VR uses computer technology [16] to generate a digital environment similar to the real environment in the visual, auditory, tactile, and other senses [17], which is used in many different fields. The operator interacts with and feels feedback from the virtual objects using specialized equipment [18]. A complete VR system consists of a stereo display device, a motion tracking device, an input device, and a computing platform. The stereo display device is usually a head-mounted display (HMD; Figure 1) [19]; another type of stereo display is the Cave Automatic Virtual Environment (CAVE; Figure 2) [20]. HMDs are more widely used than CAVEs because HMD application is more flexible and requires less space.

VR systems are divided into three immersive levels based on the degree of stimulated senses and interactions: nonimmersive system, semi-immersion system, and immersion system (Figure 3) [21]. A nonimmersive system only reproduces images on desktops; an immersion system places the user in a complete virtual environment with the support of several sensory output devices including visual devices such as HMDs, audio devices, and haptic devices [22]; and a semi-immersion system provides the user with a simulated environment between the two above.

In the implementation of VR technology, the key points are modeling and interaction. In the medical field, images from computed tomography [7,23], magnetic resonance imaging (MRI) [24], and dental scanners [25] can be used to rehabilitate virtual models. Interactions mainly contain visual interaction and tactile sensation interaction, which are actualized by display device and force feedback device, respectively.

In the medical field, VR and related technology are gradually being applied in surgical training [26] and surgical navigation systems [27]. Combining VR with medical training is a new research field that has emerged with the development of computer science, sensor technology, and automation technology. A virtual operating environment with high fidelity and real-time performance can be created through virtual simulation technology [14]. The system is created and processed by a computer, with dedicated devices such as a helmet display and a force feedback handle that allow the user to observe and interact with the scene while experiencing multisensory feedback that give a near-realistic training effect [28].

Haptic devices can receive and transmit motion signals to improve the operator's sense of reality [29], which is important for dental training assisted by VR.

To simulate the tactile sense of real dental training as much as possible, the reaction force of the virtual object is calculated using the appropriate force generation algorithm in the VR system [30]. Due to the characteristics of human touch receptors, real-time haptic rendering requires a refresh frequency of at least 1 kHz [31]. For force feedback interaction in dental skills training, several kinds of interaction algorithms have been proposed for various scenarios, including teeth preparation, scaling, and bone drilling [32,33].

There is a wide variety of dental education simulators available, each with advantages and disadvantages in terms of training content, training process, hardware device, and software design. These are briefly described in Table 1.

The DentSim system unit, consisting of a phantom head and dentoform, dental instruments, infrared sensors, infrared cameras, and two computers, was born in 1997 [34]. The infrared cameras can capture the orientation and movement of resin teeth and a handpiece so as to show the students’ work virtually on the computer screen in real time. The unit allows students to see the evaluation of their tooth preparation compared with the ideal preparation on the screen, while also providing them the ability to continue working on the resin teeth [35]. The training using DentSim is more efficient and standardized than that using traditional preclinical teaching methods [36]. The disadvantage of this unit is that it relies on physical resin teeth, which are disposable consumables.

Virtual Education System for Dentistry is a dental simulator for prosthodontics developed by the Affiliated Stomatological Hospital of Nanjing Medical University and Suzhou Digital-health Care Company. The system contains the Virtual Learning Network Platform (VLNP) and Real-time Dental Training and Evaluation System (RDTES) [37]. Prior to practical work, students are requested to learn courses on the VLNP, including reading the operational instructions and predefined criteria of crown preparation and watching the standard operational videos. Afterwards, students can perform crown preparations on the phantom head under the guidance of the RDTES; the processes and results can be recorded by the RDTES. When the students finish their preparations, the RDTES can automatically assess the procedures and results of preparation based on the predefined tooth preparation criteria. As well, the students can visually compare their own procedures and results with the predefined assessment criteria on the computer screen [37,38].

Individual Dental Education Assistant (IDEA) is a VR hand flexibility training simulator consisting of a handheld stylus that simulates a dental handpiece and provides force feedback and a computer installed with simulation software. Unlike other dental simulators, IDEA is designed to enable students to be flexible and proficient in the use of dental handpieces by practicing removing predesigned virtual materials with different shapes (eg, straight line or circle). Therefore, IDEA aims to train dental students in hand flexibility, not to train students on a particular teaching component such as crown preparation or scaling. The main advantage of the system is its evaluation system. During the training process, two parameters determine the score obtained: drilling speed and drilling accuracy. Deviation from the trajectory or to an inappropriate depth can lead to a decrease in accuracy, and this is displayed as an accuracy bar on the screen. Complete depletion of the bar means that the student fails the test [39]. Some researchers have reported that IDEA could improve students' performance in the dental skill test; in addition, it can be used to identify students with troubles with hand flexibility at an early stage that may allow for early intervention to prevent failure [40,41].

PerioSim, consisting of a stereoscopic display, a computer, and a haptic device, allows students to use virtual dental instruments to visualize and detect caries and periodontal diseases in a haptic environment [12]. The system is available online for students and allows teachers to upload different training programs, which can be downloaded and replayed by students at any time, making this system convenient and efficient [35]. Steinberg et al reported that the image display and force feedback were very realistic for teeth and dental instruments but not for gingival tissue [42].

iDental is a periodontal skill education simulator developed by Peking University School and Hospital of Stomatology and Beihang University, which can simulate periodontal examinations and treatment procedures including periodontal probing and calculus detection and removal. Unlike PerioSim, the device mainly uses a 2D monitor. However, it is equipped with an odontoscope handle, so it can be used to practice two-handed cooperative operation, making it realistic. iDental also has a basic periodontal knowledge teaching module, which enables students to review basic knowledge before operation training. A combination of the two training parts improves the teaching effectiveness [43].

Simodont Dental Trainer is a widely used teaching simulator for dental skills training that is currently available in many dental schools. It mainly includes modules for hand flexibility, cariology, crown and bridge preparations, clinical cases, and a full mouth simulation experience [13,44]. One of the highlights of the system is that an X-ray of the working tooth is attached to each individual case, which can allow students to make a diagnosis assisted by both the appearance of teeth and the X-ray films [13].

The system does have some disadvantages. It requires 3D glasses for 3D display. Excessive training on a single tooth fails to create a realistic sense of manipulation. The training process for crown preparation is single-jawed and does not fully mimic the narrow operating space of the mouth. Besides, it cannot be used to train students about positioning requirements because operation postures during training are fixed and visual angle conversion requires manual rotation of the rotary button.

The VirTeaSy project is a dental implant training simulator and is composed of VirTeaSy Scan Implant and VirTeaSy Implant Pro. VirTeaSy Scan Implant is used for implanting scheme designs by students. Radiographs are used by students to perform the implant treatment plan, including the implant’s characteristics (shape, diameter, length) and its location (location, angle, insertion depth) in the jaw. When the treatment plan is complete, it can be compared with the design planned by an expert that is stored in the database to identify where improvements could be made in their own plans. In addition, the device's database contains cases of varying difficulty, so students with different skill levels can use it to practice accordingly [45].

VirTeaSy Implant Pro is a virtual implant surgery training system, which allows students to perform surgery cases planned in VirTeaSy Scan Implant. VirTeaSy Implant Pro is supplemented by an auxiliary system that can alert students if the drilling’s location, angle, and depth are incorrect as well as if there is overheating of the bone. In addition, the VirTeaSy Implant Pro has a display through which the teacher can interact with the student in real time and assist the student as necessary. VirTeaSy Implant Pro can improve students’ skills in bone mineral density perception through force feedback, thus allowing them to perceive whether their bone density measurements in VirTeaSy Scan Implant match the reality [46]. VirTeaSy Scan Implant and Implant Pro complement each other, forming an efficient learning tool [45].

The Internet of Things–based education and learning (IDEAL) system is an oral radiology education simulator. The simulator is mainly used for teaching students to take intraoral X-ray images without using X-rays. The IDEAL system consists of a simulator cone, simulator main body, sensor, detector, and stool. Training contents comprise basic education before X-ray imaging, information on X-ray imaging techniques (such as periapical radiography, bisecting angle technique, and paralleling technique), a test bank, and an evaluation and feedback system. During the training process, students can practice taking X-ray images for different tooth positions by adjusting the angle of the X-ray tube to improve their skills in taking X-ray images. At the end of training, students submit their own imaging results to the system and receive automatic feedback and evaluations from the system. The system allows students to avoid using radiographic devices while learning to take dental films, reducing the risk of radiation exposure to both students and patients. The system is safe and affordable [47].

The Voxel-Man simulator is a VR surgical simulator that primarily consists of a 2D monitor, simulated surgical operating handle, foot pedal, and 3D glasses. At present, the device is mainly used to simulate apical surgery, such as apical resection and apical cystectomy [48].

The system provides different training modes depending on the trainees’ level, with different display interfaces and different operating instructions. The three modes to choose from are primary mode, advanced mode, and exam mode. In primary mode, the lesions and surrounding important anatomical structures (such as the alveolar nerve) are marked with bright colors. This not only helps operators to understand the anatomical characteristics of the corresponding surgical area, but also reminds them of the scope and amplitude of the operation at all times to help them avoid damaging these important anatomical structures. When the surgical instrument is close to an important anatomical structure, the system will emit a danger alarm. In advanced mode and exam mode, some of the functions and hints are turned off [48,49].

In addition, the system can record the operation process for later replay so that the operator can identify errors and mistakes to improve on [48]. The operator can stop the operation at any time and can undo a wrong step or restart a new one, saving a lot of time. Cases vary in difficulty, allowing operators with different experience levels to practice.

The effectiveness of VR systems can be evaluated using two analytical methods, qualitative and quantitative [15,43], which can be analyzed by questionnaires and comparative studies, respectively.

Dental skills training takes the form of gradual acquisition, so the proficiency of training using the simulator can improve along with increasing familiarity with the training content. Urbankova et al concluded that repeating the training led to learning the test content, which could affect the results of the study [40]. Since VR is a new technology, teachers’ sensitivity to the simulator may depend on their age, which could affect the results of the experiment. Therefore, questionnaires are often designed to eliminate the influence of these two factors [81].

Training and demonstration before the experiment, which allow objects to know the experimental flow and operational specification and eliminate the interference factors, are necessary [47]. Seymour et al tested trainees' visual ability, perception ability, and mental condition to exclude interference factors that affected experimental results [82].

To date, many pilot tests have been conducted to evaluate the validity of dental simulators in endodontics [3,83], periodontics [12,42,43,84,85], oral and maxillofacial surgery [65,86-88], dental radiography [41], prosthodontics [37,59,85], implantology [89,90], and orthodontics [90]; the specific applications of dental simulators in dental education are summarized in Table 2.

Many researchers are positive about the roles that dental simulators play in skills training. Al-Saud et al reported that students could gain basic manual dexterity at a quicker pace when they practiced skills with the guidance of experienced faculty, and training with dental simulators helped students to retain the skills they had learned [91]. A study performed by Plessas showed that students could develop a better understanding of the material by using dental simulators since they create a more varied learning environment compared with traditional training methods [74]. Suebnukarn et al evaluated the effectiveness of dental simulators in cavity preparation and concluded that haptic VR simulators are equivalent to conventional dental phantom heads in reducing operation errors [92]. Other research provided evidence that students can improve their tooth extraction skills [93], dental radiology skills [47], and implant skills [32,90] through dental simulators. Dental simulators were also reported to be able to enhance students’ confidence [94] and improve their attitudes toward patients [53] and abilities to discern and solve medical emergencies [70,95,96]. Some studies focused on students’ acceptance of dental simulators and found that most students are willing to learn with dental simulators, which would boost their enthusiasm to learn [97]. Therefore, many research studies indicated that dental simulators had the potential to be an alternative to conventional training methods [36,52,98].

However, as mentioned above, there are still some disadvantages of dental simulators that can’t be ignored, and these disadvantages may directly influence the effectiveness of dental simulators. In addition, the effectiveness of some dental simulators has not been validated [88,98]. Therefore, it is suggested that dental simulators cannot completely replace traditional skill training methods. The automated evaluation and tutorial feedback offered by dental simulators are considered to be complementary components to traditional methods [84]. Some studies concluded that a combination of traditional and virtual methods would be an optimal approach to choose in skills training [52,99,100].

By applying big data technology to medical education, we can establish statistical models by the visual analysis method [101] via educational data mining and learning analysis [102], which allow trainers to analyze and understand the learning status of trainees intuitively. Currently, big data technology has been applied in educational analysis of massive open online courses [103] and learning effectiveness prediction for medical courses [101].

A similar big data analysis platform can be set up in dental skills training. We can collect dental operation data and establish a database of correctly performed operations. Then, a scientific evaluation system to score the training results by comparing them with the correctly performed operations can be created. Trainers can identify information on general problems in operations, as well as problems experienced by the individual trainee, to analyze and predict trainees' ability to master the skills, error-prone points, and the pass rate of examinations. Based on the analysis, trainers can create or adjust different plans for all trainees to enhance their learning experience and performance. Therefore, the combination of big data and education will make it possible to deeply understand and study the training process in order to provide more trainees with quality training.

The development of 5G technology will allow transmission links with high bandwidth and low delay [104-106]. In the virtual oral training system, calculations are often performed on the server side to improve accuracy. The use of 5G technology will greatly increase the transmission rate of high-definition video data from the server host to the display device, reducing the response delay [107-109]. This will provide users with more reliable visual and tactile feedback and improve comfort [110].

Currently, the 5G quality of experience framework is proposed to solve the problem of transmitting ultra-high-definition video on the 5G network [111]. This could provide a basis for the establishment of a VR dental educational video network. Large-scale synchronous training and one-to-one demonstrations could be performed via 5G transmission to improve training efficiency.

Deep learning techniques are widely used to find rules from a large amount of data, abstract problems through neural networks, and establish input and output mapping.

In the field of haptics, deep learning has been used to obtain haptic properties of objects from their images [112], whereby force feedback devices can directly generate a sense of force feedback that mimics the real physical properties without having to manually adjust physical parameters.

We can use deep learning to obtain the force feedback properties of teeth and build different deep learning models for different parts of the teeth. Therefore, the instrument would have different force feedback in contact with normal teeth, dental caries, tooth enamel, dentin, and other areas.

Deep learning can also play a role in oral modeling. Now, deep learning has been widely used in medical image analysis and modeling [113]. Through this type of method, we can easily extract relevant regions from computed tomography or MRI data and reconstruct mesh data [114]. These technologies can be transferred to oral cavity modeling, which can greatly facilitate model construction in a virtual surgery system and quickly establish a personalized oral cavity model.

Augmented reality technology is an extension of VR technology, combining virtual images with real environments to support interaction with virtual objects in real time.

Nowadays, augmented reality technology has begun to be used in the medical field. Based on an augmented reality helmet (HoloLens by Microsoft), various applications have been developed such as surgical navigation applications [115] and intelligent medical management systems [116]. HoloLens also supports a display of high-definition video derived from high-performance computing.

The virtual tooth and the phantom head can be combined with augmented reality technology in a dental training system. Students can feel the phantom head directly with their hands and interact with a virtual tooth through a force feedback device. At the same time, high-definition images allow users to observe finer tooth details, which can greatly improve the reality and the immersiveness of the experience. The users can also observe patients from multiple views, which will greatly enhance the trainees' understanding and learning of training objectives [117,118]. Therefore, augmented reality will have a significant effect on dental education in the future.