Diagnostic Applications of Intraoral Scanners: A Systematic Review

In addition to their recognized value for obtaining 3D digital dental models, intraoral scanners (IOSs) have recently been proven to be promising tools for oral health diagnostics. In this work, the most recent literature on IOSs was reviewed with a focus on their applications as detection systems of oral cavity pathologies. Those applications of IOSs falling in the general area of detection systems for oral health diagnostics (e.g., caries, dental wear, periodontal diseases, oral cancer) were included, while excluding those works mainly focused on 3D dental model reconstruction for implantology, orthodontics, or prosthodontics. Three major scientific databases, namely Scopus, PubMed, and Web of Science, were searched and explored by three independent reviewers. The synthesis and analysis of the studies was carried out by considering the type and technical features of the IOS, the study objectives, and the specific diagnostic applications. From the synthesis of the twenty-five included studies, the main diagnostic fields where IOS technology applies were highlighted, ranging from the detection of tooth wear and caries to the diagnosis of plaques, periodontal defects, and other complications. This shows how additional diagnostic information can be obtained by combining the IOS technology with other radiographic techniques. Despite some promising results, the clinical evidence regarding the use of IOSs as oral health probes is still limited, and further efforts are needed to validate the diagnostic potential of IOSs over conventional tools.


Introduction
Intraoral scanners (IOSs) are medical devices based on 3D measurement systems that are able to capture information on the shape and size of dental arches and to reproduce 3D models of the teeth and soft tissues of the oral cavity, thus allowing complete digitalization of the mouth anatomy [1,2]. Currently, commercially available IOSs are based on different noncontact optical technologies and principles [3]. IOS technology aims to overcome some of the major limitations of traditional dental impressions, which are time-consuming and are not comfortable for patients, and to implement a fully digitalized orthodontic workflow from 2D image acquisition up to 3D modeling and treatment planning [4].
IOSs, similarly to other scanners applied in different fields, project a light source onto the dental arches, including prepared teeth and implant scan bodies [5]. The images of the oral tissues, captured by the imaging sensors, are processed by the scanning software, which generates point clouds [6] that are then triangulated by the same software, creating a 3D surface model (mesh). The 3D tissue models are the result of optical impressions and are the digital alternative to traditional plaster models [4].
These devices are, therefore, born with the aim of creating a digital 3D model of the dental arches to replace the traditional dental impression. With technological innovations Table 1. Different components of a PICO statement for the current study.

PICO Facets Considerations
Patient (P) Patients with oral cavity pathologies (e.g., caries, dental wear, periodontal diseases, oral cancer, infections) Intervention (I) Detection of oral cavity pathologies by means of an intraoral scanning device Comparison (C) Traditional methods or gold standards for the diagnosis/detection of the specific oral cavity pathology (e.g., radiographic modalities) Assessment of the performance of intraoral scanners in the diagnosis/detection of pathologies of the oral cavity compared with reference methods The main research question is focused on investigating the potential of IOSs as alternative diagnostic tools for the detection of oral cavity pathologies compared to traditional methods.
Only studies, as properly framed in the objective of this review, taking into the following aspects were considered eligible:

•
The use of intraoral scanner devices in the oral health diagnostics field; • The assessment of performance in the detection of oral cavity pathologies (e.g., caries, dental wear, periodontal diseases, oral cancer, infections); • Clear description of the diagnostic workflow; • Comparison with a reference method or gold standard.
The literature screening was carried out by considering those applications of IOSs falling in the general area of detection systems for oral health diagnostics (e.g., caries, dental wear, periodontal diseases, oral cancer). Those works mainly focused on 3D dental model reconstruction for implantology or orthodontics or prosthodontics were excluded. In particular, the following exclusion criteria were defined:

•
Papers dealing with the technical evaluation of the accuracy and precision of intraoral scanners in reconstruction systems to achieve 3D digital mapping of the oral cavity; • Papers exploiting digital models obtained from intraoral scanning to fabricate prostheses or to replace prostheses already implanted in the patient's mouth.

Information Sources and Search Strategy
A comprehensive search of the online databases PubMed, Scopus and Web of Science was conducted in English using indexed and free-text words produced in the past five years (January 2018-December 2022). Since the development of digital technology in the dental field has been limited to the last few years, the research was limited to the last five years. The following logical search query was used: ("intraoral scanner" AND "diagnos*") The query, properly adapted to each specific database accessed, was used to check the following textual fields of the documents: title, abstract, and keywords.
In addition, non-English articles, duplicates, conference papers, reviews, book chapters, and papers not available in the first phase of research were excluded.

Selection Process
The study selection process was carried out according to the PRISMA flowchart. During the screening of the databases, the duplicates were removed, and the type of publication and language were investigated, as described in the previous paragraph. Subsequently, an analysis by titles and abstracts was carried out, excluding papers that clearly did not define the diagnostic purpose of the intraoral scan. The full-text of the selected papers was then examined to verify their eligibility according to the previously defined criteria.
Abstract and full-text screening was conducted taking into consideration that this review aimed to shed light on those applications of IOSs falling in the general area of detection systems for oral health diagnostics (e.g., caries, dental wear, periodontal dis-eases, oral cancer), thus not strictly pertaining to the implantology (e.g., guided implant surgery), orthodontics (e.g., aligners, cast production, virtual patient, digital impression), or prosthodontics (e.g., digital smile design, dental restoration) fields.
The in-depth analysis of the full-texts has made it possible to identify the main diagnostic fields in the oral health area in which it is possible to use intraoral scanning, once again excluding all those articles that go beyond the scope of this systematic review.

Data Collection
The data collection was carried out based on a customized Microsoft Excel form, mainly collecting the following information: authors, title of the article, year of publication, Digital Object Identifier (DOI), and abstract. The articles were divided equally among the reviewers. Possible doubts about inclusion/exclusion and their categorization were discussed until an agreement was reached.
From the subsequent screening of full-text articles, it was possible to collect information on the type of IOS used, the use of in vivo or in vitro models, diagnostic applications in the oral health area, and software used. No contact with the authors of the manuscripts for complementary information was necessary.

Data Item Clustering
The studies were initially divided considering the diagnostic application field by identifying the diagnostic application for which the IOS would be most used. Subsequently, the number of studies using in vivo or in vitro models was investigated. The main types of IOS used in the main diagnostic fields were then analyzed by investigating the optical characteristics that allow the identification of certain conditions or pathologies of the oral cavity. Finally, the articles were clustered according to the software used, thus making it possible to evaluate the most-used software in the dental field.

Risk of Bias
To reduce any bias, the Excel form used by the reviewers to collect information was agreed on in advance, the names of the different devices were standardized, and a dedicated comments section was set up to allow any concerns to be reported.

Synthesis Methods and Analysis of the Results
From the previously described clustering, two main diagnostic applications in the oral health area have been identified for which the use of an IOS has shown promising results, in addition to other less investigated applications in the literature. The analysis is graphically represented using the Excel graphical tools, mainly through histograms and pie charts, which better highlight the numerical distribution in the different categories. In addition, tables are used to facilitate a comparison between the various devices.

Main Findings
Only those articles introducing the use of an IOS for diagnostic/detection purposes in the oral health field, as defined in the methodological section of this work, were included. Based on the eligibility criteria, the analysis was conducted by first analyzing papers by title and abstract and then by full-text. The PRISMA workflow is shown in Figure 1.
The search was limited to the last 5 years, from 2018 to 2022, and the papers included were mainly concentrated in 2020 and 2022, as shown in Figure 2.
The papers were analyzed according to several categories: aim of the study; type of IOS used; use of in vivo or in vitro models; diagnostic application; software used; and results of the studies. Table 2 summarizes these fields for each paper analyzed. The search was limited to the last 5 years, from 2018 to 2022, and the papers inclu were mainly concentrated in 2020 and 2022, as shown in Figure 2. The papers were analyzed according to several categories: aim of the study; typ IOS used; use of in vivo or in vitro models; diagnostic application; software used; results of the studies. Table 2 summarizes these fields for each paper analyzed.   The papers were analyzed according to several categories: aim of the study; IOS used; use of in vivo or in vitro models; diagnostic application; software us results of the studies. Table 2 summarizes these fields for each paper analyzed.   Through the protocol described, clinicians will be able to assess whether there will be any difference in effectiveness between the selected diagnostic methods.     Table 3 also describes the fields defined for the papers excluded from the full-text analysis to better clarify the difference.  To describe an approach to manage excessive gingival display by lengthening of the clinical crowns using a digital workflow.
Commercial TRIOS 3 In vivo Surgical digital procedure The procedure described is efficient compared with the traditional one.
Unlike the included studies, where the use of intraoral scanners in the diagnosis of pathologies of the oral cavity was discussed, the excluded studies mainly concerned the evaluation of the technical performance in the reconstruction (and not the detection) of the digital model compared with traditional methods and the description of digital planning and design flows of orthodontic and implantological treatments, etc. The purpose of the review was, therefore, mainly to evaluate the possibility of using scanners commonly used in the digital reconstruction of dental models for the diagnosis and evaluation of oral health, excluding applications that base their efficiency on a good reconstruction of the 3D model, and to conduct planning on them. In particular, evaluations of the reconstructed digital model were excluded, as they made mere comparisons of the technical parameters of the 3D model obtained with intraoral scanners and with traditional methods. In particular, Ferraro et al. [44] and Akyalcin et al. [45] compared the digital model obtained with an intraoral scanner (TRIOS (3Shape, Copenhagen, Denmark) in the first case and the Cadent iTero scanner (Align Technology, San Jose, CA, USA) in the second case with that obtained by starting from a CBCT (ProMax 3D Mid; Planmeca, Roselle, Ill) in the first case and a CS 9300 unit (Carestream Health, Atlanta, GA, USA) in the second case with differences being not clinically detectable for orthodontic applications. Fraile et al. [46] described a cross-sectional study to compare the interocclusal contact records obtained with three different digital methods (intra-and extraoral digital scanners and the T-Scan III system) with the conventional method (articulated paper), demonstrating good reliability of the scanners compared to the gold standard. Kirschneck et al. [47] compared the intraoral scan with an extraoral one and the polyether impression (reference), demonstrating better reliability of the extraoral 3D models.
In other cases, the purpose of the excluded papers was to plan an implantological, endodontic, restorative, or esthetic treatment. Davidovich et al. [48,49] presented an innovative therapeutic approach for endodontically treated teeth and treatment for the hypomineralization of molar incisors (MIH) in children using a digital workflow with an IOS and CAD/CAM for restoration, addressing the uncooperative behavior of children and enabling the preservation of the tooth structure and long-lasting restoration. Revilla-Leon et al. [50,51] and Park et al. [52] instead presented a digital workflow for esthetic rehabilitation (e.g., in case of diastema) using a combination of facial and intraoral scanners and additive manufactured (AM) clear silicone indices. DuVall [53] used an intraoral scanner and milling unit to fabricate a CAD/CAM radiographic and surgical guide for use with a Cone Beam Computed Tomography system. Ahmed et al. [54] tested the use of a digital approach to lengthen crowns, reducing the likelihood for the need for postsurgical modifications and shortening the treatment time.

Oral Conditions Diagnosed Using an IOS
Among the papers analyzing the diagnostic purpose, most investigated the ability to assess dental enamel wear, followed by articles discussing caries detection and dental plaque evaluation, as shown in Figure 3. In other cases, the purpose of the excluded papers was to plan an implantological, endodontic, restorative, or esthetic treatment. Davidovich et al. [47,48] presented an innovative therapeutic approach for endodontically treated teeth and treatment for the hypomineralization of molar incisors (MIH) in children using a digital workflow with an IOS and CAD/CAM for restoration, addressing the uncooperative behavior of children and enabling the preservation of the tooth structure and long-lasting restoration. Revilla-Leon et al. [49,50] and Park et al. [51] instead presented a digital workflow for esthetic rehabilitation (e.g., in case of diastema) using a combination of facial and intraoral scanners and additive manufactured (AM) clear silicone indices. DuVall [52] used an intraoral scanner and milling unit to fabricate a CAD/CAM radiographic and surgical guide for use with a Cone Beam Computed Tomography system. Ahmed et al. [53] tested the use of a digital approach to lengthen crowns, reducing the likelihood for the need for postsurgical modifications and shortening the treatment time.

Oral Conditions Diagnosed Using an IOS
Among the papers analyzing the diagnostic purpose, most investigated the ability to assess dental enamel wear, followed by articles discussing caries detection and dental plaque evaluation, as shown in Figure 3. Other articles included cover the evaluation of functional and structural problems, the evaluation of periodontal defects, soft tissue analysis, orthodontic diagnosis, and analysis of the nasolabial region. Finally, one paper [25] discusses the improvement of the Cone Beam Computed Tomography (CBCT) technique using intraoral scanners.
It is interesting to note that most of the papers found did not investigate the diagnostic purposes of the IOS (so they were excluded from the analysis) but mainly dealt with the use of an IOS for the evaluation of 3D models obtained through device use and treatment planning with digital flow.

In Vivo and In Vitro Studies
We intended to investigate the use of in vivo or in vitro models in the analyzed papers. It was noted that, out of the total number of papers included, the distributions of the use of in vivo versus in vitro models were almost the same (12 articles used in vitro models and 13 used in vivo models). The difference was more significant when considering only the articles dealing with the evaluation of dental wear. In this case, the use of in vitro Other articles included cover the evaluation of functional and structural problems, the evaluation of periodontal defects, soft tissue analysis, orthodontic diagnosis, and analysis of the nasolabial region. Finally, one paper [25] discusses the improvement of the Cone Beam Computed Tomography (CBCT) technique using intraoral scanners.
It is interesting to note that most of the papers found did not investigate the diagnostic purposes of the IOS (so they were excluded from the analysis) but mainly dealt with the use of an IOS for the evaluation of 3D models obtained through device use and treatment planning with digital flow.

In Vivo and In Vitro Studies
We intended to investigate the use of in vivo or in vitro models in the analyzed papers. It was noted that, out of the total number of papers included, the distributions of the use of in vivo versus in vitro models were almost the same (12 articles used in vitro models and 13 used in vivo models). The difference was more significant when considering only the articles dealing with the evaluation of dental wear. In this case, the use of in vitro models was predominant (64%). This is because, in many cases, dental wear is induced through chemical agents or mechanical techniques, and therefore, in vivo models are impractical.

Analysis of the Main Commercial IOS
Wanting to analyze the application of intraoral scanners in diagnostics, we started with a market analysis to understand which are the main IOSs on the market and to understand the technology behind 3D reconstruction. The main features of the most popular scanners on the market are shown in Table 4. In particular, data were collected by investigating the sites of the manufacturers and previous reviews comparing some intraoral scanners [55][56][57][58][59][60][61][62] and by meetings with dentists who use these devices. The survey with clinicians showed that such devices are still viewed with distrust by some who still prefer traditional impressions, and those who choose such devices for clinical practice tend to go for the top-of-the-line products from the different manufacturers. In terms of technology, almost all of them are based on 3D reconstruction technology based on structured light, that is, imaging and capturing the deformation of a projected light pattern that is deformed on the dental arch.

Technologies for Oral Diagnostics
It is of interest to assess which scanners are most widely used in the different diagnostic areas defined in Section 3.2. By analyzing the scanning devices used for diagnostic purposes in the main diagnostic fields found, it was possible to obtain the distribution shown in Figures 4 and 5. J. Imaging 2023, 9, x FOR PEER REVIEW 12 of 19

Technologies for Oral Diagnostics
It is of interest to assess which scanners are most widely used in the different diagnostic areas defined in Section 3.2. By analyzing the scanning devices used for diagnostic purposes in the main diagnostic fields found, it was possible to obtain the distribution shown in Figures 4 and 5. As expected, in caries detection, the most widely used scanners are those that, in addition to scanning for the creation of the 3D model of the dental arches, have integrated technology for caries detection. In particular, iTero Element 5D (Align Technology, Inc., Tempe, Arizona) implements NIRI technology, which uses near-infrared (light with a wavelength of 850 nm) to highlight areas of demineralization with a bright region, allowing even interproximal caries to be identified. TRIOS 4 (3Shape A/S, Copenhagen, Denmark) has a built-in fluorescent technology for caries detection, mainly on the occlusal surfaces. The only other scanner on the market that has caries detection technology is Emerald S (Planmeca, Helsinki, Finland). The latter has a dedicated tip that implements transillumination-based technology, allowing it to detect supragingival proximal carious lesions. Michou et al. [34] developed an intraoral scanner prototype that emits light at 415 nm, thus taking advantage of fluorescent light. As expected, in caries detection, the most widely used scanners are those that, in addition to scanning for the creation of the 3D model of the dental arches, have integrated technology for caries detection. In particular, iTero Element 5D (Align Technology, Inc., Tempe, Arizona) implements NIRI technology, which uses near-infrared (light with a wavelength of 850 nm) to highlight areas of demineralization with a bright region, allowing even interproximal caries to be identified. TRIOS 4 (3Shape A/S, Copenhagen, Denmark) has a built-in fluorescent technology for caries detection, mainly on the occlusal surfaces. The only other scanner on the market that has caries detection technology is Emerald S (Planmeca, Helsinki, Finland). The latter has a dedicated tip that implements transillumination-based technology, allowing it to detect supragingival proximal carious lesions. Michou et al. [35] developed an intraoral scanner prototype that emits light at 415 nm, thus taking advantage of fluorescent light. To assess dental wear, on the other hand, the most widely used intraoral scanner is certainly True Definition (3M, St. Paul, MN, USA), followed by TRIOS 3 (3Shape A/S, Copenhagen, Denmark), Cerec Omnicam (Dentsply Sirona, Bensheim, Germany) and Planscan (Planmeca, Helsinki, Finland).
The intraoral scanners used in the other identified diagnostic fields are summarized in Table 5.

Three-Dimensional Model of Evaluation Software
After obtaining the 3D model of the dental arches using intraoral scanning devices, a series of quantitative assessments must be made on the model itself to arrive at a diagnosis or assessment of oral conditions. Appropriate software was used to make these assessments in many articles reviewed, the distribution of which is shown in Figure 6. The most widely used software is Geomagic Control X (3Dsystems, Darmstadt, Germany), as shown in Figure 6.
Tempe, Arizona) implements NIRI technology, which uses near-infrared (light with a wavelength of 850 nm) to highlight areas of demineralization with a bright region, allowing even interproximal caries to be identified. TRIOS 4 (3Shape A/S, Copenhagen, Denmark) has a built-in fluorescent technology for caries detection, mainly on the occlusal surfaces. The only other scanner on the market that has caries detection technology is Emerald S (Planmeca, Helsinki, Finland). The latter has a dedicated tip that implements transillumination-based technology, allowing it to detect supragingival proximal carious lesions. Michou et al. [34] developed an intraoral scanner prototype that emits light at 415 nm, thus taking advantage of fluorescent light. Figure 5. The main intraoral scanners used for dental wear evaluation. The number of studies adopting each specific scanner for dental wear evaluation is reported on the y-axis. Figure 5. The main intraoral scanners used for dental wear evaluation. The number of studies adopting each specific scanner for dental wear evaluation is reported on the y-axis. To assess dental wear, on the other hand, the most widely used intraoral scanner is certainly True Definition (3M, St. Paul, MN), followed by TRIOS 3 (3Shape A/S, Copenhagen, Denmark), Cerec Omnicam (Dentsply Sirona, Bensheim, Germany) and Planscan (Planmeca, Helsinki, Finland).
The intraoral scanners used in the other identified diagnostic fields are summarized in Table 5.

Three-Dimensional Model of Evaluation Software
After obtaining the 3D model of the dental arches using intraoral scanning devices, a series of quantitative assessments must be made on the model itself to arrive at a diagnosis or assessment of oral conditions. Appropriate software was used to make these assessments in many articles reviewed, the distribution of which is shown in Error! Reference source not found. The most widely used software is Geomagic Control X (3Dsystems, Darmstadt, Germany), as shown in Figure 6. This prevalence was even more pronounced when considering only dental wear cases, where half of the papers made model comparisons using Geomagic Control X software, as shown in Figure 7. This prevalence was even more pronounced when considering only dental wear cases, where half of the papers made model comparisons using Geomagic Control X software, as shown in Figure 7. In fact, most evaluations conducted on dental wear were based on a comparison between 3D models before and after wear, where the use of 3D inspection and metrology software is required. Other metrology software used in [30,40] were Mountains (Digitalsurf, Besançon, France) to measure step height on a freeform surface by comparing four workflow analysis techniques and Geomagic Qualify to quickly and easily overlay, evaluate, and report deviations between designed and built parts. In addition to metrology and 3D model comparison software, point cloud processing software was also used, such as VRMesh (VRMesh studio VirtualGrid), used in [32] to evaluate scarring and asymmetry of the upper lip in surgically managed cases of unilateral cleft lip and cleft palate (UCLP). In [25], a segmented scan was conducted using Geomagic Freeform Plus (3Dsystems, Darmstadt, Germany), a 3D Design and Sculpting Software, to join the models of the arch pieces together. RStudio, with the "molarR" package [62], was also used to produce topographic parameters on which the occlusal tooth wear assessment was based in [36].
In other cases, specific software referring to the dental field was used, such as WearCompare (leedsdigitaldentistry.com, accessed on 20 June 2023) [63], a purpose-based software specifically produced to assess tooth wear; TRIOS Patient Monitoring (3Shape A/S, Copenhagen, Denmark), proprietary software from the same company that produces the TRIOS IOS, which allows the visual recording of changes over time; OrthoCAD (Cadent, Inc., San Jose, CA, USA), proprietary software from the same company that produces the iTero scanner, which allows the analysis of dental arch models, enabling measurements of the width of dental arches, such as in [22], where the authors evaluated the effect caused by removable appliances over a period of 10 months in children with malocclusion; and Maxilim software (V2.3.0, Medicim NV, Mechelen, Belgium), which is used to align the IOS models. Additionally, self-developed software was used to address specific needs, such as the evaluation of dental plaque in 3D models [20], the assessment of teeth [21], and the identification of structural and functional problems along with a carefully designed OCT hardware prototype [41].

Discussion
We systematically reviewed the possible applications of intraoral scanners in the diagnostic field. It was found that the main diagnostic application area of IOSs is the evaluation of dental wear. Dental wear affects overall health and well-being by involving structural problems (support structures and loss of the vertical dimension of occlusion) and functional problems (increased tooth sensitivity, chewing, temporo-mandibular joint dysfunction, headaches, etc.) [64]. The evaluation of tooth wear can, therefore, be very useful In fact, most evaluations conducted on dental wear were based on a comparison between 3D models before and after wear, where the use of 3D inspection and metrology software is required. Other metrology software used in [30,41] were Mountains (Digitalsurf, Besançon, France) to measure step height on a freeform surface by comparing four workflow analysis techniques and Geomagic Qualify to quickly and easily overlay, evaluate, and report deviations between designed and built parts. In addition to metrology and 3D model comparison software, point cloud processing software was also used, such as VRMesh (VRMesh studio VirtualGrid), used in [32] to evaluate scarring and asymmetry of the upper lip in surgically managed cases of unilateral cleft lip and cleft palate (UCLP). In [25], a segmented scan was conducted using Geomagic Freeform Plus (3Dsystems, Darmstadt, Germany), a 3D Design and Sculpting Software, to join the models of the arch pieces together. RStudio, with the "molarR" package [63], was also used to produce topographic parameters on which the occlusal tooth wear assessment was based in [37].
In other cases, specific software referring to the dental field was used, such as WearCompare (leedsdigitaldentistry.com, accessed on 20 June 2023) [64], a purpose-based software specifically produced to assess tooth wear; TRIOS Patient Monitoring (3Shape A/S, Copenhagen, Denmark), proprietary software from the same company that produces the TRIOS IOS, which allows the visual recording of changes over time; OrthoCAD (Cadent, Inc., San Jose, CA, USA), proprietary software from the same company that produces the iTero scanner, which allows the analysis of dental arch models, enabling measurements of the width of dental arches, such as in [22], where the authors evaluated the effect caused by removable appliances over a period of 10 months in children with malocclusion; and Maxilim software (V2.3.0, Medicim NV, Mechelen, Belgium), which is used to align the IOS models. Additionally, self-developed software was used to address specific needs, such as the evaluation of dental plaque in 3D models [20], the assessment of teeth [21], and the identification of structural and functional problems along with a carefully designed OCT hardware prototype [42].

Discussion
We systematically reviewed the possible applications of intraoral scanners in the diagnostic field. It was found that the main diagnostic application area of IOSs is the evaluation of dental wear. Dental wear affects overall health and well-being by involving structural problems (support structures and loss of the vertical dimension of occlusion) and functional problems (increased tooth sensitivity, chewing, temporo-mandibular joint dysfunction, headaches, etc.) [65]. The evaluation of tooth wear can, therefore, be very useful for preventing various disorders. Most of the papers analyzed were based on in vitro studies, as wear is induced either with chemical agents or mechanically, simulating factors that may influence tooth wear (habit-, diet-, and musculature-specific factors). The accuracy of scanners when identifying changes before and after exposure to these factors was then evaluated with 3D metrology software, as described in Section 3.6.
In particular, Kühne et al. [30] performed a comparison between the reference model acquired by noncontact white light profilometry and 3D models acquired with different scanners (TRIOS 3, Cerec Omnicam, True Definition Scanner) at three different stages of wear simulated with a diamond bur using the Geomagic Qualify 2012 version, confirming the ability of IOSs to evaluate dental wear, even considering an imprecision level of plus or minus 20 µm with respect to the profilometry. With chemically induced wear and using only the True Definition Scanner, Kumar et al. [40] concluded that while IOSs are promising for the assessment of dental wear, they may not capture small variations. In fact, after 10 min of exposure, a volume change of −0.45 mm 3 (±2.59) was noted with a high standard deviation that was too large to perform an accurate volumetric analysis, due to the dimension of the triangles created by the True Definition Scanner being 50 µm. Ille et al. [34], after three periods of tooth soda exposure, the acquisition of the arches with the Planmeca scanner, and using Geomagic Control X as the metrology software, concluded that the latter scanner is capable of detecting even small variations, detecting an average of 65 µm of dental tissue over the course of 19 h of exposure. Michou et al. [38] also evaluated wear after citric acid exposure session using TRIOS 3 and related TRIOS Patient Monitoring software, confirming the potential of this IOS for assessing tooth wear. Along with TRIOS Patient Monitoring, which is useful for assessing depth loss (mm), Machado et al. [23] used also WearCompare software to obtain the volume loss (mm 3 ) and area loss (mm 2 ), demonstrating that there are strong correlations between depth (mm 2 ) and time (r = 0.9993 p < 0.0001), volume (mm 3 ) and time (r = 0.9968, p < 0.0001), and area (mm 2 ) and time (r = 0.9475, p = 0.0003). In order to adequately assess tooth wear, it is important to define indices that can be quantitatively and objectively recognized. To date, there are indices for the visual assessment of tooth wear (such as the Basic Erosive Wear Examination (BEWE) [66]) which, although useful, are subject to bias depending on the operator's clinical experience. In fact, Travassos da Rosa Moreira Bastos et al. [31] performed a study to assess intra-and interobserver concordance based on scans taken one month from baseline, concluding that the visual evaluation based on intraoral scanning results in a lower bias with a moderate agreement for the intraoral scanner analysis (K = 0.595) using the Kappa test. However, to make the assessment more objective, Alwadai et al. [37] aimed to investigate the effectiveness of occlusal (as they are highly susceptible to wear) topographical analyses to assess the progression of simulated wear. Abrasion, in this case, was mechanically simulated using silicon carbide grind papers, and the evaluation was based on a combination of the digital impression scanner and dental topographic analysis parameters (Slope, Relief, RFI, and OPCr) calculated in RStudio with the "molaR" package. These topographic attributes were expected to decrease as induced wear increases, which was observed (e.g., the slope varied between 54.6 (±4.3) and 46.6 (±6.4) when the wear increased from 0 mm to 1.5 mm).
In vivo analyses of tooth wear are evaluated on a longer time basis. For example, in [26,36], wear was evaluated relative to baseline after 6 months and 1 year, obviously having weaker variations and taking respectively the visual evaluation and the microCT as references. Time is further extended (12 and 24 months) when wear is evaluated on lithium disilicate implant crowns and their enamel antagonists, as conducted by Stück et al. [21].
Another clinical area where scanners can provide good diagnostic results is caries detection. In this field, scanners with integrated caries detection systems based on noninvasive optical systems, described in Section 3.5, are already under development and are spreading on the market. The first scanner with integrated caries detection technology was TRIOS 4, which integrates fluorescence technology, whose diagnostic reliability was investigated in many of the articles analyzed [48][49][50]. Even near-infrared lights have the ability to locate carious lesions, and this technology has been implemented in iTero Element 5D. In this case, a greater ability to detect interproximal caries was found [27,29], even in comparison with radiography, which is considered the gold standard for these evaluations [15]. Schlenz et al. [29] made a comparison between the three previously mentioned scanners and traditional methods, finding the greatest reliability for diagnosing occlusal caries in permanent teeth with Planmeca Emerald S, which is based on transilluminescence technology. Another relevant study was carried out by Michou et al. [35], who developed an intraoral scanner prototype that emits light at 415 nm, thus taking advantage of fluorescent light. They compared it with visual-tactile, radiographic, and histological assessments of caries and showed the promising performance of fluorescent light.
Dental caries and periodontal diseases are plaque-associated conditions, so it is also important to define appropriate methods for assessing this condition. In clinical practice, plaque levels are recorded chairside using index systems that calculate the amount of plaque, despite not showing very robust results. Two of the analyzed studies aimed to investigate whether plaque can be reliably detected, quantified, and monitored on 3D models of dental arches acquired by intraoral scanners. In the first case [20], this was conducted by measuring the amount of plaque with planimetric measurements using appropriate especially designed software; in the second case [24], it was conducted by comparing visual assessments performed chairside and with the 3D model. In both cases, the IOS was proven to be an adequate tool for measuring plaque.
Intraoral scanners have also proven to be excellent tools for making assessments of structural problems in the oral cavity, such as quantifying changes in the width of the dental arches after treatment, e.g., with removable braces in children with malocclusion (orthodontic diagnostics) [22]; in the external regions, such as the evaluation of scarring and asymmetry of the upper lip in surgically managed cases of unilateral cleft lip and cleft palate (UCLP) [32]; and overall in the cranial region, creating hybrid images resulting from the crossing of several diagnostic sources, such as Cone Beam Computed Tomography (CBCT) of the skull with a 3D model of dentition derived from a scanner [25]. In the context of diagnosing functional and structural problems, there is the interesting study by Eom et al. [42], who designed and evaluated the performance response of a three-dimensional (3D) intraoral scanning probe based on optical coherence tomography (OCT), showing the ability to reconstruct both the structure and function of human teeth. The ability of intraoral scanners to allow the evaluation of structural problems is not limited to the dental structure but was also investigated by Deferm et al. [43] for the analysis of soft tissues, in terms of shape, color, and curvature.

Conclusions
From the analysis of the different papers, it emerged that the use of intraoral scanning probes represents a promising approach that could be used in the fields of orthodontics and implantology, as well as in various diagnostic fields, for structural evaluations, such as the evaluation of dental erosion, which emerged from this review as the most promising field. In the detection of caries, scanners are already considered valid and established tools on the market and are used in dental clinics. The differences between the scanners are mainly due to the 3D imaging principle, the different wavelengths used, the image acquisition principle, and the scanner wand. More research is needed to test their performance levels in the context of their differences, which appear to be established only in the case of dental caries.
In conclusion, there is a wide range of possible diagnostic applications, but the cases in which dental impression analysis can be considered the clinical standard are limited, due to the still-necessary validation compared to traditional methods.
However, the results of the studies analyzed encourage the idea that, in the near future, also thanks to further technological innovations, there is the possibility of using intraoral scanners as diagnostic devices in clinical practice, providing quantitative parameters on which to base clinical decision-making, with advantages for both the clinician and the patient.