Reliability of Extraoral Scanners in Capturing 3D Geometry for Dental Prostheses: A Systematic Review

Introduction

In the realm of dental prosthetics, the meticulous and exact acquisition of the 3-dimensional (3D) structure of the face is essential for the effective production of dental prostheses [1,2]. Traditional methods of capturing these geometries frequently depend on available resources and clinician expertise. The clinical evaluation or recording the details with impression materials or photographs, although effective, could be invasive and uncomfortable for patients [3]. Due to advances in technology, extraoral scanners have become a promising alternative for acquiring precise 3D facial geometry. They offer a noninvasive and perhaps more efficient method [4]. Extraoral scanners are used in recording the facial geometries clinically and, in the laboratory, extraoral scanners can be used for digitalizing intraoral data either by scanning the impressions or by producing casts [5]. The steps involved in the fabrication of prostheses using digital techniques require acquisition of data (scanning), computer-aided design (CAD), and computer-assisted manufacture (CAM). CAM basically has 2 methods: milling, the subtractive method, and 3D printing, the additive method [6–8].

3D extraoral facial scanning is an advanced imaging technology that captures highly detailed 3D representations of a patient’s face. The key 6 aspects and applications of 3D extraoral facial scanning are (1) noninvasiveness: the scanning process is painless and noninvasive, making it comfortable for patients; (2) detailed accuracy: 3D scans capture precise measurements of facial structures, allowing for highly detailed models of soft and hard tissues; (3) rapid imaging: it usually takes only a few minutes to complete the scan; (4) digital models: the scans create digital models that can be analyzed, manipulated, and shared with other healthcare professionals; (5) improved patient communication: patients can easily understand their condition and treatment options by viewing their 3D models; and (6) customization: the precise data allows for treatments tailored to each patient’s unique anatomy, leading to better outcomes [8,9].

The current methods of intraoral evaluation involve various advanced technologies to provide highly accurate, detailed assessments of the oral cavity. These methods are crucial for diagnosing dental conditions, planning treatments, and tracking progress over time. Intraoral scanners are some of the most advanced tools used in modern dental practices. They capture highly detailed 3D images of the teeth and surrounding tissues using noninvasive techniques. Intraoral scanners provide advantages such as comfort, accuracy, efficiency, and digital workflow. Digital photography and videos by intraoral cameras help in patient communication and record keeping. Extraoral scanners in the laboratory help in digitalizing the conventionally acquired data. Radiographic methods include cone beam computed tomography and X-rays, and occlusal evaluation with digital tools like T-Scan technology [6,7,10,11]. All these methods help in prosthesis fabrication in a better way using advanced integration methods with digital workflow using CAD/CAM technology. With CAD, the dentist or laboratory technician designs the prosthesis using computer software, based on a 3D digital scan of the patient’s mouth and face, which is captured via an intraoral scanner or extraoral facial scanner. Once the design is finalized, the data are sent to a milling machine or 3D printer, which manufactures the prosthesis with extreme precision [6,11].

The role of digital scanners is promising, as they provide better accuracy, efficiency, and comfort. An extraoral scanner or intraoral scanner is used to transfer the data of the oral cavity into computer software. Indirect digitization has been in use for quite some time [1,5,12]. Extraoral facial scanners generate precise 3D models of a patient’s face using a variety of technologies, such as structured light, laser scanning, and photogrammetry [13]. In order to create dental prostheses that fit comfortably and accurately, these scanners are made to capture the minute details of facial anatomy. The main benefit of extraoral facial scanners is their capacity to rapidly and painlessly capture large facial regions, which makes the procedure more comfortable for patients and possibly more effective for medical professionals [14]. It is crucial that these scanning technologies are reliable. Poorly fitting prostheses can cause patient discomfort and necessitate time-consuming and expensive adjustments or remakes, due to inaccurate or inconsistent scan results [15]. To assess the reliability of extraoral scanners, studies often use the intraclass correlation coefficient (ICC) as a statistical measure.

Intraclass correlation coefficient is a statistical estimate that measures the extent of agreement between at least 2 quantitative measurements. While the kappa statistic measures the extent of agreement for categorical variables, the ICC measures the extent of agreement for numerical or quantitative variables [16]. Apart from measuring the extent of agreement, ICC is also designed to measure the degree of reliability, consistency, and stability [17]. The consistency or dependability of measurements made by various observers of the same quantity is gauged by the ICC. The ICC is used to evaluate the degree of consistency between 3D measurements obtained through intraoral evaluations and those obtained through extraoral scanners [18]. A high ICC value suggests that the extraoral scanner produces accurate data that can be relied upon for clinical applications, because it shows a high degree of consistency in the measurements across various methods or observers [19].

Prior research has investigated the precision and reliability of different extraoral scanning technologies by comparing them to conventional intraoral scanning techniques. These studies frequently demonstrate elevated levels of precision and reliability for extraoral scanners, as indicated by ICC values that signify robust concurrence between the 2 techniques. Nevertheless, the diversity in study designs, sample sizes, and the specific technologies used can influence the applicability of these results [12,20,21]. Therefore, in the present study, we systematically examined the existing literature on the use and reliability of extraoral facial scanning and extraoral scanning in capturing the 3D geometry of the face, in association with intraoral evaluation for dental prosthesis applications.

Material and Methods

DEVELOPING A PROTOCOL:

This review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement guidelines of Systematic Reviews [22]. To ensure accountability, the investigation procedure was filed in the International Prospective Register of Systematic Reviews (PROSPERO-CRD42024544106) [23].

SEARCH STRATEGY:

A literature search was performed across major electronic databases, including PubMed-Medline, Web of Science, Embase, and Scopus, along with additional sources, such as Google Scholar, major journals, and cross references. A comprehensive search to identify studies related to extraoral scanners with intraoral scanners and clinical measurements for a dental prosthesis was conducted from database inception to June 2024. The search was conducted using controlled vocabulary terms, Medical Subject Heading (MeSH terms, keywords) such as “extraoral scanner”, “facial scanner”, “intraoral scanner”, “3D face scanning”, and “dental prosthesis”. No additional filters or language restrictions were kept while conducting the searches (Table 1).

DEVELOPING THE RESEARCH QUESTION:

The focused question of this study was the following: Do extraoral digital measurements by extraoral facial scanning and extraoral scanning provide reliable guidelines in association with intraoral evaluation for dental prostheses fabrication?

The following PICOS (Population, Intervention, Comparison, Outcome, Study) [22] framework was developed in accordance with the research question, as follows:

ELIGIBILITY CRITERIA:

Inclusion criteria were in vivo or in vitro studies on digital impressions utilizing extraoral scanners with intraoral scanners and clinical measurements and comparing their reliability in terms of ICCs and/or inter-examiner reliability coefficients. All the relevant studies published in the English language from January 1, 2012, to June 1, 2024 were included.

Exclusion criteria were studies involving only extraoral facial scanning/extraoral scanning or intraoral evaluation, without prosthesis fabrication; studies including impressions in implants or magnetic resonance imaging scans; in-process trials; and studies in which extraoral scan impression was recorded by material other than elastomers. Case reports, case series, reviews, letters to the editor, and conference proceedings were considered ineligible.

SELECTION PROCESS AND DATA EXTRACTION:

Two colleagues independently conducted the literature search electronically and manually. The collected data were screened using the website Covidence.org (Level 10, 446 Collins St, Melbourne VIC 3000, Australia) for full-text analysis. Also, the 2 colleagues examined the study articles and extracted data. The screening was conducted in 3 phases. First, the titles and abstracts of all papers were evaluated for eligibility. Second, in this phase, full-text screening, in which articles were selected based on title and abstract screening, were read comprehensively by the same 2 authors independently. In the third phase, the studies that met the selection criteria were finally included and reviewed for data extraction. In the event of doubt or disagreement between the reviewers, the final decision was made by a third reviewer.

All the articles were exported into Mendeley Desktop 1.19.6 software. The “check for duplicates” feature of this software was then used to identify and eliminate duplicates. Each article that was detected as a duplicate by the software was checked meticulously.

For each chosen study, the following data were retrieved from a standard form (when available): author and year of publication, sample size, population, intervention, result, and conclusion.

RISK OF BIAS ASSESSMENT:

The modified CONSORT checklist (Consolidated Standards of Reporting Trials) of items for reporting studies, as adapted by Garcia et al [24], was used to evaluate the risk of bias in the studies included. The checklist was used to assess the quality of the included articles on in vitro and in vivo studies. Originally, the CASP CONSORT checklist was designed for analyzing clinical trials. However, in 2012, a modified CONSORT checklist was published, featuring items selected to assess the reporting of in vitro studies. Garcia et al adapted this tool for their study on dental implants. Given its applicability to our included studies, we used the same tool, in which each study scores 1 for yes and 0 for no response. Studies obtaining a score of 80% or more were considered high quality in this review, whereas studies scoring between 50% and 80% were considered moderate quality, and those scoring <50% were considered low quality [24,25].

DATA SYNTHESIS:

A theme structure was used to summarize and provide a summary of the breadth and depth of the included studies. It is a method for systematically identifying, organizing, and offering insight into patterns of meaning (themes) across a dataset. In the present systematic review, the extraoral digital measurements by extraoral facial scanning and extraoral scanning in association with intraoral evaluation for dental prostheses fabrication was the main theme.

Results

STUDY SELECTION AND RESULTS:

After extensive searching, a total of 1066 studies were identified, of which 8 were duplicates. The remaining 1058 studies underwent title and abstract screening, and 16 studies were selected for full-text screening, of which 8 studies were excluded after full-text screening. Thus, a total of 8 studies that met our inclusion criteria were processed for data extraction (Figure 1).

STUDY CHARACTERISTICS:

The included studies were published between 2018 and June 2024. The study design included in-vitro design (n=4) and in vivo design (n=4). Two studies included edentulous patients, and 2 studies included dentate patients. On the other hand, 4 studies performed the intervention on model or casts. The sample size included 62 individuals, 100 sets of maxillary and mandibular dental plaster models, 1 mannequin head, 10 stone casts, and 1 edentulous model. The studies evaluated a range of scanning technologies, including custom-made static capturing systems [26,27], dual-structured light facial scanners [28], red, blue, green, and depth (RGB-D) camera-based facial scanners [29–31], extraoral scanners [26,30,32], and 3D handheld scanners [29]. Various comparison methods were used, including clinical measurements, digital calipers, direct anthropometry, and intraoral scanners. All studies used the ICC as a primary outcome measure, whereas 2 studies measured the inter-examiner reliability coefficient as well [27] (Table 2).

EDENTULOUS INDIVIDUALS: Piedra-Cascon et al [28] assessed the accuracy of a dual-structured light facial scanner and the inter-examiner reliability among 10 dentate participants with 6 soft-tissue landmarks identified on each. Inter-landmark distances were measured manually on the face using a digital caliper and digitally on 3D facial reconstructions using a dual-structured light facial scanner. While significant differences were found between manual and digital measurements, excellent inter-examiner reliability was observed. The dual-structured light facial scanner showed a mean absolute difference of 0.91±0.32 mm between manual and digital measurements, indicating good trueness and precision. The inter-examiner reliability ranged from 0.93 to 0.99, with the lowest ICC of 0.93 for 1 landmark [28]. On the other hand, Liu et al evaluated 12 edentulous participants using a custom-made static capturing system for digital facial reconstruction. Eight extraoral soft-tissue facial landmarks were identified and measured clinically and digitally. The inter-examiner reliability for each landmark varied, with mean inter-examiner reliability ranging from 0.95 to 0.98, with the lowest ICC at 0.91 for 1 landmark. The mean absolute difference between clinical and digital measurements was 1.95±0.33 mm, with some landmarks showing larger differences [29].

DENTATE INDIVIDUALS: Srinivasan et al recruited 20 fully dentate volunteers (mean age=30.0±10.7 years) for facial measurements using a digital caliper and a face scanner (Obiscanner). Measurements in maximal intercuspation and increased vertical distances (2, 4, and 6 mm) were analyzed. Strong correlations (r>0.93; P<0.001) and excellent reliability (ICC <0.90) were found for both methods. Mean differences between digital and clinical measurements ranged from 0.054±0.14 mm to 0.203±0.13 mm. Bland-Altman plots showed a 1.7% difference for vertical measurements, indicating both methods are interchangeable [33]. Kirschneck et al assessed 2 intraoral scanners (Lythos and Ormco) among 20 patients and found that Lythos intraoral scanning showed low to mediocre reliability, while alginate methods demonstrated moderate to substantial concordance with the polyether reference. Local best-fit alignments provided higher ICCs, indicating better reliability. Validity assessments showed smaller deviations for alginate methods, compared with intraoral scanning, particularly in the molar region. Pairwise comparisons revealed higher concordance between alginate methods than with intraoral scanning [26].

EDENTULOUS MODELS: Pan et al used a mannequin head, with 14 facial landmarks prelabeled. A digital vernier caliper was used for manual measurements, and an RGB-D camera-based system (Bellus3D Arc7) and a stereophotogrammetry system (3dMD face system) were used for digital measurements. Measurements were compared with the criterion standard and each other. Results showed that both systems were equivalent to the criterion standard and each other, with average absolute deviations below 1 mm and an ICC for each parameter of 0.99. Both systems demonstrated high precision and an inter-operator reliability of 0.99, indicating their accuracy and clinical acceptability [31]. Periera et al compared intraoral and extraoral scanning for assessing 3D deviations and distances between implants; a mandibular model with 4 implants was used. Scans were conducted with both types of scanners in 2 groups: scanning with scan bodies and with the device (n=10). The results indicated that intraoral scanning, with the scan bodies and with the device coupled to the scan bodies, exhibited precision and trueness in 3D deviations, when compared with extraoral scanning. Interestingly, the scanning method did not significantly impact the precision of capturing distances between implants in intraoral and extraoral scanning, although greater distance errors were noted in the scan bodies group. Furthermore, intraoral scanning with the device attached to the scan bodies accurately captured inter-implant distances within specific ranges, showing better performance than scanning with only the scan bodies, particularly for distances ranging from 9 to 22 mm and 23.5 to 40.2 mm in a crossed arch [32]. Liu et al measured the mesiodistal diameter, buccolingual diameter, and clinical crown height of 2800 teeth on plaster and 3D digital models, finding excellent reliability (ICC >0.75) and small differences, with mean biases close to zero and 95% limits of agreement within 0.5 mm [29]. Ellakany et al compared measurements from digital casts obtained via 2 intraoral scanners and 2 extraoral scanners with those from a stereomicroscope, finding no significant differences among scanners overall or in premolars and molars, but significant differences in canines (P=0.025) [30].

REPORTING OF RISK OF BIAS (QUALITY ASSESSMENT OF STUDIES):

The quality assessment of all the studies revealed high adherence to reporting standards, with a >80% score. All studies provided structured abstracts, comprehensive introductions, detailed methods, and well-reported results. Discussions addressed limitations and biases. While overall quality was high, consistent reporting of funding (n=3) and access to protocol (n=8) was not reported by some of the studies (Table 3).

Discussion

LIMITATIONS:

The present study had limitations. First, most of the studies did not individually specified the extraoral scanning limitations but instead used it for fabrication of prosthesis. Second, with an ongoing development in scanning technologies and new scanners, it is difficult to have a standardized precise system to assess the scanner’s capabilities for determining the intraoral and extraoral tissue geometry. To overcome these limitations, future research should standardize study methodologies, include diverse patient populations, investigate long-term outcomes, and assess scanning technology implementation in various clinical settings. While the included studies provide compelling evidence supporting the reliability of extraoral scanners for capturing the 3D geometry of the face, the limitations need to be considered. As the field continues to evolve, further research may focus on integrating these technologies with other digital tools and exploring their use in a wider range of clinical scenarios.

Conclusions

In the present systematic review, it can be concluded that extraoral facial scanning/extraoral scanning can digitalize workflow in association with intraoral evaluation to provide high-quality dental prostheses. The present systematic review provided robust evidence supporting the reliability of extraoral scanners in capturing the 3D geometry of the face and tissues for dental prosthesis planning in association with intraoral evaluation with intraoral scanners and clinical measurements. The high ICC values and low levels of deviation reported across studies underscore the clinical applicability of these technologies.