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Corresponding author: Dr Florian Kernen, Department of Oral and Maxillofacial Surgery, Translational Implantology, University Medical Center Freiburg, Hugstetter St 55, Freiburg 79106, GERMANY
Clinician Scientist, Department of Oral and Maxillofacial Surgery, Translational Implantology, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Anthropologist, Division of Biological Anthropology, Faculty of Medicine, University of Freiburg, Freiburg, GermanyPost-doctorate Scientist, Department of Oral and Maxillofacial Surgery, Translational Implantology, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Doctorate Candidate, Department of Oral and Maxillofacial Surgery, Translational Implantology, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Statistician, Institute of Medical Biometry and Statistics, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Associate Professor, Department of Prosthetic Dentistry, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Full Professor, Department of Oral and Maxillofacial Surgery, Translational Implantology, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Full Professor, Charité – Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Department of Oral and Maxillofacial Surgery, Berlin, GermanySenior Research Scientist, Department of Oral and Maxillofacial Surgery, Translational Implantology, Medical Center – University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
The accuracy of intraoral scanners is a prerequisite for the fabrication of dental restorations in computer-aided design and computer-aided manufacturing (CAD-CAM) dentistry. While the precision of intraoral scanners has been investigated in vitro, clinical data on the accuracy of intraoral scanning (IOS) are limited.
Purpose
The purpose of this clinical study was to determine the accuracy of intraoral scanning with different devices compared with extraoral scanning.
Material and methods
An experimental appliance was fabricated for 11 participants and then scanned intraorally and extraorally with 3 different intraoral scanners and a reference scanner. Intraoral and extraoral scans were subdivided into complete-arch and short-span scans and compared with the reference scan to assess trueness. Repeated scans in each group were assessed for precision.
Results
Precision and trueness were higher for extraoral scans compared with intraoral scans, except for complete-arch scans with 1 intraoral scanner. The median precision of short-span scans was higher (extraoral: 22 to 29 μm, intraoral: 23 to 43 μm) compared with complete-arch scans (extraoral: 81 to 165 μm, intraoral: 80 to 198 μm). The median trueness of short-span scans (extraoral: 28 to 40 μm, intraoral: 38 to 47 μm) was higher than that of complete-arch scans (extraoral: 118 to 581 μm, intraoral: 147 to 433 μm) for intraoral and extraoral scanning.
Conclusions
Intraoral conditions negatively influenced the accuracy of the scanning devices, which was also reduced for the complete-arch scans.
Clinical Implications
The accuracy of intraoral scans is a prerequisite for the fabrication of dental restorations in CAD-CAM dentistry. However, the longer the scanned distance within the dental arch, the higher the inaccuracy. Clinical intraoral scanning did not result in clinically acceptable accuracy for complete-arch scans.
Intraoral scanners (IOSs) are routinely used to provide digital information in clinical dentistry for a range of situations including diagnosis, restorative dentistry, and implant-supported restorations.
A prerequisite for intraoral scanning is the accuracy of the resulting virtual casts. Accuracy is defined by 2 different terms: precision and trueness (DIN ISO 5725-2).
DIN ISO 5725-2 Accuracy trueness and precision of measurement methods and results: a basic method for the determination of repeatability and reproducibility of a standard measurement method.
Precision defines the congruence between multiple virtual casts of the same anatomic structure. Trueness describes the congruence between the actual anatomic structure and its reproduction in a virtual cast.
The precision of intraoral scans in vivo has been assessed by repeated scanning and comparison of the resulting virtual casts.
The lower precision of intraoral scans compared with that of extraoral scans has been reported, and intraoral conditions including moisture, salivary flow, and restricted space can complicate accurate intraoral image acquisition.
Trueness may be assessed in vitro by scanning an experimental model with the respective intraoral scanner and a reference scanner and comparing the resulting virtual casts. This approach has been used to assess trueness in vitro, but the trueness of IOSs has been reported to decrease when acquiring a complete dental arch compared with a short span.
Evaluation of the trueness and precision of complete arch digital impressions on a human maxilla using seven different intraoral digital impression systems and a laboratory scanner.
Trueness in vivo has been approximated by comparing an intraoral scan with a virtual reference model produced from a conventional impression. However, the conventional impression itself introduces inaccuracy,
used an industrial scanner and IOSs to acquire the facial surface of the maxillary anterior teeth. This method captured only a short span and did not evaluate the molar area most prone to deviations or the complex acquisition of multiple surfaces. Atieh et al
focused on comparing conventional impressions with an intraoral scanner by using a reference appliance made from metal alloy. The deviation of intraoral scans was measured only in small areas distributed throughout the dental arch. Notably, in previous studies, the scans were aligned by using a best-fit algorithm including the complete surface before the assessment of the deviations. With this method, the deviations are arbitrarily averaged over the entire surface and their spatial location is not reflected. This becomes especially important when scanning larger areas, as potential inaccuracies at the end of a long span are averaged over the entire arch. This can be overcome by using a different method of alignment as implemented in the current study.
An experimental appliance comprising denture teeth and reference bodies was fabricated and scanned intraorally with multiple scanners, as well as extraorally with a reference scanner. Alignment of multiple scans was selectively performed by using reference bodies, and the denture teeth were used to assess spatial deviations. The authors are unaware of a previous study that examined the trueness and precision of multiple IOSs in vivo and assessed the spatial location of deviations. The research hypotheses of the study were that the accuracy of IOSs would be lower when used in the oral cavity and that short-span scans would be acquired with higher precision and trueness than complete-arch scans.
Material and methods
The study included intraoral scanning of voluntary participants using different IOSs and an individually fabricated experimental appliance. The study protocol was approved by the Institutional Review Board of the Medical Center—University of Freiburg, Freiburg, Germany (434/17), registered at the German Clinical Trial Register (DRKS: 00014039), and performed in accordance with the institutional research committee and the 1964 Helsinki declaration and its later amendments. Study participants were recruited in the Department of Oral and Maxillofacial Surgery, Translational Implantology in the University Medical Center Freiburg during July 2017 and July 2018. Participants gave written consent to the study before inclusion.
Participants with partial edentulism in both posterior regions of either the maxilla (n=6) or the mandible (n=5) were selected (Table 1). This specific anatomic situation was required for the placement of the experimental appliance and the arrangement of scan objects. Table 1 displays the selected jaw and number and regions of the test surfaces. Participants with limited mouth opening or hard and soft tissue defects, including extended scar tissues in the oral cavity, were excluded.
Table 1Study population with selected jaw, regions, and number of test surfaces
Participant
Jaw
Regions of Test Surfaces
1
Mandible
First premolar left, second premolars, first molars
2
Maxilla
Second premolar left, first molars
3
Mandible
Second premolars
4
Maxilla
First premolars, second premolars, first molars
5
Mandible
First premolar right, second premolars, first molars
6
Maxilla
Second premolar left, first molars
7
Mandible
First premolar left, second premolar left, first molars
8
Maxilla
First premolar left, second premolars, first premolars
9
Mandible
First premolars, second premolars
10
Maxilla
First premolars right, second premolars, first molars
The experimental appliance was manufactured by using a pink acrylic resin base plate, denture teeth, and cylindrical reference bodies. A stone cast of the respective jaw of each participant was poured from a conventional irreversible hydrocolloid (Pluralgin Super; Pluradent) impression. A visible light-polymerized denture base material (Megatray Basisplatte; Megadenta Dentalprodukte GmbH) was adapted to cover the dentate and edentulous areas of the arch. A geometric pattern of indentations was created on the denture base material to achieve a morphology that was easily captured by the surface scanner. Cast denture teeth (SR Ivocron; Ivoclar AG) were fixed with an adhesive (Palapress; Kulzer GmbH) to the base plate in both the premolar and molar areas. Three cylindrical reference bodies (Straumann Mono Scanbody, Art.032.041; Institut Straumann AG) were fixed with the adhesive (Palapress) in the area of the second molars bilaterally and in the anterior region of the baseplate (Fig. 1). The experimental appliance was designed to imitate the intraoral anatomy, including the test surface and the pink acrylic resin base plate, and host reference bodies for data registration and evaluation.
Figure 1A, Individually fabricated experimental appliance with reference objects (R1, R2, R3) and test surfaces represented by acrylic resin denture teeth (T). B, Occlusal view of inserted appliance.
A reference data set of the experimental appliance was established with a desktop optical scanner (S600 Arti; Zirkonzahn) with a manufacturer specified precision of ≤10 μm. The accuracy of the acquired data was evaluated by using a coordinate measuring machine (CMM) (DS 10; Renishaw). The positions of the reference objects (R1-R3) were acquired with this high precision tactile scanner (accuracy <5 μm) and used for verification.
Comparison of the acquired optical scanner data with the CMM data showed a median deviation of 22 μm when using a best-fit registration algorithm.
The experimental appliance was scanned on the stone cast with the following IOSs: TRIOS3, version 1.6.10.1 (3Shape A/S) (TR); CEREC Omnicam, version 4.6, (Dentsply Sirona) (OC); True Definition, version 5.4 (3M) (TD). Each scan was performed by 1 operator (F.K.) using the manufacturer’s recommended scanning path. Each scanner was used 3 times for extraoral scanning.
The respective experimental appliance was placed in the mouth of the participant and cheek retractors and a saliva ejector were applied. Passivity was confirmed and the appliance was fixed by using a denture adhesive (Blend-a-dent Plus Haftcreme; Oral-B) Intraoral scans were acquired by the same operator using the 3 scanning devices, and 3 scans were made with each IOS. Caution was used to scan only the intraoral device and not the participant’s teeth because of the risk of movement of the experimental appliance. The scheme for data acquisition and evaluation are displayed in Figure 2.
Figure 2Number of scans and comparisons for assessment of precision and trueness.
Standard tessellation language (STL) files of the reference scans, extraoral scans, and intraoral scans were imported into a 3D modeling software program (Meshmixer; Autodesk Inc). The reference bodies and test surfaces were cropped to prepare the scan for evaluation. The reference bodies were used for the registration of multiple scans and were therefore essential for the assessment of the spatial location of the deviations. Registration was defined as the 3D alignment of multiple scans using common surface information. The protocol for registration is displayed in Table 2. To elaborate the critical mass of information for an accurate registration, a randomly chosen scan was registered multiple times. Randomization was achieved by using a simple randomization sheet in Microsoft Excel (Microsoft Corp) and applying it to all members of the study population. Registration was performed with 1 reference body (R1), 2 reference bodies (R1, R2), and 3 reference bodies (R1, R2, R3), respectively. The comparison showed no differences in registration accuracy. Therefore, the final assessment of deviations was performed by using 1 reference body (R1 or R3) at each distal site. The test surfaces were used for the measurement of deviations between scans.
Table 2Protocol for registration of multiple scans and evaluation of deviations between surfaces and interpretation of results with regard to scan length
The registration of only 1 selected scan body was performed by using a best-fit registration algorithm based on an iterative closest point search procedure.
The scan body surfaces were first aligned according to their principle axis, and then an iterative closest point search was performed. This was done by finding the closest point on the target surface. Correspondences pointing in the wrong direction (normal vectors) were discarded. Of the remaining corresponding points, those further away than the 90th percentile of distances were discarded to avoid invalid registration results. Based on these correspondences, the reference was iteratively rotated to the target surface. Finally, the transformation was applied to the entire scan.
To evaluate the mesh discrepancies, the (unsigned) distance for each vertex in the region of interest on the reference mesh to the surface of the aligned scans was recorded. This resulted in tens of thousands of error values per alignment that were averaged per vertex over all iterations and then accumulated over all participants.
Mesh alignment and error assessment were performed by using the mathematical and statistical platform R and specifically the R-packages Rvcg, Morpho, and mesheR.
The primary end point was the precision of intraoral and extraoral scanning with 3 different scanners, evaluated by comparing the 3 consecutive scans in each group with 1 randomly selected scan as the reference. The secondary end point was the trueness of intraoral and extraoral scanning with 3 different IOSs, assessed by aligning the 3 consecutive scans in each group with the virtual reference model.
Statistical analysis of the median deviations for each test group was performed by using a mixed linear regression model and Bonferroni correction after pairwise comparisons (Stata Statistical Software: StataCorp. 2017. Release 15; StataCorp LLC) (α=.05).
Results
A total of 11 participants were enrolled in the study. Three extraoral and 3 intraoral scans with each of the 3 IOSs resulted in 9 extraoral scans and 9 intraoral scans per participant.
The median precision of extraoral scanning of short spans was 29 μm (TD), 22 μm (TR), and 23 μm (OC) (Fig. 3, Table 3). The median precision of extraoral scanning of long spans was 165 μm (TD), 81 μm (TR), and 103 μm (OC) (Fig. 3, Table 3).
Figure 3Box plot for precision of TD, TR, and OC when used for extraoral scanning. Values of deviation given in μm. Boxplots do not show outliers, that is, whiskers restricted to maximum length of 1.5 times interquartile range.
The median precision for intraoral scanning of short spans was 31 μm (TD), 23 μm (TR), 43 μm (OC) (Fig. 4, Table 3). The median precision of intraoral scanning of long spans was 153 μm (TD), 80 μm (TR), and 198 μm (OC) (Fig. 4, Table 3).
Figure 4Box plot for precision of IOSs TD, TR, and OC when used for intraoral scanning. Values of deviation given in μm. Boxplots do not show outliers, that is, whiskers restricted to maximum length of 1.5 times interquartile range.
The median trueness of extraoral scanning of short spans was 40 μm for TD, 28 μm for TR, and 36 μm when using OC (Table 3). The median trueness of extraoral scanning of long spans was 581 μm (95th percentile: 1387 μm) (TD), 132 μm (TR), and 118 μm (OC) (Table 3).
The median trueness of intraoral scanning of short spans was 47 μm (TD), 38 μm (TR), and 45 μm (OC) (Fig. 5, Table 3). The median trueness of intraoral scanning of long spans was 433 μm (TD), 147 μm (TR), and 198 μm (OC) (Fig. 6, Table 3).
Figure 5Trueness of intraoral scanning of TD, TR, and OC for short span. Values of deviation given in μm. Boxplots do not show outliers, that is, whiskers restricted to maximum length of 1.5 times interquartile range.
Figure 6Trueness of intraoral scanning of TD, TR, and OC for complete-arch. Values of deviation given in μm. Boxplots do not show outliers, that is, whiskers restricted to maximum length of 1.5 times interquartile range.
The precision of the IOSs was significantly different for extraoral and intraoral long-span scans. Pairwise comparison showed significantly higher precision for extraoral long-span scans with TR compared with TD (P=.005) and intraoral long-span scans with TR compared with OC (P<.001) and TR compared with TD (P<.001). The intraoral and extraoral precision of short-span scans was significantly higher than for long-span scans for all scanners (P<.001).
The trueness of the IOSs was significantly different except for intraoral short-span scans (P=.87). Pairwise comparison showed lower trueness of TD compared with TR (P<.001) and OC (P<.001) for extraoral and intraoral long-span scans, and extraoral short-span scans. The intraoral and extraoral trueness of short-span scans was significantly higher than for long-span scans for all scanners (P<.001).
Discussion
The results of the present clinical study supported the hypotheses that the accuracy of IOS would be lower when used in the oral cavity and that short-span scans would be acquired with higher precision and trueness than complete-arch scans. The accuracy of intraoral scans in vivo was significantly lower than for extraoral scans using 3 different intraoral scanning devices. The authors are unaware of a previous study that has investigated the precision and trueness of multiple IOSs in vivo. Median deviations of the trueness of long-span intraoral scans in vivo totaled 433 μm. Partially edentulous participants were enrolled in this study, as the experimental appliance required sufficient space for the arrangement of scan objects within the dental arch. Denture base material with a dull surface was chosen based on Schnuth and Buerakov
Evaluation of the trueness and precision of complete arch digital impressions on a human maxilla using seven different intraoral digital impression systems and a laboratory scanner.
The precision of long-span scans was lower in this study compared with the previous studies, presumably because of the method of alignment. The reported precision of long-span scans in vitro was 30 to 42 μm (OC) compared with the present findings of 103 μm (OC).
The precision of long-span scans in vivo was 41 μm (TR), 46 and 71 μm (OC), and 52 μm (TD) compared with 80 μm (TR), 198 μm (OC), and 153 μm (TD) in the present study. However, the study confirmed the previous findings that long-span intraoral scans in vivo have lower precision than extraoral model scans.
An intraoral scanner incrementally acquires single images that are stitched together into a virtual model. Minor displacements in every stitching may add up to a relevant deviation.
Optical desktop scanners project a light or laser pattern on the complete object surface and acquire its reflection for virtual model creation. A subsequent stitching of single images, potentially prone to error, is therefore not necessary.
Previous studies on the trueness of scanning long spans in vitro have been inconclusive. Values between 33 and 119 μm (OC) and 32 and 70 μm (TR) have been reported.
Evaluation of the trueness and precision of complete arch digital impressions on a human maxilla using seven different intraoral digital impression systems and a laboratory scanner.
Therefore, the deviations within this study with 132 μm (TR) are higher than those previously reported and with 118 μm (OC) comparable with those of reported by previous studies.
The study found an exceptionally lower trueness for the extraoral and intraoral scanning of long spans with 1 scanner (TD) compared with the other devices. The authors are unaware of previous studies on the trueness of long-span scans with this intraoral scanner. Notably, the trueness of short-span scans was comparable with that of the other IOSs. The reasons for the unexceptional high deviations (median 581 for trueness extraoral/433 μm for trueness intraoral) could be explained by the design of the experimental appliance. The respective scanner might not adequately acquire the composite design (including teeth, reference bodies, and edentulous areas) of the long span to render a complete-arch virtual model.
The trueness of IOSs in vivo has only been assessed in 2 previous studies.
used an industrial optical scanner to acquire the facial surface of the anterior teeth and premolars of the maxilla in study participants and compared them with images acquired with intraoral scans, Atieh et al
used a test object in the shape of a denture that was scanned with 1 intraoral scanner (OC) and with an optical reference scanner to create a virtual reference model. In addition to the restriction of only scanning 1 surface of anterior teeth, Nedelcu et al
reported a mean deviation between long-span intraoral scans and the reference model (trueness) of 46 μm. However, they recorded deviations in the molar areas of ≥140 μm in 60% of the intraoral scans and ≥250 μm in 28% of the intraoral scans.
Alignment of the intraoral scans was achieved with the registration of only 1 selected scan body. This allowed the consideration of spatial deviations on test surfaces (teeth). The alignment based solely on the reference bodies ensures that the error was not arbitrarily averaged over the entire scan, as this would mask the effect of the subsequent “stitching” of partial scans as performed by all 3 scanner models. This was hypothesized to lead to an increased error relative to the size of the scan area. Previous studies aligned scans using the complete model surface. The same surface was consecutively used to evaluate deviations.
This procedure masks the spatial allocation of the deviation by arbitrarily averaging the error over the entire surface and might therefore be a contributing factor to the divergent results.
A threshold for the clinically acceptable accuracy of impressions or intraoral scans is currently lacking. The marginal fit of a dental restoration could be used as a measure for acceptable accuracy. The fit of dental restorations, however, is not only dependent on the impression or scan accuracy but also on the complete workflow with impressions or scans as a first step, followed by the manufacture of the cast and the restoration and its intraoral delivery. Each step in the workflow might introduce an error that adds to the marginal discrepancies. Values for marginal discrepancies of prosthetic devices between 18 and 119 μm have been stated.
The American Dental Association states that the proper fit of a fixed prosthesis ranges from 25 to 40 μm (ADA No. 8, ADA 1970/71). The trueness of all the IOSs tested in this study was lower, with deviations well above 100 μm for quadrant and complete-arch impressions.
In this study, no different scanning paths were evaluated for the respective IOSs. However, the manufacturer recommendations for scanning paths were followed. Previous in vitro studies have shown that the scanning path does not significantly influence the accuracy of quadrant scans, that the accuracy of complete-arch scans depends on the scanning path, and that manufacturer recommendations are better than individual scanning protocols.
Furthermore, it must be taken into account that the experimental appliance did show 1 possible intraoral morphological situation in every participant.
Conclusions
Based on the findings of this clinical study, the following conclusions were drawn:
1.
Intraoral scanning showed lower accuracy compared with extraoral scanning.
2.
The accuracy of IOSs was negatively influenced by the length of the scanned distance.
3.
Intraoral scanning resulted in clinically unacceptable accuracy for virtual models of long-span scans.
Acknowledgments
The authors thank Siegbert Witkowski for his advice and support with the design and production of the experimental appliance and to Johannes Wietschorke for his support with the conduction of clinical procedures.
Accuracy trueness and precision of measurement methods and results: a basic method for the determination of repeatability and reproducibility of a standard measurement method.
Evaluation of the trueness and precision of complete arch digital impressions on a human maxilla using seven different intraoral digital impression systems and a laboratory scanner.
Supported by a grant (ORF42001) of the Oral Reconstruction Foundation, Basel, Switzerland. The funders had no role in study design, data collection, and analysis, decision to publish, or preparation of the paper.
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