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Corresponding author: Dr Kyu-Bok Lee, Department of Prosthodontics, School of Dentistry, Advanced Dental Device Development Institute, Kyungpook National University, 2177 Dalgubuldaero, Jung-gu, Daegu 41940, REPUBLIC OF KOREA
Various strategies for intraoral scanners (IOSs) can be used to scan the oral cavity. However, research on the scan range that can be clinically is lacking.
Purpose
The purpose of this in vitro study was to compare the 3-dimensional (3D) distortion of complete arch scans as part of the scan strategy and analyze the clinically recommended scan range.
Material and methods
A computer-aided design (CAD) reference model was obtained with an industrial scanner. A CAD test model was obtained by using 6 IOSs (TRIOS2, TRIOS3, CS3500, CS3600, i500, and Primescan) to apply 2 scan strategies and 2 dental laboratory scanners (DOF and E1) (N=15). All the teeth were segmented in the reference model by using 3D inspection software (Geomagic control X). The 3D analysis was performed by aligning the test model to the reference model and evaluating the root mean square values of all segmented teeth. The Mann-Whitney U-test was performed for a statistical comparison of the 2 scan strategies (α=.05), the Kruskal-Wallis test (α=.05) was used to compare the scanners, and the Mann-Whitney U-test and Bonferroni correction method were used as post hoc tests (α=.0017).
Results
The 8 scanners obtained significant differences in the root mean square values of all teeth (P<.001). The root mean square value of IOSs increased from the left maxillary second molar to the right maxillary second molar. The difference in the 2 scan strategies showed different patterns depending on the IOS.
Conclusions
Scan strategy 2 improved the accuracy of the IOSs. TRIOS2 and CS3500 are for single crowns; TRIOS3, CS3600, and i500 are for short-span prostheses; and Primescan is for long-span prostheses.
Clinical Implications
The scan strategy used in the present study improved the accuracy of complete arch scans with intraoral scanners; the TRIOS3, CS3600, and i500 increased the recommended scan range (within 100 μm) with the improved scan strategy.
The development of intraoral scanners (IOSs) has facilitated many dental applications by digitizing tasks previously performed manually through a complex process.
Accuracy of a chairside intraoral scanner compared with a laboratory scanner for the completely edentulous maxilla: an in vitro 3-dimensional comparative analysis.
Because the virtual cast is composed of the coordinates of the 3D point cloud, to evaluate the absolute distance for all point clouds in the scanned area, the 3D accuracy has been evaluated by calculating the root mean square (RMS) value.
Other previous studies have reported less accuracy than that achieved using conventional methods, with increased scanning range, such as for a complete arch prosthesis.
However, further research on this subject is required to accumulate clinical evidence.
In previous studies, because the IOS could scan only 1 or 2 teeth at a time in one direction in the oral cavity, many scan strategies for complete arch scanning have been proposed to improve accuracy.
However, the optimum scan strategy for the most accurate scan has not yet been established. The recommended scan range of an IOS for fixed dental prostheses has been reported in the first part of the present study.
However, research on the recommended scan range for a fixed dental prosthesis as part of various scan strategies and additional scanners is lacking.
The purpose of this study was to evaluate the RMS value of the complete arch scanned with 6 IOSs by using 2 scan strategies and to propose a recommended scan range for fixed dental prostheses in 6 IOSs. The null hypotheses were that the RMS value would not differ based on the scan strategy or based on the type of scanner used.
Material and methods
The sample size of this study was determined under the following conditions (N=15 per scanner; actual power=99.25%; power=99%; α=.05) by applying the results of 5 pilot experiments to a power analysis software program (G∗Power v3.1.9.2; Heinrich-Heine-Universität Düsseldorf). A reference model was fabricated by replicating a typodont (ANA-4; frasaco GmbH) with dental stone and scanned with an industrial scanner (Solutionix C500; MEDIT) with a resolution of 2×5 megapixels and a blue light-emitting diode. A computer-aided design (CAD) reference model (CRM) was obtained. Before the study, the industrial scanner was verified for accuracy to within 5 μm via calibration as per the manufacturer's recommendations.
Six IOSs (TRIOS2; 3Shape A/S, TRIOS3; 3Shape A/S, CS3500; Carestream Dental, CS3600; Carestream Dental, i500; MEDIT, and Primescan; Dentsply Sirona) and 2 dental laboratory scanners (DOF; FREEDOM HD and E1; 3Shape A/S) were used to obtain CAD test models (CTMs) (N=15). After the scanning of the left maxillary second molar was done preferentially, 2 scan strategies were applied (Fig. 1). In scan strategy 1, the occlusal and incisal surfaces from the left maxillary second molar to the right maxillary second molar were scanned, and then, the lingual and buccal surfaces were scanned (Fig. 1A). With scan strategy 2, scanning was first performed from the left maxillary second molar to the right maxillary canine (Fig. 1B) and then from the left maxillary canine to the right maxillary second molar (Fig. 1C). All the scanning procedures using 8 scanners were performed by 1 experienced investigator (K.S.) under the same scanning conditions (ambient light, temperature, and humidity). The 2 scan strategies were evaluated after the same investigator had achieved sufficient proficiency. The CTMs obtained from the 8 scanners were extracted as standard tessellation language files for the 3D analysis.
Figure 1Scan strategy. A, scan strategy 1 (1 to 3). B, C, scan strategy 2 (1 to 10).
To evaluate the recommended scan range of the fixed dental prosthesis, 3D analysis was performed by using the same method as in part 1 of the present study.
All the teeth of the complete arch were segmented in CRM by using a 3D inspection software program (Geomagic control X v2019.0.1; 3D Systems). First, CRM and CTM performed initial alignment based on all the segmented teeth of CRM, and finally, optimized alignment (best-fit alignment) was performed based only on the left maxillary second molar where the scan was first started. In the process of calculating the RMS value, the maximum deviation was designated as 1 mm for outlier handling. Based on 662 100 points (area, 2832 mm2) in all the segmented teeth of CRM, all corresponding points in the CTMs (sampling rate, 100%) were calculated. The RMS values were derived for 3D analysis as per all point clouds of the segmented teeth of CRM, and the following formula was used:
where represents the measurement point of of the CRM, is the measurement point of of the CTM, and n is the number of all points measured in each analysis. The RMS value indicated the absolute distance between the point cloud of CRM and the point cloud of CTM; thus, an RMS value close to zero indicated 3D accuracy.
The 3D comparison was shown as a color difference map at a specified range of ±1.0 mm (20 color segments) and a clinically recommended scanning range of ±100 μm (green). Each divided tooth yielded 3D analysis values. All the divided teeth were analyzed together to obtain the overall RMS values.
All the RMS values of all segmented teeth were analyzed by using a statistical software program (IBM SPSS Statistics, v25.0; IBM Corp). The Mann-Whitney U-test was performed to compare the 2 scan strategies (α=.05); the Kruskal-Wallis test (α=.05), the Mann-Whitney U-test, and the Bonferroni correction method were used as post hoc tests (α=.0017) to compare the scanners. In addition, factorial ANOVA with aligned rank transform was performed to verify the influence of the parameters (α=.05). Aligned rank transform ANOVA is a nonparametric approach to factorial ANOVA, which can analyze interaction effects.
The results of a Kruskal-Wallis test indicated a difference in the RMS values of all the teeth by scanners (P<.001, Table 1). As per the results of the post hoc test, in the left maxillary second molar, the 6 IOSs were not significantly different in terms of the RMS values of the laboratory scanners (P>.05, Table 1). From the left maxillary first molar, TRIOS2 and CS3500 were significantly different in terms of the RMS values of the laboratory scanners (P<.05, Table 1). From the right maxillary central incisor, 6 IOSs were significantly different from the RMS values of laboratory scanners (P<.05. Table 1). From the right maxillary canine to the right maxillary second molar, there was no significant difference between the Primescan and laboratory scanners (P>.05, Table 1). In terms of the overall RMS value, 6 IOSs were significantly different from the RMS values of laboratory scanners (P<.05, Table 1). As seen in Figure 2, the RMS value of IOSs increased from the left maxillary second molar to the right maxillary second molar.
Table 1Comparison of 3D complete arch displacement from first scanned tooth by using 6 intraoral scanners and 2 dental laboratory scanners
Significance determined using Kruskal-Wallis H test, P<.05. Different letters indicate significant differences among scanner groups obtained using Mann-Whitney U-test and Bonferroni correction method, P<.0017.
Significance determined using Mann-Whitney U-test, P<.05.
<.001
<.001
.412
.838
.202
.116
—
3D, 3-dimensional; IR, improvement rate; LC, left canine; LCI, left central incisor; LFM, left first molar; LFP, left first premolar; LLI, left lateral incisor; LSM, left second molar; LSP, left second premolar; RC, right canine; RCI, right central incisor; RFM, right first molar; RFP, right first premolar; RLI, right lateral incisor; RSM, right second molar; RSP, right second premolar; SS, scan strategy.
a Significance determined using Mann-Whitney U-test, P<.05.
b Significance determined using Kruskal-Wallis H test, P<.05. Different letters indicate significant differences among scanner groups obtained using Mann-Whitney U-test and Bonferroni correction method, P<.0017.
Figure 2Comparison of RMS value per tooth according to scanner. A, scan strategy 1. B, scan strategy 2. LC, left canine; LCI, left central incisor; LFM, left first molar; LFP, left first premolar; LLI, left lateral incisor; LSM, left second molar; LSP, left second premolar; RC, right canine; RCI, right central incisor; RFM, right first molar; RFP, right first premolar; RLI, right lateral incisor; RMS, root mean square; RSM, right second molar; RSP, right second premolar.
The difference in the 2 scan strategies showed different patterns depending on the IOS (Table 1, Fig. 3). TRIOS3 showed the greatest decrease in the RMS values as per the scan strategy, and the Mann-Whitney U-test found a significant difference between the 2 scan strategies for all teeth except the right maxillary central incisor (P<.05, Table 1, Fig. 3). In contrast, as per the results of the Mann-Whitney U-test, CS3600 showed no significant difference in the 2 scan strategies for all the teeth (P>.05, Table 1). As shown in Figure 3, 6 IOSs showed that the RMS value decreased in scan strategy 2 as compared with that in scan strategy 1.
Figure 3Comparison of RMS value per tooth according to scan strategy. A, TRIOS2. B, TRIOS3. C, CS3500. D, CS3600. E, i500. F, Primescan. LC, left canine; LCI, left central incisor; LFM, left first molar; LFP, left first premolar; LLI, left lateral incisor; LSM, left second molar; LSP, left second premolar; RC, right canine; RCI, right central incisor; RFM, right first molar; RFP, right first premolar; RLI, right lateral incisor; RMS, root mean square; RSM, right second molar; RSP, right second premolar.
The results of a factorial ANOVA found that 2 scan strategies, 6 IOSs, and the evaluated teeth influenced the RMS value, as indicated by the presence of interaction effects (P<.001, Table 2). Figure 4 shows the 3D distortion of the complete arch scans from the IOSs.
Table 2Result of ANOVA of scan strategy, intraoral scanner, and evaluated teeth
Figure 4Comparison of color difference map according to scanner. A, TRIOS2. B, TRIOS3. C, CS3500. D, CS3600. E, i500. F, Primescan. G, DOF. H, E1. SS, scan strategy.
The present study aimed to compare the 3D arch distortion of the IOSs as per the scan strategy and analyze the clinically recommended scan range. The null hypothesis was partially rejected as the 2 scan strategies had significant differences (P<.05; Table 1). Different patterns were identified with different IOSs (Table 1; Fig. 3). For the TRIOS3, a significant difference was found between the 2 scan strategies in all teeth (P<.05; Table 1) except for the right central incisor. However, for the CS3600, no significant differences were found between the 2 scan strategies in any of the teeth (P>.05; Table 1). The null hypothesis regarding lack of difference as per type of the scanner was rejected (P<.001; Table 1), based on no difference in the RMS values depending on the type of the scanner used. Two scan strategies, 6 IOSs, and the evaluated teeth influenced the RMS value (P<.001; Table 2), and 3D distortion was identified with the intraoral scanners from the tooth where the scan first started to the contralateral tooth (Fig. 3).
Previous studies have reported 3D arch distortion in IOSs.
evaluated accuracy by measuring the distance between cylinder landmarks. The accuracy of 5 IOSs (Omnicam Sirona, Itero Element, Planmeca Emerald, CS 3600, and TRIOS3) was evaluated, and the distortion of a complete arch scan determined. Alzahrani et al
evaluated the 3D accuracy with the scan range (sextant and quadrant) as a variable and reported differences in the results of the 2 IOSs tested (TRIOS3 and Emerald). For Emeral, the accuracy decreased as the scan range increased, but for TRIOS3, the scan range did not affect the accuracy.
However, in the present study, the accuracy deteriorated as the scan range of TRIOS3 increased (Table 1). The conflicting results may be owing to the different 3D analysis methods used. In the present study, only the tooth (left second molar) on which the scan was started were aligned, each tooth was segmented, and the RMS value was analyzed. Previous studies have reported relatively lower RMS values than those reported in the present study because the entire scanned area was aligned and subjected to 3D analyses.
The only reason the tooth (left second molar) on which the scan was started in the present study was aligned and 3D analyzed was to propose a scan range for fixed dental prostheses.
In addition, different study models were used, and the accuracy of complete arch scanning was not assessed, making it difficult to compare with the results of the present study. In part 1 of the present study,
the 3D arch distortion of the IOSs was assessed. In the present study, the 3D arch distortion of the recently released IOS (i500 and Primescan) was determined, indicating that 3D arch distortion may vary as per the type of scan strategy.
Scan accuracy for fixed dental prostheses has been assessed.
reported that IOSs influence the marginal and internal fit of fixed dental prostheses. Moreover, clinically acceptable scan accuracy to within 100 μm for fixed dental prostheses has been reported.
In the present study, TRIOS2 and CS3500 showed a clinically acceptable scan range with 3 teeth from the left second molar to the left second premolar (Table 1). TRIOS3, CS3600, and i500 showed different allowable scan ranges as per the scan strategy (Table 1). TRIOS3 had a range of 4 teeth in the scan strategy 1; however, the range of 8 teeth in scan strategy 2 increased the clinically acceptable scan range (Table 1). The CS3600 and i500 ranged from 7 teeth in scan strategy 1 to 8 teeth in scan strategy 2 (Table 1). Therefore, TRIOS3 could have the same scan range as CS3600 and i500 with scan strategy 2. Primescan showed results that were different from those with other IOSs. Primescan showed a clinically acceptable scan range of 7 teeth from the left second molar to the left central incisor (Table 1). In addition, Primescan showed an acceptable scan range of 3 teeth from the right second premolar to the right second molar (Table 1). This difference may be because the scanner software program compensates for the distortion error of the complete arch, and further research on this subject is warranted. Considering the clinically acceptable scan range, TRIOS2 and CS3500 can be recommended for single prosthesis scans; TRIOS3, CS3600, and i500 can be recommended for short-span prostheses; and Primescan can be recommended for long-span prostheses. However, additional studies are needed to verify this by fabricating actual fixed dental prostheses.
The IOS can scan only 1 or 2 teeth in 1 direction in the oral cavity; therefore, the intraoral scan of the complete arch performs the process of aligning and merging the scanned area in the software program of the IOS.
analyzed the 3D accuracy by using 3 scan strategies with 2 IOSs (i500 and TRIOS3); in the present study, 3D analysis was performed by applying the scan strategy that showed the best accuracy (Fig. 2A). Latham et al
evaluated the 3D accuracy by applying 4 scan strategies with 4 IOSs (Omnicam, Emerald, iTero Element, TRIOS3). A specific scan strategy for 4 IOSs showed the best accuracy. Therefore, in the present study, the scan strategy that showed the best accuracy in the study by Latham et al
was applied (Fig. 2B, C). Additional studies that use scan strategies other than that used in the present study are required.
Limitations of the present study included its in vitro design that was different from the actual oral environment, with different ambient light, temperature, humidity, and movement.
Clinical studies are required to overcome these limitations. The scanning strategy recommended by the manufacturer of each intraoral scanner was not applied. Primescan recommends a complete arch scanning strategy in the order of the lingual, occlusal, and buccal regions. TRIOS recommends the same scanning strategy applied in the present study (Fig. 1A). Carestream and i500 did not recommend a scanning strategy in the manufacturer's manual. Therefore, it is necessary to apply various scanning strategies in an additional study.
Conclusions
Based on the findings of this in vitro study, the following conclusions were drawn:
1.
The 2 scan strategies of 6 IOSs used in the present study produced complete arch distortion from the tooth where the scan started.
2.
Scan strategy 2 reduced the distortion of the complete arch scanned using the IOSs, and the accuracy of the complete arch in TRIOS3 was significantly improved.
3.
The recommended scanning range (within an accuracy of 100 μm) in TRIOS3, CS3600, and i500 increased with a scan strategy 2.
4.
As per the clinically acceptable scanning range for fixed dental prostheses, TRIOS2 and CS3500 are recommended for single prostheses, TRIOS3, CS3600, and i500 are recommended for short-span prostheses, and Primescan is recommended for long-span prostheses.
Acknowledgments
The authors thank the researchers in the Advanced Dental Device Development Institute, Kyungpook National University, for their time and contributions to the study.
Accuracy of a chairside intraoral scanner compared with a laboratory scanner for the completely edentulous maxilla: an in vitro 3-dimensional comparative analysis.
Supported by the Industrial Strategic Technology Development Program (10062635, New hybrid milling machine with a resolution of less than 10 μm development, using open CAD/CAM S/W integrated platforms for 1-day prosthetic treatment of 3D smart medical care system) funded by the Ministry of Trade, Industry, and Energy (MOTIE, Korea); and the industrial infrastructure program of laser industry support, which is funded by the Ministry of Trade, Industry, and Energy (MOTIE, Korea, N0000598).
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