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Comparison of digital intraoral scanner reproducibility and image trueness considering repetitive experience
RESEARCH AND EDUCATION
Comparison of digital intraoral scanner reproducibility and image trueness considering repetitive experience Jung-Hwa Lim, DDS, MSD,a Ji-Man Park, DDS, MSD, PhD,b Minji Kim, DDS, MSD, PhD,c Seong-Joo Heo, DDS, MSD, PhD,d and Ji-Yun Myung, DDS, MSDe Because computer-aided design ABSTRACT and computer-aided manuStatement of problem. Because the digital workflow can begin directly in the oral cavity, intraoral facturing (CAD-CAM) technolscanners are being adopted in dental treatments. However, studies of the relationship between the ogy was introduced to dentistry experience of the practitioner and the accuracy of impression data are needed. in the early 1980s, the in-office Purpose. The purpose of this clinical study was to investigate the effect of the experience curve on fabrication of inlays, onlays, changes in trueness when a patient’s complete dental arch is scanned. crowns, and fixed partial denMaterial and methods. Twenty dental hygienists with more than 3 years of experience in dental tures, as well as implant surgical clinical practice (group 1 had 3 to 5 years; group 2 had >6 years) were recruited to learn to operate 2 guides and immediate interim intraoral scanner systems. All learners scanned the assigned patient’s oral cavity 10 times during the implant crowns have been experience sessions. Precision was calculated as the mean deviation among all superimposition possible.1-5 Intraoral scanners combinations from the 10 scanned data sets of each learner [n=10C2=45]. Trueness was evaluated have enabled the acquisition of by superimposing the 10 consecutive intraoral scan data onto the impression scan data from each patient’s rubber impression body (n=10). The acquired images were processed and analyzed using data directly from the oral 6 a 3-dimensional analysis software. For statistical analysis, the independent 2-sample t test and cavity. repeated measures ANOVA were performed (a=.05). Studies of the accuracy of intraoral scanners have been Results. The mean precision of the Trios scanner was greater than that of the iTero (Trios, 52.30 mm; iTero, 60.46 mm; P<.01). The iTero group showed an improvement in trueness upon repeated performed with individual 7-11 experience (P<.05), whereas the Trios group did not (P>.05). In the iTero group but not in the Trios abutments, CAD-CAM group, the length of clinical experience influenced the change of trueness as a result of repeated 12-14 restorations, and quadrantexperience (P<.05). In terms of the scanned region, the results for trueness were better for the and complete-arch scans.15-27 maxillary arch than the mandibular arch with repeated scanning in the iTero group (P<.05). Among those studies, Ender Conclusions. The single-image based system required repeated learning sessions for effective et al15 compared the trueness clinical application. The newer system offered better trueness and precision and was less likely and precision of conventional to be influenced by the length of clinical career or the region being scanned. (J Prosthet Dent impressions versus CEREC 2017;-:---) Bluecam and LAVA COS optical impressions from a complete-arch model and reintraoral scanners and reported that, except for 1, system, ported similar accuracy for digital and conventional all intraoral scanning systems showed comparable levels methods. Patzelt et al20 digitized a model with 14 preof trueness and precision. Ender et al21 compared the pared abutments with an industrial scanner and 4 accuracy of 4 conventional with that of 4 digital
Supported by Industrial Technology Innovation Program funded by Ministry of Trade, Industry and Energy award 10053907. a Predoctoral student, Graduate School of Clinical Dentistry, Ewha Womans University, Seoul, Republic of Korea. b Clinical Associate Professor, Department of Prosthodontics & Dental Research Institute, Seoul National University Gwanak Dental Hospital, Seoul, Republic of Korea. c Assistant Professor, Department of Orthodontics, Graduate School of Clinical Dentistry, Ewha Womans University, Seoul, Republic of Korea. d Professor, Department of Prosthodontics & Dental Research Institute, Seoul National University, Seoul, Republic of Korea. e Predoctoral student, Graduate School of Clinical Dentistry, Ewha Womans University, Seoul, Republic of Korea.
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Clinical Implications Variations in the trueness of an intraoral scanner depend on the experience of the practitioner. This is an important clinical consideration before using digital impression making as an alternative to traditional impression making.
complete-arch impressions and concluded that the intraoral scanning systems showed higher local deviations and were no more accurate than conventional impressions. Renne et al24 evaluated the accuracy of 6 intraoral scanning systems in both quadrant and complete arches and found that the orders of trueness and precision of the systems were different for the 2 test arrangements. However, because most of these studies were based on models, they have limited application for clinical practice. Ender et al25 recently conducted an in vivo study and compared the precision of conventional with digital methods for complete-arch impressions. They concluded that the digital technique yielded higher local deviations within the complete-arch cast; however, they produced equal and higher precision than irreversible hydrocolloid materials. In these studies, the participants were already experienced with optical impression systems in order to reduce the risk of operator bias. As a result, how operator experience influences data accuracy could not be evaluated. Birnbaum and Aaronson28 stated that users of digital intraoral scanners would require time and education to develop new skills in order to provide a fast and convenient restoration with the best fit. Typically, learning curves in dental students show a trend of initial enhancement of performance ability as knowledge or skills are accumulated through repeated practice; without the addition of new skills, the curve is slowed after a certain time, despite additional practice.28,29 In the field of general medicine, many studies have investigated the learning curve of the system users when new technologies are introduced.30-34 However, few studies of the learning curve in dentistry have been done.35-38 The enhancement of skill with a certain device or clinical method can reduce chair time and increase treatment quality. Recently, studies have reported the scan time, preference, and accuracy of digital and conventional impression methods,24,39,40 but the present authors are unaware of studies confirming the level of improvement in proficiency and data accuracy as a result of repeated intraoral scanning. In this study, we investigated the precision and trueness of 2 digital intraoral scanners with different scanning systems after repeated scanning by clinically experienced dental hygienists who scanned the complete
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arch of the oral cavity. The impact of clinical experience and scanned region on the trueness of the scanned images was also evaluated. The null hypothesis was that the increase in skill from repeated scanning practice would not influence the accuracy of optical impression data. MATERIAL AND METHODS This study was approved by the institutional review board at Ewha Womans University Medical Center Mokdong Hospital. The selection criteria for the dental hygienist participants were as follows: clinical experience of 3 to 5 years (short-career group) or more than 6 years (long-career group) in the dental field; a clear understanding of the purpose of the study; voluntary consent; and ability to carry out a 4-day experience course. The 4 test individuals who took part in this study had Angle class I occlusion characterized by mild crowding (<3 mm). A total of 24 dental hygienists applied for this study. Four of them did not attend the appointment after 1 or 2 visits and were counted as dropouts as they could not complete the experience process. A total of 20 hygienists were thus included, 12 of whom belonged to the short-career group and 8 to the long-career group. The mean age of each participant group was 27.9 ±0.8 years of age for the short-career group and 31.1 ±1.7 years of age for the long-career group. All participants were female and had no experience with optical scanning. The scheduling of the participants’ first visits and the test individuals’ distribution for scanning systems were conducted based on a list which was prefabricated by a third person. At the first visit, 2 hygienists were distributed to 2 impression systems by simple randomization. The iTero (Align Technology) and Trios (3Shape) intraoral scanners were used according to the manufacturer’s instructions. The iTero uses a red laser beam and parallel confocal imaging technology to capture up to 100 000 points of laser light with an accuracy of <20 mm, according to the manufacturer’s data. The Trios scanner is operated on the confocal principle, with the video bitrate image capturing method based on the real-time rendering technique. The participants scanned at the dental clinic for 4 consecutive days. To investigate the difference in precision and trueness of the 2 intraoral scanners, the participants scanned the maxillary and mandibular arch of a single test individual 10 times each using 1 assigned scanner as follows. On the first day, either of the 2 scanners (iTero and Trios) and 1 test individual were assigned. After receiving training in the theory and practice of the assigned scanner, the participants scanned the dental arch of the assigned individual twice. On the second and third days, the dental arch of the same individual was scanned 3 times each, and on the fourth
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Table 1. Precision of scan images obtained with iTero and Trios (mm) Mean ±SD Scanner Variable
iTero (n=12)
Trios (n=8)
F
P
Average
60.5 ±32.9
52.3 ±45.5
4.414
<.001
649.6 ±239.1
451.1 ±255.1
16.792
<.001
Maximum
Statistically significant at P<.05. Precision measured by polygon deviation between 2 of 10 images, which totals 45 pairs for each participant.
day, the experiment was concluded after rescanning the same individual twice. To create reference images for the assessment of trueness, impressions of the maxillary and mandibular arch of the 4 test individuals were made with a polyether impression material (Soft Monophase; 3M ESPE), and an intaglio scan was performed using a calibrated desktop scanner (7Series; Dental Wings Inc). The scanning strategy was standardized by instructing the participants to follow the manufacturer’s recommendation. In the iTero group, parts of the dental arch were captured and superimposed to complete an image, with overlapping sufficient to ensure that the image stitching process would not generate errors. The scanning direction was to start from the left of the mandible, proceed clockwise across the complete arch one quadrant at a time, and follow by scanning for bilateral occlusion. The automatic capture function was used for continuous image acquisition. In the Trios group, the scanning direction was the same as that followed by the iTero group. For smooth recognition of the scanning area, the occlusal surface was recommended as the starting point for scanning. All images from 20 participants were transformed to the standard tessellation language (STL) file format. For the measurement of precision, the images acquired from 10 scans by each participant with the assigned intraoral scanner were paired, and the mean and maximum deviations of these 45 pairs were calculated (n=10C2=45). To compare trueness, the reference image was superimposed one by one on a total of 10 images acquired by participants in the order of scanning. Subsequently, the differences between polygons were compared, and the variations in trueness with repeated experience were assessed (n=10). After images were initially superimposed with an auto align command through the scanner inspection software (Geomagic Verify v4.1.0.0; 3D Systems Inc), irregular parts of the gingiva, including the buccal vestibule, beyond 2 to 3 mm apical from the gingival margin were cut out to make the fine registration more accurate. Trimmed images were registered again using a best-fit align command to match the more than 60 000 points composing each image. Deviations between triangular polygons formed by the points composing 2 superimposed images were computed and the distance data of all superimpositions were summarized and imported into a statistical program. Lim et al
Figure 1. Color map of precision of each scanner. Color map set from −300 to +300 mm. Color index of color map indicates increasing deviation trend toward red (+) and indicates decreasing deviation trend toward blue (−). Image shown is color map of dental arch overlapped for 2 images.
Collected data were analyzed using a statistical software (IBM SPSS Statistics v20.0; IBM Corp). To determine the differences in the deviations according to the scanner, the independent 2-sample t test was conducted to examine the significance of the precision data. For comparisons of trueness according to repeated experience, the repeated measures ANOVA was used to test for differences between the scanners, clinical experience of the participants, and the scanning region within the same scanner group. The significance of time-dependent changes and the interaction between the scanner group and time variables were examined by within-subject tests. Differences between the scanner groups were examined by between-subject tests. The significance of the 10 consecutive scans and 4 visits in the scanner group and the difference between the scanner groups at each time point were examined by post hoc Bonferroni test (a=.05 for all tests). RESULTS Results of the independent 2-sample t test showed that the precision of the participants’ skill in the Trios group was significantly better than that of the iTero group (P<.001) (Table 1; Fig. 1). Deviations between the scanned images of the dental arch and the reference image used to examine trends in the trueness of 10 consecutive scans are shown in Table 2. Significant differences in trueness were found between the scanners and with repeated experience (P=.005 and P=.047). The difference between the scanners for each scan was statistically significant, with the high mean trueness generally shown in the iTero group. Within the iTero group, the results of the repeated measures ANOVA showed that there were differences THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 2. Trueness of 10 consecutive images obtained with iTero and Trios (mm) Mean ±SD Sessions/Visits Scanner
T1
T2
T3
T4
T5
T6
F (P) T7
T8
T9
T10
Time
Group
Time×Group
iTero (n=12)
99.8 ±19.7 98.8 ±18.6 93.0 ±21.7 95.3 ±18.8 96.0 ±27.0 90.2 ±19.8 91.9 ±24.0 93.5 ±22.7 87.7 ±16.9 87.5 ±23.9 1.996 (.041) 8.713 (.005) 0.791 (.625) 1,2>6,7,9,10
Trios (n=8)
80.9 ±15.7 84.3 ±16.6 79.3 ±7.9 78.8 ±10.8 83.2 ±13.3 75.3 ±9.4 80.0 ±12.5 79.9 ±10.6 81.2 ±11.1 79.8 ±13.6 .986 (.454)
t value
3.205
2.519
2.388
3.188
1.758
2.803
1.810
2.229
1.365
1.154
1.929
P
.003
.016
.022
.003
.087
.008
.078
.032
.180
.256
.047
Visit 1
Visit 2
Visit 3
Visit 4
iTero (n=12)
99.3 ±17.6
94.8 ±18.0
91.9 ±19.5
87.6 ±18.33
5.896 (.001) 8.479 (.006) 1.944 (.126) 1>2, 3>4
Trios (n=8)
82.6 ±13.7
80.4 ±7.2
78.4 ±9.2
80.5 ±11.81
.818 (.491)
t value
3.192
3.013
2.57
1.365
4.381
P
.003
.005
.014
.180
.006
T1-T10, session number. Statistically significant at P<.05. Trueness measured by polygon deviation between reference data and intraoral scan image.
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between the first and second scans, and the sixth, seventh, eighth, ninth, and tenth scans, with a general tendency for improved trueness with repeated practice (P=.041). No similar statistically significant trend change was found in the Trios group (P=.454) (Fig. 2). Examination of the changes in trends in trueness for each of the 4 daily visits also revealed significant differences between the scanners and the visits (both, P=.006) (Table 1). In the iTero group, the trend changes in trueness were statistically significant (P<.001) and generally tended to show improvement. No statistically significant differences were found in the trends for each scanning visit in the Trios group (P=.491). Changes in trends of trueness measured at the 4 scanning visits according to clinical experience are shown in Table 3. Significant differences were found between the 2 clinical experience subgroups using the iTero scanner with repeated experience (P=.001). The shortcareer subgroup showed a statistically significant difference between the first 3 days and the fourth day, while the long-career subgroup showed a statistically significant difference between the first day and the last 3 days (P=.014 and P=.012). The mean deviation also decreased in both iTero subgroups. In the Trios group, no statistically significant difference was observed between the 2 clinical experience subgroups (Fig. 3). Changes in trends of trueness measured at the 4 scanning visits according to the scanned regions of the maxillary and mandibular arch are shown in Table 4. At each of the 4 scanning visits in the iTero group, a difference was found between the maxillary and mandibular arch, with better trueness in the maxillary arch with the repeated experience (P=.002). The trueness of the maxillary arch was improved with repeated scanning visits (P=.046). Within the Trios group, the difference between the maxillary and mandibular arch was not statistically significant, nor was the change in trueness according to each scanning visit (Fig. 4).
90 85 80 75 70 65 60
Visit 1 T1
T2
Visit 2 T3
T4
Visit 3 T5
T6
T7
Visit 4 T8
T9
T10
Session Number iTero
Trios
Figure 2. Trueness of 10 consecutive images obtained with iTero and Trios.
DISCUSSION In this study, the null hypothesis was rejected because the trueness of optical impression data showed significant improvement with repeated scanning practice in the iTero group. The clinical experience and the scanned region affected the trueness of the scanned images of the iTero digital intraoral scanner but not that of the Trios system. This study was different from other similar studies in several aspects. First, this study was conducted in the oral cavities in a dental office environment rather than on a model. Second, the relationship between the skill in scanning and the trueness of images was assessed with an experience curve. In other studies, the participants already had intraoral scanner experience,20,38 and thus the skill of the users could not be ruled out as a factor affecting the accuracy of the scanner. Last, the reference images in this study were made with impression scanning. Although model scanning is typically performed to obtain the reference image, an error is generated because
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Table 3. Trueness of images based on clinical experience according to visits (mm) Mean (±SD) Visits Scanner iTero
Trios
F (P)
Experience (y)
Visit 1
Visit 2
Visit 3
Visit 4
Time
Group
Time×Group
3-5 (n=6)
100.0 (18.1)
100.0 (18.7)
95.8 (18.0)
86.5 (15.8)
4.103 (.014) 1,2,3>4
.429 (.519)
2.119 (.106)
>6 (n=6)
98.6 (17.9)
89.6 (16.5)
87.9 (20.9)
88.6 (21.2)
4.230 (.012) 1>2,3,4
t value
.187
1.447
.998
-.278
6.182
P
.853
.162
.329
.783
.001
3-5 (n=6)
81.5 (13.4)
80.2 (7.5)
79.3 (10.2)
79.8 (11.3)
.230 (.875)
.050 (.826)
.608 (.614)
>6 (n=2)
86.1 (16.1)
81.0 (7.4)
75.8 (4.7)
82.5 (14.8)
.645 (.605)
t value
-.574
-.184
.652
-.374
1.326
P
.575
.857
.525
.714
.279
Statistically significant at P<.05.
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Trueness (µ µm)
of the expansion of the gypsum. DeLong et al5 measured the data accuracy after using a tabletop scanner to scan both the poured stone cast and the impression body from a steel standard model. They reported that the impression scan (13 ±3 mm) was nearly twice as accurate as the cast scan (24 ±2 mm). As the scope of studies on digital intraoral scanners has expanded, various methods of verifying the trueness of the digital scan image have been used. van der Meer et al16 used a cylinder to evaluate the errors in distance and angle, and Grünheid et al38 measured the distance by creating a reference ball in the model. The precision of the iTero measured in this study was 60.5 mm and slightly higher than the 50-mm figure reported by Flügge et al17 and the 36.4-mm figure reported by Ender and Mehl.21 This seems to be caused by the difference between a clinical and in vitro study. Compared with the in vivo precision of the iTero and Trios systems reported by Ender et al,25 the trueness in this study, acquired at the tenth practice where scanning experience had been accumulated, was still high. This could be explained by the fact that 2 dentists, already expert in operating the intraoral scanner, participated in the study by Ender and Mehl,21 whereas the hygienists in this study did not have any experience and came to learn. Unlike Ender et al,25 who superimposed 2 images among data sets of the same group and measured deviations to assess the precision, we superimposed scanned images onto the reference image to assess the trends in trueness.26 However, errors in the desktop scanner used to acquire the impression scan during such a process cannot be ruled out.23 When the deviation patterns were evaluated from the color map, the iTero tended to produce images that were bigger in general and had higher deviations in the molar region. Ender and Mehl21 suggested that this pattern in the iTero could be attributed to single-image stitching. In addition, the iTero scans the quadrant arch separately, and the image stitching is completed by collecting right and left quadrant images at the anterior region.
90 85 80 75 70 65 60
Visit 1 T1
T2
Visit 2 T3
T4
Visit 3 T5
T6
T7
Visit 4 T8
T9
T10
Session Number iTero
3-5 y >6 y
Trios
3-5 y >6 y
Figure 3. Trueness of 10 consecutive images based on clinical experience obtained with iTero and Trios.
Therefore, if the scanning of the anterior region is not accurate, the phenomenon of arch expansion at the posterior region is more likely to occur. For the Trios system, the range of the deviations was smaller than for the iTero. Ender et al25 reported in their in vivo study that 1 quadrant began to distort the distal-to-canine region in the video-based system, whereas local deviation was shown with increasing deformation in the distal direction in the single image-based system. For the quantified evaluation, the deviation between reference and intraoral scan data at the buccal and palatal reference points on second molars was measured in this study. From this linear measurement, the mean buccolingual displacement was 307.1 mm expansion in the iTero group and 32.7 mm expansion in the Trios group (Fig. 5). To see the trends in trueness according to the skill of the scanner user, the images acquired from the 10 scans were sequentially superimposed onto the reference image in the order of scanning. While the trueness values improved in both scanner groups, only the change in the iTero group was significant. These results may indicate
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Table 4. Trueness of images based on scanning region according to visits (mm) Mean (±SD) Visits Scanner iTero
Trios
Region
F (P)
Visit 1
Visit 2
Visit 3
Visit 4
Time
Group
Time×Group
Maxillary (n=12)
94.9 (19.7)
90.9 (21.2)
88.7 (22.4)
83.4 (15.8)
2.975 (.046) 1>2,3>4
1.434 (.244)
.075 (.973)
Mandibular (n=12)
103.8 (14.8)
98.7 (14.0)
95.00 (16.5)
91.8 (20.3)
2.779 (.056)
t−value
-1.251
-1.064
-.785
-1.136
5.659
P
.224
.299
.441
.268
.002
Maxillary (n=8)
87.7 (15.8)
80.6 (5.7)
78.4 (8.5)
82.8 (12.1)
1.584 (.223)
.777 (.393)
1.63 (.197)
Mandibular (n=8)
77.6 (9.6)
80.3 (8.9)
78.5 (10.3)
78.1 (11.9)
.352 (.788)
t value
1.547
.090
-.020
.787
.852
P
.144
.930
.984
.445
.473
Statistically significant at P<.05.
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Trueness (µ µm)
that there was a learning effect due to the repeated scanning. In contrast, while the Trios group did not show a significant difference in trueness over 10 scans, overall it offered better trueness and precision than the iTero group. From the complete-arch in vivo study by Ender et al,25 the precision of older systems like iTero was reported to be lower than with newer systems like Trios. Given these results, the Trios appears to require less training time than the iTero, making it easier to use in clinical practice. However, the experience accumulated in the process of adapting to the machine through repeated experience might effect a change in the trueness potential of the iTero. The length of clinical experience influenced the effect of repeated experience that was evident only in the iTero group. The long-career group showed a significantly reduced deviations in the first scanning on the second day and a relatively stable curve up to the final scanning. However, the short-career group showed a trend in trueness that consistently improved with practice, without a sharp decrease in deviations. Although this was not analyzed statistically, it may be that the longcareer group had become skilled relatively faster than the short-career group. In studies of actual patients rather than models, clinical experience in terms of the ability to react to the patients’ movements, saliva or tongue, which might interfere with the scanning, must be considered as a factor affecting trueness.25 The ability to react to the patient-specific factors is stronger in the long-career group, and it seems that this aspect was expressed in the system with the higher level of difficulty. On the contrary, depending on the kind of system, the experienced operator also can produce less accurate digital impressions if he or she rotates the scanner frequently to capture a wider area.3 Comparing the scanned regions, a statistically significant difference was also found between the maxillary and mandibular arch in the iTero group. When the scanning experience of the participants had accumulated the most, trueness was better in the maxillary arch than
95 90 85 80 75 70 65 60
Visit 1 T1
T2
Visit 2 T3
T4
Visit 3 T5
T6
T7
Visit 4 T8
T9
T10
Session Number iTero
Lower arch Upper arch
Trios
Lower arch Upper arch
Figure 4. Trueness of 10 consecutive images based on scanning region obtained with iTero and Trios.
the mandibular arch. Conversely, Flügge et al17 reported better trueness for the mandibular arch, whereas Ender reported that the deviations were larger in the maxillary arch because the labial surface of the maxillary anterior teeth had to be approached at more difficult angles.18 The different outcomes produced with iTero in this study might have been due to the participants’ reported difficulty with the approach of the tip to the lingual surface of the mandibular anterior teeth; here the size of the arch was relatively smaller than that of the maxillary arch because of the volume of the wand. The shape of an experience curve that graphically represents the phenomenon of learning is diverse, but normally there comes a point where the accumulation of knowledge and skills obtained through experience reveals learner competence.29 While the results generally showed a declining trend in the curve of the graph, the learners in this study performed too few scans to determine the point where the curve might have become gradual. The participants only worked with one of the
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CONCLUSIONS Based on the findings of this clinical study, the following conclusions were drawn:
A
1. Repeated experience, clinical experience, and the scanned region all affected the trueness of the scanned images of the single-image-based system. Consequently, users require repeated experience opportunities for effective clinical application. 2. The video-based system offered better trueness and precision and was less likely to be influenced by the length of clinical career and the region being scanned. 3. Although clinicians cannot afford to introduce every latest device, newer versions seem to be more accurate and could more easily be applied to clinical practice. REFERENCES
Second Molar Deviation at Both Sides (µm)
B 300
Right max 2nd molar Buccal Right max 2nd molar Palatal Left max 2nd molar Palatal Left max 2nd molar Buccal
250 200 150 100 50 0 –50
iTero
Trios
–100 –150
C
Figure 5. Linear measurement for evaluation of arch distortion. A, Reference points at buccal and palatal surfaces of maxillary second molars. B, Displacement between reference and intraoral scan data measured at reference points. C, Amount and direction of linear displacement in buccolingual direction shows aspect of distortion.
scanning systems, and the test was not repeated by switching the participants and the systems. As a result, a risk of selection bias is possible. In addition, our attempt to investigate the influence of the user’s clinical experience on scanner trueness by dividing the participants into long and short-career subgroups was hampered by the participants’ overall inexperience with digital intraoral scanners, which are currently not widely available. Future studies should refine the skill level of the selected participants and gather a larger dataset of user experience opportunities to find the point at which learners become competent with digital intraoral scanners. Lim et al
1. Andreiotelli M, Kamposiora P, Papavasiliou G. Digital data management for CAD/CAM technology. An update of current systems. Eur J Prosthodont Restor Dent 2013;21:9-15. 2. van Steenberghe D, Glauser R, Blomback U, Andersson M, Schutyser F, Pettersson A, et al. A computed tomographic scan-derived customized surgical template and fixed prosthesis for flapless surgery and immediate loading of implants in fully edentulous maxillae: a prospective multicenter study. Clin Implant Dent Relat Res 2005;7(suppl 1):S111-20. 3. Giménez B, Özcan M, Martínez-Rus F, Pradíes G. Accuracy of a digital impression system based on active triangulation technology with blue light for implants: effect of clinically relevant parameters. Implant Dent 2015;24: 498-504. 4. Strub JR, Rekow ED, Witkowski S. Computer-aided design and fabrication of dental restorations: current systems and future possibilities. J Am Dent Assoc 2006;137:1289-96. 5. DeLong R, Heinzen M, Hodges JS, Ko CC, Douglas WH. Accuracy of a system for creating 3D computer models of dental arches. J Dent Res 2003;82:438-42. 6. Schoenbaum TR. Decoding CAD/CAM and digital impression units. Dent Today 2010;29:144-5. 7. Nedelcu RG, Persson AS. Scanning accuracy and precision in 4 intraoral scanners: an in vitro comparison based on 3-dimensional analysis. J Prosthet Dent 2014;112:1461-71. 8. Kim SY, Kim MJ, Han JS, Yeo IS, Lim YJ, Kwon HB. Accuracy of dies captured by an intraoral digital impression system using parallel confocal imaging. Int J Prosthodont 2013;26:161-3. 9. Lee JJ, Jeong ID, Park JY, Jeon JH, Kim JH, Kim WC. Accuracy of singleabutment digital cast obtained using intraoral and cast scanners. J Prosthet Dent 2017;117:253-9. 10. Carbajal Mejía JB, Wakabayashi K, Nakamura T, Yatani H. Influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions. J Prosthet Dent 2017 Feb 17. doi: 10.1016/j.prosdent.2016.10.021. [Epub ahead of print]. 11. Park JM. Comparative analysis on reproducibility among 5 intraoral scanners: sectional analysis according to restoration type and preparation outline form. J Adv Prosthodont 2016;8:354-62. 12. Seelbach P, Brueckel C, Wöstmann B. Accuracy of digital and conventional impression techniques and workflow. Clin Oral Investig 2013;17: 1759-64. 13. Brawek PK, Wolfart S, Endres L, Kirsten A, Reich S. The clinical accuracy of single crowns exclusively fabricated by digitalworkflowethe comparison of two systems. Clin Oral Investig 2013;17:2119-25. 14. Sakornwimon N, Leevailoj C. Clinical marginal fit of zirconia crowns and patients’ preferences for impression techniques using intraoral digital scanner versus polyvinyl siloxane material. J Prosthet Dent 2017 Feb 17. doi: 10.1016/ j.prosdent.2016.10.019. [Epub ahead of print]. 15. Ender A, Mehl A. Full arch scans: conventional versus digital impressions: an in-vitro study. Int J Comput Dent 2011;14:11-21. 16. van der Meer WJ, Andriessen FS, Wismeijer D, Ren Y. Application of intraoral dental scanners in the digital workflow of implantology. PLoS One 2012;7:e43312. 17. Flügge TV, Schlager S, Nelson K, Nahles S, Metzger MC. Precision of intraoral digital dental impressions with iTero and extraoral digitization with
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the iTero and a model scanner. Am J Orthod Dentofacial Orthop 2013;144: 471-8. Ender A, Mehl A. Influence of scanning strategies on the accuracy of digital intraoral scanning systems. Int J Comput Dent 2013;16:11-21. Ender A, Mehl A. Accuracy of complete-arch dental impressions: a new method of measuring trueness and precision. J Prosthet Dent 2013;109:121-8. Patzelt SB, Emmanouilidi A, Stampf S, Strub JR, Att W. Accuracy of full-arch scans using intraoral scanners. Clin Oral Investig 2014;18:1687-94. Ender A, Mehl A. In-vitro evaluation of the accuracy of conventional and digital methods of obtaining full-arch dental impressions. Quintessence Int 2015;46:9-17. Su TS, Sun J. Comparison of repeatability between intraoral digital scanner and extraoral digital scanner: An in-vitro study. J Prosthodont Res 2015;59: 236-42. Anh JW, Park JM, Chun YS, Kim M, Kim M. A comparison of the precision of three-dimensional images acquired by 2 digital intraoral scanners: effects of tooth irregularity and scanning direction. Korean J Orthod 2016;46:3-12. Renne W, Ludlow M, Fryml J, Schurch Z, Mennito A, Kessler R, et al. Evaluation of the accuracy of 7 digital scanners: An in vitro analysis based on 3-dimensional comparisons. J Prosthet Dent 2016 Dec 23. doi: 10.1016/j. prosdent.2016.09.024. [Epub ahead of print]. Ender A, Attin T, Mehl A. In vivo precision of conventional and digital methods of obtaining complete-arch dental impressions. J Prosthet Dent 2016;115:313-20. Ender A, Zimmermann M, Attin T, Mehl A. In vivo precision of conventional and digital methods for obtaining quadrant dental impressions. Clin Oral Investig 2016;20:1495-504. Atieh MA, Ritter AV, Ko CC, Duqum I. Accuracy evaluation of intraoral optical impressions: A clinical study using a reference appliance. J Prosthet Dent 2017 Feb 17. doi: 10.1016/j.prosdent.2016.10.022. [Epub ahead of print]. Birnbaum NS, Aaronson HB. Dental impressions using 3D digital scanners: virtual becomes reality. Compend Contin Educ Dent 2008;29:498-505. Chambers D. Learning curves: what do dental students learn from repeated practice of clinical procedures? J Dent Educ 2012;76:291-302. Sammon J, Perry A, Beaule L, Kinkead T, Clark D, Hansen M. Robot-assisted radical prostatectomy: learning rate analysis as an objective measure of the acquisition of surgical skill. BJU Int 2010;106:855-60. Feldman LS, Cao J, Andalib A, Fraser S, Fried GM. A method to characterize the learning curve for performance of a fundamental laparoscopic simulator task: defining “learning plateau” and “learning rate.” Surgery 2009;146:381-6. Blavier A, Gaudissart Q, Cadiere GB, Nyssen AS. Comparison of learning curves and skill transfer between classical and robotic laparoscopy according
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to the viewing conditions: implications for training. Am J Surg 2007;194: 115-21. Yohannes P, Rotariu P, Pinto P, Smith AD, Lee BR. Comparison of robotic versus laparoscopic skills: is there a difference in the learning curve? Urology 2002;60:39-45. Prasad SM, Maniar HS, Soper NJ, Damiano RJ Jr, Klingensmith ME. The effect of robotic assistance on learning curves for basic laparoscopic skills. Am J Surg 2002;183:702-7. Gurusamy K, Aggarwal R, Palanivelu L, Davidson BR. Systematic review of randomized controlled trials on the effectiveness of virtual reality training for laparoscopic surgery. Br J Surg 2008;95:1088-97. Spaun GO, Zheng B, Martinec DV, Arnold BN, Swanstrom LL. A comparison of early learning curves for complex bimanual coordination with open, laparoscopic, and flexible endoscopic instrumentation. Surg Endosc 2010;24: 2145-55. Kim J, Park JM, Kim M, Heo SJ, Shin IH, Kim M. Comparison of experience curves between two 3-dimensional intraoral scanners. J Prosthet Dent 2016;116:221-30. Grünheid T, McCarthy SD, Larson BE. Clinical use of a direct chairside oral scanner: an assessment of accuracy, time, and patient acceptance. Am J Orthod Dentofacial Orthop 2014;146:673-82. Yuzbasioglu E, Kurt H, Turunc R, Bilir H. Comparison of digital and conventional impression techniques: evaluation of patients’ perception, treatment comfort, effectiveness and clinical outcomes. BMC Oral Health 2014;14:10. Lee SJ, Gallucci GO. Digital vs. conventional implant impressions: efficiency outcomes. Clin Oral Implants Res 2013;24:111-5.
Corresponding author: Dr Ji-Man Park Department of Prosthodontics Dental Research Institute Seoul National University Gwanak Dental Hospital 1 Gwanak-ro, Gwanak-gu, Seoul REPUBLIC OF KOREA Email: [email protected] Acknowledgments The authors thank Dr Im Hee Shin for providing statistical analysis for this study. Copyright © 2017 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Comparison of digital intraoral scanner reproducibility and image trueness considering repetitive experience Jung-Hwa Lim, DDS, MSD,a Ji-Man Park, DDS, MSD, PhD,b Minji Kim, DDS, MSD, PhD,c Seong-Joo Heo, DDS, MSD, PhD,d and Ji-Yun Myung, DDS, MSDe Because computer-aided design ABSTRACT and computer-aided manuStatement of problem. Because the digital workflow can begin directly in the oral cavity, intraoral facturing (CAD-CAM) technolscanners are being adopted in dental treatments. However, studies of the relationship between the ogy was introduced to dentistry experience of the practitioner and the accuracy of impression data are needed. in the early 1980s, the in-office Purpose. The purpose of this clinical study was to investigate the effect of the experience curve on fabrication of inlays, onlays, changes in trueness when a patient’s complete dental arch is scanned. crowns, and fixed partial denMaterial and methods. Twenty dental hygienists with more than 3 years of experience in dental tures, as well as implant surgical clinical practice (group 1 had 3 to 5 years; group 2 had >6 years) were recruited to learn to operate 2 guides and immediate interim intraoral scanner systems. All learners scanned the assigned patient’s oral cavity 10 times during the implant crowns have been experience sessions. Precision was calculated as the mean deviation among all superimposition possible.1-5 Intraoral scanners combinations from the 10 scanned data sets of each learner [n=10C2=45]. Trueness was evaluated have enabled the acquisition of by superimposing the 10 consecutive intraoral scan data onto the impression scan data from each patient’s rubber impression body (n=10). The acquired images were processed and analyzed using data directly from the oral 6 a 3-dimensional analysis software. For statistical analysis, the independent 2-sample t test and cavity. repeated measures ANOVA were performed (a=.05). Studies of the accuracy of intraoral scanners have been Results. The mean precision of the Trios scanner was greater than that of the iTero (Trios, 52.30 mm; iTero, 60.46 mm; P<.01). The iTero group showed an improvement in trueness upon repeated performed with individual 7-11 experience (P<.05), whereas the Trios group did not (P>.05). In the iTero group but not in the Trios abutments, CAD-CAM group, the length of clinical experience influenced the change of trueness as a result of repeated 12-14 restorations, and quadrantexperience (P<.05). In terms of the scanned region, the results for trueness were better for the and complete-arch scans.15-27 maxillary arch than the mandibular arch with repeated scanning in the iTero group (P<.05). Among those studies, Ender Conclusions. The single-image based system required repeated learning sessions for effective et al15 compared the trueness clinical application. The newer system offered better trueness and precision and was less likely and precision of conventional to be influenced by the length of clinical career or the region being scanned. (J Prosthet Dent impressions versus CEREC 2017;-:---) Bluecam and LAVA COS optical impressions from a complete-arch model and reintraoral scanners and reported that, except for 1, system, ported similar accuracy for digital and conventional all intraoral scanning systems showed comparable levels methods. Patzelt et al20 digitized a model with 14 preof trueness and precision. Ender et al21 compared the pared abutments with an industrial scanner and 4 accuracy of 4 conventional with that of 4 digital
Supported by Industrial Technology Innovation Program funded by Ministry of Trade, Industry and Energy award 10053907. a Predoctoral student, Graduate School of Clinical Dentistry, Ewha Womans University, Seoul, Republic of Korea. b Clinical Associate Professor, Department of Prosthodontics & Dental Research Institute, Seoul National University Gwanak Dental Hospital, Seoul, Republic of Korea. c Assistant Professor, Department of Orthodontics, Graduate School of Clinical Dentistry, Ewha Womans University, Seoul, Republic of Korea. d Professor, Department of Prosthodontics & Dental Research Institute, Seoul National University, Seoul, Republic of Korea. e Predoctoral student, Graduate School of Clinical Dentistry, Ewha Womans University, Seoul, Republic of Korea.
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Clinical Implications Variations in the trueness of an intraoral scanner depend on the experience of the practitioner. This is an important clinical consideration before using digital impression making as an alternative to traditional impression making.
complete-arch impressions and concluded that the intraoral scanning systems showed higher local deviations and were no more accurate than conventional impressions. Renne et al24 evaluated the accuracy of 6 intraoral scanning systems in both quadrant and complete arches and found that the orders of trueness and precision of the systems were different for the 2 test arrangements. However, because most of these studies were based on models, they have limited application for clinical practice. Ender et al25 recently conducted an in vivo study and compared the precision of conventional with digital methods for complete-arch impressions. They concluded that the digital technique yielded higher local deviations within the complete-arch cast; however, they produced equal and higher precision than irreversible hydrocolloid materials. In these studies, the participants were already experienced with optical impression systems in order to reduce the risk of operator bias. As a result, how operator experience influences data accuracy could not be evaluated. Birnbaum and Aaronson28 stated that users of digital intraoral scanners would require time and education to develop new skills in order to provide a fast and convenient restoration with the best fit. Typically, learning curves in dental students show a trend of initial enhancement of performance ability as knowledge or skills are accumulated through repeated practice; without the addition of new skills, the curve is slowed after a certain time, despite additional practice.28,29 In the field of general medicine, many studies have investigated the learning curve of the system users when new technologies are introduced.30-34 However, few studies of the learning curve in dentistry have been done.35-38 The enhancement of skill with a certain device or clinical method can reduce chair time and increase treatment quality. Recently, studies have reported the scan time, preference, and accuracy of digital and conventional impression methods,24,39,40 but the present authors are unaware of studies confirming the level of improvement in proficiency and data accuracy as a result of repeated intraoral scanning. In this study, we investigated the precision and trueness of 2 digital intraoral scanners with different scanning systems after repeated scanning by clinically experienced dental hygienists who scanned the complete
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arch of the oral cavity. The impact of clinical experience and scanned region on the trueness of the scanned images was also evaluated. The null hypothesis was that the increase in skill from repeated scanning practice would not influence the accuracy of optical impression data. MATERIAL AND METHODS This study was approved by the institutional review board at Ewha Womans University Medical Center Mokdong Hospital. The selection criteria for the dental hygienist participants were as follows: clinical experience of 3 to 5 years (short-career group) or more than 6 years (long-career group) in the dental field; a clear understanding of the purpose of the study; voluntary consent; and ability to carry out a 4-day experience course. The 4 test individuals who took part in this study had Angle class I occlusion characterized by mild crowding (<3 mm). A total of 24 dental hygienists applied for this study. Four of them did not attend the appointment after 1 or 2 visits and were counted as dropouts as they could not complete the experience process. A total of 20 hygienists were thus included, 12 of whom belonged to the short-career group and 8 to the long-career group. The mean age of each participant group was 27.9 ±0.8 years of age for the short-career group and 31.1 ±1.7 years of age for the long-career group. All participants were female and had no experience with optical scanning. The scheduling of the participants’ first visits and the test individuals’ distribution for scanning systems were conducted based on a list which was prefabricated by a third person. At the first visit, 2 hygienists were distributed to 2 impression systems by simple randomization. The iTero (Align Technology) and Trios (3Shape) intraoral scanners were used according to the manufacturer’s instructions. The iTero uses a red laser beam and parallel confocal imaging technology to capture up to 100 000 points of laser light with an accuracy of <20 mm, according to the manufacturer’s data. The Trios scanner is operated on the confocal principle, with the video bitrate image capturing method based on the real-time rendering technique. The participants scanned at the dental clinic for 4 consecutive days. To investigate the difference in precision and trueness of the 2 intraoral scanners, the participants scanned the maxillary and mandibular arch of a single test individual 10 times each using 1 assigned scanner as follows. On the first day, either of the 2 scanners (iTero and Trios) and 1 test individual were assigned. After receiving training in the theory and practice of the assigned scanner, the participants scanned the dental arch of the assigned individual twice. On the second and third days, the dental arch of the same individual was scanned 3 times each, and on the fourth
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Table 1. Precision of scan images obtained with iTero and Trios (mm) Mean ±SD Scanner Variable
iTero (n=12)
Trios (n=8)
F
P
Average
60.5 ±32.9
52.3 ±45.5
4.414
<.001
649.6 ±239.1
451.1 ±255.1
16.792
<.001
Maximum
Statistically significant at P<.05. Precision measured by polygon deviation between 2 of 10 images, which totals 45 pairs for each participant.
day, the experiment was concluded after rescanning the same individual twice. To create reference images for the assessment of trueness, impressions of the maxillary and mandibular arch of the 4 test individuals were made with a polyether impression material (Soft Monophase; 3M ESPE), and an intaglio scan was performed using a calibrated desktop scanner (7Series; Dental Wings Inc). The scanning strategy was standardized by instructing the participants to follow the manufacturer’s recommendation. In the iTero group, parts of the dental arch were captured and superimposed to complete an image, with overlapping sufficient to ensure that the image stitching process would not generate errors. The scanning direction was to start from the left of the mandible, proceed clockwise across the complete arch one quadrant at a time, and follow by scanning for bilateral occlusion. The automatic capture function was used for continuous image acquisition. In the Trios group, the scanning direction was the same as that followed by the iTero group. For smooth recognition of the scanning area, the occlusal surface was recommended as the starting point for scanning. All images from 20 participants were transformed to the standard tessellation language (STL) file format. For the measurement of precision, the images acquired from 10 scans by each participant with the assigned intraoral scanner were paired, and the mean and maximum deviations of these 45 pairs were calculated (n=10C2=45). To compare trueness, the reference image was superimposed one by one on a total of 10 images acquired by participants in the order of scanning. Subsequently, the differences between polygons were compared, and the variations in trueness with repeated experience were assessed (n=10). After images were initially superimposed with an auto align command through the scanner inspection software (Geomagic Verify v4.1.0.0; 3D Systems Inc), irregular parts of the gingiva, including the buccal vestibule, beyond 2 to 3 mm apical from the gingival margin were cut out to make the fine registration more accurate. Trimmed images were registered again using a best-fit align command to match the more than 60 000 points composing each image. Deviations between triangular polygons formed by the points composing 2 superimposed images were computed and the distance data of all superimpositions were summarized and imported into a statistical program. Lim et al
Figure 1. Color map of precision of each scanner. Color map set from −300 to +300 mm. Color index of color map indicates increasing deviation trend toward red (+) and indicates decreasing deviation trend toward blue (−). Image shown is color map of dental arch overlapped for 2 images.
Collected data were analyzed using a statistical software (IBM SPSS Statistics v20.0; IBM Corp). To determine the differences in the deviations according to the scanner, the independent 2-sample t test was conducted to examine the significance of the precision data. For comparisons of trueness according to repeated experience, the repeated measures ANOVA was used to test for differences between the scanners, clinical experience of the participants, and the scanning region within the same scanner group. The significance of time-dependent changes and the interaction between the scanner group and time variables were examined by within-subject tests. Differences between the scanner groups were examined by between-subject tests. The significance of the 10 consecutive scans and 4 visits in the scanner group and the difference between the scanner groups at each time point were examined by post hoc Bonferroni test (a=.05 for all tests). RESULTS Results of the independent 2-sample t test showed that the precision of the participants’ skill in the Trios group was significantly better than that of the iTero group (P<.001) (Table 1; Fig. 1). Deviations between the scanned images of the dental arch and the reference image used to examine trends in the trueness of 10 consecutive scans are shown in Table 2. Significant differences in trueness were found between the scanners and with repeated experience (P=.005 and P=.047). The difference between the scanners for each scan was statistically significant, with the high mean trueness generally shown in the iTero group. Within the iTero group, the results of the repeated measures ANOVA showed that there were differences THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 2. Trueness of 10 consecutive images obtained with iTero and Trios (mm) Mean ±SD Sessions/Visits Scanner
T1
T2
T3
T4
T5
T6
F (P) T7
T8
T9
T10
Time
Group
Time×Group
iTero (n=12)
99.8 ±19.7 98.8 ±18.6 93.0 ±21.7 95.3 ±18.8 96.0 ±27.0 90.2 ±19.8 91.9 ±24.0 93.5 ±22.7 87.7 ±16.9 87.5 ±23.9 1.996 (.041) 8.713 (.005) 0.791 (.625) 1,2>6,7,9,10
Trios (n=8)
80.9 ±15.7 84.3 ±16.6 79.3 ±7.9 78.8 ±10.8 83.2 ±13.3 75.3 ±9.4 80.0 ±12.5 79.9 ±10.6 81.2 ±11.1 79.8 ±13.6 .986 (.454)
t value
3.205
2.519
2.388
3.188
1.758
2.803
1.810
2.229
1.365
1.154
1.929
P
.003
.016
.022
.003
.087
.008
.078
.032
.180
.256
.047
Visit 1
Visit 2
Visit 3
Visit 4
iTero (n=12)
99.3 ±17.6
94.8 ±18.0
91.9 ±19.5
87.6 ±18.33
5.896 (.001) 8.479 (.006) 1.944 (.126) 1>2, 3>4
Trios (n=8)
82.6 ±13.7
80.4 ±7.2
78.4 ±9.2
80.5 ±11.81
.818 (.491)
t value
3.192
3.013
2.57
1.365
4.381
P
.003
.005
.014
.180
.006
T1-T10, session number. Statistically significant at P<.05. Trueness measured by polygon deviation between reference data and intraoral scan image.
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Trueness (µm)
between the first and second scans, and the sixth, seventh, eighth, ninth, and tenth scans, with a general tendency for improved trueness with repeated practice (P=.041). No similar statistically significant trend change was found in the Trios group (P=.454) (Fig. 2). Examination of the changes in trends in trueness for each of the 4 daily visits also revealed significant differences between the scanners and the visits (both, P=.006) (Table 1). In the iTero group, the trend changes in trueness were statistically significant (P<.001) and generally tended to show improvement. No statistically significant differences were found in the trends for each scanning visit in the Trios group (P=.491). Changes in trends of trueness measured at the 4 scanning visits according to clinical experience are shown in Table 3. Significant differences were found between the 2 clinical experience subgroups using the iTero scanner with repeated experience (P=.001). The shortcareer subgroup showed a statistically significant difference between the first 3 days and the fourth day, while the long-career subgroup showed a statistically significant difference between the first day and the last 3 days (P=.014 and P=.012). The mean deviation also decreased in both iTero subgroups. In the Trios group, no statistically significant difference was observed between the 2 clinical experience subgroups (Fig. 3). Changes in trends of trueness measured at the 4 scanning visits according to the scanned regions of the maxillary and mandibular arch are shown in Table 4. At each of the 4 scanning visits in the iTero group, a difference was found between the maxillary and mandibular arch, with better trueness in the maxillary arch with the repeated experience (P=.002). The trueness of the maxillary arch was improved with repeated scanning visits (P=.046). Within the Trios group, the difference between the maxillary and mandibular arch was not statistically significant, nor was the change in trueness according to each scanning visit (Fig. 4).
90 85 80 75 70 65 60
Visit 1 T1
T2
Visit 2 T3
T4
Visit 3 T5
T6
T7
Visit 4 T8
T9
T10
Session Number iTero
Trios
Figure 2. Trueness of 10 consecutive images obtained with iTero and Trios.
DISCUSSION In this study, the null hypothesis was rejected because the trueness of optical impression data showed significant improvement with repeated scanning practice in the iTero group. The clinical experience and the scanned region affected the trueness of the scanned images of the iTero digital intraoral scanner but not that of the Trios system. This study was different from other similar studies in several aspects. First, this study was conducted in the oral cavities in a dental office environment rather than on a model. Second, the relationship between the skill in scanning and the trueness of images was assessed with an experience curve. In other studies, the participants already had intraoral scanner experience,20,38 and thus the skill of the users could not be ruled out as a factor affecting the accuracy of the scanner. Last, the reference images in this study were made with impression scanning. Although model scanning is typically performed to obtain the reference image, an error is generated because
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Table 3. Trueness of images based on clinical experience according to visits (mm) Mean (±SD) Visits Scanner iTero
Trios
F (P)
Experience (y)
Visit 1
Visit 2
Visit 3
Visit 4
Time
Group
Time×Group
3-5 (n=6)
100.0 (18.1)
100.0 (18.7)
95.8 (18.0)
86.5 (15.8)
4.103 (.014) 1,2,3>4
.429 (.519)
2.119 (.106)
>6 (n=6)
98.6 (17.9)
89.6 (16.5)
87.9 (20.9)
88.6 (21.2)
4.230 (.012) 1>2,3,4
t value
.187
1.447
.998
-.278
6.182
P
.853
.162
.329
.783
.001
3-5 (n=6)
81.5 (13.4)
80.2 (7.5)
79.3 (10.2)
79.8 (11.3)
.230 (.875)
.050 (.826)
.608 (.614)
>6 (n=2)
86.1 (16.1)
81.0 (7.4)
75.8 (4.7)
82.5 (14.8)
.645 (.605)
t value
-.574
-.184
.652
-.374
1.326
P
.575
.857
.525
.714
.279
Statistically significant at P<.05.
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105 100 95
Trueness (µ µm)
of the expansion of the gypsum. DeLong et al5 measured the data accuracy after using a tabletop scanner to scan both the poured stone cast and the impression body from a steel standard model. They reported that the impression scan (13 ±3 mm) was nearly twice as accurate as the cast scan (24 ±2 mm). As the scope of studies on digital intraoral scanners has expanded, various methods of verifying the trueness of the digital scan image have been used. van der Meer et al16 used a cylinder to evaluate the errors in distance and angle, and Grünheid et al38 measured the distance by creating a reference ball in the model. The precision of the iTero measured in this study was 60.5 mm and slightly higher than the 50-mm figure reported by Flügge et al17 and the 36.4-mm figure reported by Ender and Mehl.21 This seems to be caused by the difference between a clinical and in vitro study. Compared with the in vivo precision of the iTero and Trios systems reported by Ender et al,25 the trueness in this study, acquired at the tenth practice where scanning experience had been accumulated, was still high. This could be explained by the fact that 2 dentists, already expert in operating the intraoral scanner, participated in the study by Ender and Mehl,21 whereas the hygienists in this study did not have any experience and came to learn. Unlike Ender et al,25 who superimposed 2 images among data sets of the same group and measured deviations to assess the precision, we superimposed scanned images onto the reference image to assess the trends in trueness.26 However, errors in the desktop scanner used to acquire the impression scan during such a process cannot be ruled out.23 When the deviation patterns were evaluated from the color map, the iTero tended to produce images that were bigger in general and had higher deviations in the molar region. Ender and Mehl21 suggested that this pattern in the iTero could be attributed to single-image stitching. In addition, the iTero scans the quadrant arch separately, and the image stitching is completed by collecting right and left quadrant images at the anterior region.
90 85 80 75 70 65 60
Visit 1 T1
T2
Visit 2 T3
T4
Visit 3 T5
T6
T7
Visit 4 T8
T9
T10
Session Number iTero
3-5 y >6 y
Trios
3-5 y >6 y
Figure 3. Trueness of 10 consecutive images based on clinical experience obtained with iTero and Trios.
Therefore, if the scanning of the anterior region is not accurate, the phenomenon of arch expansion at the posterior region is more likely to occur. For the Trios system, the range of the deviations was smaller than for the iTero. Ender et al25 reported in their in vivo study that 1 quadrant began to distort the distal-to-canine region in the video-based system, whereas local deviation was shown with increasing deformation in the distal direction in the single image-based system. For the quantified evaluation, the deviation between reference and intraoral scan data at the buccal and palatal reference points on second molars was measured in this study. From this linear measurement, the mean buccolingual displacement was 307.1 mm expansion in the iTero group and 32.7 mm expansion in the Trios group (Fig. 5). To see the trends in trueness according to the skill of the scanner user, the images acquired from the 10 scans were sequentially superimposed onto the reference image in the order of scanning. While the trueness values improved in both scanner groups, only the change in the iTero group was significant. These results may indicate
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Table 4. Trueness of images based on scanning region according to visits (mm) Mean (±SD) Visits Scanner iTero
Trios
Region
F (P)
Visit 1
Visit 2
Visit 3
Visit 4
Time
Group
Time×Group
Maxillary (n=12)
94.9 (19.7)
90.9 (21.2)
88.7 (22.4)
83.4 (15.8)
2.975 (.046) 1>2,3>4
1.434 (.244)
.075 (.973)
Mandibular (n=12)
103.8 (14.8)
98.7 (14.0)
95.00 (16.5)
91.8 (20.3)
2.779 (.056)
t−value
-1.251
-1.064
-.785
-1.136
5.659
P
.224
.299
.441
.268
.002
Maxillary (n=8)
87.7 (15.8)
80.6 (5.7)
78.4 (8.5)
82.8 (12.1)
1.584 (.223)
.777 (.393)
1.63 (.197)
Mandibular (n=8)
77.6 (9.6)
80.3 (8.9)
78.5 (10.3)
78.1 (11.9)
.352 (.788)
t value
1.547
.090
-.020
.787
.852
P
.144
.930
.984
.445
.473
Statistically significant at P<.05.
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110 105 100
Trueness (µ µm)
that there was a learning effect due to the repeated scanning. In contrast, while the Trios group did not show a significant difference in trueness over 10 scans, overall it offered better trueness and precision than the iTero group. From the complete-arch in vivo study by Ender et al,25 the precision of older systems like iTero was reported to be lower than with newer systems like Trios. Given these results, the Trios appears to require less training time than the iTero, making it easier to use in clinical practice. However, the experience accumulated in the process of adapting to the machine through repeated experience might effect a change in the trueness potential of the iTero. The length of clinical experience influenced the effect of repeated experience that was evident only in the iTero group. The long-career group showed a significantly reduced deviations in the first scanning on the second day and a relatively stable curve up to the final scanning. However, the short-career group showed a trend in trueness that consistently improved with practice, without a sharp decrease in deviations. Although this was not analyzed statistically, it may be that the longcareer group had become skilled relatively faster than the short-career group. In studies of actual patients rather than models, clinical experience in terms of the ability to react to the patients’ movements, saliva or tongue, which might interfere with the scanning, must be considered as a factor affecting trueness.25 The ability to react to the patient-specific factors is stronger in the long-career group, and it seems that this aspect was expressed in the system with the higher level of difficulty. On the contrary, depending on the kind of system, the experienced operator also can produce less accurate digital impressions if he or she rotates the scanner frequently to capture a wider area.3 Comparing the scanned regions, a statistically significant difference was also found between the maxillary and mandibular arch in the iTero group. When the scanning experience of the participants had accumulated the most, trueness was better in the maxillary arch than
95 90 85 80 75 70 65 60
Visit 1 T1
T2
Visit 2 T3
T4
Visit 3 T5
T6
T7
Visit 4 T8
T9
T10
Session Number iTero
Lower arch Upper arch
Trios
Lower arch Upper arch
Figure 4. Trueness of 10 consecutive images based on scanning region obtained with iTero and Trios.
the mandibular arch. Conversely, Flügge et al17 reported better trueness for the mandibular arch, whereas Ender reported that the deviations were larger in the maxillary arch because the labial surface of the maxillary anterior teeth had to be approached at more difficult angles.18 The different outcomes produced with iTero in this study might have been due to the participants’ reported difficulty with the approach of the tip to the lingual surface of the mandibular anterior teeth; here the size of the arch was relatively smaller than that of the maxillary arch because of the volume of the wand. The shape of an experience curve that graphically represents the phenomenon of learning is diverse, but normally there comes a point where the accumulation of knowledge and skills obtained through experience reveals learner competence.29 While the results generally showed a declining trend in the curve of the graph, the learners in this study performed too few scans to determine the point where the curve might have become gradual. The participants only worked with one of the
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CONCLUSIONS Based on the findings of this clinical study, the following conclusions were drawn:
A
1. Repeated experience, clinical experience, and the scanned region all affected the trueness of the scanned images of the single-image-based system. Consequently, users require repeated experience opportunities for effective clinical application. 2. The video-based system offered better trueness and precision and was less likely to be influenced by the length of clinical career and the region being scanned. 3. Although clinicians cannot afford to introduce every latest device, newer versions seem to be more accurate and could more easily be applied to clinical practice. REFERENCES
Second Molar Deviation at Both Sides (µm)
B 300
Right max 2nd molar Buccal Right max 2nd molar Palatal Left max 2nd molar Palatal Left max 2nd molar Buccal
250 200 150 100 50 0 –50
iTero
Trios
–100 –150
C
Figure 5. Linear measurement for evaluation of arch distortion. A, Reference points at buccal and palatal surfaces of maxillary second molars. B, Displacement between reference and intraoral scan data measured at reference points. C, Amount and direction of linear displacement in buccolingual direction shows aspect of distortion.
scanning systems, and the test was not repeated by switching the participants and the systems. As a result, a risk of selection bias is possible. In addition, our attempt to investigate the influence of the user’s clinical experience on scanner trueness by dividing the participants into long and short-career subgroups was hampered by the participants’ overall inexperience with digital intraoral scanners, which are currently not widely available. Future studies should refine the skill level of the selected participants and gather a larger dataset of user experience opportunities to find the point at which learners become competent with digital intraoral scanners. Lim et al
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Corresponding author: Dr Ji-Man Park Department of Prosthodontics Dental Research Institute Seoul National University Gwanak Dental Hospital 1 Gwanak-ro, Gwanak-gu, Seoul REPUBLIC OF KOREA Email: [email protected] Acknowledgments The authors thank Dr Im Hee Shin for providing statistical analysis for this study. Copyright © 2017 by the Editorial Council for The Journal of Prosthetic Dentistry.
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