Impact of diﬀerent scanning strategies on the accuracy of ... · digital impressions involving a single dental quadrant, but accuracy may be aﬀected as the scan area increases.3-6

International Journal of Computerized Dentistry 2019;22(4):307–319 307

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Leandro Passos, Sergio Meiga, Vinicius Brigagão, Alexandre Street

Impact of different scanning strategies on the accuracy of two

current intraoral scanning systems in complete-arch impressions:

an in vitro study

Abstract

Aim: To determine the scanning strategy that obtains the most accurate results for two intraoral scanners (IOS) in com-plete-arch digital impressions. Scan time was evaluated and correlated with scan strategies. Materials and method: A custom model used as the refer-ence standard was fabricated with teeth having dentin- and enamel-identical refractive indices simulating natural denti-tion. A reference scan of the custom typodont was obtained using an ATOS III Triple Scan 3D optical scanner. Two IOS set-ups – Omnicam v 5.1.0 and Primescan v 5.0.2 – were used for complete-arch scanning, each using 13 scanning strategies, obtaining 260 digital files (n = 10 per group), recording each scan time, converting all experimental scans to standard tes-sellation language (STL) format, and using a comprehensive metrology program to compare the reference standard scan with the experimental scans. Statistical analyses utilized Welch’s unequal variances t test. Results: Group M exhibited the lowest trueness and precision values (P < 0.05) for Primescan (47.5% of the average among all other groups) and the lowest trueness value (P < 0.05) for Omnicam (53.4% of the average among all other groups), where group B exhibited the lowest precision value (65.6% of the average among all other groups) with P < 0.05. Primescan featured a better trueness index (4.79 μm) than that of Omni-cam (19.13 μm), with a statistically significant difference (P < 0.00001). Primescan, group M, also featured a better pre-cision index (4.67 μm) than Omnicam, group B (16.75 μm), with a statistically significant difference (P < 0.00001). Conclusion: For both IOS systems, group M provided the low-est scanning times. For trueness and precision of com-plete-arch scans, group M was the dominant scanning strat-egy in Primescan, while there was no dominant strategy in Omnicam. Group M had the best scanning time for both IOS systems.

Keywords: CAD/CAM, intraoral scanner, digital impression, accuracy, full-arch dental impression, standard tessellation language

Introduction

In the last decade, in computer-aided design/computer-aided manufacturing (CAD/CAM) systems, an increasing number of intraoral scanners (IOS) based on different technologies have been released onto the market.1 These systems were able to promote restorations based on the three-dimensional (3D) image capturing of dental preparations, from which restor-ations were manufactured and real models were produced based on the digital capture of information by stereolithogra-phy or milling.2

Currently, the main concern with IOS systems is to increase their accuracy; the risk of errors is low for short-span digital impressions involving a single dental quadrant, but accuracy may be affected as the scan area increases.3-6 This could be a challenging situation as the prosthetic workflow is becoming increasingly faster and a great variety of diagnoses or rehabilitation options can be achieved through com-plete-arch digital impressions.

Accuracy comprises precision and trueness (ISO 5725-1).7 Precision describes the closeness of repeated measurements (samples) to each other. Therefore, a scanner with higher preci-sion indicates a more repeatable, or less disperse, and consist-ent scan. Trueness describes the extent to which a measure-ment deviates from the actual dimensions of the measured object. Therefore, a scanner with high trueness indicates that the scanner delivers a result that is close or equal to a reference recognized as the true dimensions of the object being scanned.8 Thus, accuracy is vital in ensuring the sufficient fit of dental restorations and the correct articulation of the models.

The type of material being scanned also has a significant impact on the accuracy of the scanner,9 and several studies have used metal or polymeric materials with a refractive index (RI) different from the RI of the tooth structure used for scanning.10 These studies may not provide reliable informa-tion on the accuracy of a scanner designed to be used intraorally.11,12 A recent study13 showed that, using teeth that approximate the optical properties of natural teeth, new developments in terms of hardware and/or software are capable of providing major improvements related to impres-

International Journal of Computerized Dentistry 2019;22(4):307–319308

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sion accuracy for current IOS devices. Notwithstanding, it remains unclear whether the scanning strategy of these IOS devices can affect the definitive accuracy of digital impres-sions, and if so, which strategy is best.

This study was designed to determine, in complete-arch digital impressions, the scanning strategy that obtains the most accurate result in terms of trueness and precision for two current IOS systems. The null hypothesis for each IOS sys-tem was that no difference exists between the available scan-ning strategies. Finally, this study was also designed to deter-mine the best IOS system in terms of accuracy. Here, the null hypothesis was that no difference exists between the best scanning strategies of each IOS system.

Materials and methods

A custom master model was fabricated in the following manner. Twelve maxillary typodont teeth of a dental simula-tor (Versys Flex Model; P-Oclusal Produtos Odontológicos, Brazil) were prepared for complete coverage ceramic crowns in accordance with the principles of Rosenstiel et al15 with the following specifications: a slight continuous 1 mm modified shoulder finish line that followed the scal-loped gingiva; 6 to 10 degrees combined convergence angle; a functional cusp bevel; 1.5 to 2 mm of occlusal reduction; 1 to 1.5 mm of axial reduction; and an overall rounded and smooth finish. The left and right maxillary first molars were replaced by implant analogs. A TiBase (BO 4.1 L; Dentsply Sirona, Germany) and the appropriate scan body were fixed to each analog.

Telio CAD (TC) polymethyl methacrylate (Low Translucen-cy shade A3, sizes B55 and A16 Large; Ivoclar Vivadent, Liech-tenstein) restorations and implant restorations (left and right maxillary first molar abutments) were fabricated and bonded onto the typodont and TiBases, respectively. An airborne-par-ticle abrasion (CoJet Sand, 40 μm; 3M ESPE) of the intaglio surfaces of the restorations and TiBases was performed before the cementation process using a self-etching self-ad-hesive resin cement (Calibra Universal; Dentsply Sirona)16 (Fig 1). The TC material was chosen to simulate tooth sub-stance because its RI (1.49) lies within the range of enamel (1.63) and dentin (1.54).17,18

A master reference scan of the master model was obtained from an industrial 3D scanning company (Vtech Consulting) through an ATOS II Triple Scan (GOM GmbH, Germany) and an industrial structured blue light 3D scanner, complying with ISO 12836 and shown to be accurate up to 3 μm, with a preci-sion of 2 μm for jaw-sized scans.10,17,19,20

Two IOS setups comprising hardware and software com-binations were used in this study: Omnicam, with Cerec v 5.1.0 software (Dentsply Sirona), and Primescan, with Cerec v 5.0.2 software (Dentsply Sirona). Scans of the complete arch were repeated 10 times per group (n = 10 samples) using 13 scanning strategies to obtain a total of 260 digital files. Both IOS systems were previously calibrated according to the manufacturer’s guidelines.

The same trained investigator carried out the strategies (Fig 2), which were as follows:

Exterior–interior (A): First, the vestibular surfaces, starting with the left second molar and continuing until the right second molar, returning via the occlusal surfaces and finally sweeping once over the palatal surfaces.2

Interior–exterior or recommended by manufacturer 1 (B): First, the palatal surfaces, starting with the left second molar and continuing until the right second molar, return-ing via the occlusal surfaces and finally sweeping once over the vestibular surfaces.

Occlusal–exterior (C): First, the occlusal surfaces, starting with the left second molar and continuing until the right second molar, returning via the palatal surfaces and finally sweeping once over the vestibular surfaces.

Occlusal–interior (D): First, the occlusal surfaces, starting with the left second molar and continuing until the right second molar, returning via the vestibular surfaces and finally sweeping once over the palatal surfaces.2,14

Sequential or arbitrary (E): Sequential scanning of the three surfaces of each tooth (occlusal, vestibular, and pal-atal), performing an S-shaped movement from the right

Fig 1 Customized complete-arch maxillary cast with teeth made from polymethyl methacrylate (Telio CAD; Ivoclar Vivadent) that served as the reference cast.


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Fig 2 Graphic representation of the 13 scan strategies evaluated (A to M).

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second molar in all directions and without returning to the starting point.2,14

Quadrants–interior (F): First, the occlusal surfaces, starting with the right central incisor and continuing toward the right second molar, returning via the vestibular surfaces and then the palatal surfaces. Second, the occlusal surfac-es, starting with the left central incisor and continuing until the left second molar, returning via the vestibular surfaces and then the palatal surfaces.2

Quadrants–exterior (G): First, the occlusal surfaces, start-ing with the right central incisor and continuing toward the right second molar, returning via the palatal surfaces and then the vestibular surfaces. Second, the occlusal sur-faces, starting with the left central incisor and continuing until the left second molar, returning via the palatal sur-faces and then the vestibular surfaces.

Sextants (H): First, the occlusal surfaces, starting with the right second molar and continuing until the right first pre-molar, returning via the vestibular surfaces and then the palatal surfaces. Second, the occlusal surfaces, starting with the right canine and continuing until the left canine, returning via the vestibular surfaces and then the palatal surfaces. Third, the occlusal surfaces, starting with the left first premolar and continuing until the left second molar, returning via the vestibular surfaces and then the palatal surfaces.2

Modified–ortho (I): First, the palatal surfaces, starting with the left second molar occlusal and continuing until the palatal surfaces until the right second molar, then sweep-ing to its occlusal. Returning to the left second molar, starting at the occlusal and continuing until the midline. Returning to the left second molar, starting at the occlusal and sweeping to the vestibular surface, continuing until the midline. Next to the right second molar, starting at the occlusal and continuing until the midline. Returning to the right second molar, starting at the occlusal surface and sweeping to the vestibular surface, continuing until the midline.

Linear–experimental 1 (J): First, the palatal surfaces, start-ing with the left second molar occlusal, continuing toward the palatal until the right second molar and sweeping to its occlusal. Returning to the left second molar, starting at the occlusal, continuing until the opposite canine. Next to the right second molar, starting at the occlusal, continu-ing until the opposite canine. Returning to the left second molar, starting at the occlusal and sweeping to the vestib-ular surface, continuing until the opposite canine. Return-ing to the right second molar, starting at the occlusal and

sweeping to the vestibular surface, continuing until the opposite canine.

Linear–experimental 2 (K): First, the palatal surfaces, start-ing with the left second molar occlusal, continuing toward the palatal until the right second molar and sweeping to its occlusal. Returning to the left second molar, starting at the occlusal, continuing until the right second molar. Returning to the left second molar, starting at the occlusal and sweeping to the vestibular surface, continuing until the opposite canine. Returning to the right second molar, starting at the occlusal and sweeping to the vestibular surface, continuing until the opposite canine.

Recommended by manufacturer 2 (L): Start with the oral surface of the anterior teeth and move the scanner in the oral direction along the quadrant. Move the scanner over the distal tooth to the vestibular side and track the first quadrant to the anterior teeth. Gently tilt the scanner approximately 30 degrees in the coronoapical direction. Move the scanner for the second quadrant. Then scan the anterior teeth from canine to canine in the coronoapical direction. Ensure that both the labial surface and the oral surfaces are visible. Extend this third scan to locations where you can view the scan holes.

Linear–continuous experimental 3 (M): First, the palatal surfaces, starting with the left second molar occlusal, con-tinuing toward the palatal until the right second molar, returning via the occlusal surfaces until the left second molar and sweeping to its vestibular surface, continuing until the opposite canine. Starting again with the right second molar occlusal and sweeping to the vestibular sur-face, continuing until the opposite canine.

Each sample (scan file) was exported into a binary standard tessellation language (STL) file in high resolution through both software programs using the appropriate manufactur-er’s recommended conversion. The STL files were compared with the master STL file using a 3D analysis software program (GOM Inspect 2018 Hotfix 6, Rev 117418, Build 2019-04-11). Using a best fit alignment, this software aligned the scans with the master scan made with the ATOS II Triple Scan. The average distance values from the master scan were recorded for each of the 10 samples per group (A to M). A separate op-erator recorded the time taken during the scanning with a digital stopwatch, and all times were averaged.

To measure the trueness within each group, the average distance among the 10 samples was evaluated from the ATOS master scan image, which represented the reference (or true) model. To measure precision, the same approach was used


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but with the centroid (average) image of each group as refer-ence. The difference between the best scanning strategy – the one with the lowest sample average – and the rest of them was statistically evaluated with the Welch’s unequal variances t test (n = 10) for each IOS system, for both trueness and precision indices, using Microsoft Excel v 16.16.11.

Results

In Table 1, sample data and statistics for the deviations from the ATOS master scan image obtained with Primescan are depicted per group. The sample average per group is used as the trueness index. Therefore, the lower the value of the index, the higher the trueness. As can be seen, group M exhibits the best trueness value, at 47.5% of the average among all other groups (average of groups A to L equals 10.08 μm), and 29.6% lower than the second-best group (A).

Group M exhibited the lowest value for the trueness index, with statistically significant differences at a significance level of 0.05. It is worth noting, however, that group A presented a borderline P value of 0.048, indicating that further tests should be carried out with a larger sample size to confirm the result for the difference between groups A and M. Figure 3 shows Table 1 data as a boxplot diagram.

In Table 2, sample data and statistics for the deviations from the centroid of each group with Primescan are depicted per group, using the sample average per group as the preci-sion index. Therefore, the lower the value of the index, the higher the precision. As can be seen, group M also exhibited the best precision value, at 41.6% of the average among all other groups (average of groups A to L equals 11.23 μm), and 33.6% lower than the second-best group (D). Group M exhib-ited the lowest value for the precision index, with statistically significant differences at a significance level of 0.05. Figure 4 shows Table 2 data as a boxplot diagram.

Table 1 Primescan sample data and statistics for trueness (μm)

Average deviation (μm) – Primescan

Sample A B C D E F G H I J K L M

1 6.27 8.18 10.96 6.85 11.07 14.73 18.99 3.12 16.97 9.79 9.77 9.87 3.95

2 5.78 2.06 7.65 13.76 10.26 8.29 8.17 7 9.88 9.65 20.36 10.19 0.71

3 9.86 7.21 11.05 6.25 6.72 8.86 11.1 11.7 9.78 10.52 14.42 5.92 3.28

4 3.48 7.24 11.23 9.49 7.51 9.87 14.81 8.55 5.31 9.1 12.65 10 5.49

5 2.67 9.62 13.6 6.01 6.97 6.31 15.25 6.46 7.36 12.01 10.59 9.64 5.1

6 10.08 7.57 8.98 9.51 16.37 10.29 13.28 7 6.46 7.23 11.29 8.93 4.83

7 7.24 14.87 7.67 8.85 17.09 12.27 12.69 10.56 8 9.78 12.25 9.86 8.31

8 6.62 10.42 9.96 10.12 11.88 11.55 13.22 7.22 16.13 11.78 17.84 11.43 5.4

9 11.83 9.26 14.1 10.19 13.63 9.05 11.55 7.69 13.34 10.17 11.55 8.55 5.1

10 4.23 11.95 10.33 7.8 11.48 11.16 11.14 7.26 13.9 10.79 13.3 10.88 5.77

Trueness

(μm)

A B C D E F G H I J K L M

Mean 6.81 8.84 10.55 8.88 11.30 10.24 13.02 7.66 10.71 10.08 13.40 9.53 4.79

Median 6.45 8.72 10.65 9.17 11.28 10.08 12.96 7.24 9.83 9.98 12.45 9.87 5.10

Difference of means

2.01 4.04 5.76 4.09 6.50 5.44 8.23 2.86 5.92 5.29 8.61 4.73

P value 0.048129 0.002723 0.000004 0.000256 0.000129 0.000014 0.000001 0.004175 0.000729 0.000001 0.000003 0.000008


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In Table 3, sample data and statistics for the deviations from the ATOS master scan image obtained with Omnicam are depicted per group, using the sample average per group

as the trueness index. As can be seen, group M exhibits the best trueness value at 53.4% of the average among all other groups (average of groups A to L equals 35.85 μm), and 40.2%

Table 2 Primescan sample data and statistics for precision (μm)

Average deviation (μm) – Primescan


1 8.08 17.07 14.33 4.42 9.19 16.99 27.73 3.08 18.46 18.59 4.97 6.93 7.43

2 6.72 4.59 8.15 13.57 6.53 4.67 5.45 8.11 4.64 18.87 15.92 8.12 2.31

3 14.58 9.58 16.45 2.14 4.02 7.59 15.3 12.56 11.72 8.3 9.88 4.73 4.04

4 4.98 14.88 13.9 6.46 5.15 9.95 26.93 7.36 5.74 17.78 8.86 8.13 5.1

5 10.27 6.89 15.93 5.54 3.78 6.47 26.17 6.09 8.13 19.2 7.59 7.94 5.06

6 9.05 15.18 6.42 8.08 12.48 15.67 23.76 5.24 8.51 15.1 6.84 6.64 4.05

7 4.41 19.74 10.28 7.76 13.24 13.93 17.46 11.55 9.29 17.68 8.61 7.66 7.97

8 12.87 12.33 12.25 7.55 8.05 14.97 18.92 6.59 17.35 15.4 14.28 10.08 3.47

9 14.52 13.46 20.15 9.83 10.52 6.99 18.6 8.56 14.1 17.27 11.65 7.22 1.81

10 6.79 20.56 18.91 5.03 9.62 11.83 14.61 5.36 13.61 13.97 12.33 10.54 5.5

Precision

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Mean 9.23 13.43 13.68 7.04 8.26 10.91 19.49 7.45 11.16 16.22 10.09 7.80 4.67

Median 8.57 14.17 14.12 7.01 8.62 10.89 18.76 6.98 10.51 17.48 9.37 7.80 4.56

Difference of means

4.55 8.75 9.00 2.36 3.58 6.23 14.82 2.78 6.48 11.54 5.42 3.13

P value 0.0024 0.0002 0.00004 0.0318 0.0057 0.0007 0.00003 0.0123 0.0008 0.0000001 0.0003 0.0007

Fig 3 Primescan boxplot for trueness (μm). Fig 4 Primescan boxplot for precision (μm).


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lower than the second-best group (K). Group M exhibited the lowest value for the trueness index, with statistically signifi-cant differences at a significance level of 0.05. It is important to emphasize the highly significant difference presented by group M from the other groups, with P values lower than 0.0001. Figure 5 shows Table 3 data as a boxplot diagram.

In Table 4, sample data and statistics for the deviations from the centroid of each group with Omnicam are depicted per group, using the sample average per group as the preci-sion index. In this case, group B exhibited the best precision value, at 65.6% of the average among all other groups (aver-age of group A and groups C to L equals 25.53 μm), and 20.4% lower than the second-best group (A). Group B exhibited the lowest value for the precision index, with statistically signifi-cant differences at a significance level of 0.05. Figure 6 shows Table 4 data as a boxplot diagram.

Figure 7 compares the best results from each system regarding trueness. From this figure, it is clear that Primescan

features the best index (4.79 μm against 19.13 μm featured by the best Omnicam group – values extracted from Tables 1 and 3), as corroborated by a P value lower than 0.00001.

Table 3 Omnicam sample data and statistics for trueness (μm)

Average deviation (μm) – Omnicam


1 25.53 33.13 39.01 32.71 38.01 37.02 44.67 33.24 24.5 30.02 30.69 33.95 18.19

2 32.09 35.09 40.67 38.98 38.1 43.82 44.98 37.89 24.93 30.05 30.14 36.14 12.92

3 34.83 32.16 34.93 30.33 37.82 34.05 44.4 36.9 28.13 30.58 29.96 33.99 13.42

4 37.43 37.14 39.3 49.31 37.39 45.36 49.15 36.56 34.04 38.85 32.07 37.44 25.93

5 29.99 36.77 36.58 41.21 41.59 42.5 40.21 37.33 36.01 33.75 32.24 29.42 21.85

6 35.68 40.36 41.6 36.59 36.78 45.13 38.41 31.62 34.08 34.24 31.06 28.81 15.35

7 33.49 37.08 42.85 43.35 34.73 47.38 41.79 34.69 27.08 36.09 34.79 31.26 24.8

8 30.19 33.25 38.36 40.23 36.16 43.41 37.07 34.04 34.08 29.37 29.36 32.77 22.53

9 30.06 41.31 37.52 37.82 30.79 40.71 41.54 34.8 36.86 29.49 33.06 33.42 19.98

10 30.39 36.28 31.11 39.16 36.79 40.19 36.47 32.41 43.85 33.31 32.19 29.64 16.34

Trueness

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Mean 31.97 36.26 38.19 38.97 36.82 41.96 41.87 34.95 32.36 32.58 31.56 32.68 19.13

Median 31.24 36.53 38.69 39.07 37.09 42.96 41.67 34.75 34.06 31.95 31.57 33.10 19.09

Difference of means

12.84 17.13 19.06 19.84 17.69 22.83 22.74 15.82 13.23 13.44 12.43 13.55

P value 0.000001 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000027 0.000001 0.000003 0.000001

Fig 5 Omnicam boxplot for trueness (μm).


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Table 4 Omnicam sample data and statistics for precision (μm)

Average deviation (μm) – Omnicam


1 16.2 13.21 25.96 17.27 30.86 23.98 26.1 23.26 16.47 27.81 22.99 25.21 23.81

2 20.82 15.03 24.58 21.4 31.15 29.4 32.97 26.04 14.91 29.17 22.61 24.07 21.09

3 22.01 20.07 21.89 15.42 25.17 19.78 37.16 26.33 20.65 27.34 21.84 22.23 8.49

4 24.97 17.65 25.83 30.49 22.7 33.31 41.03 26.18 24.25 30.94 26.4 25.96 33.19

5 21.8 17.68 25.19 23.55 34.27 28.82 34.3 23.78 29.35 31.99 27.84 18.86 25.21

6 23.38 17.9 28.82 22.11 31.47 29.95 34.98 23.91 26.35 26.58 26.95 17.46 17.86

7 23.65 17.75 29.95 28.49 30.14 32.54 34.13 23.65 17.38 34.8 27.38 21.27 26.45

8 18.63 13.01 19.53 29.12 33.15 30.4 31.13 22.33 28.31 23.46 17.96 21.18 26.69

9 19.82 19.34 25.21 18.79 25.72 23.97 33.83 23.56 27.32 17.55 26.87 24.24 25.8

10 19.25 15.88 20.4 24.05 36.49 25.24 29.44 23.22 32.32 29.95 27.2 22.98 22.43

Precision

(μm)


Mean 21.05 16.75 24.74 23.07 30.11 27.74 33.51 24.23 23.73 27.96 24.80 22.35 23.10

Median 21.31 17.67 25.20 22.83 31.01 29.11 33.98 23.72 25.30 28.49 26.64 22.61 24.51

Difference of means

4.30 7.98 6.32 13.36 10.99 16.76 7.47 6.98 11.21 8.05 5.59 6.35

P value 0.000713 0.0000073 0.0020771 0.0000003 0.0000030 0.00000001 0.0000004 0.0029514 0.0000087 0.0000056 0.0000720 0.0073977

Fig 6 Omnicam boxplot for precision (μm). Fig 7 Primescan versus Omnicam boxplot for trueness (μm).

In Figure 8, the same pattern is illustrated, with the best Primescan group (M) featuring a better precision index than the best Omnicam group (B). In this figure, the precision index for the third best group (M) is also provided for the pur-

poses of comparison. It is noteworthy that the P values for the differences between Primescan group M (4.67 μm – see Table 2) and Omnicam groups M (23.10 μm – see Table 4) and B (16.75 μm – see Table 4) were both lower than 0.00001.

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In Tables 5 and 6, the data and similar statistical analyses are presented for the scanning times for both Primescan and Omnicam. Results confirm that group M provided the lowest scanning times for the two IOS systems. For Primescan, group L is the only group for which the null hypothesis is not reject-ed below a significance level of 0.05. Figures 9 and 10 show the scanning time data from Tables 5 and 6 as boxplots, re-spectively.

Discussion

The primary goal of this study was to determine the scanning strategy that obtains the best accuracy results in terms of trueness and precision for Primescan and Omnicam systems. For Primescan, the null hypothesis used to assess the statis-tical differences between the lowest index and the indices of the rest of the groups (below a significance level of 0.05) for both trueness and precision (see Tables 1 and 2) was rejected.

Additionally, the best indices for trueness and precision were obtained with strategy M. Group M is therefore the best scan-ning strategy for Primescan.

Fig 8 Primescan versus Omnicam boxplot for precision (μm).

Table 5 Primescan scan time data (s)

Time (s) – Primescan


1 27.15 37.21 45.07 38.64 52.38 39.85 39.39 37.65 23.1 41.18 25.89 19.95 16.94

2 28.68 40.31 38.57 33.6 60.12 42.72 45.72 27.52 26.34 42.08 33.35 22.64 20.01

3 34.83 27.59 37.76 28.85 48.31 45.71 48.74 37.72 23.77 37.04 26.94 28.35 16.39

4 29.99 28.35 39.59 38.46 56.43 50.64 43.67 33.11 24.88 30.26 35.71 22.75 18.96

5 26.75 28.27 36.36 39.9 50.02 41.62 47.81 28.62 23.03 29.66 23.46 19 21.77

6 25.5 36.85 39.63 34.39 51.86 50.86 42.98 28.05 23.1 33.52 26.85 19.9 17.3

7 25.25 27.18 43.47 46.54 64.33 50.82 46.12 30.02 23.96 29.76 22.64 22.12 17.62

8 23.15 28.65 41.49 42.64 50.96 55.11 41.88 26.79 34.24 29.09 20.52 27.11 18.98

9 27.61 37.83 43.39 33.69 72.72 51.73 41.95 26.31 27.71 31.58 25.87 16.15 21.71

10 23.29 41.99 38.86 31.59 42.08 51.37 50.88 25.54 26.59 33.82 22.82 16.55 20.45

Statistics A B C D E F G H I J K L M

Mean 27.22 33.42 40.42 36.83 54.92 48.04 44.91 30.13 25.67 33.80 26.41 21.45 19.01

Median 26.95 32.75 39.61 36.43 52.12 50.73 44.70 28.34 24.42 32.55 25.88 21.04 18.97

Difference of means

8.21 14.41 21.41 17.82 35.91 29.03 25.90 11.12 6.66 14.79 7.39 2.44

P value 0.000006 0.000013 < 0.000001 < 0.000001 < 0.000001 < 0.000001 < 0.000001 < 0.000001 0.000053 0.000001 0.000432 0.055091

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Table 6 Omnicam scan time data (s)

Time (s) – Omnicam


1 28.83 25.5 54.34 60.77 73.01 57.19 51.03 45.28 36.49 65.96 45.31 25.89 18.08

2 23.95 24.01 60.77 55.41 72.35 58.29 44.69 43.07 44.39 57.85 40.48 29.34 16.52

3 34.61 28.28 71.04 56.87 78 61.97 43.68 38.52 36.52 59.77 44.43 24.84 16.76

4 25.17 33.18 67.15 62.92 73.28 67.8 48.99 41.85 44.33 54.14 35.16 23.18 18.92

5 28.09 27.49 55.15 63.98 78.42 67.18 45.82 40.98 47.98 59.77 31.05 29.86 21.11

6 25.37 27.05 57.17 64.54 74.18 62.25 46.81 44.37 40.69 63.51 38.98 30.21 23.48

7 24.6 23.6 52.19 75.03 67.85 60.63 36.26 41.54 37.69 58.26 33.19 24.84 24.3

8 29.13 22.75 62.91 51.23 71.01 68.29 38.51 37.18 44.67 58.93 29.17 30.19 20.76

9 26.5 25.38 51.19 80.48 78.83 70.45 43.56 46.87 46.64 63.1 29.48 23.99 20.38

10 29.24 27.53 61.52 52.28 69.48 68.72 47.06 42.91 51.15 58.52 30.51 28.18 20.43

Statistics A B C D E F G H I J K L M

Mean 27.55 26.48 59.34 62.35 73.64 64.28 44.64 42.26 43.06 59.98 35.78 27.05 20.07

Median 27.30 26.28 58.97 61.85 73.15 64.72 45.26 42.38 44.36 59.35 34.18 27.04 20.41

Difference of means

7.48 6.40 39.27 42.28 53.57 44.20 24.57 22.18 22.98 39.91 15.70 6.98

P value 0.000011 0.000043 < 0.000001< 0.000001< 0.000001< 0.000001< 0.000001< 0.000001< 0.000001< 0.000001 0.0000038 0.0000105

Fig 9 Primescan scan time boxplot (s). Fig 10 Omnicam scan time boxplot (s).

For Omnicam, the null hypothesis was also rejected (below a significance level of 0.05), but here group M per-formed best for trueness, and group B for precision. There-fore, no dominant strategy emerged for Omnicam. Notwith-

standing, it is worth noting that the magnitude of the results – the differences in absolute terms – for trueness are significantly higher for group M; samples are almost disjoint-ed from the rest of the groups, as can be seen in the boxplot

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Passos et al

of Figure 5. Regarding precision, group M is an average group. Hence, as no dominant strategy emerged in this bi-cri-terion selection, from a clinical perspective, group M could be chosen due to its significantly better performance in terms of trueness.

The second goal of this study was to determine which sys-tem offers the best performance. Figures 7 and 8 show that, in terms of trueness and precision, the best strategy for Pri-mescan outperforms that for Omnicam, an outcome also cor-roborated by P values lower than 0.00001.

This study used a master model composed of a material with an RI similar to that of tooth structure, but despite mod-eling several variables it did not replicate a clinical situation and is limited by the failure to simulate saliva; soft tissue; patient movement; humidity in the oral environment and multiple substrates, including amalgam, ceramic, cast metal, composite resin, dentin, and enamel.10 Further in vitro and in vivo studies are necessary to determine whether these differ-ent variables affect accuracy.

Additionally, it is generally acknowledged that operator experience influences accuracy;21 an inexperienced user is not able to generate results that are suitable for analysis. This study used only one experienced investigator to perform all the scans with both IOS systems.

In this study, we sought to evaluate the greatest num-ber of descriptions of techniques available in the literature, performing inverse described strategies to verify their pos-sible influence on the results. In parallel, new techniques were created, with the idea that a small number of twists/twirls in camera movements (keeping scanning as linear as possible) would generate a smaller number of oscillations in terms of image movement during the acquisition. Inter-ruptions in scanning were indicated at specific points (at the median line) due to the need for a union of hemiarches (mostly for the vestibular surface), where continuous scan-ning would not be feasible; for that reason, crossing scan-ning in such areas was proposed and tested, as described in techniques J to M above. Therefore, it is worth mentioning the ‘in vivo’ aspect for the practitioner when shifting from left to right in the jaws. For the mandible, the sitting pos-ition of the examiner should be at 12 o’clock. For palatal surfaces, the examiner should shift the tip of the camera toward the distal and, occlusally, should also tilt from ‘fore-hand’ to ‘backhand,’ trying to keep the tip of the camera distal. In the buccal ‘crossing-zone,’ the examiner should guide a slight twist of the patient’s head in the opposite direction to the camera tip, avoiding it moving toward the incisal or apical direction during scanning in both hemi-

arches. For the mandible, the sitting position of the exam-iner should be at 9 o’clock to perform the scans precisely as described for the maxilla.

While the time required to complete a complete-arch intraoral scan might influence a practitioner’s choice, this study considered only the time required for the complete acquisition of the arch, even if certain areas needed comple-mentation. Such scan supplementation and software render-ing of the models being performed in the sequence were not timed. It is worth noting that, regardless of the technique used, Omnicam presented a greater need for scanning com-plementation compared with Primescan.

A recent study13 using a customized model simulating in vivo-like conditions in terms of tooth surfaces and optical characteristics evaluated the accuracy of new and actual digi-tal and conventional impression methods in vitro for com-plete- and partial-arch areas. The results support the asser-tion that new hardware and/or software developments are able to provide major improvements in terms of impression accuracy for IOS devices. Although – due to variations in study design such as different materials, different software versions, different numbers of teeth being analyzed, and dif-ferent methods in analysis software being used – a direct comparison with this published result is difficult, the results of that study agree with the present study regarding the true-ness and precision of Primescan IOS.

There is a need for further research and also the develop-ment of a standardized method for evaluating and compar-ing different IOS systems. Due to the difficulty of accounting for software versions, scanning substrate variability, and arch configurations, conclusions can be drawn only for the exact scanning scenario outlined in this article.

Conclusion

Within the limitations of this in vitro study, the results provide statistically significant evidence that for trueness and preci-sion of complete-arch scans, group M was the dominant scanning strategy in Primescan. In Omnicam, in terms of true-ness and precision, there was no dominant strategy. Regard-ing the scanning times, group M was the optimal strategy for both IOS systems.

Finally, regarding the comparison between IOS systems, Primescan provided the best results for trueness and preci-sion indices. Further in vivo studies are needed to validate the accuracy of this IOS device under clinical conditions, which cannot be fully replicated in vitro.


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Acknowledgment

The authors thank Vtech Consulting, especially Ms. Isabela Quadros, for the help and assistance with the ATOS II Triple Scan scanner.

References

1. Brawek PK, Wolfart S, Endres L, Kirsten A, Reich S. The clinical accura-cy of single crowns exclusively fabricated by digital workflow – the comparison of two systems. Clin Oral Investig 2013;17:2119–2125.

2. Medina-Sotomayor P, Pascual-Moscardó A, Camps I. Accuracy of four digital scanners according to scanning strategy in complete-arch impressions. PLoS ONE 2018;13:e0202916.

3. Ender A, Mehl A. In-vitro evaluation of the accuracy of conventional and digital methods of obtaining full-arch dental impressions. Quin-tessence Int 2015;46:9–17.

4. Patzelt SB, Emmanouilidi A, Stampf S, Strub JR, Att W. Accuracy of full-arch scans using intraoral scanners. Clin Oral Investig 2014;18:1687–1694.

5. Seelbach P, Brueckel C, Wöstmann B. Accuracy of digital and conven-tional impression techniques and workflow. Clin Oral Investig 2013;17:1759–1764.

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7. International Organization for Standardization. Accuracy (trueness and precision) of measurement methods and results – Part 1: General principles and definitions (ISO 5725-1:1994). Berlin: Beuth Verlag GmbH, 1997.

8. Ender A, Mehl A. Accuracy of complete-arch dental impressions: a new method of measuring trueness and precision. J Prosthet Dent 2013;109:121–128.

9. 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;112:1461–1471.

10. Renne W, Ludlow M, Fryml J, et al. Evaluation of the accuracy of 7 digital scanners: An in vitro analysis based on 3-dimensional com-parisons. J Prosthet Dent 2017;118:36–42.

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Passos et al

Einfluss verschiedener Scanning-Strategien auf die Genauigkeit der

Ganzkieferabformung bei zwei intraoralen Scannern: Eine In-vitro-Studie

Schlüsselwörter: CAD/CAM, intraoraler Scanner, digitale Abformung, Präzision, Ganzkieferabformung, Standard Tessellation Language

Zusammenfassung

Ziel: Ermittelt werden sollte die Scanning-Strategie, die bei der digitalen Ganzkieferabformung mit zwei intraoralen Scan-nern (IOS) zu den genauesten Ergebnissen führt. Dazu wurde die Aufnahmezeit evaluiert und mit den Aufnahmestrategi-en korreliert. Materialien und Methoden: Als Referenzstandard wurde ein Modell angefertigt, dessen Zähne mit Dentin und Schmelz identische Brechungsindizes aufwiesen und somit die natürliche Bezahnung imitierten. Mit dem optischen Scanner ATOS III Triple Scan 3D wurde eine Referenzaufnahme des Typodonts angefertigt. Die Ganzkieferabformung erfolgt mit zwei IOS-Geräten: Omnicam v. 5.1.0 und Primescan v. 5.0. Beide führten 13 Scanning-Strategien durch, erzeugten 260 digitale Dateien (n = 10 pro Gruppe), zeichneten die jeweilige Abtastzeit auf, wandelten alle experimentellen Scans in Standard Tessellation Language (STL) um und verglichen die experimentellen Scans mithilfe eines komplexen Messprogramms mit dem Referenzstandardscan. Die statistische Auswertung erfolgte mit dem Welch-t-Test für ungleiche Varianzen.Ergebnisse: Die Gruppe M erzielte die niedrigsten Werte für Genauigkeit und Präzision (p < 0,05) des Primescan (47,5 % des Durchschnittswertes aller anderen Gruppen) und den niedrigsten Wert für die Genauigkeit (p < 0,05) des Omnicam (53,4 % des Durchschnittswertes aller anderen Gruppen). Gruppe B hingegen erreichte mit p < 0,05 den niedrigsten Prä-zisionswert (65,6 % des Durchschnittswertes aller anderen Gruppen) des Omniscan. Primescan erzielte einen besseren Genauigkeitsindex (4,79 μm) als Omnicam (19,13 μm), dieser Unterschied war statistisch signifikant (p < 0,00001). Pri-mescan erzielte in der Gruppe M einen besseren Genauigkeitsindex (4,67 μm) als Omnicam in der Gruppe B (16,75 μm). Auch dieser Unterschied war statistisch signifikant (p < 0,00001). Schlussfolgerung: Bei beiden IOS-Systemen war die Abtastzeit in der Gruppe M am kürzesten. Bei der Genauigkeit und Präzision der Ganzkieferabformung war die Gruppe M beim Primescan die beste Scanning-Strategie, während Omnicam keine überlegene Scanning-Strategie aufwies. Bei beiden IOS-Systemen war die Abtastzeit in Gruppe M am besten.

Leandro Passos, DDS, MSc, PhD

Associate Professor, Department of Prosthetic Dentistry, Federal Fluminense University, Health Institute of Nova Friburgo, School of Dentistry, Rio de Janeiro, BrazilVisiting Professor, Department of Dentistry, University of Alberta, Edmonton Clinic Health Academy, Edmonton, Canada

Sergio Meiga, DDS

Private Practice, Rio de Janeiro, Brazil

Vinicius Brigagão, DDS, MSD, PhD

Associate Professor, Department of Prosthodon-tics, Grande Rio University, School of Dentistry, Rio de Janeiro, Brazil

Alexandre Street, MSD, PhD

Associate Professor, Electrical Engineering Department, Pontifical Catholic University of Rio de Janeiro, School of Engineering, Rio de Janeiro, Brazil

Leandro Passos

Address Dr. Leandro Passos, Department of Prosthetic Dentistry, Federal Fluminense University, Health Institute of Nova Friburgo, School of Dentistry, Rua Doutor Silvio Henrique Braune, 22, 28625-650, Nova Friburgo, Rio de Janeiro, Brazil; Tel: Landline: +55 22 2528-7166, Mobile: +55 21 98128-5002; E-Mail: [email protected], [email protected]

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