Effect of scanning-aid agents on the scanning accuracy in specially designed metallic models: A laboratory study.

The advent of intraoral scanning methods has caused a paradigm shift in dentistry. However, despite their many advantages, intraoral scanners cannot accurately recognize the metallic surfaces of prothesis. Therefore, this experiment was designed to verify the effect of scanning-aid agents on the scanning accuracy using metallic reference models. Three different types of metallic reference models (inlay, onlay, and bridge) were specially designed and produced using a milling machine to simulate intraoral dental restorations. Three experimental groups (application of ScanCure, IP Scan Spray, and VITA Powder Scan Spray) were set up and scanned images (each n = 5) were acquired using the I500® intraoral scanner. The reference datasets were established by a 3D design that reflected the deviations between the measured distances and previously planned distances on the reference models. All acquired experimental datasets were digitally superimposed and compared with the reference datasets. Intragroup comparisons (precision, n = 10) were also performed. The root mean square (RMS) values of trueness in the ScanCure and IP groups were significantly more accurate than those of the VITA group in the inlay and onlay reference models (p < 0.05). Notably, in the bridge reference model, the liquid-type ScanCure group showed the highest accuracy of trueness, with statistical significance (p < 0.05). However, the RMS values of precision were not significantly different among the groups. These findings suggest that liquid-type scanning agents can be effectively used to obtain more accurate scan images of intraoral metallic dental restorations.

Introduction

The development of intraoral scanning technology has caused a paradigm shift from conventional impression techniques to direct digital impression making [1,2]. With computer-aided design (CAD) / computer-aided manufacturing (CAM) systems and clinically reliable intraoral scanners, practitioners can easily obtain intraoral scanned images to fabricate the prosthesis digitally [3]. Additionally, as the accuracy of conventional impressions affects the final fit of the dental prosthesis, precise digital scanned images are a prerequisite for making clinically acceptable prostheses [4]. Although intraoral scanning methods have several advantages such as saving storage space, patient convenience, and freedom from distortion errors in impression material, there are also crucial limitations in digital image acquisition in intraoral environments [5,6].

First, intraoral scanners used in the dental field cannot accurately recognize the metallic surfaces of the prostheses. Because scanners obtain digital information by interpreting the diffusion of light from the surface of objects, the shiny or translucent properties of the metallic surface interfere with the matching of the point of interest by the software due to overexposure [7,8]. This results in incorrectly scanned images and consequently, in an improperly fitted prosthesis. Therefore, to create a homogeneous antireflective surface, titanium dioxide powder is often applied before intraoral scanning [9]. Of course, this additional step can be uncomfortable for both patients and clinicians [10]. The thickness of the scanning powder applied to the objects can vary and is affected by the proficiency of the operator [11]. However, the software of the intraoral scanner itself can compensate for these differences [12]. Liquid-type coating agents can achieve a more uniform coating thickness, and therefore, a more accurate scanned image, than the powder-type agents [13].

Second, errors from the increased scan distance are inevitable with intraoral scanners [14]. The stitching or matching process is necessary to form the whole scanned image because the field of view of the intraoral scanner is too small to obtain the total image at once, especially of the full dental arch [15]. Thus, stitching errors are accumulated in the software proportional to the scan distance [16]. These also results in an inappropriate final prosthesis. To minimize these scanning errors over a long span, such as a full dental arch, several factors are considered. For instance, not only the matching algorithms of software, but also the sensor quality of the intraoral scanner must be improved [17]. Scanning strategies such as scanning path, environment control, and application of scanning-aid agents are used to obtain accurate scanned images [10,18].

As mentioned above, reflective metallic surfaces are challenging to scan using intraoral scanners. In addition, the accuracy of the scanned image may be affected by the extent of the scan. Therefore, to reduce the reflective properties of the metallic surface and increase the accuracy in the case of a long span, surface treatments, such as the application of scanning-aid agents, are frequently suggested. A previous study reported that when a resin-based full-arch model was scanned with the application of scanning-aid materials, the precision of root mean square (RMS) values was improved [18].

The purpose of this study was to evaluate the influence of scanning-aid materials on the scanning accuracy in specially designed metallic models that imitate intraoral dental restorations, such as inlay, onlay, and bridge. To compare the scanning accuracy, 3D superimposition with the best-fit algorithm was used.

Materials and methods

Reference models and scanning-aid materials

Three different types of metallic reference models were designed using CAD software (Solidworks 2016, Dassault Systèmes SolidWorks Corp., Waltham, MA, USA) and milled using a 5-axis milling machine (VARIAXIS i-600, Yamazaki mazak Corp., Aichi, Japan) (Fig 1). The metallic material used for milling the reference model was an aluminum alloy. These reference models simulated different crown shapes for dental restorations: inlay, onlay, and bridge. The bridge model was designed by connecting one inlay and one onlay form with one crown form (Fig 1C).

Fig 1

Specially designed reference models.

A—inlay model; B—onlay model; C—bridge model.

Three different types of scanning-aid agents were used: Vita Powder Scan Spray (Vita Zahnfabrik, Germany), IP Scan spray (IP-Division, Haimhausen, Germany), and ScanCure (SC-80, ODS Co., Incheon, Korea). The VITA and IP spray were powder-type agents, while ScanCure was liquid. VITA Powder Scan Spray is a spray-on, titanium dioxide free and blue colored pigment suspension with ethanol and isobutane. IP Scan Spray contains titanium dioxide with ethanol, propane, butane, and isobutane. And the ScanCure contains titanium dioxide and ethanol.

Because the application procedure of scanning-aid materials is sensitive to operator’s proficiency, one skilled prosthodontist applied the materials on the model as follows: the liquid type of ScanCure was applied by one brush stroke at a time in all surfaces of model, powder type of VITA and IP scan spray was applied at a same distance (5 inch) and angulation (45 degree) of the spray tip from the specimen with the same time (2 seconds) to make an uniform thin layer of powder.

Acquisition of digital data

Because it was difficult to acquire the scanned images of the untreated metallic reference models (Fig 2), the reference datasets were obtained using a 3D design, which reflected the errors between the measured distances and the previously planned distances on the reference models. The distances on the fabricated reference models were calculated using a laser scanner (HERO7106, Dukin Corp.,Daejeon, Korea).

Fig 2

Scanned images with a model scanner (Identica T500®).

A—inlay model; B—onlay model; C—bridge model.

The experimental datasets of the three groups (application of Vita Powder Scan Spray(VITA), IP Scan spray(IP), and ScanCure) were obtained using the I500 intraoral scanner (Medit Co., Seoul, Korea) with the different reference models (inlay, onlay, and bridge). Five scan images were obtained for each experimental group, and all scanned datasets were converted into the standard tessellation language (STL) data format.

The scanning procedure was executed by a skilled prosthodontist. The remnants of the scanning-aid materials after scanning were removed using organic solvents and an air compressor water gun. The scanning-aid agents were then reapplied for each scanning cycle.

Three-dimensional analysis

All experimental STL datasets were compared with the reference dataset (trueness, n = 5) using the best-fit superimposition method of the 3D analysis software (Geomagic Control X®, 3d systems, Rock Hill, USA). Intragroup comparisons were performed for each experimental group (precision, n = 10). The alignment setting value was determined to produce minimal error based on least square regression with a set tolerance of ± 0.03 mm and a maximum tolerance range of ± 0.3 mm. The mean and standard deviation of the experimental groups were measured using the root mean square (RMS) value.

Statistical analysis

Statistical analysis was performed using the SigmaPlot 14.0TM (Systat Software Inc., San Jose, CA, USA) program. Differences between groups in trueness, precision were evaluated by Kruskal–Wallis test. Pairwise comparisons were performed through the Mann-Whitney test in the case of significant difference according to the Kruskal–Wallis test. The level of significance (α = 0.05) was adjusted according to the Bonferroni correction method.

Results

Trueness

In each reference model, five datasets (trueness, n = 5) for each group (ScanCure, IP, and VITA) were superimposed with the reference datasets in the 3D analysis software (Fig 3). The mean and standard deviation (SD) of the RMS values for each group were measured. The results are summarized in Table 1.

Fig 3

3D superimposition color maps of three experimental groups in three reference models.

A—inlay model; B—onlay model; C—bridge model (tolerance range ± 30 μm).

Table 1

The RMS values of three experimental groups (ScanCure, IP, VITA) in each reference model (mean ± SD).

Trueness	n	ScanCure (μm)	IP (μm)	VITA (μm)
Inlay	5	46.06 ± 2.56	45.46 ± 1.80	62.74 ± 2.73
Onlay	5	45.34 ± 3.94	47.10 ± 2.82	69.10 ± 4.15
Bridge	5	64.62 ± 4.27	84.38 ± 1.80	98.94 ± 1.11

In the inlay and onlay reference models, the RMS values of the ScanCure and IP groups were significantly lower than those of the VITA group (p < 0.05) (Fig 4A and 4B). In the bridge reference model, the ScanCure group had the lowest RMS value compared to the other groups, and the IP group had a significantly lower RMS value than the VITA group (p < 0.05) (Fig 4C).

Fig 4

Box plots of the RMS values for three experimental groups (Scancure, IP, VITA) in three reference models.

A–inlay; B–onlay; C—bridge.

Precision

Similarly, the RMS values (precision, n = 10) for each group were measured using a combination of the two different intragroup datasets. The mean and standard deviation of the RMS values are shown in Table 2.

Table 2

The RMS values of three experimental groups (ScanCure, IP, VITA) in each reference model (mean ± SD).

Trueness	n	ScanCure (μm)	IP (μm)	VITA (μm)
Inlay	10	35.21 ± 8.39	31.50 ± 14.26	31.98 ± 6.91
Onlay	10	35.07 ± 8.51	30.26 ± 8.82	35.49 ± 8.15
Bridge	10	50.68 ± 8.49	49.34 ± 12.23	58.75 ± 12.66

Except for the RMS values of trueness, there were no statistically significant differences among the three experimental groups with all types of reference models (p > 0.05) (Fig 5).

Fig 5

Box plots of the RMS values for three experimental groups (Scancure, IP, VITA) in three reference models.

A—inlay model; B—onlay model; C—bridge model.

Discussion

In terms of trueness, the ScanCure and IP groups had significantly lower RMS values than the VITA group with the inlay and onlay reference models; with the bridge model, the RMS value of the ScanCure group was the lowest among the three groups with a statistically significant difference. These results may be explained by the properties of each scanning-aid material. Unlike the powder-spray types (IP, VITA), ScanCure is a liquid-paint type agent. Thus, ScanCure can be applied to hard-to-reach surfaces of the model as well [13]. In addition, the properties of surface tension, contact angle, and viscosity of the liquid components help maintain a uniform thickness on the surface [19]. In the ScanCure group, the uniformity of the coating layer was confirmed by sight; the layer was also maintained well on the metallic reference models (Fig 6).

Fig 6

Application of ScanCure® agent on the three reference models.

A—inlay model; B—onlay model; C—bridge model.

Furthermore, the VITA group had the largest RMS values among the three groups with all the reference models, with a statistically significant difference. When using the powder-type agents, the total amount that is sprayed on the models may vary even when the same operator applies them under the same conditions (same angle, distance, time, etc.). This is because the design and size of the injection nozzles are different for each agent (Fig 7). The VITA agent had a larger ejection hole size than the IP agent (Fig 7C and 7F). This difference led to a larger amount of the VITA agent being applied; this explains the greater scanning errors seen in the VITA group compared to the IP group. In addition, the powder-type agents were more susceptible to other external conditions (e.g., the size of the object to be applied, operator, nozzle design, spraying time, distance, and angle) than the liquid type.

Fig 7

Designs of ejection nozzle and spraying hole in powder-type agents.

A—without ejection nozzle in VITA; B—with ejection nozzle in VITA; C—spraying hole in VITA; D—without ejection nozzle in IP; E—with ejection nozzle in IP; F—spraying hole in IP.

Precision is defined as the reproducibility of a scanned image under the same conditions [20]. Unlike trueness, there were no statistically significant differences in the RMS values of precision even though the IP group showed the highest precision among the three groups with all the reference models. As described above, the trueness values were significantly different among the groups because the total amount and uniformity of the applied materials were different. However, in each group, the hardware design of the scanning-aid materials was sufficient to reproduce the results of the scanned images. The reproducibility and accuracy of the outcomes were affected by the operator [21]. This means that highly skilled prosthodontists can reduce the errors in intragroup comparisons. In addition, in all three experimental groups, the RMS values of the bridge reference model were larger than those of the inlay and onlay reference models. This suggested that the error accumulates as the scan area increases because the intraoral scanner acquired the digital image of the object using the stitching algorithm [16].

The limitation of this study was that it did not reflect real clinical situations (e.g., accessibility of scanner in the mouth, patient’s compliance, humidity environment such as saliva, blood, etc.). Moreover, it is suggested that other types of intraoral scanners such as confocal-type (e.g., 3Shape Trios®) and not the triangulation-type (Medit I500®) should be used. Further studies using the full-arch reference model are necessary to verify the effect of the scan distance on the scanning accuracy.

Conclusions

The liquid-type scanning-aid agent showed a significantly lower RMS value of trueness than the other powder-type agents with the metallic bride reference model. This suggests that liquid-type scanning-aid agents can be applied uniformly on the entire prosthetic surface by the brushing technique, especially when large areas need to be scanned. In addition, it is relatively difficult to control the amount of powder-type agent applied. (Separate the paragraphs)

Based on our findings, we recommend that liquid-type scanning agents should be used to obtain more accurate scan images of the metallic surfaces of dental restorations in clinical practice.

23 Feb 2022

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Reviewer #1: The authors submit an article "Effect of Scanning-Aid Agents on the Scanning Accuracy in Specially Designed

Metallic Models: A Laboratory Study".

The study compares three types of sprays that are applied to the metal surface of dental prostheses and subsequently scanned with an intraoral scanner. Is there often a need in dental practice to scan metal surfaces of prostheses with an intraoral scanner? What other significant difference is there used between sprays (other than liquid type and powder type)?

The work scans the metal surface of prostheses - wouldn't it be enough to sandblast the surface to eliminate the shiny surface?

Cobalt chromium alloy or titanium alloy, is usually used in dental practice. Why did the authors use an aluminum alloy? It has no surface properties other than cobalt chromium alloy or titanium alloy?

In the article, you state that you used the I500 intraoral scanner for scanning - what technology does it scan with? Optical light? Laser? What is the accuracy and speed of scanning? Have you taken into account Trueness and Precision of the intraoral scanner itself when evaluating the data obtained?

The article is clearly written, described in detail Materials and Methods. The Trueness and Precision data are evaluated in the Results chapter.

In Conclusion, it is stated that it is better to use a liquid type spray.

This is certainly important information for dental practice, but I still miss the point, the main idea of the article. Is it a comparison of sprays or a comparison of metallic reference models (inlay, onlay, and bridge)? Why is it important for the dental technician / dentist which spray is better?

Reviewer #2: The report is well written and easy to understand. Accurate results have been achieved. The information is also accurately illustrated. The authors work with modern technologies in design. Although the SolidWorks software is not the latest version (version 2016 is listed), it is strong enough functionally.

Not as a remark, but as a recommendation:

- If possible, use institutional contact emails.

- The conclusion is too small. It should increase the size. (Also in conclusion) The most important points should be noted in separate paragraphs.

Reviewer #3: This is a fascinating study about the effect of scanning-aid agents on scanning accuracy in specially designed metallic models. Hence, the study adds to the current knowledge in this relatively new field and is definitely of clinical significance. However, it is still not clear to me the following matters as far as I read the manuscript.

This study aimed to evaluate the effect of different scanning-aid agents on the scanning performance of intraoral scanners. However, the description of ScanCure in the test group is insufficient. It is a liquid, so the authors need to describe more about it, including the ingredients. Also, there should be an explanation of how the agent from each group is applied. In what order and how many times these were applied or sprayed on the model. It is also necessary to describe how much powder was sprayed each time. The distance and angulation of the spray tip from the specimen are essential.

In the 'Acquisition of Digital Data' section, the authors say that it was difficult to obtain a reference dataset, so it was obtained using 3D design, but it is not easy to understand. First of all, HERO7106 equipment is a contact scanner known as a Coordinate Measuring Machine (CMM), not a laser scanner. As I understand it, to scan with a laser scanner, the spray must be applied no matter how industrial-level accurate scanners are, so the quality of scan data is no different from that of the test group intraoral scanner. By reflecting the dimensions of each block measured by CMM in the design data (mentioned as 3D design in the manuscript), the data was transformed into the same size as the real milled. And then, this modified data was used as reference data. I want to check whether I understood well. In addition, no matter how accurate measured values are reflected as design data, it is thought that there might still be errors. How did the authors try to overcome this part?

Tables 1 and 2 do not contain information on statistical results. In the discussion section, the authors mentioned trueness, but more analysis on precision results should follow. Although there was no statistical difference, the ScanCure group showed high precision, which should be added to the manuscript. The first sentence of the second paragraph in the discussion section says '~ VITA group had lowest RMS values,' but it seems to be the opposite. In the middle of the third paragraph, 'A hardware design of the products was sufficient to reproduce the results,' it is unclear whether the products refer to sprays/liquid, intraoral scanner, or specimen, so it is difficult to understand the intention of the authors.

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Reviewer #1: No

Reviewer #2: Yes: Professor (Associate) Tihomir Dovramadjiev PhD Eng.

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29 Mar 2022

Dear editor

Manuscript Number: PONE-D-21-40935

“Effect of Scanning-Aid Agents on the Scanning Accuracy in Specially Designed Metallic Models: A Laboratory Study"

We thank the reviewers for their comments, and we are grateful for the opportunity to provide further revisions to our paper. We changed our manuscript according to the reviewers’ comments and recommendations. We are trying to adequately address each of the points made by the reviewers. We would be very thankful if you could please reconsider a thoroughly revised manuscript. We highlighted the changes made in the manuscript by using a different color font (red): see correction marked form file, and explained details in this letter.

Response to Reviewer #1 Comments

The authors submit an article "Effect of Scanning-Aid Agents on the Scanning Accuracy in Specially Designed Metallic Models: A Laboratory Study". The study compares three types of sprays that are applied to the metal surface of dental prostheses and subsequently scanned with an intraoral scanner. Is there often a need in dental practice to scan metal surfaces of prostheses with an intraoral scanner? What other significant difference is there used between sprays (other than liquid type and powder type)?

Response: Thank you for your valuable comment. Scanning-aid materials used in this study are often necessary in daily clinical practice because the metallic shiny surface of prostheses(ex; metal crown or restoration) prevent the intraoral scanner from recognizing and obtaining the surface images properly. Composition and color are different between scanning-aid materials. According to each product information, ScanCure and IP Scan Spray contain titanium dioxide as their main components and have a white color, whereas VITA Powder Scan Spray is a spray-on, titanium dioxide free and blue colored pigment suspension.

The work scans the metal surface of prostheses - wouldn't it be enough to sandblast the surface to eliminate the shiny surface?

Response: Thank you for your valuable comment. The dental sandblasting is usually used to increase bonding strength during the cementation of prosthesis by mechanically roughening the surface. However, if it is used to eliminate the shiny surface of prosthesis in patient’s mouth, it adversely affects the polished surface of prosthesis which is essential for oral hygiene care. Also, the powder in sandblasting rather than the specialized scanning-aid agents can cause more discomforts for patients.

3. Cobalt chromium alloy or titanium alloy, is usually used in dental practice. Why did the authors use an aluminum alloy? It has no surface properties other than cobalt chromium alloy or titanium alloy?

Response: Thank you for your valuable comment. Due to the limitation of material selection in making the specially designed metallic models, we are focused on simulating the shiny surface of metallic model which interferes the scanner’s recognition of scanned images

As you mentioned, further studies with cobalt chromium alloy or titanium alloy in real patient’s mouth are necessary to reflect the real clinical situation.

4. In the article, you state that you used the I500 intraoral scanner for scanning - what technology does it scan with? Optical light? Laser? What is the accuracy and speed of scanning? Have you taken into account Trueness and Precision of the intraoral scanner itself when evaluating the data obtained?

Response: Thank you for your valuable comment. The I500 intraoral scanner uses video-type scanning based on triangulation technology with optical light. The accuracy is described by two measurement methods: trueness and precision. Trueness refers to the deviation of the measured value from the original value whereas precision refers to the closeness between repeated measured value. The speed of scanning was evaluated by measuring the time to obtain the total scanned image. We have considered the trueness and precision of the intraoral scanner by the product information which is based on the experiments from their own company and several published journals. Further studies using other types of intraoral scanners such as confocal-type type (e.g., 3Shape Trios®) are necessary to verify the accuracy between intraoral scanners.

5. The article is clearly written, described in detail Materials and Methods. The Trueness and Precision data are evaluated in the Results chapter. In Conclusion, it is stated that it is better to use a liquid type spray. This is certainly important information for dental practice, but I still miss the point, the main idea of the article. Is it a comparison of sprays or a comparison of metallic reference models (inlay, onlay, and bridge)? Why is it important for the dental technician / dentist which spray is better?

Response: Thank you for your valuable comment. The main point of this article is that how different types of scanning-aid materials affect the scanning accuracy using an intraoral scanner in specially designed metallic models. Inlay, onlay and bridge models were individually evaluated because we wanted to know if the same results of accuracy of scanning-aid materials were obtained in the cases of different shapes and distances. Particularly, in the bridge model, there were significantly differences of the trueness among the three different scanning-aid materials. It is important for the dental staff to know which spray is better because the accuracy of scanned image affects the success of the long-term prognosis of prosthesis

Response to Reviewer #2 Comments

The report is well written and easy to understand. Accurate results have been achieved. The information is also accurately illustrated. The authors work with modern technologies in design. Although the SolidWorks software is not the latest version (version 2016 is listed), it is strong enough functionally.

Response: Thank you for your favorable review for our manuscript.

Not as a remark, but as a recommendation:

- If possible, use institutional contact emails.

Response: Thank you for your valuable comment. “limdds@snu.ac.kr and bjkim016@snu.ac.kr” is the email of the Seoul National University.

- The conclusion is too small. It should increase the size. (Also in conclusion) The most important points should be noted in separate paragraphs.

Response: Thank you for your valuable comment. The following sentence was separated from the original paragraph as suggested by the reviewer.

Line 248: “Based on our findings, we recommend that liquid-type scanning agents should be used to obtain more accurate scan images of the metallic surfaces of dental restorations in clinical practice.”

Response to Reviewer #3 Comments

This is a fascinating study about the effect of scanning-aid agents on scanning accuracy in specially designed metallic models. Hence, the study adds to the current knowledge in this relatively new field and is definitely of clinical significance. However, it is still not clear to me the following matters as far as I read the manuscript.

This study aimed to evaluate the effect of different scanning-aid agents on the scanning performance of intraoral scanners. However, the description of ScanCure in the test group is insufficient. It is a liquid, so the authors need to describe more about it, including the ingredients. Also, there should be an explanation of how the agent from each group is applied. In what order and how many times these were applied or sprayed on the model. It is also necessary to describe how much powder was sprayed each time. The distance and angulation of the spray tip from the specimen are essential.

Response: Thank you for your valuable comment. The following paragraphs were added in materials and methods section as suggested by the reviewer.

Line 106: “VITA Powder Scan Spray is a spray-on, titanium dioxide free and blue colored pigment suspension with ethanol and isobutane. IP Scan Spray contains titanium dioxide with ethanol, propane, butane, and isobutane. And, ScanCure contains titanium dioxide and ethanol.”

Line 110: “Because the application procedure of scanning-aid materials is sensitive to operator’s proficiency, one skilled prosthodontist applied the materials on the model as follows: the liquid type of ScanCure was applied by one brush stroke at a time in all surfaces of model, powder type of VITA and IP scan spray was applied at a same distance (5 inch) and angulation (45 degree) of the spray tip from the specimen with the same time (2 seconds) to make an uniform thin layer of powder.”

In the 'Acquisition of Digital Data' section, the authors say that it was difficult to obtain a reference dataset, so it was obtained using 3D design, but it is not easy to understand. First of all, HERO7106 equipment is a contact scanner known as a Coordinate Measuring Machine (CMM), not a laser scanner. As I understand it, to scan with a laser scanner, the spray must be applied no matter how industrial-level accurate scanners are, so the quality of scan data is no different from that of the test group intraoral scanner. By reflecting the dimensions of each block measured by CMM in the design data (mentioned as 3D design in the manuscript), the data was transformed into the same size as the real milled. And then, this modified data was used as reference data. I want to check whether I understood well. In addition, no matter how accurate measured values are reflected as design data, it is thought that there might still be errors. How did the authors try to overcome this part?

Response: Thank you for your valuable comment. It is clearly correct what you understood as mentioned above. We wanted to make the reference dataset which was not affected by the scanning-aid materials. Of course, there should be inevitable error because it was not obtained by direct scanning. However, we try to reduce the error as possible by reflecting many measured data in the designing software.

Tables 1 and 2 do not contain information on statistical results. In the discussion section, the authors mentioned trueness, but more analysis on precision results should follow. Although there was no statistical difference, the ScanCure group showed high precision, which should be added to the manuscript. The first sentence of the second paragraph in the discussion section says '~ VITA group had lowest RMS values,' but it seems to be the opposite. In the middle of the third paragraph, 'A hardware design of the products was sufficient to reproduce the results,' it is unclear whether the products refer to sprays/liquid, intraoral scanner, or specimen, so it is difficult to understand the intention of the authors.

Response: Thank you for your valuable comment. Fig 4 and 5 are box plots which include data in Table 1 and 2 respectively.

As confirmed in Fig 5 and Table 2, the IP group showed the highest precision among the three groups with all the reference models although there was no statistical difference. The following sentence was revised in the discussion section.

Line 221: “Unlike trueness, there were no statistically significant differences in the RMS values of precision even though the IP group showed the highest precision among the three groups with all the reference models.”

The following sentence was revised to correct our mistake.

Line 204: “Furthermore, the VITA group had the largest RMS values among the three groups with all the reference models, with a statistically significant difference.”

The products what we meant were scanning-aid materials. The following sentence was revised to clarify our intention.

Line 225: “However, in each group, the hardware design of the scanning-aid materials was sufficient to reproduce the results of the scanned images.”

Submitted filename: Y-J Lim Response to Reviewers.docx

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14 Apr 2022

Effect of Scanning-Aid Agents on the Scanning Accuracy in Specially Designed Metallic Models: A Laboratory Study

PONE-D-21-40935R1

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Please, prior to publication, correct the captions of Figs. 4 & 5 as indicated by reviewer 1

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Reviewer #2: Yes

Reviewer #3: Yes

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Reviewer #2: Yes

Reviewer #3: Yes

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Reviewer #3: The authors have amended and modified the text adequately with my suggestions for improvement and, to the best of my understanding, also proposed by the other referees. The part that still needs additional correction is that the figure legends of Figures 4 and 5 are identical. Please state that Fig. 4 indicates trueness, and Fig. 5 describes precision. I strongly recommend that this manuscript be published in PLOS ONE.

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Reviewer #1: No

Reviewer #2: Yes: Professor (Assoc.) Tihomir Dovramadjiev PhD Eng.

Reviewer #3: No

27 Apr 2022

PONE-D-21-40935R1

Effect of Scanning-Aid Agents on the Scanning Accuracy in Specially Designed Metallic Models: A Laboratory Study

Dear Dr. Lim:

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