Forces and Moments Generated by Direct Printed Aligners During Bodily Movement of a Maxillary Central Incisor

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for submitting your work to the Applied Sciences Journal. I found your research really interesting. Please see below my comments.

1. Introduction.
   Your introduction is concise. I would advise you to write a paragraph on 3D printing in Orthodontics and possibly other applications as well. 
2. Materials and methods.
   Did you do any sample size calculation? How did you decide to have a total of 60 aligners?
   Line 77. Please provide the manufacturers' instructions for fabricating thermoformed aligners. 
3. Results
   What is the power of your results?  Did you run a Post-hoc power analysis? If not, you can include it now. This will make your results more valuable. 
4. Well written
5. Conclusions
   I would advise you to write the conclusion in a single paragraph and avoid using numbers. 

Author Response

Comment 1: Introduction.
Your introduction is concise. I would advise you to write a paragraph on 3D printing in Orthodontics and possibly other applications as well. 
Response 1: Thank you for your recommendation. We have added a paragraph in the introduction section dedicated to 3D printing in orthodontics, including other applications.

Comment 2: Materials and methods.
Did you do any sample size calculation? How did you decide to have a total of 60 aligners?
Line 77. Please provide the manufacturers' instructions for fabricating thermoformed aligners. 
Response 2: Thank you for your questions. The sampling was performed by convenience, but we performed a post-hoc power analysis which is described in the Materials and Methods section. The result from this analysis is in the Results section. Briefly, using 10 aligners, we achieved a power of 88%, indicating that we had sufficient power to assess the difference between the groups.
The manufacturers’ instructions for ATMOS (PET-G) and Zendura FLX (TPU) foils of 0.76-mm (0.030”) thickness are to use the Biostar machine setting of Code 123 and Code 162, respectively. Code 123 equates to heating for 30 sec at 220^oC, and pressure forming at 5.8 bar, and then cooling for 120 sec. Code 162 equates to heating for 50 sec at 220^oC, and pressure forming at 5.8 bar, and then cooling for 60 sec.

Comment 3: Results
What is the power of your results?  Did you run a Post-hoc power analysis? If not, you can include it now. This will make your results more valuable. 
Response 3: Thank you for this opportunity to improve the scientific rigor of our manuscript. We have included the results from the post-hoc power analysis.  

Comments 4: Well written 
Response 4: We appreciate your feedback.

Comments 5: Conclusions
I would advise you to write the conclusion in a single paragraph and avoid using numbers. 
Response 5: Thank you for your recommendation. The numbered points have been combined into a single paragraph.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper is well written. This novel finding provides an important contribution to orthodontic treatment using aligner. However, the following points need to be cleared.

 

Abstract

Page 1, line 9: “in vitro” should be written in italics.

 

Materials and methods

Page 3, line 81: It is described that “For DPA (Figure 3), uDesign® 6.0 was used to generate aligner files with 0.50-mm thickness and 0.05-mm offset from tooth surfaces, …”. Why was it set to 0.05 mm thickness and 0.05 mm offset from tooth surfaces? If you have quoted something, you should cite the references.

 

Page 6, line 123: Can you describe the calculation method of M/F ratio in detail for reader’s understanding?

 

Results

Table 2: Under the 0.5 mm activation conditions, Fx is lingual direction in ATMOS® and Zendura FLX®, but My is facial. Normally, if force is applied in the lingual direction, the moment is also considered to be in the lingual direction. Can you confirm it again?

 

Table 3: Under the conditions of this experiment, it seems strange that the right central incisor and left lateral incisor receive force toward the lingual direction. Can you confirm it again?

 

Discussion

Page 12, line 218: Is “Forces and moments from planned lingual bodily movement” typo?

 

Page 14, line 295: It is described “When the activation was further increased to 0.50 mm, the force felt by the lateral incisor changed to opposite directions with ATMOS® and Zendura FLX® (0.54 N and 0.41 N, respectively).” Why do you think the direction of the force changed when the amount of activation increased? Can you describe in detail for reader’s understanding?

Author Response

Comment 1: Abstract Page 1, line 9: “in vitro” should be written in italics.
Response 1: Thank you for your feedback. We have corrected this error.

Comment 2: Materials and methods
Page 3, line 81: It is described that “For DPA (Figure 3), uDesign® 6.0 was used to generate aligner files with 0.50-mm thickness and 0.05-mm offset from tooth surfaces, …”. Why was it set to 0.05 mm thickness and 0.05 mm offset from tooth surfaces? If you have quoted something, you should cite the references.
Response 2: Thank you for your questions. A 0.5 mm thickness is representative of clinically relevant thickness in clear aligner therapy. During the thermoforming process, a 0.76-mm thickness foil would typically produce clear aligners of thickness in the range of 0.38 mm to 0.69 mm (Bucci et al. 2019). The thickness of Invisalign trays also range 0.566 – 0.644 mm (Mantovani et al. 2021). In 3D-printed aligners, 0.5 mm is a typical thickness reported in the literature (Baik et al. 2024). An offset of 0.05 mm from the tooth surface is to allow ease of removal from the dentition, and this setting has also been reported in the literature (Zinelis et al. 2022).

• Bucci R, Rongo R, Levatè C, et al. Thickness of orthodontic clear aligners after thermoforming and after 10 days of intraoral exposure: a prospective clinical study. Prog Orthod. 2019;20(1):36. Published 2019 Sep 9. doi:10.1186/s40510-019-0289-6
• Mantovani E, Parrini S, Coda E, et al. Micro computed tomography evaluation of Invisalign aligner thickness homogeneity. Angle Orthod. 2021;91(3):343-348. doi:10.2319/040820-265.1
• Baik JC, Choi YK, Cho Y, et al. Evaluation of different designs of 3D printed clear aligners on mandibular premolar extrusion using force/moment measurement devices and digital image correlation method. Korean J Orthod. 2024;54(6):359-373. doi:10.4041/kjod24.016
• Zinelis S, Panayi N, Polychronis G, Papageorgiou SN, Eliades T. Comparative analysis of mechanical properties of orthodontic aligners produced by different contemporary 3D printers. Orthod Craniofac Res. 2022 Aug;25(3):336-341. doi: 10.1111/ocr.12537. Epub 2021 Oct 4. PMID: 34569692; PMCID: PMC9544566.

Comment 3: Page 6, line 123: Can you describe the calculation method of M/F ratio in detail for reader’s understanding? 
Response 3: Thank you for your question. The faciolingual moment (My) and faciolingual force (Fx) were first transformed to a new reference point, the facial axis of the clinical crown (FA point) of the central incisor. Then, the My was divided by Fx to obtain the M/F ratio.

Comment 4: Results
Table 2: Under the 0.5 mm activation conditions, Fx is lingual direction in ATMOS® and Zendura FLX®, but My is facial. Normally, if force is applied in the lingual direction, the moment is also considered to be in the lingual direction. Can you confirm it again?
Response 4: We appreciate your detailed observation. Let us suppose that a single lingual force is applied at the labial surface of the incisor. This would indeed generate a lingual force at the center of resistance of the tooth, as well as a crown-lingual moment. However, this is true of a single point contact (i.e. a roller support).
With a clear aligner, the full-coverage plastic can create simultaneous contacts anywhere along the surface of the crown. With planned lingual translation of the incisor, the aligner is expected to cause mechanical displacement of the tooth in the lingual direction, where the crown rotates around a pivot, such as the center of resistance. The tooth would then contact the aligner against the linguo-incisal surface, and the combined distributed forces on the entire crown can conceivably produce a net moment at the center of resistance to rotate the crown facially, while the center of resistance feels a net lingual force.

Comment 5: Table 3: Under the conditions of this experiment, it seems strange that the right central incisor and left lateral incisor receive force toward the lingual direction. Can you confirm it again?
Response 5: Thank you for your observations. Clear aligner mechanics differ from that of fixed appliances. Fixed appliances have discrete points of force application which permit the elastic deformation of the appliance to be modeled and understood, while clear aligners present practical challenges in rationalization of its force system due to multiple simultaneous contacts and complex elastic deformations occurring throughout the appliance.
We know that a clear aligner, when seated onto the dentition, must be in equilibrium, where all forces and moments are balanced. But the contact points are numerous and it is difficult to identify which areas of the dentition are experiencing stress at a given time. It is possible that the combined lingual forces against the anterior teeth were balanced by mesial forces applied to the posterior teeth.

Comment 6: Discussion
Page 12, line 218: Is “Forces and moments from planned lingual bodily movement” typo?
Response 6: Thank you for your attention to this error. This should have been a subsection heading.

Comment 7: Page 14, line 295: It is described “When the activation was further increased to 0.50 mm, the force felt by the lateral incisor changed to opposite directions with ATMOS® and Zendura FLX® (0.54 N and 0.41 N, respectively).” Why do you think the direction of the force changed when the amount of activation increased? Can you describe in detail for reader’s understanding?
Response 7: Thank you for your questions. If a lingual force is applied, then it certainly follows that the adjacent teeth feel a facial force. We may hypothesize that this only occurs if the plastic is sufficiently elastic or is dimensionally accurate to the setup.
We found that with DPA, a lingual force on the central incisor resulted in a facial force on the adjacent lateral incisor, as one might expect.
With TFM, the lateral incisor felt a lingual force, perhaps because the aligner applied lingual forces against both teeth simultaneously, because the material was too stiff, or was dimensionally inaccurate due to limitations inherent to the thermoforming process. With increased activation, perhaps the contact geometry changed in such a way that the lingual surface of the lateral incisor received higher stress, leading to facial force.
Clear aligner therapy is inherently unpredictable, but we may cautiously suggest that ensuring a closer fit between the aligner and the dentition may lead to improved predictability of the force system. However, our data did not capture real-time stress and strain behavior across the dentition and it may be inappropriate to include the above rationalization into the body of the article. Finite element analysis or digital image correlation may be better suited to answer these questions.

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for the opportunity to review the manuscript entitled “Effect of Denture Adhesives on the Surface Roughness and Hardness of Denture Base Resins.” This study addresses the physical effects of prolonged denture adhesive exposure on different denture base materials fabricated by heat curing, 3D printing, and CAD-CAM milling. The topic is clinically relevant and potentially beneficial to the understanding of material performance in removable prosthodontics. However, several aspects of the study—particularly in terms of experimental design, data interpretation, and depth of discussion—require improvement. The manuscript would benefit from a thorough revision according to the comments provided below to enhance its scientific rigor and clarity.

1. Although this is an in vitro study, the authors did not sufficiently discuss how their findings translate into clinical practice. Please elaborate on the practical implications of the results for treatment planning, activation increments, or the need for auxiliary attachments.
2. While the data suggest that TC-85 generates lower and more predictable force systems, the manuscript lacks mechanistic explanations. The authors should elaborate on how TC-85's material characteristics (e.g., elastic modulus, shape memory effect) contribute to its biomechanical performance.
3. The discussion refers to "unintended forces and moments" but lacks quantitative analysis to assess their clinical impact. The authors should consider providing metrics such as angular deviation or peak off-axis forces to support their conclusion regarding TC-85’s superiority.
4. Although the M/F ratios are calculated and compared with established criteria, the clinical relevance in the absence of attachments remains unclear. Should future studies incorporate attachment design to realistically evaluate bodily movement feasibility?
5. The conclusion section contains some redundancy. It could be improved by condensing the content and focusing on key findings and clinical recommendations—e.g., whether high activation levels should be avoided or DPA systems should be preferred in practice.

Author Response

Comment 1: Although this is an in vitro study, the authors did not sufficiently discuss how their findings translate into clinical practice. Please elaborate on the practical implications of the results for treatment planning, activation increments, or the need for auxiliary attachments.
Response 1: We are grateful for this opportunity to improve the clinical relevance of our study. As you correctly point out, this is an in vitro study, and we remain cautious in making broad clinical recommendations based on our results. We have followed your suggestions and made changes to the manuscript to further elaborate on the clinical implications of our results, including activation increments and the need for composite attachments and pressure points to produce force systems conducive to challenging tooth movements such as translation.

Comment 2: While the data suggest that TC-85 generates lower and more predictable force systems, the manuscript lacks mechanistic explanations. The authors should elaborate on how TC-85's material characteristics (e.g., elastic modulus, shape memory effect) contribute to its biomechanical performance. 
Response 2: We greatly appreciate your insightful feedback. We have elaborated on TC-85’s material characteristics which contribute to the biomechanical performance of the aligners printed using the resin.

Comment 3: The discussion refers to "unintended forces and moments" but lacks quantitative analysis to assess their clinical impact. The authors should consider providing metrics such as angular deviation or peak off-axis forces to support their conclusion regarding TC-85’s superiority.
Response 3: Thank you for your recommendations. We are committed to strengthening the scientific rigor of our manuscript. We have provided quantitative metrics comparing the directional fidelity of DPA versus TFM.

Comment 4: Although the M/F ratios are calculated and compared with established criteria, the clinical relevance in the absence of attachments remains unclear. Should future studies incorporate attachment design to realistically evaluate bodily movement feasibility? 
Response 4: Thank you for your suggestion. Per the results of our study, it is evident that force system conducive to translation is difficult to accomplish using aligners alone regardless of the material. Our future studies will investigate the role of composite attachments and pressure points in enhancing the force system of challenging tooth movements using clear aligners.

Comment 5: The conclusion section contains some redundancy. It could be improved by condensing the content and focusing on key findings and clinical recommendations—e.g., whether high activation levels should be avoided or DPA systems should be preferred in practice. 
Response 5: Thank you for your suggestions. We have added clinical recommendations to the conclusion section and reduced redundancy.

Reviewer 4 Report

Comments and Suggestions for Authors

In the manuscript entitled “Forces and Moments Generated by Direct Printed Aligners During Bodily Movement of a Maxillary Central Incisor” the authors aimed to present an in vitro study investigating the forces and moments generated by direct printed aligners (DPA) compared to thermoformed aligners (TFM) during bodily movement of a maxillary central incisor. The study addresses a relevant topic in orthodontics, particularly with the increasing interest in direct printing technologies. I have only some minor suggestions to improve the quality of the manuscript:

• While the materials (PET-G, TPU, TC-85 resin) are mentioned, further details on their specific properties (e.g., modulus of elasticity, hardness) relevant to force delivery would be beneficial for comparison.
• Clarify how the 0.25 mm and 0.50 mm bodily displacements were precisely achieved and maintained during the measurements.

• Expand the discussion on the clinical implications of lower forces and moments, particularly regarding patient comfort, root resorption, and the potential for more predictable tooth movement with direct printed aligners.
• Consider adding a brief section on the limitations of in vitro studies and how these findings might translate (or not) to the complex in vivo environment, including factors like saliva, temperature, and masticatory forces.

• Ensure consistency in reporting units and decimal places throughout the results section and in any tables/figures.

Author Response

Comment 1: While the materials (PET-G, TPU, TC-85 resin) are mentioned, further details on their specific properties (e.g., modulus of elasticity, hardness) relevant to force delivery would be beneficial for comparison.
Response 1: Thank you for your recommendation. We have further elaborated on the relevant material properties which impact the biomechanical characteristics of DPA versus TFM.

Comment 2: Clarify how the 0.25 mm and 0.50 mm bodily displacements were precisely achieved and maintained during the measurements.
Response 2: We appreciate the opportunity to clarify this important experimental detail. The 0.25 mm and 0.50 mm bodily displacements were planned movements programmed into the clear aligners at the setup stage. However, the actual physical displacement of the resin teeth after the manufactured aligners were adapted to the dentition would likely differ from the planned movements.

Comment 3: Expand the discussion on the clinical implications of lower forces and moments, particularly regarding patient comfort, root resorption, and the potential for more predictable tooth movement with direct printed aligners.
Response 3: Thank you for your recommendation. We have expanded on the clinical relevance of our findings, including the implication of force magnitudes and force consistency on patient comfort, root resorption and predictability of tooth movement.

Comment 4: Consider adding a brief section on the limitations of in vitro studies and how these findings might translate (or not) to the complex in vivo environment, including factors like saliva, temperature, and masticatory forces.
Response 4: Thank you for your recommendation. Our in vitro design is indeed limited by the lack of factors such as saliva and masticatory forces, and the revised manuscript discusses these limitations.

Comment 5: Ensure consistency in reporting units and decimal places throughout the results section and in any tables/figures.
Response 5: Thank you for your recommendation. Maintaining the highest level of reporting quality is our priority. We have verified the tables, figures and the text to ensure consistency in the decimal places, reporting to the hundredths place value.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for making all the suggested changes. I think your work is truly outstanding, and it is great to be published.