Design of a Metal 3D Printing Patient-Specific Repairing Thin Implant for Zygomaticomaxillary Complex Bone Fracture Based on Buttress Theory Using Finite Element Analysis

Round 1

Reviewer 1 Report

The paper presents an interesting method for designing and modeling of zygomaticomaxillary bone fracture repair implant. The results can add to the current knowledge on the design of the craniofacial implants, but the quality of the manuscript must be significantly improved for potential publication.

Here are some general comments for the authors:

The grammar and English structure of the whole manuscript needs to be significantly improved.

Organization of the figures needs revision to follow the results: the figures provided and referred to in the methodology are presenting some of the results as well, which are later discussed in the results section….

The limitations of the study is not presented and discussed well.

Specific comments:

Page 2 , line 63, please explain what are the clinical requirements for the ZMC structural design and function.

 Page 2, line 65, Please describe more thoroughly the clinical advantages that the topology optimization-based designs have presented in the craniofacial repair outcomes.

Also, please discuss previous and recent designs of the ZMC implants, and their limitations.

Page 2, line 67. Please revise the English.

Page 2, line 86, “…loads was applied…”

Page 3, line 94, …to optimize… and to reduce…

Page 3, line 94, Please explain why the volume needs to be reduced in the optimized model…

Page 3, line 100, describe in details, what are the variables of the equation….

Page 3, line 102, revise the English

Page 4, line 117, describe  the buttress theory. Revise the English. Please provide quantitative measures to show sufficient resemblance of the HS model and IFS model load bearing distribution.

Page 5, line 122, revise the structure of the sentence.

Page 5, line 151, revise the English, incomprehensible

Page 8, line 205. How are the percentage variations calculated? Percentage to what value? The structure of the sentence is very hard to understand. What is meant to be delivered? Please revise.

Reference the described results to the associated figure/table.

Line 206, the structure of the sentence needs to be revised to properly communicate the results.

Line 208, The results are not clearly presented. What is the main finding from this set of stress distribution analysis? What is the stress distribution in the implant for PRST and mini-plate? They are missing in the model (figure 7). The orbital plate used on the model is also missing from these analysis.  The results of the tensile and compression stress distribution must be presented and explained clearly.

Page 8, table 2. What are the units of the presented data? Also, revise the table to assign what the first and second row for each model data presents.

Page 10, line 261, “…whose stress exceeds? Or do not exceed?”

Page 11, line 283, 30 um in diameter?

Author Response

Please see attached Word file.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear authors,

The manuscript "Design of a metal 3D printing patient-specific repairing thin implant for zygomaticomaxillary complex bone fracture based on buttress theory using topology optimization and finite element analysis" refers to the development of a fine repairing implant specific for the patient of the zygomatic-maxillary complex through the use of models. With this study, it was possible to optimize the topology to obtain a hollow skeleton model with structure and volume optimized. It is a work that brings interesting novelty for the implants construction. I think it is a good job to be published in this Journal, but I have some details to suggest to improve before acceptance.

Title => it is too long! Put a shorter title.  

Line 23 => space between the number and the unit (250 N)

Keywords => try to use words that it is not present in the title

Line 75 => "Material" and not "Materials"

Line 84 => add the word "respectively" for the parameters elastic modulus and Poisson’s ratio

Line 85, line 102, and others => space between the number and the unit

Line 98 and 99 => explain what is each variable mentioned here.

 

 

 

Author Response

The manuscript "Design of a metal 3D printing patient-specific repairing thin implant for zygomaticomaxillary complex bone fracture based on buttress theory using topology optimization and finite element analysis" refers to the development of a fine repairing implant specific for the patient of the zygomatic-maxillary complex through the use of models. With this study, it was possible to optimize the topology to obtain a hollow skeleton model with structure and volume optimized. It is a work that brings interesting novelty for the implants construction. I think it is a good job to be published in this Journal, but I have some details to suggest to improve before acceptance.

 

General comment:

1. Title => it is too long! Put a shorter title.

Response: Thanks for your comment. The title was revised to “Design of a metal 3D printing patient-specific repairing thin implant for zygomaticomaxillary complex bone fracture based on buttress theory”

 

2. Line 23 => space between the number and the unit (250 N)

Response: Thanks for your suggestion. This sentence was revised.

 

3. Keywords => try to use words that it is not present in the title

Response: Thanks for your suggestion. The keywords were modified to “Bony supporting; patient matched; 3D printing; topology optimization; finite element method”.

 

4. Line 75 => "Material" and not "Materials"

Response: Thanks for your suggestion. This word was corrected.

 

5. Line 84 => add the word "respectively" for the parameters elastic modulus and Poisson’s ratio

Response: Thanks for reminding. This sentence was revised.

 

6. Line 85, line 102, and others => space between the number and the unit

Response: Thanks for reminding. This sentence was revised.

 

7. Line 98 and 99 => explain what is each variable mentioned here.

Response: Thanks for your comment. We explained each variable as follows:

V=the volume of design structure

=design variable

=von-Mise stress in each element

=maximum allowable stress

 

 

Reviewer 3 Report

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

The manuscript presents a finite element model with optimized geometry to a set of “buttress pillars” that

eliminates the low stress regions of the model. The work sounds interesting. The authors have published a

similar work in https://doi.org/10.1142/S0219519419400256. I believe the work can be published if the

major concerns are addressed. Considering this is a biomechanical work, I believe that the mechanical part

needs more elaboration on different subjects, before it can be published. Here are my comments:

 

General comments

1. I do not understand the main goal of the work. A finite element model is developed and it goes through

FEM optimization to remove the volumes that “do not need to exist” (line 114).

Response: Thanks for your comment. The buttress structure is the basic PSRT implant shape design structure, which gives the PSRT implant a lightweight shape and allows the implant to restore the mid-face mastication force transfer pattern to increase mid-face structure stability.

 

2. The optimization only includes one type of load (compressive force on teeth, somehow modeling the chewing process), but what happens if the model is exposed to another type of load, for example, punch-like pressure to the face or impact forces? If these loads are considered, the topographic optimization will result in a different HS model. In fact, the HS model presented here is quite weak against pressure-like loads as all buttress pillars will turn to bending beam-like structure. So, this model is not really usable as a biomechanical representation of facial bones, as it only responds OK to one type of load.

Response: Thanks for your comment. This article focuses on chewing function restoration in patients after using the PRST implant to reconstruct the mid-face fracture. The other impact force should be ignored.

 

3. The word buttress theory is misleading. In solid mechanics, such a theory (with that name) is unheard
4. The authors have used it in another publication, https://doi.org/10.1142/S0219519419400256. There is no explanation of what this theory expresses and how it is formulated. The general idea is that if you remove low stress regions from a structure under mechanical load, the response of the new system to the same load is not affected that much. There is a section in the manuscript, 2.2, but the theory is not explained there. This is a key point in the manuscript and needs more elaboration. The verification mentioned in this part of the manuscript does not help that much either. The maximum stress in HS model is 18.3% higher than solid model, which is not as negligible as it is thought.

Response: Thanks for your comment. The buttress theory is an important theory for reconstruction and restoring stability to the mid-face in the plastic surgery division. The supporting facial bony structure can be classified as vertical and horizontal buttresses. The vertical buttresses consist of the paired nasomaxillary (NM) and Zygomaticomaxillary (ZM) buttress which can provide the bony support required for mastication and protect the surrounding thin bone.

 

Reference: Stanley, R.B. Jr. Buttress fixation with plates. Oper. Tech. Otolayngol. Head Neck Surg. 1995, 6, 97-103.

 

4. Expression before Figure 2 is confusing. It is mentioned that areas with stress below 20 MPa were removed, but Figure 2 shows that blue areas (that are removed in HS model) have the stress value around

4-5 MPa and are removed in Figure 2d. The HS model includes regions with stresses at levels well below

20MPa.

Response: Thanks for your comment. Using topology optimization analysis, the unnecessary support structure in the IFS model was removed to obtain the remaining supporting structure, i.e. HS model. In the HS model, the same occlusal forces were applied again to verify the topology optimization analysis that the unnecessary structure was removed and the remaining structure can support the occlusal forces in mid-face. The result showed that when 75% of the volume was removed from the IFS model, the remaining structural (HS model) bone stress was merely 18.3% higher than the IFS model stress.

 

5. One major problem that is needed to be addressed is the fracture criteria used in the model (for bone and the implant materials). Apparently, the two materials (it is actually three, because bone part is divided

into two different material types) would have different failure criteria. The authors need to use a failure

criteria for each material and could sue one of the three classical failure theories (max. normal stress,

maximum shear stress, maximum distortion energy) or modified versions of them for bone and titanium.

 

Just presenting the maximum normal stress (first and third principal stresses) is not sufficient and does

not represent the actual strength of the structure.

 

Response: Thanks for your comment. Bone is a brittle material, so the failure strength of bone is presented in principal stress (maximum and minimum principal stress), while the implant is a homogeneous isotropic ductile material. The failure strength of the implant is presented in von-Mises stress, which are common criteria in biomechanics.

 

6. While the authors explain the contact between screws’ head and the implant surface, it is not clear how much of compressive stress (Hertz stress) is created between the screw and the bone part. This is

something critical to explore as the entire force will be transferred to the bone section via side walls of

the screws.

Response: Screws and bones are assumed as bonded to simulate screw/bone interfaces in which the screws are anchored well and the forces continuously transmitted to the mid-face.

 

7. In line 282, it is mentioned that “for validation of the feasibility of this study” an actual model is manufactured and shown in Figure 8, but there is no further verification. Since the goal of the manuscript

is the topology optimization and FEM simulation, the only way to validate this is to test an actual case, but there is no test presented here.

Considering the complication of the system under investigation (variable material properties on bones), and the topology optimization, I believe the only way to verify that the entire work is credible is to run some actual test and either determine the stresses under certain load (using strain gauges) or to increase the load until the system fails and compare the actual and simulated fracture stresses.

One of the testing points is the compressive stress at the intersection of the screws’ side walls and the bone. It is not clear how this is simulated, and considering the complexity of the bone’s porous material, this might be a critical point in the design and needs experimental verification.

Response: Thanks for your comment: This study focused on design bone plate feasibility in combination with the lightweight and mid-face occlusal force transmission function using topology optimization. Firstly, this study verified that the PSRT implant could restore mid-face stability using the finite element method results. In future work, we can simulate the occlusal forces and muscle tension on the model reconstructed with PSRT implant in the biomechanical test, and measure the bone and the implant using strain gauges for verification.

 

There are also minor changes, as explained below:

Minor changes

1. The manuscript is well-written and may contain few typos. I found one in line 94: “was used to optimize”.。

Response: Thanks for your comments. This sentence was revised.

 

2.It is more convenient to express Young’s modulus of Titanium in GPa (line 155).

Response: Thanks for your comments. This sentence was revised.

 

3. In Table 1, the variation % is wrong. Since the actual value of the stress is for IFS model, the error

should be calculated using that number (26.46-22.37)/22.37 = 18.3%.

Response: Thanks for your comments. This sentence was revised.

 

4. Data in Table 2 is quite confusing. There are 2 rows for each case, but the negative and positive numbers are not explained, so I did not understand what those numbers represent. The table should be revised in a better format, so the reader understands it.

Response: Thanks for your comments. The Table 2 was modified.

 

 

 

 

 

Round 2

Reviewer 1 Report

The comment are included in red in the following response document. 

3. The limitations of the study is not presented and discussed well.

Response: Because the chewing condition occurs clinically most often, it was applied to simulate occlusal force. Structural optimization was obtained for explaining the buttress theory under chewing forces. However, this could not represent actual individual occlusion situations. The PSRT will undergo animal trials and clinical trials in the future to confirm whether the PSRT plate can restore the mid-face mechanical performance.

 

Please include the discussion of the study limitations in the Discussion section.

 

Specific comments:

1. Page 2, line 63, please explain what are the clinical requirements for the ZMC structural design and function.

Response: Traditionally, clinical requirements for the ZMC fracture involved resting bone restoration and fixation using a mini-plate after the mid-face reconstruction surgery. However, geometric restoration demands, lightweight structure and primary stability are increased in the current mid-face fracture treatment protocol.

 

Include this explanation in the manuscript in the introduction section.

 

2. Page 2, line 65, Please describe more thoroughly the clinical advantages that the topology optimization-based designs have presented in the craniofacial repair outcomes.

Response: As shown in figure 1(c), the occlusal forces were transmitted upward through the Nasomaxillary buttress and zygomaticomaxillary buttress to the mid-face. We found these main supporting structures on the mid-face of the intact bone with bone thickness expressed after performing the topology optimization. If the bone plate was designed according to the mid-face main supporting structure, it could replace the intact bone to support and transmit the occlusal forces in mid-face.

 

Include the description in the manuscript text in introduction

 

3. Also, please discuss previous and recent designs of the ZMC implants, and their limitations.

Response: The traditional min-plate provides a fixation function only to the resting bone, but the mid-face stability after surgery is not considered, which may lead to plate deformation or screw loosening, making an asymmetrical mid-face appearance.

 

It is informative to include this information in the manuscript. Please add.

7. Page 3, line 94, Please explain why the volume needs to be reduced in the optimized model…

Response: Thanks for your comment. Using topology optimization analysis, the main mid-face supporting structure from the HS model was obtained as the design criteria for the PSRT implant design, so that the PRST implant with lightweight structure could restore the mechanical performance and increase mid-face stability.

Include the explanation in the manuscript

 

8. Page 3, line 100, describe in details, what are the variables of the equation….

 

 

 

 

Response:

V=the design structure volume

=design variable

=von-Mise stress in each element

=maximum allowable stress

 

 is the objective function. This function represents the volume (V) that is being minimized for best performance.  is the characteristic that the solution needs to be satisfied. The element whose stress is below the minimum allowable stress that can be removed.

Include the details in the manuscript

 

10. Page 4, line 117, describe the buttress theory.

Response: Thanks for your comment. Buttress theory: The supporting facial bony structure can be classified as vertical and horizontal buttresses. The vertical buttresses consist of the paired nasomaxillary (NM) and Zygomaticomaxillary (ZM) buttress which can provide the bony support required for mastication and protect the surrounding thin bone.

Reference: Stanley, R.B. Jr. Buttress fixation with plates. Oper. Tech. Otolayngol. Head Neck Surg. 1995, 6, 97-103.

Include the description of the bitterness theory in the manuscript in associated section

 

12. Please provide quantitative measures to show sufficient resemblance of the HS model and IFS model load bearing distribution.

Response: The stress values of 1~6 points were extracted from the HS model and compared with the IFS model, the variation between the two groups was found to be 0.16%~11.42%, indicating that the stress distribution of the HS model is similar to the IFS model.

 

 

 

 

 

 

 

 

 

 

 

 

Provide the comparison of the two models in the supplementary data.

 

15. Page 8, line 205. How are the percentage variations calculated? Percentage to what value? The structure of the sentence is very hard to understand. What is meant to be delivered? Please revise.

Response: Thanks for your comment. As the control group, minimum/maximum principal stress at the 1st~6th points was recorded to be compared with the PSRT implant group and the mini-plate group individually. The difference between the values should be expressed as a percentage, as shown in Figure 6. Because the frontal process was subjected to compressive stress, the stress value at 1st to 3rd point on each model was compared with the minimum principal stress, while the tensile stress was applied to the Zygomatic process, so the stress value at 4th to 6th point was compared with the maximum principal stress. The stress variations between the IFS and PSRT implant models were much smaller than the stress variations between the IFS model and mini-plate group, which showed that the mid-face stress distribution reconstructed by the PSRT implant was similar to the stress distribution in the IFS model.

 

Please concise this explanation and include in the associated section in the manuscript

18. Line 208, The results are not clearly presented. What is the main finding from this set of stress distribution analysis? What is the stress distribution in the implant for PRST and mini-plate? They are missing in the model (figure 7).

Response: Figure 8 and Figure 7 should be compared together. It is difficult to observe the stress distribution of the three-dimensional structure just using the stress distribution in Figure 8. Figure 8 shows that the PSRT model has a similar stress trend as the IFS model, which means the mid-face will have high stability with the PSRT implant reconstruction.

Include the explanation in the manuscript to clarify the results.

 

19. The orbital plate used on the model is also missing from these analysis. The results of the tensile and compression stress distribution must be presented and explained clearly.

Response: Thanks for your comment. In order to simulate the actual clinical reconstruction situation, the orbital plate was used on the PSRT model and the mini-plate model. The orbital plate did not affect the overall stress distribution trend of the PSRT model and mini-plate model, so the stress distribution of the orbital plate was not presented in the results.

 

Include the explanation in the manuscript

 

Author Response

The comment are included in red in the following response document. 

3. The limitations of the study is not presented and discussed well.

Response: Because the chewing condition occurs clinically most often, it was applied to simulate occlusal force. Structural optimization was obtained for explaining the buttress theory under chewing forces. However, this could not represent actual individual occlusion situations. The PSRT will undergo animal trials and clinical trials in the future to confirm whether the PSRT plate can restore the mid-face mechanical performance.

Please include the discussion of the study limitations in the Discussion section.

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 12, line 301).

 

Specific comments:

1. Page 2, line 63, please explain what are the clinical requirements for the ZMC structural design and function.

Response: Traditionally, clinical requirements for the ZMC fracture involved resting bone restoration and fixation using a mini-plate after the mid-face reconstruction surgery. However, geometric restoration demands, lightweight structure and primary stability are increased in the current mid-face fracture treatment protocol.

Include this explanation in the manuscript in the introduction section.

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 2, line 62).

 

2. Page 2, line 65, Please describe more thoroughly the clinical advantages that the topology optimization-based designs have presented in the craniofacial repair outcomes.

Response: As shown in figure 1(c), the occlusal forces were transmitted upward through the Nasomaxillary buttress and zygomaticomaxillary buttress to the mid-face. We found these main supporting structures on the mid-face of the intact bone with bone thickness expressed after performing the topology optimization. If the bone plate was designed according to the mid-face main supporting structure, it could replace the intact bone to support and transmit the occlusal forces in mid-face.

Include the description in the manuscript text in introduction

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 2, line 69).

 

 

3. Also, please discuss previous and recent designs of the ZMC implants, and their limitations.

Response: The traditional min-plate provides a fixation function only to the resting bone, but the mid-face stability after surgery is not considered, which may lead to plate deformation or screw loosening, making an asymmetrical mid-face appearance.

It is informative to include this information in the manuscript. Please add.

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 1, line 35).

 

7. Page 3, line 94, Please explain why the volume needs to be reduced in the optimized model…

Response: Thanks for your comment. Using topology optimization analysis, the main mid-face supporting structure from the HS model was obtained as the design criteria for the PSRT implant design, so that the PRST implant with lightweight structure could restore the mechanical performance and increase mid-face stability.

Include the explanation in the manuscript

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 4, line 148).

8. Page 3, line 100, describe in details, what are the variables of the equation…. 

Response:

V=the design structure volume

=design variable

=von-Mise stress in each element

=maximum allowable stress

 is the objective function. This function represents the volume (V) that is being minimized for best performance.  is the characteristic that the solution needs to be satisfied. The element whose stress is below the minimum allowable stress that can be removed.

Include the details in the manuscript

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 3, line 106).

10. Page 4, line 117, describe the buttress theory.

Response: Thanks for your comment. Buttress theory: The supporting facial bony structure can be classified as vertical and horizontal buttresses. The vertical buttresses consist of the paired nasomaxillary (NM) and Zygomaticomaxillary (ZM) buttress which can provide the bony support required for mastication and protect the surrounding thin bone.

Reference: Stanley, R.B. Jr. Buttress fixation with plates. Oper. Tech. Otolayngol. Head Neck Surg. 1995, 6, 97-103.

Include the description of the bitterness theory in the manuscript in associated section

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 4, line 127).

 

12. Please provide quantitative measures to show sufficient resemblance of the HS model and IFS model load bearing distribution.

Response: The stress values of 1~6 points were extracted from the HS model and compared with the IFS model, the variation between the two groups was found to be 0.16%~11.42%, indicating that the stress distribution of the HS model is similar to the IFS model.

Provide the comparison of the two models in the supplementary data.

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 8, line 221, table 1).

15. Page 8, line 205. How are the percentage variations calculated? Percentage to what value? The structure of the sentence is very hard to understand. What is meant to be delivered? Please revise.

Response: Thanks for your comment. As the control group, minimum/maximum principal stress at the 1st~6th points was recorded to be compared with the PSRT implant group and the mini-plate group individually. The difference between the values should be expressed as a percentage, as shown in Figure 6. Because the frontal process was subjected to compressive stress, the stress value at 1st to 3rd point on each model was compared with the minimum principal stress, while the tensile stress was applied to the Zygomatic process, so the stress value at 4th to 6th point was compared with the maximum principal stress. The stress variations between the IFS and PSRT implant models were much smaller than the stress variations between the IFS model and mini-plate group, which showed that the mid-face stress distribution reconstructed by the PSRT implant was similar to the stress distribution in the IFS model.

Please concise this explanation and include in the associated section in the manuscript

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 8, line 221).

18. Line 208, The results are not clearly presented. What is the main finding from this set of stress distribution analysis? What is the stress distribution in the implant for PRST and mini-plate? They are missing in the model (figure 7).

Response: Figure 8 and Figure 7 should be compared together. It is difficult to observe the stress distribution of the three-dimensional structure just using the stress distribution in Figure 8. Figure 8 shows that the PSRT model has a similar stress trend as the IFS model, which means the mid-face will have high stability with the PSRT implant reconstruction.

Include the explanation in the manuscript to clarify the results.

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 9, line 234).

19. The orbital plate used on the model is also missing from these analysis. The results of the tensile and compression stress distribution must be presented and explained clearly.

Response: Thanks for your comment. In order to simulate the actual clinical reconstruction situation, the orbital plate was used on the PSRT model and the mini-plate model. The orbital plate did not affect the overall stress distribution trend of the PSRT model and mini-plate model, so the stress distribution of the orbital plate was not presented in the results.

Include the explanation in the manuscript

 

Response: Thanks for your comments. These sentences have been included in the revised manuscript (Page 9, line 239).

Reviewer 3 Report

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

 

1. The authors mentioned the load they consider is to simulate chewing, but this needs to be emphasized that the model does not considered other loads (maybe for simplicity? Or any reason the authors consider).

Response:

Yes, only vertical loading condition was considered in our simulation to simplify the complexity of chewing function because vertical occlusal force applied on the molar was the main component transmitting from tooth to maxillary during chewing period [2].

 

Reference: Janovic, A.; Saveljic, I.; Vukicevic, A.; Nikolic, D.; Rakocevic, Z.; Jovicic, G.; Filipovic, N.; Djuric, M. Occlusal load distribution through the cortical and trabecular bone of the human mid-facial skeleton in natural dentition: a three-dimensional finite element study. Ann. Anat. 2015, 170, 16-23.

 

2. The optimization process still unclear and the authors did not make changes to address that. This si what I asked in my previous review: It is mentioned that areas with stress below 20 MPa were removed, but Figure 2 shows that blue areas (that are removed in HS model) have the stress value around 4-5 MPa and are removed in Figure 2d. The HS model includes regions with stresses at levels well below 20MPa. Apparently the claim that areas with stresses above 20 MPa are kept and the rest are removed is not correct, or I am missing something. There is no revision in this area and it needs to be fixed.

Response:

The unstressed volumes on the “IFS model” were removed to generate the “HS model” with supporting-structure after topology optimization analysis. This HS model likes the intact buttress supporting structure to share and transmitted occlusal force of the mid-face. When the HS model again to receive the same load, the occlusal forces would be redistributed to the HS model and the stress of each element was recalculated, so the element on the HS model may be subjected to stress less than 20 MPa.

 

3. It seems the assumption for contact will miss the contact stress and is too simplified. Considering the bone structure is a composite (with variable material and mechanical properties) bonded contact may result in significant error and should be verified or fixed.

Response:

In this study, the interfacial fixation between the screws and bone was assumed to be the press-fit and bonded condition (displacement continue) was use to mimic the osseointegration completed. Friction contact condition was used to simulate the interfacial adaption between the screw head and the PSRT implant. To understand the contact stress even in bone/screw or screw/PSRT implant interfaces might be importance for micro-mechanics but is not the goal of our global optimization analysis owing numerical convergence consideration.

 

4. To explain Figure 8, the authors have expressed that this paper is only about the modeling. There are to points to address: (i) considering the complexity of the bone structure and the simplifications the authors have used in the simulation, it is difficult to verify the simulation results unless there is experimental verification, so I highly recommend the test verification to be included in the work. (ii) if the authors do not want (or cannot) include the experimental work, then figure 8 does not add any value to the paper and should be removed, because it is now misleading that there is a fabricated platform, but there is no test. I still believe that an experimental verification is necessary to verify the simulation results, and without the test data, considering inconsistency and major simplifications, it is hard to accept the simulation results.

 

Response:

1. The main objective of this study is to design a lightweight implant which could restore the mid-face fracture and transmit occlusal forces according to the intact buttress structure. The obtained PSRT implant was attempted to manufacture by metal 3D printing for further clinical application owing to the complexity of the printing process such as printing direction, laser power, supporting structure design, etc. for thin objects fabrication were needed overcome. Fig. 9 illustrated its feasibility of the 3D-printing manufactured process.
2. Thanks for the Reviewer’s comments. We know the importance of performing an experiment to validate the result of FE analysis. However, the physical skull bone model (geometry and material property) was difficulty to fabricate due to it was reconstructed from a real patient's CT medical image. Cadaver skull models with ZMC fracture might be used to perform a validated experiment in the future based on this study, but enough cadaver skull collection and how to create a standardized ZMC fracture pattern on the skull model may be the first step at this stage.

 

 

Round 3

Reviewer 3 Report

The authors have not responded to two of my main comments:

1. The topology optimization is set to remove any element with stress of below 20 MPa (line 133-116 of the text), but when we look at the stress simulation results shown in Figure 6a, only red areas are above 20 MPa. This means that in the contour plot the entire blue to orange areas should be removed which is not logical and the Figure 6b also does not approve that. Therefore, there is a serious issue with accepting the results of this optimization and simulation, and the authors need to clarify that.
2. As I mentioned in previous report, there are so many simplifications in the FEM model that it needs to be experimentally verified, and without experimental verification, it is very difficult (if not impossible) to understand how accurate or correct the results are.
3. The authors should remove Figure 9 from the content. It does not add any extra value and it is rather misleading, because they have mentioned (in line 293) the fabrication of the sample is for "validation of the study". The study is FEM simulation and its validation is only possible by experiments and fabrication alone does not validate the work.
4. I believe the authors should clearly respond to the comments and not repeat what is already in the manuscript. The need to address the above comments before the paper can be accepted.

Author Response

Comment:

The topology optimization is set to remove any element with stress of below 20 MPa (line 133-116 of the text), but when we look at the stress simulation results shown in Figure 6a, only red areas are above 20 MPa. This means that in the contour plot the entire blue to orange areas should be removed which is not logical and the Figure 6b also does not approve that. Therefore, there is a serious issue with accepting the results of this optimization and simulation, and the authors need to clarify that.

Response:

The elements removed in the IFS model were based on the topology optimization calculation results of these elements whose “von- Mises stress” is less than 20MPa and to generate the HS model with supporting-structure. However, the stress patterns shown in Figure 6 were “maximum principal stress” due to bone structure is prone to brittle materials, so Figure 6 cannot use 20MPa as a value indicator of whether the element was deleted. Also the sentence on Line 116 was revised to “The von Mise Stress value of 20 MPa was setup as the minimum….” to avoid confusion.  

 

Comment:

As I mentioned in previous report, there are so many simplifications in the FEM model that it needs to be experimentally verified, and without experimental verification, it is very difficult (if not impossible) to understand how accurate or correct the results are.

Response:

1. We agree the Reviewer’s comments, i.e. it is very difficult to understand how accurate or correct the FE analysis results are and the importance of performing an experiment to validate the result of FE analysis. However, the physical skull bone model (geometry and material property) was difficulty to fabricate due to it was reconstructed from a real patient's CT medical image. Cadaver skull models with ZMC fracture might be used to perform a validated experiment in the future based on this study, but enough cadaver skull collection and how to create a standardized ZMC fracture pattern on the skull model may be the first step at this stage.
2. In order to more closely reflect the focus of the structure optimization of the skull to support mid-face occlusal force transfer of this study, the title was revised to “Design of a metal 3D printing patient-specific repairing thin implant for zygomaticomaxillary complex bone fracture based on buttress theory: a finite element analysis” to mean this study is devoted to the analysis and design of the PSRT implant.

 

Comment:

The authors should remove Figure 9 from the content. It does not add any extra value and it is rather misleading, because they have mentioned (in line 293) the fabrication of the sample is for "validation of the study". The study is FEM simulation and its validation is only possible by experiments and fabrication alone does not validate the work. I believe the authors should clearly respond to the comments and not repeat what is already in the manuscript. The need to address the above comments before the paper can be accepted.

Response:

We agree the Reviewer’s comments, Figure 9 and the related content of the verification experiment were deleted in the revised manuscript.