Potential Applications of Additive Manufacturing in Intervertebral Disc Replacement Using Gyroid Structures with Several Thermoplastic Polyurethane Filaments

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

  1-The title mentions "various TPU filaments," but the study only tests four; consider rewording to more accurately reflect the scope or include a justification for this selection.

  2-  The methods section states "at least 20 measurement points were collected" for microscopy, but does not specify the number of samples measured, which is crucial for reproducibility.

   3- The results describe printing defects qualitatively (e.g., "some drop formation and slight stringing"); please discuss how these defects could be prevented.

  4- Please elaborate on the mechanical analysis more; this section seems vague. For instance, it seems that in the compression test, results show only 2-3 samples per filament met the target force range, which is a very low yield; the discussion should address this high rate of design failure and its implications.

 5- Please use a summarized table to quickly assess relative mechanical performance.

  6- Please discuss the specific surgical or implantation challenges for such TPU Gyroid structures.

 7- It would be better if  the conclusion states future works  and the study's limitations due to using simple flat discs 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present an update on their prior work developing TPU-based gyroid structures through an increased filament library and by using different 3D printers. 4 different TPU filaments were optimized and evaluated for potential uses to replace intervertebral discs. They work to present the structural and mechanical characteristics for the different filaments under different design constraints to identify potential combinations that could serve as a suitable replacement. The authors however lack critical details and gaps within the present report that must be addressed.

 

Major Revisions:

1. The Authors indicate two separate printers were used, but no indication when each printer was used and why these two were chosen. Printers, even from the same company, will vary in hardware and software used to produce the final prints. Even the authors themselves mentioned the Prusa having more trouble with some stinging occurring, but still claim the prints were flawless. This could have drastically altered the performance.
2. The two printers used, have no specifications attached to them. While an entire rundown of the printers is needed, something as critical as if any modifications were made to the printers or if they were “stock”. It is also very important to know what size nozzle was used to print the structures. This would then drastically alter the layer heights and print resolution.
3. There are no print parameters included in the main manuscript and no indication of it in the SI. The authors simply state they tested different print temperatures to optimize the print settings, but typically there is much more that goes into it than that. If they just wanted to test different print temperatures than they should state that instead. Also, even if the print settings were maintained across all prints, it is important to include those even if they are in the SI.
4. This issue carries over to the stress-strain curves where the ISO EN 527 standards were followed, but no indication for print settings were provided. Were they 100% infill, how many walls, what was the layer height, etc.?
5. The software used to design the structures was not well discussed. For typical “gyroid” structures there are multiple patterns that can be chosen and different patterning parameters that will dictate the final structure.

 

Minor Revisions:

1. The abstract is too long. Not sure of the exact word count, but it can be condensed especially the methods section.
2. The introduction was 1 complete section, and it would help to break it up into distinct sections. Such as:
   1. Line 63 – “A new…”
   2. Line 77 – “Since 2010…”
   3. Line 90 – “A previous study…”
3. The authors discuss many aspects of the TPU disk replacements, but never once mention anything about bioactivity or integration within the human body. This would be of significant concern for the feasibility of the implant replacement current standards. They do not need to add in bioactivity experiments, but there should be something in the literature discussing this. If not, then it could potentially be an extremely novel addition to the field.
4. Slicer software used was Prusa-branded, but was this for both printers?
5. Tables 2, 3, 4, and 6 could be placed into SI as they are not needed for the main body of the manuscript.
6. The figure and table captions could be improved to include more details relevant to the data being presented. Replicate number, statistical method testing, if data was mean with SD or SEM, etc.
7. Figure 1 could use a reference dimension to give the printed structures scale. They also are very difficult to determine what is being evaluated. It could possibly help to have a zoom in, or just present 1 and have the other in SI zoomed in.
8. Table 3 could be presented better in a column or group column format using some form of a normalization or %difference. Then, statistical differences could be drawn over which were superior this would then beef up lines 235 – 243.
9. I might have missed this, but why are the disks so large for the compression tests? Was this to mirror the previous study?
10. Figure 10 the y axis is misspelled and there is no labeling or indication what each set of bars refer to. I assume they are to the chosen print parameters from Table 6.
11. Conclusion section, line 387 the authors refer to figure 4 but I believe they are wanting to refer to the appendix figure.
12. Line 396, the concern over the Prusa printer having issues with the TPU filament was odd and open ended. How did it struggle, but then still produce flawless prints.
13. Line 200, in vivo should be italicized.

 

In conclusion, this is a very cool publication and could provide greater insight into the applicability of 3D printed orthopedics. More work must be done to clarify decisions made and better organization/presentation of the results would further strengthen the impact.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors report an interesting study, which is a continuation of their previous work. However, it requires some improvement before publication.

Suggestions:

1. The abbreviation “TPU” in the title should be replaced with the full meaning “thermoplastic polyurethane”
2. The sentence on lines 67-70 should be revised by providing the exact values. The years (2021 and 2022) in question have already passed. So, the sentence should not be in future tense “expected to increase” or “will remain”. It is generally inappropriate when describing events or data that occurred entirely in the past.
3. The introduction could be enhanced further by commenting on previous studies involving TPU rather than focusing only on what the authors have done.
4. If possible, add the constant printing speed value on line 164.
5. The authors should check this sentence on lines 387-388 “This is illustrated in Figure 4, which shows a cross-section of a printed part”. The Figure 4 is about “Stress-Strain Curves” not cross-section/dimension.
6. “Table A 1” should be mentioned in the text.
7. Conclusion should be revised. The first sentence with the phrase “comprehensive test matrix” is not entirely the case. Other parameters such as thermal, and fluid dynamics properties were not evacuated in the current work, so it cannot be a “comprehensive” work. Please, include the limitation of the study in this section.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors made a good attempt to address the previous set of comments. However, there are still missing details that should be included.

Major Revisions:

1. The Authors indicate two separate printers were used, but no indication when each printer was used and why these two were chosen. Additionally, there is still no inclusion of printer settings within the manuscript. Things like layer height, wall and top/bottom counts, etc. These will drastically alter the final product. Furthermore, for elastic filament there are additional critical parameters like retract speed and distance. In the current state, no reader would be able to replicate the printer setup and settings. A print speed of 10 mm/s is thrown in, but no reason or emphasis on why this value was chosen. Was this optimized?
2. This issue carries over to the stress-strain curves where the ISO EN 527 standards were followed, but no indication for print settings were provided. How many walls, what was the layer height, etc.?

Minor Revisions:

1. Tables 2, 3, 4, and 6 could be placed into SI as they are not needed for the main body of the manuscript.
2. The figure and table captions could be improved to include more details relevant to the data being presented. Replicate number, statistical method testing, if data was mean with SD or SEM, etc.
3. Table 3 could be presented better in a column or group column format using some form of a normalization or %difference. Then, statistical differences could be drawn over which were superior this would then beef up lines 235 – 243.

In conclusion, the authors addressed many of my comments and with more details in the methods on print settings should be ready for publishing. Again, a lot of the tables and such can be moved to the SI to make the manuscript easier to digest. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf