PONE-D-24-27653A dual-plane fluoroscope to track joint kinematics during dynamic daily activitiesPLOS ONE

Dear Dr. Planta,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

I have struggled to secure a 2nd review for this article, and so have relied on the extensive review of the first reviewer. The reviewer was generally positive about the article but highlight areas that require more clarity and challenged some of the methods used. Please see comments below. 

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Aliah Faisal Shaheen

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PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This paper introduces the novel iteration of a double fluoroscope mounted on a robot to measure accurate knee kinematics during dynamic activities. The study goal is to present the device and validate the knee position tracking during various activities with healthy participants. Such system is very relevant for the field and is very promising in terms of future applications. This is a high quality paper, well written and easy to follow. However, I still have two major concerns, the first one regarding the metrics used to assess the quality of the tracking and the second one on the shared data.

Major concerns:

Metrics - The metric used is the distance between the knee centre and the centre of the field of view (FoV) of each fluoroscope. The study shows multiple timepoints where the knee centre is on the edge of one or the other fluoroscope’s FoV. If the knee centre is at the edge of the FoV, the image measured by the fluoroscope will not contain the whole view of the knee, e.g. a femoral condyle or part of the tibial plateau may be missing. And, if this is the case on both views at the time the view of the anatomical structure may be quite limited.

My concern is the following: is it possible to estimate accurately the pose of the femur or tibia with only partial view of the bone on one or two images? If this is the case, the authors should elaborate on this in the manuscript with appropriate references. If this is not the case, the results may overestimate the quality of the tracking and authors may have to use a metric estimating whether the volume of the knee is within the volume of acquisition during the various tasks, instead of 2 separate 2D metrics.

Data - The authors made a great effort in publishing the data, however, they are currently in proprietary format and requires MATLAB licences to be used. This goes against the FAIR (Findability, Accessibility, Interoperability, and Reusability) Data Principles, please update the data format to match these recommendations.

Minor concerns & Comments:

General questions:

- Although this may not be the focus of research, could the authors elaborate on potential applications of this system to other joints?

- The study mention the tibio-femoral joint, could this tool be used to track the patello-femoral joint as well? If so, it would be interesting to mention it for reader not familiar with dual fluoroscopy.

- What is the weight of the full system? Did you have to reinforce the screed of the room where the system is?

- Since the emitters is on one side of the walkway, is possible to measure either knee when walking or can the system only measure the knee that is closest/farthest to the emitter?

- The title and legend of the figure seems to be missing. In PlosOne they are generally reported after the paragraph where they are mentioned first.

Specific remarks:

- P2L26: “our system only used the real-time estimates of the joint to achieve its tracking”. Do you mean “joint centre”? if so, please specify to help the understanding.

- P2L30: “tasks with high variability between trials”, could you provide 1 or more examples to clarify what you mean?

- P2L31: The “However” seems irrelevant, please rephrase.

- P3L99: How was the threshold of 50mm chosen?

- P4EQ1: The admissible range of the motor torque mentioned L123 is not reported.

- P5L165: The four hours include also preparation of setup and cleaning or just the time with the participant?

- P7L242: ”Therefore there need to be another effect causing differences in performance tracking between subjects”. Does the authors have some hypotheses on the other potential sources of errors?

- Figure 5 represent the distribution in percentage of the distance to the centre of the FoV?

- Are figure 10,11,12,13 mentioned in the manuscript?

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Reviewer #1: Yes: Xavier Gasparutto

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PONE-D-24-27653R1A dual-plane fluoroscope to track joint kinematics during dynamic daily activitiesPLOS ONE

Dear Dr. Planta,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process. I appreciate that you have already gone over one round of reviews already, we had to ask a different reviewer to look at the manuscript as the original reviewer wasn't available and they have provided a constructive review with further suggestions for improvements (see below).  

Please submit your revised manuscript by Jun 28 2025 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Aliah Faisal Shaheen

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Partly

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: The authors present a first-of-its-kind dual fluoroscope setup for tracking the entire gait cycle during walking and stair ascent/descent. The experimental advance is noteworthy, and the authors are commended for developing an impressive setup for tracking knee joint motion in healthy participants and later in patients following total knee arthroplasty (TKA). The manuscript provides sufficient detail to describe the system setup and, if others had the resources, replicate the work. (Of particular note is the description for the tracking of knee joint position and controller.)

On the other hand, the potential advance of this setup is clouded by the lack of gold standard measurements to compare tracking results against. Knee position tracking is an understandable target, but the authors’ setup may perform worse than single plane system or static dual fluoroscopy systems due to movement of the emitters and intensifiers.

Furthermore, the knee centre location was not observed on fluoroscope images, further shadowing the potential advancement described in the manuscript. For example, knee joint positions on the trailing edge of the fluoroscopes will have very little bone in the image for registration, but knee joint positions towards the leading edge will have much of the bone available for image registration.

Another concern is the simulated cones for determining the fluoroscopes’ overlap. While the authors are commended for performing this calculation, the edges of the image intensifiers do not always yield usable image data for image registration. An object with the simulated cones dimensions, or close to it, imaged with both fluoroscopes, even statically, would lend confidence to the authors’ calculations. In the manuscript, all references to staying in the fluoroscopes’ FOV needs to include that the FOV was simulated.

Also of concern, but less so, the references to ‘all gait speeds’ refers to a narrow, self-selected band of speeds in healthy participants. Patients following TKA may walk at slower speeds and, more importantly, with higher variability, which, again, brings into question the advancement described by the authors. While the authors acknowledged the lowest gait speed for a single trial, separating the trials into different groups would have made their point about variable speeds much stronger.

Please change all references to ‘tracking’ with ‘knee joint position tracking’, because in the field of fluoroscopy tracking often refers to model-based tracking. And accuracy of tracking typically refers to comparisons to a gold standard for joint translations and rotations.

Specific Comments

Abstract

As referenced in the general comments, please clarify that the FOV was simulated, and that no images were acquired.

Introduction

No comments. Well written.

Methods

For lines 82-85, “As the radius of the intensifier sensor is 158mm, this setup (Fig 2) results in a capture volume with a length of 305mm in horizontal direction and a length of 445mm in lateral direction, and a length of 248mm in vertical direction at the intersection of the beam cone symmetry axes …”, were these theoretical values? Or experimental measures? Oftentimes the image intensifiers do not have usable image data all the way to the edge. Either way, the authors are commended for performing and reporting these calculations/measurements.

On line 87, “The knee centre is tracked using an active marker tracking system (Fig 3):….” Was the active marker placed at a standard distance up the thigh? And did the tracking include an directionality to determine if the knee was bent or straight?

The ‘Optical knee tracking’ section is noteworthy for the details provided on the experimental implementation. The text describes potential ‘rejected’ positions. Do the authors have an estimate for how often this occurred?

Three to four hours for acquisition seems rather long if no fluoroscope images were acquired. As alluded to the in the last comment, did the authors experience setbacks with the system that would not be expected in a traditional motion capture setup?

Results & Discussion

Could the authors describe the results in Figure 5 in more detail? The text merely states that the tracking varied substantially between subjects. Also of note in Figure 5, it’s not clear what the y axis is plotting from the caption. Distance from the projected knee joint position to the center of the image intensifier?

Lines 278-279, “leading to the negative values in the vertical plots of Fig 5.” The values in the plots look all positive. Are the authors referring to the positions plotted on the image intensifiers? Also, why did this not lead to the knee joint center position being outside the FOV, if the knee centre was lower the safety distance?

Could the authors explain this statement in greater detail? Lines 307-308, “This study is limited by the fact that the X-ray system was not used due to ethical considerations.” Isn’t the point of the system to use x-rays? It would be understandable to assess tracking of the knee joint position to confirm performance prior to exposing participants to x-rays. But it’s not clear if that was what the authors are implying.

Figures and Tables

Figure 1 is a nice zoomed-out view of the system, but, being the first figure, labels for the different components would benefit the readers who are not as familiar with fluoroscope systems.

The authors are applauded for the impressive polar plots in Figure 6 that clearly demonstrated the results.

For Figures 5, 6, 8, 10, and 11, it would help the reader interpret the images to know which direction the participants were moving in. This was mentioned in the text but not in the captions nor figures.

Conclusions

Same as the abstract, please insert ‘simulated’ prior to FOV, and clarify that no x-ray images were acquired. Mentioning that the knee joint position was tracked with skin markers is also necessary.

The last statement needs to be removed or clarified, as the high variability is between tasks and not necessarily between trials. Between trials implies the same participant completed the task differently, which the authors did not necessarily show.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

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[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

A dual-plane fluoroscope to track joint kinematics during dynamic daily activities

PONE-D-24-27653R2

Dear Dr. Planta,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Kind regards,

Aliah Faisal Shaheen

Academic Editor

PLOS ONE

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