-->PONE-D-26-07309-->-->A Preliminary Comparison of Prosthetic Socket Liner Strain Determined Using Digital Image Correlation and Finite Element Analysis-->-->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

Reviewer #2: Partly

Reviewer #3: Partly

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

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: No

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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

Reviewer #2: Yes

Reviewer #3: 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

Reviewer #2: Yes

Reviewer #3: 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: The manuscript presents a novel and valuable study that compares digital image correlation (DIC) and finite element analysis (FEA) for measuring strain in prosthetic liners. The work is well motivated, methodologically detailed, and demonstrates promising agreement between experimental and computational strain estimates. However, the reviewer identifies several important limitations requiring major revision.

Key Strengths

First known attempt to measure liner strain in vivo under realistic loading conditions.

Strong methodological transparency in DIC setup, socket design, and FEA workflow.

Good numerical agreement between DIC and FEA strain patterns, supporting the feasibility of this validation approach.

Major Concerns

•Single participant, limiting generalizability.

•Simplified soft‑tissue modelling (linear elastic instead of hyperelastic).

•Limited DIC field-of-view, restricting strain measurement to a small region.

•Anatomical approximations due to generic bone models and tape‑based scaling.

•Artificial experimental conditions, including lubricant use and glare from the transparent socket.

•Simplified loading conditions with several neglected moments and assumptions.

Required Revisions

•Provide quantitative error metrics between DIC and FEA.

•Justify participant selection and modelling assumptions.

•Expand discussion of optical and mechanical limitations affecting DIC accuracy.

•Clarify why hyperelastic models failed, and better explain strain discrepancies.

•Temper conclusions to reflect the preliminary, single‑subject nature of the study.

Suggested improvement

Lines 19–37

•Comment: The abstract is clear and well structured, but it should acknowledge the single participant design explicitly, as this strongly limits generalizability.

•Suggestion: Add 1–2 sentences describing key limitations upfront to set realistic expectations.

Lines 40–81

•Lines 49–58: You highlight variability in FEA stress predictions due to a lack of validation. This is an excellent justification. However, more explanation of typical error magnitudes or clinical relevance thresholds would strengthen the rationale.

•Line 60: DIC is presented as widely accepted; consider citing more prosthetics-specific DIC applications to position the study within the field.

•Lines 74–81: The two research goals are clearly stated. However, consider emphasising that this is an exploratory single-subject feasibility study, not a validation study.

Lines 85–122

•Lines 85–89: Single participant; demographic justification is absent. Authors should state why this individual was chosen (e.g., typical limb geometry, availability).

•Line 100: Speckle pattern creation using fabric dye and thickener requires justification—does this alter liner compliance or friction properties?

Lines 105–122

•Lines 105–118: The two tasks (vertical loading, half step) are simple and controlled. However, they do not reflect everyday gait dynamics. Authors should justify why a more realistic gait was excluded.

•Lines 118–122: Because only two DIC cameras were available, separate trials were required for anterior and lateral views. This creates potential alignment inconsistencies between datasets and should be acknowledged earlier.

Lines 132–145

•Lines 137–140: More detail is needed on the DIC reconstruction error, stereocalibration accuracy, and noise filtering.

•Lines 140–145: The strain measurement region was extremely limited. This methodological constraint strongly weakens the validity of comparing full field FEA results with partial-field DIC data.

Lines 150–190

•Lines 155–157: Bone anatomy derived from an open-source model and scaled using tape measurements introduces significant geometric uncertainty. Consider quantifying potential anatomical mismatch.

•Lines 166–170: Soft tissues modelled as linear elastic contradict well-established nonlinear, viscoelastic behaviour. A more rigorous rationale is needed.

•Lines 183–190: The loading simplifications (no frontal/transverse plane moments, assumed COP alignment, no inertial effects) reduce ecological validity. Authors should evaluate how these simplifications might bias results.

Lines 199–234

•Lines 200–212: The agreement between DIC and FEA strains is well presented. However, statistical error metrics (e.g., RMSE, correlation, Bland–Altman plots) are missing.

•Lines 211–218: For the half-step task, a substantial discrepancy appears in the lateral minimum and maximum strains. Authors should explain this more clearly rather than briefly noting it.

•Figures: Clear, but inconsistent colour scaling between DIC and FEA maps makes direct visual comparison difficult.

Lines 235–286

•Lines 248–263: The authors compare their strain results to prior DIC studies. Suggest adding discussion on why magnitude differences occur (e.g., liner vs skin measurements).

•Lines 273–278: The explanation of task sensitivity is insightful; however, the discussion should address how the simplified load case compares to actual gait loading.

•Lines 282–285: The authors acknowledge the limitation of linear tissue modelling, but the dismissal of hyperelastic models requires more detail—what specific instabilities occurred?

Lines 288–316

1.Repeatability: No intra-trial variability analysis or repeated measures.

2.Liner nonuniformity: Real prosthetic liners vary locally in thickness and stiffness, unlike the modelled uniform shell.

3.Effect of lubricant and clear socket: The test conditions differ materially from real-world prosthesis use.

4.Region mismatch: The limited DIC area may not correspond perfectly to the FEA region assessed.

Lines 319–324

•Strong concluding summary; however, avoid overgeneralizing given the single participant nature.

•Suggest softening claims about “good agreement,” emphasising that findings are preliminary.

Reviewer #2: Review Comments to the Author:

The manuscript presents a preliminary comparison of prosthetic liner strains measured using digital image correlation (DIC) and predicted using finite element analysis (FEA) in a unilateral transtibial prosthesis user. The study addresses an important and longstanding challenge in prosthetic biomechanics: experimental validation of FEA models used to estimate interface mechanics. The integration of DIC with subject-specific FEA modeling under load-bearing conditions is novel and technically interesting.

Overall, the study demonstrates methodological feasibility and is clearly written. However, several issues related to validation rigor, experimental design, and interpretation must be addressed before the conclusions can be considered adequately supported.

Major Comments:

1. Calibration vs. Validation

The manuscript states that three values of Young’s modulus were evaluated and that 0.3 MPa was selected because it provided the closest match to DIC measurements. However, this approach constitutes calibration rather than independent validation. After tuning material properties to match DIC, the manuscript then concludes that FEA and DIC exhibit “good agreement.”

To strengthen the scientific rigor:

• Clearly distinguish between model calibration and model validation.

• Provide quantitative error metrics (e.g., RMSE, mean absolute error, percent difference, spatial correlation).

• Present strain comparisons for all tested material properties to show how sensitive agreement is to stiffness selection.

• Avoid language implying independent validation unless supported by independent data.

2. Sample Size and Generalizability

The study includes only one participant. While acceptable for a feasibility study, this limits the strength of conclusions. Anatomical variability, tissue properties, and socket fit vary substantially across individuals.

The manuscript should:

• Explicitly frame the study as a single-subject feasibility or proof-of-concept study.

• Avoid generalizable claims regarding prosthetic socket design.

• Clearly acknowledge that findings may be subject-specific.

3. Quantitative Assessment of Agreement

Agreement between DIC and FEA is described largely qualitatively (“good agreement”) and supported by selected peak strain values. However:

• No mesh convergence study is reported for FEA.

• No statistical or spatial comparison metrics are included.

I strongly recommend adding:

• Quantitative comparison metrics.

• A spatial difference map between DIC and FEA strain fields.

• Mesh refinement justification.

4. Modeling Assumptions and Sensitivity

The FEA model includes several simplifying assumptions:

• Linear elastic soft tissues

• Frictionless liner–socket interface

• Simplified loading (sagittal plane only)

• Assumed center-of-pressure alignment

• Non-subject-specific bone geometry

These assumptions may substantially influence strain distribution. While limitations are acknowledged, the manuscript would benefit from:

• A brief sensitivity analysis (e.g., effect of friction coefficient or stiffness variation on strain magnitude).

• Clarification of how loads were applied (to bone nodes, rigid body, distributed load).

• Discussion of how assumptions may bias agreement with DIC.

5. Limited DIC Measurement Region

The measurable strain region is approximately 7 cm in height and 8 cm circumferentially. This represents a limited portion of the liner surface.

The manuscript should:

• Clarify what percentage of total liner surface was analyzed.

• Discuss whether high-strain regions outside this window may have been missed.

• Ensure that claims of agreement are limited to the measured region.

Minor Comments:

• Provide additional DIC processing details (subset size, step size, smoothing, calibration error).

• Ensure identical color scales are used when comparing DIC and FEA strain maps.

• Soften statements such as “validate FEA model results” to reflect the preliminary nature of the study.

• Consider depositing raw DIC data and model inputs in a public repository to align with PLOS data transparency standards.

Reviewer #3: The present study demonstrates the use of DIC to measure prosthesis liner strains of a unilateral transtibial prosthesis user and conducts a preliminary comparison of these strains with those predicted from an FEA model.

The use of DIC has certain challenges as compared to the other experimental techniques, such as:

1. Unstable Speckle Pattern-Sweat or skin stretching alters the speckle pattern, Prosthetic socket friction can erase or distort the pattern, Long walking trials degrade pattern quality. It will be useful mentioning how a Stable Speckle pattern was assured for this study.

2. Secondly Walking involves Dynamic Motion and leads to Motion blurring if cameras are not high speed. Also, Synchronization errors between cameras and gait cycle phases. How are these Issues addressed?

3. Another major drawback of using DIC is that for minor changes in prostheses alignment cause large changes in stress distribution, which makes DIC results difficult to generalize. How is the alignment issue handled in the present study?

4. For a prostheses user, there is a movement of skin moves relative to the underlying bone and muscle known as soft tissue artifact. This phenomenon is known as soft tissue artifact. How present technique addresses this issue and what is the error percentage for the same.

Also, there are some studies done in past that uses DIC for Prostheses Socket Pressure Measurement. Refer to the following study -Evaluation of the influence of cyclic loading on a laser sintered transtibial prosthetic socket using Digital Image Correlation (DIC) by Saey et al. Annu Int Conf IEEE Eng Med Biol Soc, 2019

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: Yes: Amit Kumar Singh

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A preliminary comparison of prosthetic socket liner strain determined using digital image correlation and finite element analysis

PONE-D-26-07309R1

Dear Dr. Philen,

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.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Alessandra Aldieri

Academic Editor

PLOS One

Additional Editor Comments (optional):

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 #1: All comments have been addressed

Reviewer #2: All comments have been addressed

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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 #1: Yes

Reviewer #2: Yes

**********

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

Reviewer #1: Yes

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 #1: Yes

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 #1: Yes

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 #1: Thank you for addressing the reviewers' comments. The rebuttals to the reviewer comments are itemized individually. The inclusion of the document with track changes enabled the reviewers to verify the actions taken by the authors in response to their comments.

Reviewer #2: The authors have responded thoroughly and constructively to the comments raised during the initial review. The revised manuscript has been substantially improved, particularly in the following aspects:

* The study is now appropriately framed as a preliminary, single-participant feasibility investigation rather than an independent validation study.

* Quantitative comparison metrics between DIC and FEA have been added, including distribution-based analyses and supporting sensitivity studies.

* Data and model inputs have been made publicly available in accordance with the journal's data-sharing requirements.

* The limitations of the methodology and the assumptions underlying the finite element model are now clearly acknowledged and discussed.

* The conclusions have been appropriately softened to reflect the preliminary and subject-specific nature of the findings.

While some limitations remain, they are inherent to the feasibility nature of the study and have been transparently addressed by the authors. I do not believe that further revision would substantially improve the scientific contribution of the manuscript.

In my opinion, the manuscript is technically sound, the conclusions are appropriately supported by the data presented, and the work provides a novel and valuable methodological contribution to the field of prosthetic biomechanics.

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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.

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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 #1: No

Reviewer #2: Yes: Norwahida Yusoff

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