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6 Oct 2020
PONE-D-20-28531
A user-centric approach towards the design, development, and test of an affordable dynamic prosthetic foot
PLOS ONE
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Reviewers' comments:
Reviewer's Responses to Questions
Comments to the Author
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Reviewer #1: Partly
Reviewer #2: Yes
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2. Has the statistical analysis been performed appropriately and rigorously?
Reviewer #1: No
Reviewer #2: Yes
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Reviewer #1: Yes
Reviewer #2: Yes
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Reviewer #1: Yes
Reviewer #2: 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 and work aim to address an important gap in prosthetic practice in LMICs by designing a new dynamic foot. The work involves a number of design and testing stages, as would be typically needed for the development of prosthetic componentry. The design process is in itself complex, and not all facets should be considered as research. While I can appreciate the work that has gone into developing the foot, the novelty is not entirely clear. Moreover, the paper appears to lack adequate rigour and details in many facets of the work as detailed below.
Abstract:
While it may be true that the prototype foot “showed a better performance and acceptance by users compared to a SACH foot”, did it perform adequately well to be deemed a dynamic foot as was the goal? The paper needs to clearly present this here, and in the discussion section.
Introduction:
The introduction does not adequately address the state of the science. What are the benefits of dynamic feet compared to SACH, and why is this critical for LMICs? Reasons other than affordable technology such as a lack of prosthetists should be acknowledged as a part of the limited access to prostheses in LMICs. There exist highly affordable feet such as the Niagara foot, and this background into existing solutions is not adequately acknowledged. In fact the Niagara foot is priced well below $100. Information about the manufacturing costs of feet needs to be dealt with and presented. What are the clinical and user issues with SACH feet? These limitations need to be presented.
Methods:
The authors define and use data relating to pitch angles and GRFs in the assessment and design of the prototype foot. Please provide some justification and citations to support that these metrics are in fact important aspects of dynamic foot design. Relating to this, the authors need to very clearly define and if applicable quantify what constitutes the differences between different foot classifications (K1 vs K2 vs K3). Please more clearly define what distinguishes a K3 foot from a K2 and K1 foot in terms of the functional or mechanical properties.
The FEA lacks detail about constraints, loading, meshing, convergences etc.
The optimization on line 225+ is vaguely described, and information is needed about what was actually done.
Line 292 – what is the 2% threshold based on?
Results:
The removal of 14 of 44 days is a significant amount (1/3 of data) and this could bias the results. How did the authors know that data were manipulated? What is the basis for removing data related to extreme weather? Monsoons are a normal occurrence in Vietnam and should therefore not be excluded. More so, did the monsoon days effect one foot more than the other?
Please provide some justification that bouts of walking are a good measure of prosthetic performance. The differences in bouts could be attributed to other factors. People in places such as Vietnam use walking as a primary means of mobility. They need mobility to pursue ADLs and other necessary tasks, so it seems unlikely that they could elect to walk more or less in light of these necessities, especially considering that the SACH foot appeared to work quite well for them. Please provide an explanation or hypothesis for these found differences.
Were the questionnaire results statistically significant? Please provide stats. Are the differences clinically significant?
Discussion and conclusions:
Need to explain the novelty and contributions of this work more clearly.
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The cost aspect needs to be discussed in comparison to other existing foot technologies designed for LMICs
Minor:
Line 37 – remove ‘both’ since referring to 3 items
Line 73 – spelling of ‘toe’ is incorrect
Line 92 – need to specify the models and manufacturers of feet
Line 385 – what is 11’142?
Caption of Figure 6 stating the 13 and 5 participants is confusing and it is not clear how this relates to the actual sample size of 11 participants.
In table 1, the numbers do not make sense. For example, for the pitch angle at 50%, both the P3C and PK4 have the same value (22), yet they are noted as being significantly different. That is not possible. Also, the table is confusing, as it is not clear whether results are better or worse than the PK4. The direction matters and should be clear from the table or other figure.
Reviewer #2: GENERAL COMMENTS:
This paper presents a multi-disciplinary approach for designing and testing an affordable passive prosthetic foot suitable for the developing world context. It aims to create an affordable high mobility dynamic prosthetic foot to replace the currently distributed low mobility solid-ankle cushioned heel prostheses. The authors outline the design framework starting with design requirements, set through stakeholder discussions and gait studies of able-bodied walkers. Following with the development of the prosthetic foot design through material testing, structural optimization, manufacturing analysis, and mechanical testing. Lastly, they validated the designed prototype through gait studies, additional mechanical testing and a field trial.
The work described here has a lot of significance to the field in terms of the design approach as well as the resulting device. It is definitely worthy of a publication. However the work requires major revisions since additional information would be required to: support all the claims (mainly on the expected level of performance of the prosthetic foot prototype), justify the design and methodology choices (Why were this set of structural optimization target chosen, compared to standard measures such as roll-over shape or push off work? Can a foot that exhibit a certain load displacement behavior be categorized as fulfilling the WHO d4602 or K3 level performance? How close do you need to get to the P_K4 prosthetic foot to fulfill your requirements), and discussions on the gait analysis results would benefit from being put in perspective with existing literature (whole body propulsion, loading rate and peak vertical loads at heel strike, limitations of prosthetic boots usage compared to people with amputation, center of pressure progression, ankle moment etc…). In addition, the title of the work could be misleading as it refers to ‘user-centric’ approach but throughout the design process nor for the design requirements (given by the ICRC and WHO guidelines) were people with amputation consulted nor included in the development. People with amputation were included after the design of the prosthesis was completed. Lastly, minor revisions regarding the spelling, grammar, and phrasing issues would improve on the clarity of the work.
SPECIFIC COMMENTS:
Abstract:
Line 42: What specific aspects about the prosthetic foot performance was improved? By how much?
Introduction:
The authors should specify the level of performance of the current ICRC SACH foot to put in contrast with the target foot’s performance. What specifically about the current ICRC SACH foot prevents it to meet the listed requirements?
Line 54: How about the Niagara foot, which meets the cost requirement, through bolt attachement requirement as well as the K2-K3 level activity? (Ziolo and Bryant 2002, Wellens 2011).
Methods
Project Framework:
Line 94: The selected commercial prosthetic feet models should be specified to allow for reproducibility of the experiments, instead of abbreviated codes (P_K3_C).
Line 101: Which part of the in-lab biomechanical and mechanical evaluations where used to improve the model?
In-lab characterization of commercial prostheses:
Line 116: How long was the habituation period? Was all the tests for each participant with the commercial feet conducted on the same day?
Line 141: How were the shank, fore and rear foot frames defined? I assume the rear foot frame was defined using the three markers at the heel, the forefoot frames using the three marker at the forefoot and the shank frame, the four markers at the prosthetic ankle?
Line 145: Why were the pitch angle and flexion angle considered instead of the more traditional ankle angle or shank angle?
Line 151: Why were the stance phase behavior at 30%, 50% and 100% of BW specifically chosen instead of others gait events/frames?
Line 158: the x-axis of Fig 3b seems mislabeled.
Design, materials and simulation:
Line 165: was the shell included in the optimization/modelling of the prosthesis?
Line 168: How were the target performance outlined in d4602 translated into specific design requirements as the d4602 only outlines general walking aspects.
Line 170: The term ‘best gait pattern’ should be clarified. Which aspects of the gait pattern were targeted? Does this term refer as getting as close as the K4 foot? How close?
Line 182-183: the blade’s elasticity and strength were optimized but what were the optimization targets? Was the blade optimized independently of the foam and ankle part?
Line 184: How much eversion/inversion was set as the target?
Line 188: It is mentioned here that the foam density was used to accommodate different user weights, but only one foam density is later presented? Was the designed varied for user weights?
‘Different materials’ (line 193) and ‘ specimens’ (line 199) should include the details of the material that were tested/part of the selection process.
Line 220: Why was a quasi-static FE model chosen insead of a dynamic FE model that would represent the prosthetic foot loading cycle and loading rate of the ISO tests?
Line 224-228: Why was only the compression test of the selected commercial feet chosen as target for the optimization instead of the gait cycle tests? Furthermore, the gait cycle tests included walking activities such as stairs, ramps and side to side stepping. Were these represented too in the compression tests? Was the objective equally weighting the deformation, reaction force and moment ? Is the reaction force prescribed along with the plate orientation or was it an objective?
Mechanical compression tests
Should this section be presented before the design sections since these results were used as target during the design process?
Line 241:’Feet hysteresis were quantified and evaluated’ should these also appear in the results section?
Line 245: How were the different pitch angles selected?
Field Testing
Were these subjects given the same foot designed for an 80kg user as stated in the design section?
A copy of the adapted PEQ should be provided as supplemental information.
Line 292: How was the threshold of 2% selected? Is that related to a statistical analysis?
Results
Prototype design
Line 319: What was the result of the optimization, how close did the prototype design get to the target objectives?
Line 343: How were the stress level/fatigue resistance values set?
Line 363: Figure 8a caption refers to P_SACH but the graph represents P_PRO. It is unclear in Fig 8.b what test was conducted? Was the vertical displacement the same for all of the prostheses and the corresponding load was measured? The methods section only refers to loading cases being prescribed and not displacements. Was it from the biomechanical tests?
Figure 8 and 9: Why was only one P_K3 represented in these tests? Which one is it?
Discussion
Line 453: the P_PRO exhibit a more progressive motion of the center of pressure compared to the P_SACH but more abrupt compared to the other commercial feet? Does that allow the P_PRO to meet this requirement as there is a more abrupt change compared to the 3 higher level commercial feet?
Line 456: What explains the higher energy restitution after fatigue compared to the new state and how does it compare to the ther prosthetic feet? Was this test conducted with the foam and cosmetic shell?
Line 471: Does the difference between the P_SACH and P_PRO in terms of overall satisfaction significant?
Does the ankle moment, loading and displacement of the P_PRO enables the device to fulfill the k3 level requirement?
What were the limitations of the study? Did the prosthetic boot gait study match the walking pattern of prosthetic users?
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Reviewer #1: No
Reviewer #2: No
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The International Committee of the Red Cross supports a worldwide program of prosthetic fitting and rehabilitation. In this context, a prosthetic foot was developed and widely distributed in least developed countries.
Prospective, randomized, double-blind, controlled study.
To compare patient satisfaction and energy expenditure during ambulation between a low-cost prosthetic foot designed with a polypropylene keel (CR-Equipements™ solid ankle cushion heel, International Committee of the Red Cross) to a well-recognized solid ankle cushion heel foot with a wooden keel (solid ankle cushion heel foot, Otto Bock).
A total of 15 participants with unilateral transtibial amputation were evaluated using the two prosthetic feet in a randomized prospective double-blind crossover study. Main outcomes were patient satisfaction questionnaires (Satisfaction with Prosthesis Questionnaire and prosthetic foot satisfaction) and energy expenditure (oxygen consumption—mL/kg/min, oxygen cost—mL/kg/m, and heart rate—bpm).
There were no significant differences between the two prosthetic feet for satisfaction and energy expenditure.
The low-cost solid ankle cushion heel foot with polypropylene keel provides comparable satisfaction and similar energy expenditure as the solid ankle cushion heel foot with wooden keel.
The results of this study support the application and widespread use of the CR-Equipements™ solid ankle cushion heel foot. From a cost-effectiveness standpoint, patients are well satisfied and exhibit similar outcomes at a substantially lower cost.
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