A Comparison of Two Surgical Treatment Methods for Atlantoaxial Instability in Dogs: Finite Element Analysis and a Canine Cadaver Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this research efforts the authors have compared three patient-specific implants for stabilizing biomechanical instability in small-breed dogs using a FEA supported by a cadaveric surgical time assessment.  The literature search is adequate, the alloy Ti-6Al-4V is the biocompatible standard, the cadaver study is well documented with statistical analysis, and the discussion covers the limitations of the cadaver study.  The following changes need to be made to clarify the material-based results and the overall flow.

1) The paper is written like a thesis.  The structure of the introduction needs to me modified to fit a paper format.

2) Figures need to be revisted.

2a) There is a redundancy of Figures. For example, Figures 1,2 and 3 should be removed or go into supplementary information as Figures 7 and 8 already describe the concepts.

2b) There are no dimensions on any of the CAD drawings.  There should be at least one Figure set with dimensions (i.e Figure 4).

2c) Figures 9 a b c should be combined into a single set (please remove bars unless you have standard deviations) showing the stress/strain curve as the plots are redundant and there are already too many figures.  What is the cause of the outlier in for stabillizer (row 3 first plot in red)?  Why is there a fourth figure on the first rows?

2d) The figure descriptions and captions do not explain the observations in general.  What do the arrows describe (regions of the Von Mises criteria failing?).  

3) What are the anticipated results in larger animal models based on additional literature review? Is there a mechanical parameter that you can vary to assess this projection and include in the discussion for future efforts?

Author Response

Dear reviewer,
Thank you for taking the time to provide your valuable comments.

In this research efforts the authors have compared three patient-specific implants for stabilizing biomechanical instability in small-breed dogs using a FEA supported by a cadaveric surgical time assessment.  The literature search is adequate, the alloy Ti-6Al-4V is the biocompatible standard, the cadaver study is well documented with statistical analysis, and the discussion covers the limitations of the cadaver study.  The following changes need to be made to clarify the material-based results and the overall flow.

• The paper is written like a thesis.  The structure of the introduction needs to me modified to fit a paper format.

Answer: The Introduction section was rewritten

• Figures need to be revised.

2a) There is a redundancy of Figures. For example, Figures 1,2 and 3 should be removed or go into supplementary information as Figures 7 and 8 already describe the concepts.

Answer: done

2b) There are no dimensions on any of the CAD drawings.  There should be at least one Figure set with dimensions (i.e Figure 4).

Answer: done

2c) Figures 9 a b c should be combined into a single set (please remove bars unless you have standard deviations) showing the stress/strain curve as the plots are redundant and there are already too many figures.  What is the cause of the outlier in for stabillizer (row 3 first plot in red)?  Why is there a fourth figure on the first rows?

Answer: The fourth figure presents the third cervical vertebra (C3), which was analyzed only in one stabilization configuration (ventral C1–C3 approach). An appropriate description has been added to the horizontal axis of each figure. In addition, the observed outlier in the stress-strain curve has been explicitly explained in the Discussion section, where its transitional nature and the influence of model geometry and boundary conditions on the temporary local non-linearity are described.

2d) The figure descriptions and captions do not explain the observations in general.  What do the arrows describe?

Answer: The arrows indicate the locations of maximum and minimum values of displacement, strain, and stress observed in the model under the applied loading conditions (corresponding descriptions have been added to the figure captions).

• What are the anticipated results in larger animal models based on additional literature review? Is there a mechanical parameter that you can vary to assess this projection and include in the discussion for future efforts?

Answer: For more claarity, we added a paragraph in the Discussion section:

Based on additional literature and fundamental mechanical considerations, it can be anticipated that larger animal models, e.g., larger and heavier dogs, would be primarily subjected to higher bending and torsional moments at the C1-C2 segment, resulting from increased head mass and longer lever arms. Consequently, if implant geometry and screw dimensions were kept unchanged, higher peak stresses in both the implant and the surrounding bone, as well as larger displacements, would be expected, approximately scaling with the applied load within a linear-elastic framework, as observed in the present 5-25 N simulations. However, larger animals typically provide greater bone cross-sectional areas and wider safe corridors, allowing the use of larger-diameter screws and/or thicker plates, which substantially increase construct stiffness and load-bearing capacity. Supporting evidence is provided by Cabreira et al. [9], who developed a patient-specific dorsal implant using ∅ 1.7 mm locking screws and reported a maximum implant stress of approximately 425 MPa (well below the material yield strength) together with small displacements under multi-directional loading (flexion, extension, lateral bending, and torsion). These findings suggest that scaling the construct to larger animal models does not necessarily compromise mechanical safety margins, provided that implant geometry and fixation parameters are appropriately adapted to the increased loading conditions.

Reviewer 2 Report

Comments and Suggestions for Authors

The present paper offers a technically sound and clinically useful comparison of three patient-specific fixation approaches for the treatment of atlanto-axial instability in toy breed dogs, combining finite element modelling with cadaveric fixation. The similarities among the specimens, especially with respect to geometric parameters, solver used, and loading condition, are a strength of the study and offer a comparative examination of the dorsal and ventral fixation approaches.

The process of finite element analysis is carried out in details, with particular attention to the meshing, the definition of contact, and the treatment of boundary conditions. One of the most noticeable features of this study is the quality of the figures that are very effectively drawn, easy to understand, informative, and properly scaled for comparison.

The addition of a cadaveric aspect to the model improves the manuscript. The observation that dorsal implantation is quicker than ventral implantation is informative. However, the limitations of the procedural model, especially the fact that it does not include pathology, the presence of bleeding, and the reduction phase, need to be addressed with greater clarity. The fidelity of the soft tissue environment of live surgery cannot be simulated.

Some aspects deserve clarification or additional discussion:

The fact that the simulation does not model cyclic loading and fatigue limits the discussion of long-term results, especially regarding the dorsal group, where the bone stress is maximal. This must be made more explicit. The topic of arthrodesis is given comparatively short shrift, although it is a very important consideration when assessing results for atlantoaxial instability. The fact that the dorsal model does not provide fusion functionality is certainly worthy of more than a mention.

Even if the tone of the manuscript is very precise, there are also some instances where the language used is, I would say, promotional, for example, the use of the words “remarkable fidelity” or the phrase “high translational value”.

The manuscript has several strengths and innovative aspects, representing a valuable contribution once the discussion on methodological limitations will be better defined and the topic of clinical relevance will get a balanced treatment.

In my opinion, the paper can be accepted for publication after the minor adjustments mentioned.

Author Response

Dear reviewer,
Thank you for taking the time to provide your valuable comments.

The present paper offers a technically sound and clinically useful comparison of three patient-specific fixation approaches for the treatment of atlanto-axial instability in toy breed dogs, combining finite element modelling with cadaveric fixation. The similarities among the specimens, especially with respect to geometric parameters, solver used, and loading condition, are a strength of the study and offer a comparative examination of the dorsal and ventral fixation approaches.

The process of finite element analysis is carried out in details, with particular attention to the meshing, the definition of contact, and the treatment of boundary conditions. One of the most noticeable features of this study is the quality of the figures that are very effectively drawn, easy to understand, informative, and properly scaled for comparison.

The addition of a cadaveric aspect to the model improves the manuscript. The observation that dorsal implantation is quicker than ventral implantation is informative. However, the limitations of the procedural model, especially the fact that it does not include pathology, the presence of bleeding, and the reduction phase, need to be addressed with greater clarity. The fidelity of the soft tissue environment of live surgery cannot be simulated.

Answer: For more clarity, the Discussion section was rewritten

Some aspects deserve clarification or additional discussion:

The fact that the simulation does not model cyclic loading and fatigue limits the discussion of long-term results, especially regarding the dorsal group, where the bone stress is maximal. This must be made more explicit. The topic of arthrodesis is given comparatively short shrift, although it is a very important consideration when assessing results for atlantoaxial instability. The fact that the dorsal model does not provide fusion functionality is certainly worthy of more than a mention.

Answer: For more clarity, we added such a limitation in the Discussion section:

This study found that dorsal instrumentation can be installed more quickly and that its mechanical performance is comparable to that of ventral instrumentation. However, the simulation does not model cyclic loading or fatigue limits. Additionally, arthrodesis of the C1–C2 junction is not possible when the spinal stabilizer is inserted dorsally. Consequently, the long-term effectiveness of this construct may be limited. Clinical studies are therefore needed to verify whether C1–C2 arthrodesis is necessary to maintain long-term stability of the atlantoaxial junction with the proposed dorsal stabilizer design. 

Even if the tone of the manuscript is very precise, there are also some instances where the language used is, I would say, promotional, for example, the use of the words “remarkable fidelity” or the phrase “high translational value”.

Answer: The manuscript has been carefully revised to remove language that could be perceived as promotional. Expressions such as “remarkable fidelity” and “high translational value” have been replaced with more neutral, descriptive terms to better reflect the scientific scope and tone of the study.

The manuscript has several strengths and innovative aspects, representing a valuable contribution once the discussion on methodological limitations will be better defined and the topic of clinical relevance will get a balanced treatment.

Answer: For more clarity, the Revision section has been rewritten.

In my opinion, the paper can be accepted for publication after the minor adjustments mentioned.

 

Reviewer 3 Report

Comments and Suggestions for Authors

The authors proposed a paper with the aim to choose an implant which reflect the preferred load-transfer pathway as well as anatomical or surgical constraints that may limit ventral access, for the atlantoaxial instability in toy and small-breed dogs with risk of complications, using different methodologies: computed tomography, segmentation, virtual reduction, CAD/CAM design, and finite element analysis. The following comments are raised to improve the quality of the present paper.

 

-The “Introduction” section is divided into different subchapters, where the authors only discuss the 9 references related to the main topic. It is suggested to cite and discuss more literature relevant to the topics: implants stabilization; other related methodologies as clinical, computational and/or experimental, for example. Some of the references are recommended: DOI:10.3390/biomedinformatics3020020 .  DOI:10.1186/s12891-016-1312-4

 

-According to the presented numerical model, why the authors only use a linear static analysis? Why only use isotropic material properties?

 

-About the obtained value of 255.51 MPa presented in figure 10 a) for the ventral stabilizer, can you explain better this peak value or if not relevant according to the range of stress pattern? To compare the maximum equivalent stress, it will better if additional information related with the yield stress of the materials could be introduced.

 

-Authors mentioned that ‘’Cancellous bone was not modeled explicitly because, in toy-breed atlantoaxial anatomy, its volume fraction is small relative to the cortical shell.’’ It is possible to make this argument validated according with other studies.

 

-In the end, authors need to explain the limitations of the study (all that you may find). If it is possible, authors should increase their discussion with previous research and highlight how their study is providing a different approach to what has been done. May be clinical highlight would be encouraged to improve the significance of this paper.

Author Response

Dear reviewer,
Thank you for taking the time to provide your valuable comments.

The authors proposed a paper with the aim to choose an implant which reflect the preferred load-transfer pathway as well as anatomical or surgical constraints that may limit ventral access, for the atlantoaxial instability in toy and small-breed dogs with risk of complications, using different methodologies: computed tomography, segmentation, virtual reduction, CAD/CAM design, and finite element analysis. The following comments are raised to improve the quality of the present paper.

-The “Introduction” section is divided into different subchapters, where the authors only discuss the 9 references related to the main topic. It is suggested to cite and discuss more literature relevant to the topics: implants stabilization; other related methodologies as clinical, computational and/or experimental, for example. Some of the references are recommended: DOI:10.3390/biomedinformatics3020020 .  DOI:10.1186/s12891-016-1312-4

-According to the presented numerical model, why the authors only use a linear static analysis? Why only use isotropic material properties?

Answer: A linear static analysis was employed because the primary objective of the numerical model was the comparative assessment of the mechanical behavior of different stabilization variants, rather than the prediction of long-term material damage, fatigue phenomena, or nonlinear tissue response. Moreover, this approach is consistent with previously published finite element studies on atlantoaxial stabilization, allowing for a direct and meaningful comparison of the present results with existing literature. Isotropic material properties were employed to simplify the model and maintain methodological consistency across all analyzed constructs. For Ti-6Al-4V implants, the assumption of isotropy is standard practice in finite element analyses of medical devices and is justified by the material’s macroscopic homogeneity. Cortical bone was also modeled as an isotropic material, as the analysis focused on the global distribution of stresses and strains around the screw–bone interface, rather than on local microdamage mechanisms or bone remodeling processes. Importantly, the same material assumptions were applied to all stabilization variants, ensuring that these simplifications do not compromise the validity of the relative comparisons, which constituted the main objective of the study.

-About the obtained value of 255.51 MPa presented in figure 10 a) for the ventral stabilizer, can you explain better this peak value or if not relevant according to the range of stress pattern? To compare the maximum equivalent stress, it will better if additional information related with the yield stress of the materials could be introduced.

Answer: The value of 255.51 MPa shown in Figure 10a represents a local maximum of equivalent (von Mises) stress occurring in the most highly loaded region of the ventral stabilizer plate under the 25 N load case. Such stress peaks are typical in numerical analyses and arise from stress concentrations at load-transfer regions and geometric discontinuities, including areas around screw holes, thickness transitions, fillets, and contact zones where the main force path is closed. We agree with the reviewer that a clearer interpretation of the results requires reference to the material properties. Therefore, we will supplement the manuscript with information on the yield strength of the Ti-6Al-4V alloy, to clearly demonstrate that the observed maximum stresses remain within the safe elastic operating range of the material.

-Authors mentioned that ‘’Cancellous bone was not modeled explicitly because, in toy-breed atlantoaxial anatomy, its volume fraction is small relative to the cortical shell.’’ It is possible to make this argument validated according with other studies.

Answer: For more clarity, we added such a sentence in the Discussion section:

This study found that dorsal instrumentation could be installed more quickly than ventral instrumentation, with comparable mechanical performance. However, the simulation does not model cyclic loading or fatigue limits. Also, cancellous bone was not modelled explicitly because its volume fraction is relatively small compared to the cortical shell of the C1 and C2 vertebrae in toy breeds [9, 26]. For this reason, the available veterinary literature does not provide sufficient information to enable an extended load simulation to be carried out. It was also not possible to promote arthrodesis of the C1–C2 junction when the spinal stabilizer was inserted dorsally, which could be a significant clinical issue. Consequently, the long-term effectiveness of this construct may be limited. Therefore, clinical studies are necessary to verify whether C1–C2 arthrodesis is required to maintain the long-term stability of the atlantoaxial junction with the proposed dorsal stabilizer design. 

-In the end, authors need to explain the limitations of the study (all that you may find). If it is possible, authors should increase their discussion with previous research and highlight how their study is providing a different approach to what has been done. May be clinical highlight would be encouraged to improve the significance of this paper.

Answer: For more clarity, we added such a sentence in the Discussion section:

This study found that dorsal instrumentation could be installed more quickly than ventral instrumentation, with comparable mechanical performance. However, the simulation does not model cyclic loading or fatigue limits. Also, cancellous bone was not modelled explicitly because its volume fraction is relatively small compared to the cortical shell of the C1 and C2 vertebrae in toy breeds [9, 26]. For this reason, the available veterinary literature does not provide sufficient information to enable an extended load simulation to be carried out. It was also not possible to promote arthrodesis of the C1–C2 junction when the spinal stabilizer was inserted dorsally, which could be a significant clinical issue. Consequently, the long-term effectiveness of this construct may be limited. Therefore, clinical studies are necessary to verify whether C1–C2 arthrodesis is required to maintain the long-term stability of the atlantoaxial junction with the proposed dorsal stabilizer design. 

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for addressing the comments.  The technical merit is much more transparent.