Round 1

Reviewer 1 Report

Thank you for allowing me to review your interesting paper on understanding whole body compensatory alignment in differing spinopelvic sagittal balances. 

Abstract: 

- KF as an acronym is not spelt out prior in line 16 

- unfortunately unlike the title the conclusion is quite vague. The title suggest that knee flexion is recruited in order to compensate for decompensated spinopelvic sagittal balance - BUT neither the results or conclusions clearly show this in the abstract. The title appears misleading at this point. 

Introduction 

- good backgroun information- however the 1st and 2nd paragraph could be shorter. 

- the purpose of this study was to: 

1. understand whole body alignment changes/differences between compensated and decompensated

2. KF cut off that indicates decompesnated spinopelvic alignment 

Unfortunately the abstract purpose and the purpose in the introduction is quite different. Abstract states "demonstrate involvement of whole body compensatory alignment" this very much appears to imply causation. However the purposes in the introduction are much more about identifying differences and investigating correlations rather than causation. Please clarify 

Methodology 

- line 99 - reference for the cut off of 4cm for compensated and decompensated please 

- how correlation strength is defined is not conveyed in the methodology. please kindly provide numbers that reflect- very strong, strong, moderate, negligable  -- this needs to be clarified and descriptions of correlations written in a uniform manner - phrases such as "stronger correlation (line 229)" unfortunately dont make sense.

- besides above, the methodolgy is sound and clear 

Results 

- 253 compensated vs 41 decompensated 

- KF and SVA r = 0.396 

- most parameters and demographics differed between group c and d 

- lack of ideal LL was compensated for with TK/SS/PT/KF/AA regardless of group - as statisitically significant

- I would recommend table 2 have the standardised strength of correlation written so it is easy to follow 

- KF 8.4 degrees correlated with a decompesnated pts. Underpowered for any causality but the correlation with ROC is present 

Discussion 

- line 177-180 makes it appear to sound as if there were any differences between the groups BUT rather table 2 shows they recruit quite similarly? All statisticalyl significant? This sentence in line 177-180 sounds misleading

- line 193 -195 - sure there was a greater correlation but statistically it was signficant for both group C and D. Can we then really say the greater correlation genuinely reflects recruitment?

- line 195 -196 - please refrain from using absolute words such as "definitive." if it truly was definitive, there would be no basis for this study. 

- line 202 - despite a weak correlation it was still statistically significant?

- further limitations would be the underpowered nature of group D?

conclusion

-  line 224-229 feels awkward. this is the conclusions, please kindly rephrase to be more clear for your point and how it answers the first aim of your study. 

Line 230-233 - I can agree with as this is a conclusion that answers the inital purpose of this paper. 

Author Response

Response to Reviewer 1 Comments

Reviewer 1:

Thank you for allowing me to review your interesting paper on understanding whole body compensatory alignment in differing spinopelvic sagittal balances.

We deeply appreciate your professional review on our manuscript. According to your suggestions, we made extensive corrections to our original draft, and the detailed responses are listed below.

Abstract:

- KF as an acronym is not spelt out prior in line 16

Response: Thank you for pointing this out. We have added the correct spelling as follows.

“knee flexion (KF)” (line 16)

- unfortunately unlike the title the conclusion is quite vague. The title suggest that knee flexion is recruited in order to compensate for decompensated spinopelvic sagittal balance - BUT neither the results or conclusions clearly show this in the abstract. The title appears misleading at this point.

Response: While the content of this article indicates that patients with decompensated global spinopelvic balance are more strong correlation between the loss of lumbar lordosis and lower limb compensatory parameters, including the knee, we acknowledge that the original title does not accurately represent our study design, as you point out. We have made a major change to the title as follows.

Introduction

- good backgroun information- however the 1st and 2nd paragraph could be shorter.

Response: Following the comments, the overall text was shortened by omitting sentences that are repetitive of the content of the first and second paragraphs. Please review the revised lines 29-64 text.

- the purpose of this study was to:

1. understand whole body alignment changes/differences between compensated and decompensated
2. KF cut off that indicates decompesnated spinopelvic alignment

Unfortunately the abstract purpose and the purpose in the introduction is quite different. Abstract states "demonstrate involvement of whole body compensatory alignment" this very much appears to imply causation. However the purposes in the introduction are much more about identifying differences and investigating correlations rather than causation. Please clarify

Response: Thank you for your definitive point. In accordance with your comment, we have made the following changes to the background of the abstract.

“The aim of this study was to investigate the differences in the involvement of whole-body com-pensatory alignment in different conditions of spinopelvic sagittal balance (compen-sated/decompensated).” (lines 10-12)

Methodology

- line 99 - reference for the cut off of 4cm for compensated and decompensated please

Response: We used the threshold for global balance (anterior spinal tilt) in the definition of spinal deformity proposed by Schwab et al. To clarify this, the following changes were made and a reference was added.

“The cutoff for SVA was the threshold for anterior spinal inclination in the spinal deformity classification proposed by Scwab et al.” (lines 116-118)

- how correlation strength is defined is not conveyed in the methodology. please kindly provide numbers that reflect- very strong, strong, moderate, negligable  -- this needs to be clarified and descriptions of correlations written in a uniform manner - phrases such as "stronger correlation (line 229)" unfortunately dont make sense.

Response: The criterion for strength of association is described in the “Statistical analysis” section (lines 140-141). For clarity, we have described this criterion in Table 2.

- besides above, the methodolgy is sound and clear

Results

- 253 compensated vs 41 decompensated

- KF and SVA r = 0.396

- most parameters and demographics differed between group c and d

- lack of ideal LL was compensated for with TK/SS/PT/KF/AA regardless of group - as statisitically significant

- I would recommend table 2 have the standardised strength of correlation written so it is easy to follow

Response: We appreciate your helpful suggestion. As noted above, we describes the criteria for correlative strength in Table 2.

- KF 8.4 degrees correlated with a decompesnated pts. Underpowered for any causality but the correlation with ROC is present

Response: We added a new limitation of this study regarding lack of statistical power at the end of the discussion.

Discussion

- line 177-180 makes it appear to sound as if there were any differences between the groups BUT rather table 2 shows they recruit quite similarly? All statisticalyl significant? This sentence in line 177-180 sounds misleading

Response: In response to the comment, the wording has been changed from "the lower extremity parameters are newly correlated" to "the correlation is getting stronger" to clarify the intent as follows.

“Comparative analysis of the compensatory and decompensatory groups revealed that during the decompensatory phase of spinopelvic sagittal balance, the standing posture is maintained by more intense recruitment of whole-body compensatory parameters from the spine to the feet, excluding craniocervical and cervical alignment.” (lines 198-202)

- line 193 -195 - sure there was a greater correlation but statistically it was signficant for both group C and D. Can we then really say the greater correlation genuinely reflects recruitment?

Response: A sentence describing the extent to which the compensatory mechanisms of the lower extremity (knee and ankle joints) were correlated with the Lack of iLL. The following changes were made to clarify the follows.

“In group D with decompensated spinopelvic sagittal balance, we found a moderate correlation between Lack of iLL and recruitment of KF, and a weak correlation between Lack of iLL and ankle dorsiflexion in the maintenance of a standing posture.” (lines 214-217)

- line 195 -196 - please refrain from using absolute words such as "definitive." if it truly was definitive, there would be no basis for this study.

Response: In accordance with your comment, we have made the following changes.

“KF has been considered one of the effective compensatory mechanisms, and previous studies have shown that ankle flexion is also an important compensatory mechanism, especially in the elderly.” (lines 217-219)

Also removed "definitive" on line 38 for this.

- line 202 - despite a weak correlation it was still statistically significant?

Response: We rechecked, and our statistical analysis showed this to be correct, so We'll leave it at that.

- further limitations would be the underpowered nature of group D?

Response: As mentioned above, we added a limitation about the lack of statistical power at the end of the discussion.

conclusion

-  line 224-229 feels awkward. this is the conclusions, please kindly rephrase to be more clear for your point and how it answers the first aim of your study.

Response: We appreciate your valuable suggestions. In following your comment, I have rewritten the conclusion section (lines 269-280) to be more in line with the text.

Line 230-233 - I can agree with as this is a conclusion that answers the inital purpose of this paper.

We appreciate once again the detailed review and beneficial comments. We believe that the revised manuscript will be considered for publication.

Reviewer 2 Report

This study revealed a compensatory mechanism of knee flexion angle for whole spine sagittal alignment and found an interesting correlation between loss of ideal lumbar lordosis and knee flexion angle. This study is important but may have some points to clarify:

1. Decompensated mechanisms mean that these patients may have difficulty walking or standing. Can they be named as healthy subjects?  Why do you use SVA>=4 to discriminate between compensated/decompensated groups? (Is there any literature support?)

2. Decompensated group is significantly older than the compensated group? Will this affect the interpretation of the study results? Please discuss this part.

3. I think the recruitment of ankle angle into this study is also necessary because it seems significant in the correlations with loss of iLL and SVA. I think that it also has an essential effect on the compensatory mechanism. (You may need to find the cutoff point of AA).

4. What are the clinical applications for your study findings? Please discuss more on this part.

There were some grammar errors needing to be corrected.

Author Response

Response to Reviewer 2 Comments

This study revealed a compensatory mechanism of knee flexion angle for whole spine sagittal alignment and found an interesting correlation between loss of ideal lumbar lordosis and knee flexion angle. This study is important but may have some points to clarify:

1. Decompensated mechanisms mean that these patients may have difficulty walking or standing. Can they be named as healthy subjects? Why do you use SVA>=4 to discriminate between compensated/decompensated groups? (Is there any literature support?)

Response: We deeply appreciate your professional review on our manuscript. According to your suggestions, we made corrections to our original draft.

In response to the comment, we have changed the term "healthy subjects" to a more appropriate term for the participants in this study. (lines 12, 26, 82)

Additionally, as for the criterion of SVA>4cm, we used the threshold for global balance (anterior spinal inclination) in the definition of spinal deformity proposed by Schwab et al. To clarify this, the following changes were made and a reference was added.

“The cutoff for SVA was the threshold for anterior spinal inclination in the spinal deformity classification proposed by Schwab et al.” (lines 116-118)

2. Decompensated group is significantly older than the compensated group? Will this affect the interpretation of the study results? Please discuss this part.

Response: Thank you for your critical comments. We agree that the difference in age between the two groups may affect the comparative analysis in this study. Spinopelvic sagittal imbalance is considerably related to aging, which can be resolved by future multivariate analysis with a sufficient number of patients to exclude age-related effects (e.g., muscle weakness, OA in the lower extremity joints). We added this discussion to the limitation. The following text has been added.

“Future multivariate analyses, including a larger number of cases, may be necessary to discern the association between global sagittal imbalance and knee flexion, as well as to evaluate the influence of age-related factors (e.g., gluteal muscle strength and joint OA) on this association.” (lines 263-267)

3. I think the recruitment of ankle angle into this study is also necessary because it seems significant in the correlations with loss of iLL and SVA. I think that it also has an essential effect on the compensatory mechanism. (You may need to find the cutoff point of AA).

Response: Thank you for your important comments. Which of these two parameters reflects spinal sagittal balance is not the focus of this study. We think that the two parameters are substitutable because AA is strongly correlated with KF (r: 0.774, 95%CI: 0.723-0.815, p<0.0001 in our analysis). Attach is a dot plot of KF and AA.

We acknowledge that both parameters are useful. However, in this study, the KF results are presented for the following two reasons.

・KF is more strongly (moderate) correlated with SVA in Group D in the results of this study.

・An use of KF as an index for evaluating global spinopelvic balance offers practical advantages. Facilities without access to EOS can still incorporate the knee in whole spine radiographs, making KF a more convenient option.

4. What are the clinical applications for your study findings? Please discuss more on this part.

Response: We appreciate your important point of view. It is not considered to be an absolute indicator because spinopelvic sagittal balance is compensated by variable alignment throughout the whole body, including the knee, and the KF value may be blurred by factors that directly affect the knee joint, such as knee joint range of motion and muscle strength. Even with that limitation, the relationship between the KF and spinopelvic sagittal balance demonstrated in this study has the potential to screen for spinopelvic imbalance with x-rays targeting the knee joint, and to provide a convenient measure of whole-body sagittal balance assessment in ASD patients with difficulty assessing pelvic parameters (which are included in the SVA measurement points) due to obesity or deformities caused by degeneration. The following text has been added to the discussion regarding this comment.

“These findings, including the KF cutoff and the observed relationship between KF and global spinopelvic sagittal alignment, have the potential for screening patients under-going knee plane radiographs to identify spinopelvic sagittal imbalance conditions. Moreover, we believe that these findings will serve as a more convenient index for evaluating global balance and assessing treatment outcomes in patients with spinal deformity, particularly in cases where there may be challenges in recognizing radio-graphic parameters in the sacral pelvic region [17, 18]. However, the compensatory mechanisms of the whole body consist of many alignments of mobility [3, 5], and it is controversial whether a single variable such as KF accurately reflects an individual patient's state of balance maintenance.” (lines 245-254)

We appreciate once again the detailed review and beneficial comments. We believe that the revised manuscript will be considered for publication.

Author Response File: Author Response.pdf

Reviewer 3 Report

The study examines from a quantitative point of view the role of knee flexion as a compensatory mechanism for spino-pelvic sagittal imbalance.

The attempt was to define a threshold numerical value potentially useful in defining both imbalance magnitude and treatment outcomes.

The topic is certainly interesting. So many uncertainties still exist in the understanding of spino-pelvic balance, and studies like this one provide useful contributions. The study is methodologically very well conducted. The article is well written and clear in all its sections. 

Nevertheless, in my opinion, it is conceptually wrong to assume that a single value expressed in degrees and disengaged from the other variables that define sagittal balance can work as a threshold value. I think this must be researched in the relationship with the other variables. In particular, the significance of pelvic incidence cannot be neglected. In fact, pelvic incidence defines individual potential for compensation by pelvic retroversion. Likewise, depending on its value, pelvic incidence could also determine a different contribution of knee recruitment as further compensation mechanism. In particular, I would hypothesize that the greater the pelvic compensation potential, the slower and more gradual the progressive recruitment of knee flexion. However, the mean value of pelvic incidence between group C and group D was similar, so it can be asserted that this study confirms that knee recruitment is proportional to the extent of sagittal imbalance. Nevertheless, little can be said beyond this. I think these aspects should be widely included in the discussion.

Author Response

Response to Reviewer 3 Comments

The study examines from a quantitative point of view the role of knee flexion as a compensatory mechanism for spino-pelvic sagittal imbalance.

The attempt was to define a threshold numerical value potentially useful in defining both imbalance magnitude and treatment outcomes.

The topic is certainly interesting. So many uncertainties still exist in the understanding of spino-pelvic balance, and studies like this one provide useful contributions. The study is methodologically very well conducted. The article is well written and clear in all its sections.

Nevertheless, in my opinion, it is conceptually wrong to assume that a single value expressed in degrees and disengaged from the other variables that define sagittal balance can work as a threshold value. I think this must be researched in the relationship with the other variables. In particular, the significance of pelvic incidence cannot be neglected. In fact, pelvic incidence defines individual potential for compensation by pelvic retroversion. Likewise, depending on its value, pelvic incidence could also determine a different contribution of knee recruitment as further compensation mechanism. In particular, I would hypothesize that the greater the pelvic compensation potential, the slower and more gradual the progressive recruitment of knee flexion. However, the mean value of pelvic incidence between group C and group D was similar, so it can be asserted that this study confirms that knee recruitment is proportional to the extent of sagittal imbalance. Nevertheless, little can be said beyond this. I think these aspects should be widely included in the discussion.

Response: We deeply appreciate your professional review on our manuscript. Following are our comments on your review and the changes we have made to the manuscript regarding it.

We acknowledge that there are some limitations to a single value threshold for sagittal spinopelvic imbalance as you have pointed out. Standing balance is maintained by variable alignment changes throughout the whole-body, and we agree that standing balance cannot be assessed by knee flexion alone.

The result of our study is not that KF is indicative of spinopelvic imbalance itself, but rather that whole-body compensatory parameters correlate more strongly with lack of lumbar lordosis in subjects with imbalance.

In the relatively healthy subjects in this study, KF detected spinopelvic sagittal imbalance with reasonably high sensitivity (89%). The strength of the results of this study is that we were able to show that this KF threshold has the potential to be used as a screening to detect spinopelvic sagittal imbalance. This threshold value may be useful for screening for spinal disease using radiograph focused on the knee (due to the small imaging area, it is necessary to construct parameters to approximate the KF). Additionally, it can provides a convenient measure of global sagittal balance assessment in ASD patients with difficulty assessing pelvic parameters (which are included in the SVA measurement points) due to obesity or deformities caused by degeneration.

Since there are various factors that affect compensatory parameters, including KF, in individual cases, such as the PI you noted, We have to conclude from the results of this study that any further evaluation of these parameters is ultimately left to the surgeon.

We have made the major changes to discuss these points to the Discussion (lines 245-254) and the Conclusion (lines 269-280) section.

Once again, thank you for your evaluation of our manuscript and your thoughtful comments on the alignment. We kindly request the opportunity to evaluate the changes made to our manuscript and to further assess their suitability for publication.

Round 2

Reviewer 1 Report

Nil further

Reviewer 3 Report

Thank you, in my opinion the authors have modified the text satisfactorily.