Reliability of unconventional torso anthropometry using a three-dimensional scanner in Peruvian children and adolescents

Summary
The manuscript by Delgado et al. describes the application of a 3D scanner to assess body surface area and body volume in Peruvian children and adolescents in the age of 6 to 17 years. The study included 82 children and adolescents and authors used concordance correlation coefficients to assess ‘reliability’ of body surface area and body volume estimation, and ROC analyses to assess ‘validity’ of the volume-to-area ratio as compared to the manually measures waist-to-height ratio. Authors conclude that the 3D scanner ‘provides a reliable but inaccurate alternative for area and volume torso measurement’.

With regard to the growing interest in finding new technologies and novel markers to assess the individual anthropometry as potential risk factor, the study deals with an interesting topic and promising measurement technique. However, there are major concerns about the study protocol used and the presentation of the study in the manuscript, especially in terms of the methods.

Please see detailed comments below.  

Major comments
Introduction

1. Although touching an interesting field, i.e., the assessment of 3D body volume and body surface area, I think that the scientific impact and predictive value of assessing body surface area and volume, and, thus, the rational for the study and the potential big advantage that 3D body scanners may possess, was not make clear. One decisive point is only mentioned shortly in the discussion section: the potential of 3D scanners to be a ‘diagnostic tool for obesity’. In this respect, also the ‘unconventionality’ (see title) of the measures ‘surface area’ and ‘volume’ was unfortunately not pointed out, although this would clearly highlight the scientific relevance of the manuscript. Further, a short overview over current methods to assess body surface area and volume, and their shortcomings would underline the value of 3D body scanners.

2. The scanner technique used by the authors is different from many of the currently used stationary 3D body scanners (e.g., the Vitus Smart XXL), not only due to the portability of the scanner but also due to the technique underlying the scanner. However, in the introduction and discussion section, the authors cite studies using such other stationary scanning devices to point at the ‘accuracy’ and ‘precision’ of 3D scanners, and authors further compared their findings with other stationary scanning devices. Discussing differences between (and limitations of) the current (portable) and other (stationary) scanning techniques would help to point at the ‘novelty’ of the technology and further to meaningfully interpret the current findings.

Methods
3. Overall, the statistical methods used, are reported insufficiently, so that I believe that analyses presented are not reproducible by others.

4. Further, the scan protocol is not made clear. Persons were scanned twice – what was the time interval between measures? Some more information about the scanning technique is needed to understand the procedure and study (e.g. What kind of image is created? A whole body image? Which/how many anthropometric measures are determined? The authors mentioned using a 3D design software – is it specific for the body scanner? The authors report that scanning device captures pictures by ‘rotating around the subject’ – was rotating based on a manual movement by the study personnel or was it performed automatically by the device?).

5. Further, could the authors please explain how they defined reliability (short-term vs. long-term reliability, i.e., technical reliability of the device vs. true variability/stability of the measure) and validity? This is currently not made clear to the reader and, thus, the aim and methods are not really clear.

6. Why did the authors use rho_c to assess reliability and ROC analyses to investigate the agreement of the volume-to-area and the waist-to-height ratio (which is their concept to proof validity)? Did the authors also thought about using ICC and Bland Altman plots instead?

7. Typically, persons being investigated are scanned with only wearing underwear and a bathing cap. In the present study, children and adolescents were scanned wearing loose clothing as presented in Figure 1. I am not sure if this makes sense, when aiming to investigate reliability and validity of the torso in terms of body surface area and volume. The authors themselves discuss that for ‘comparison purposes [...] some uniformity is required in the amount of clothes’. Uniformity, however, was not fulfilled in the present study.

8. There were huge differences in the scanning procedures between sexes: boys were scanned ‘in a standing position with the arms at the sides and the palms forward’, while girls were scanned ‘in a standing position, with their arms over their heads, holding their hair’. However, it is important to consider that previous studies indicate the significant impact of the arm position on abdominal measures assessed by 3D body scanners.

9. It is not clear to me, why captured 3D images were edited so extensively (‘The acquired images were rebuilt as objects without texture [...] images were manually reshaped [...] in order to exclude hair in girls or arms in boys and girls, retaining only the torso’). Typically, researchers using 3D scanning techniques evaluate whole body images. Could the authors please explain the rational/need for this editing? Why was it not possible to evaluate the whole body image? Further, was there any standard operating procedure defining the ‘manual’ editing to make the process less subjectively and, therefore, reproducible?

10. Similarly, the differences in editing 3D images between sexes seem to be problematic in my point of view (‘images were manually reshaped [...] in order to exclude hair in girls or arms in boys and girls’). Probably, authors wanted to account for (typically) longer/more voluminous hair in girls than in boys. However, did the authors consider excluding hair in both sexes or using bathing caps to reduce bias due to sex differences in data handling?

11. Regarding editing of images, in the methods the authors report that ‘images were manually reshaped [...] in order to exclude hair in girls or arms in boys and girls’, while in the abstract and discussion they say that ‘images were manually reshaped to exclude head, hair, arms, and legs’ – how was reshaping done? If heads were excluded, exclusion of hair (and the description of exclusion of hair in girls in the methods) seems to be unnecessary.

12. I am not sure about using the waist-to-height ratio as standard reference to assess validity of the 3D scanner-based volume-to-area ratio, since both ratios reflect different anthropometric information on body shape. However, what is the predictive value of the waist-to-height ratio in terms of health risk? Pointing at this issue may support the authors’ use of this ratio to assess validity. Nevertheless, in my point of view, the concept used here may only assess validity indirectly, since the waist-to-height ratio is not a commonly used standard reference to assess body surface area or volume (for the latter, air displacement plethysmography may be the better ‘gold standard’). Pointing at this limitation would be beneficial for the manuscript.

13. The ROC analysis is not quite clear to me. Figures show three dots for each curve only – could the authors please be more precise about the analyses, underlying data, and choosing cut points. What is meant by ‘certain values of this method’? Typically, the ROC shows sensitivity and 1-specifity over a range of cut points. However, in the results, figures suggest that the ROC is only shown for the cut points 44 and 48 for the volume-to-area ratio – or how can the figures be interpreted? Further, calculation of AUC is not reported in the methods section.

Results
14. Results are partly described sparsely and do not always represent the ‘whole picture’ of results shown in the tables (see minor comments for details).

15. The AUC observed is 0.5707 and 0.6383. An acceptable discrimination is typically considered for AUC >0.7. Could the authors provide an evaluation and interpretation of the observed AUC and an underlying definition for such evaluation?

Discussion
16. The discussion section would strongly improve, when authors may more discuss their own results. Currently, their own results are hardly discussed.

17. The authors did several sub-group analyses but did not really discuss differences found between measures, sexes, age groups, and ‘obesity’ groups, or probable reasons for different findings.

18. Strengths and limitations should be addressed in more detail, also with regard to the comments made on the scanning protocol and data handling.

19. Finally, I am not sure about the conclusion the authors made. It is concluded that 3D scanners provide “a reliable but inaccurate alternative” – Is it really an alternative if the measurement is inaccurate? Further, if at all, what it is an alternative for (see also comment #1)?

Abstract
20. In the background section, authors mention that 3D scanners allow measuring and displaying body surface and shape with ‘high precision’. However, is investigating the ‘precision’ not one of the aims of the authors’ study (in terms of reliability)?

21. In the background section, authors state that investigating ‘unconventional’ torso measures was one of the current aims? For me, it is not clear what is meant by ‘unconventional’. Probably, body area and volume are meant. However, both are first mentioned in the methods section. I would further recommend to shortly make clear the predictive value of measuring body surface area and volume, and, thus, the potential big advantage of 3D body scanners.

22. In the methods section, there is no information about statistics. Thus, the following results are hardly to follow and to interpret. E.g. What scan protocol was applied (obviously authors scanned persons twice – what was the time interval between measures)? How did the authors define reliability (short-term vs. long-term reliability, technical reliability of the device vs. true variability/stability of the measure) and how was it analyzed (obviously by using rho)? How was validity defined and analyzed (obviously by using ROC analyses with the waist-to-height ratio as standard reference)?

23. In the methods section, there is no information about age range and sample size included.

24. In the results section, I do not think that it is useful to report mean body surface area and volume as averaged over all ages, as ages included ranged from 6 to 17 years (as mentioned inthe main text), therefore, encompassing a wide range of body weight, height, and shape.

25. Please refer to major comment #19.

Minor comments
Introduction
1. In the introduction section, authors say that 3D scanning is a ‘time saving procedure’ but in the discussion section they say that scanning requires around 100 seconds, with the scanner being ‘extremely sensitive to movements’. In my point of view, 100 seconds is quite a long time for standing motionlessly, especially for children. Further, ‘time saving’ would only make sense for me if knowing the number of measures assessed in this time period and if knowing the time required for alternative measurements.

2. First paragraph, second sentence and elsewhere: ‘3D’ instead of ‘three-dimensional’

3. Second paragraph, first sentence: please check wording.

Methods
4. Second paragraph, first sentence: What does the numbers in parenthesis stand for? What data was the basis for the sample size calculation?

5. Second paragraph: Please check the numbers on participants recruited. In the abstract and results section, authors report to have recruited 100 participants (54 females and 46 males), while here, authors report to have recruited 82 participants (36 females and 46 males). Obviously, the difference may be explained by the 18 participants excluded due to incomplete pictures as mentioned in the results section. However, please align numbers on recruited (vs. included/assessed) participants.

6. Did the authors consider a selection bias due to the recruitment of participants from only 2 private schools being proximate to the institute/researchers in charge?

7. Fourth paragraph: I find it a bit confusing that authors say that training in usage of the scanner was not required, while reporting the way of training done in the present study at the same time.

8. Sixth paragraph: ‘gold standard’ instead of ‘gold-standard’

9. How did the authors define ‘accuracy’?

10. Was there any definition for interpreting rho_c, e.g., as ‘poor’ or ‘good’?

11. Last paragraph: Analyses were also performed stratified by waist-to-height ratio.

Results
12. First paragraph: ‘not be repeated’ instead of ‘not be repeated twice’

13. Obviously, only girls were excluded due to incomplete scanning – was there any reason for that?

14. Second paragraph, last sentence: Maybe you want to add the information that for other comparisons (volume and younger ages) in girls, coefficients were weak.

15. Third paragraph: ‘the reliability coefficient for area and volume’ instead of ‘the reliability coefficient for obesity’ (since the term ‘obesity’ is used by the authors with regard to waist-to-height ratio)

16. Third paragraph, last sentence: Authors say that rho_c was ‘>0.8 in every comparison [...] at any age’. However, this is not true for several sub-group analyses, e.g., in children for area and waist-to-height ratio ≤0.5, rho_c was 0.6000.

17. Fourth paragraph: This finding is interesting but it is not further discussed. What is the implication of this result?

18. Table 1: Here, waist-to-height ratio is abbreviated by using ‘W/H’, while in the text, authors use ‘WHtR’. Please align.

19. Tables 1 to 3: the highest age included is 17 years; thus, I would recommend changing age group ‘15-18 years’ to ’15-17 years’.

20. Tables 1 to 3: What do the p-values stand for?

21. In addition to figure 3, a table showing sensitivity, specificity and AUC for all assessed cut points for the volume-to-area ratio would help to interpret results.

22. Figures 2 and 3: Since all other analyses were made separately for sexes, age groups, and waist-to-height groups, I think that providing stratified figures for the different groups would be more in line with the other results and would, thus, help to interpret results as a whole.

Discussion
23. First paragraph: Does these statements are based on the authors’ results or based on previous research?

24. Fourth paragraph: I would recommend to use ‘waist-to-height ratio <0.5’ or ‘waist-to-height ratio ≤0.5’ instead of ‘obese’ and ‘non-obese’, respectively, to facilitate following the discussion with regard to the results.

25. I do not understand the explanation that reshaping ‘smaller images’ may be more bias-sensitive and, thus, may have caused the finding that rho_c was larger for waist-to-height ratios >0.5 than for ratios ≤0.5. The editing of images was not made in the torso area and the differences in the size of the images may not have been that huge to fully explain differences in rho_c between waist-to-height ratios >0.5 vs. ≤0.5. Did the author thought about other reasons, why rho_c was larger for waist-to-height ratios >0.5 than for ratios ≤0.5?

26. Did the authors thought about the generalizability of their findings, keeping in mind that the study population was recruited from 2 private schools in Lima?

27. Fifth paragraph: I do not think that this paragraph is that relevant for the discussion without putting these previous findings in context with the current findings; see major comment 2.

28. Sixth paragraph: ‘Strong reliability’ does not hold true for all sub-groups assessed in the current study, e.g., rho_c was mainly weak in girls and for waist-to-height ratios ≤0.5.

29. Sixth paragraph, third sentence onwards: I would recommend moving this part into the background section, since it is the basic topic of the current study and highlights its scientific relevance.

30. Eighth paragraph: I do not think that this is a strength of the current study, since it was its aim.

Conclusion
31. First sentence: ‘provide’ instead of ‘provides’

Abstract
32. In the methods section, last sentence, please delete one of the two left parentheses.

33. In the method section, please introduce the abbreviations rho_c, SD, and ROC.