Foot morphology of normal, underweight and overweight children

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1 (2008) 32, & 2008 Macmillan Publishers Limited All rights reserved /08 $ PEDIATRIC HIGHLIGHT Foot morphology of normal, underweight and overweight children M Mauch, S Grau, I Krauss, C Maiwald and T Horstmann Department of Sports Medicine, Medical Clinic, University of Tuebingen, Tuebingen, Germany Objective: Due to the fact that there is a global increase in obesity, knowledge about the impact of obesity on the development of a child s foot is of great importance for orthopaedic and paediatric physicians with regard to prevention, clinical treatment and management. Therefore, the purpose of this study was to investigate the influence of body mass on the development of a child s foot based on a foot type classification. Methods: The feet of 1450 boys and 1437 girls aged 2 14 years were measured using a three-dimensional (3D) foot scanner (Pedus, Human Solutions Inc., Germany) in a bipedal upright position. Twelve relevant 3D foot measures were recorded, as well as the children s age, gender, height and mass. Factor analysis of principal components was used to obtain a smaller number of independent and standardized variables. The variables were used for cluster analysis to classify the children s feet. Results: Five foot types were identified: flat, robust, slender, short and long feet. There were significant differences among foot types with respect to the children s body mass index. Normal weight children displayed an almost equal distribution of all foot types throughout childhood. Flat and robust feet were more common in overweight children, whereas underweight children showed more slender and long feet. Conclusion: The influence of excess, as well as deficient mass could be verified for the comprehensive foot morphology based on a foot type classification. Subsequently, foot discomfort as a result of various musculoskeletal disorders may develop. In turn, this might keep the children from being active and therefore reinforce the risk of developing obesity. However, there is still a lack of information regarding these relationships, which needs to be determined. This knowledge may help prevent orthopaedic foot problems and injuries. (2008) 32, ; doi: /ijo ; published online 15 April 2008 Keywords: foot type; foot shape; children; cluster analysis Introduction Over the past few decades, the number of overweight and obese children has been increasing worldwide. In Europe, 20% of the children are overweight, one-third of these are obese and by 2010, one in ten children will be obese. 1 Obesity and overweight go along with other complications, such as cardiovascular diseases, and also musculoskeletal disorders, particularly of the lower limbs and feet which are exposed to the additional mass daily. 2,3 A child s foot is at particular risk due to external influences throughout childhood, such as weight gain, as a child s musculoskeletal Correspondence: Dr M Mauch, Department of Sports Medicine, Medical Clinic, University of Tuebingen, Silcherstrasse 5, Tuebingen 72076, Germany. marlene.mauch@med.uni-tuebingen.de Received 10 October 2007; revised 18 March 2008; accepted 19 March 2008; published online 15 April 2008 system is in an ever-changing process of developing and maturing. The consequences of childhood obesity on the development of the musculoskeletal system, particularly of the feet, have received little attention in earlier studies. 2,4 Therefore, it is of great interest to examine the extent to which overweight as well as underweight can alter foot shape during childhood development. As the foot is the base of support for the chain of motion and body posture, it is postulated that obese children suffer an increased risk of developing pathologies and injuries due to increased loading, such as a flattening of the medial longitudinal arch (MLA). This is supported by findings of different studies reporting an increased mechanical loading in plantar pressures in obese individuals. 5 8 Some effects of obesity on different foot dimensions were found in children who displayed significantly larger foot dimensions in terms of broader, taller and thicker feet in comparison to nonobese children. 9 In addition, the studies indicate a general

2 trend of obese children having flatter feet than their non-obese peers, which was evident by an increased Chippaux Smirak Index (CSI) and decreased arch angle in obese children. 5,9 13 Unfortunately, current studies examining the effects of obesity on the morphology of children s feet have been limited in terms of subject number and age ranges. They focused on single foot dimensions, mostly plantar measures, which do not allow a comprehensive three-dimensional (3D) foot shape. In addition, there are no studies, which examine the influence of underweight on foot morphology. Although accurate 3D capturing methods, such as foot scanners, have become available in recent years, there is still a lack of information about 3D foot data. The new methods provide comprehensive and reproducible information about foot morphology, as well as help to assess a large number of subjects economically. In addition, fast data processors allow for better and more complex statistical analyses. The purpose of the present study was to examine foot morphology in its comprehensive 3D shape. Therefore, the aim was to classify children s feet into different foot types based on 3D foot measures, and further to determine the effect of body mass index (BMI) on the prevalence of these foot types. Assessment of data from a large population of children aims to provide reliable information about these influential factors on the development throughout childhood. Methods Subjects Feet of 2887 children (1450 boys; 1437 girls), aged 2 14 years, were measured in several German preschools, primary and secondary schools. Before commencing the study, all procedures and methods were approved by the Ethics Committee of the Medical Faculty of the Eberhard-Karls- University of Tuebingen, Germany (no. 352/2004). Body mass index Each child s height and mass were measured, and BMI was calculated using the Quetelet Index, body mass divided by height squared (kg m 2 ). 14 Children were classified by their BMI score according to percentile range cut-off points, which allow for age and gender. 15 This study did not differentiate between overweight and obese: 2257 (79%) of the children were normal weight, 456 (16%) overweight and 158 (5%) underweight. No mass or height values were available for 16 children who were therefore excluded from further analysis (Table 1). Foot anthropometrics The foot measures were taken using a 3D foot scanner (Pedus, Human Solutions Inc., Germany), with the children standing in a bipedal upright position. All measurements Table 1 Body BMI categories subdivided into age groups of the children Classification according to BMI Age Underweight Normal Overweight Total Total Dropouts 16 Overall 2887 Abbreviation: BMI, body mass index. Subgroups with nx10 in all BMI categories (6 13 years) were included in the subsequent cluster analysis. For 16 subjects no mass or height values were available and therefore BMI could not be calculated. included both left and right feet. Data were analysed using a customized version of ScanWorX SL1 (Human Solutions Inc.). Anatomical landmarks of foot were defined manually by the same investigator. Twelve foot measures in the forefoot, midfoot and hind foot areas were taken as follows: foot length, ball-of-foot length, outside ball-of-foot length, toe length, ball-of-foot width and circumference, heel width, ball angle, dorsal arch height, arch angle, CSI and Staheli Index (SI) (Table 2). Intra-tester measurement error was quantified by analysing five different children s feet four times by the same investigator. Root mean square error was calculated for different foot measures. 16 Results were between 0.5 mm and 2 mm for metric measures, and 1.31 and 1.41 for angle measures. The mean values were calculated for each foot measure per child using both feet. However, if one of the scans was not valid, then the measures of the valid foot scan were used. The sizeable influence of foot length on all other length measures was eliminated by normalizing the data to foot length and generating percent values. Statistical analysis Data were examined by multivariate and descriptive analyses using the software package SPSS 14.0 for Windows. Cluster analysis was applied, as it was considered to be a suitable method for classifying feet into different foot types. 17 The feet were classified so that each foot is very similar to others in the same cluster with respect to some predetermined selection criteria, such as foot measures. A combination of 1069

3 1070 Table 2 Description of foot measurements used for classifying feet into different types Foot measure Foot length (FE FT 1 ) Description Distance between FOOT END (FE; heel) and FOOT TIP (FT 1 ; anterior point of the most protruding toe) along the medial tangent of the foot across point B1 und point Hm (foot measuring line; y-axis) Ball-of-foot length (FE B1) Distance between FE and the first metatarsophalangeal protrusion (MTP; B1) Outside ball-of-foot length (FE B5) Distance between FE and the fifth MTP (B5) parallel to the foot measuring line (FE FT 1 ) Toe length (BM FT 2 ) Ball-of-foot width (B1 B5) Distance between the bisected ball line (B1 B5) (BM) and FOOT TIP (FT 2 ; anterior point of the most protruding toe) Connection line between first MTP joint (B1) and fifth MTP joint protrusion (B5; ball line) Ball circumference (B1 B5) Maximum circumference over the first (B1) and fifth MTP joint protrusion (B5) Plantar arch width (AW 1 AW 2 ) Narrowest section of the plantar medial longitudinal arch (AW 1 AW 2 ) Used for the indices CSI and SI (see below) Heel width (HW 1 HW 2 ) Widest part of the heel (plantar print; HW 1 HW 2 ) parallel to the ball line (B1 B5) Ball angle (BA) Angle between the horizontal (901 to foot measuring line) and diagonal ball line (B1 B5) in B1 Arch angle (AA) Dorsal arch height (DA 1 DA 2 ) Angle between line connecting the first MTP joint (B1) and the medial heel (Hm) with a secnd line drawn to the apex of the medial longitudinal arch concavity (AP) Height of medial dorsal junction of the foot and leg to the floor Indices CSI: Chippaux-Smirak-Index (%) ((AW 1 AW 2 )/(B1-B2) 100) SI: Staheli-Index (%) ((AW 1 AW 2 )/ (HW 1 HW 2 ) 100) Quotient between plantar arch width (AW 1 AW 2 ) and ball-of-foot width (B1 B2) Quotient between plantar arch width (AW 1 AW 2 ) and heel width (HW 1 HW 2 ) hierarchical and non-hierarchical methods was used. First, Ward s method (hierarchical technique) established the number of clusters and profiles of the cluster centres. The results were then optimized by a non-hierarchical method (SPSS: QUICK CLUSTER). 17,18 Before carrying out the cluster analysis procedure based on different foot measures, it is indispensable to use a small number of independent and standardized variables. Therefore, the 11 foot measures (normalized to foot length) were reduced using factor analysis of principal components. Varimax rotation was done, allowing for eigenvalues after rotation being greater than one. The differences among various foot types regarding BMI for the total sample were tested, using analysis of variance (ANOVA). In addition, cross tabulation for different age groups and BMI categories (underweight, normal and overweight) was applied and tested using w 2 -Test and Cramer-V. Results Factor analysis Four principal components (factors) were identified which account for 88% of the variance, semantically described by arch (including arch angle, CSI and SI), volume (including ball-of-foot width and circumference, heel width and dorsal arch height), angle (including outside ball-of-foot length and

4 ball angle) and length (including ball-of-foot length and toe length). If one or more of the 11 foot measures were not available for a subject, because of missing values in the scan, no factor could be generated for this subject and therefore this subject needed to be excluded from subsequent cluster analysis. Cluster analysis The standardized and independent factors, arch, volume, angle and length, were used for cluster analysis. The aim of using cluster analysis was to obtain the greatest differences possible among groups. For this reason, the factor angle was excluded from further cluster analysis, because it yielded no statistically significant differences among the calculated clusters. Five clusters, which represent the different foot types, were identified on the basis of the remaining three factors. The profiles of the different foot types were described by their optimized cluster centre (QUICK CLUSTER) as follows (Figure 1): (1) Flat feet (n ¼ 405; 15%): This foot type was mainly characterized by its flattened MLA, meaning a small arch angle, as well as a high CSI and SI; the factors volume and length displayed average z-values of around 0.1, indicating a medium characteristic of these foot dimensions within this foot type. (2) Slender feet (n ¼ 638; 23%): The feet in this cluster were characterized by their very small volume, which means they had narrow ball and heel widths and a low dorsal arch height. In addition, they displayed long toes and a relatively high arch. (3) Robust feet (n ¼ 433; 15%): In contrast to slender feet, this foot type had a fairly large volume and rather short toes with an average arch. (4) Short feet (n ¼ 562; 20%): This foot type was characterized by a short hind foot proportion and a long forefoot indicated by its short ball-of-foot length. Compared to other clusters, it displayed a relatively high arch and a rather high volume. (5) Long feet (n ¼ 760; 27%): The long feet cluster comprised the largest of the five groups. These feet were mainly characterized by their long ball-of-foot length and short toe length. In the other dimensions, volume and arch, this cluster showed average z-values of around 0.3, indicating a medium characteristic of these variables. Subgroup analysis: foot types and BMI There were statistically significant differences among the foot types with respect to children s BMI (F ¼ 16.96; p(f)o0.005). Children with robust feet displayed the highest BMI value, whereas children with slender feet displayed the lowest. The mean BMI over all foot types was ±2.58 kg m 2 (Table 3). After looking at the values for all subjects, the distribution of the foot types regarding the BMI categories was assessed for three different age groups (I: 6 9 years; II: 8 11 years; III: years). Only children between the ages of 6 and Flat Feet Slender Feet Robust Feet Short Feet Long Feet Cluster Centres [z-values] Arch Volume Length Arch Volume Length Arch Volume Length Arch Volume Length Arch Volume Length Figure 1 Profiles of five foot types. The x axis shows the three principal components; the y axis shows z-values of the cluster centres of each cluster, whereby large positive or negative z-values correspond to an increased or decreased dominance of the factor characteristic within the particular foot clusters, a value around zero represents a medium characteristic.

5 1072 Table 3 Number, percentage and BMI values (mean and s.d.) for the different foot types Foot types n % Mean (kg m 2 ) S.D. Flat feet Slender feet Robust feet Short feet Long feet Total Dropouts 89 Overall 2887 ANOVA F (4,2791) ¼ 16.96; p(f)o0.005 Of the 2887 children, 89 needed to be excluded from the conducted cluster analysis, since one or more of the eleven foot measures were not available for these children (dropouts). Analysis of variance (ANOVA) was calculated to test the differences between the five foot types with respect to the mean BMI. years were analysed, so that each age and BMI category included at least 10 subjects (see Table 1). Overlapping age group categories were chosen to achieve a more homogenous distribution across the three BMI categories. Therefore, children from 8 to 11 years were randomly assigned to age groups I, II or III. The proportion of the foot types in each age group was set to 100%, to meet the uneven distribution of underweight, normal and overweight children within the age groups. This allowed a more differentiated consideration of the differences among the foot types. Figure 2 shows the distribution of different foot types for underweight, normal and overweight children. There was a change in foot morphology regarding different foot types in normal weight children. As age increased, the proportion of slender and long feet also increased, whereas the proportion of flat, robust and short feet decreased. This development of foot morphology appeared different for underweight and overweight children. In fact, there were obvious differences. Flat feet were less frequent in underweight children and more frequent in overweight children. Differences were even more pronounced for robust and slender feet. The frequency of slender feet in overweight children in the age groups I, II and III was 5, 22 and 13%, respectively. The proportion of slender feet for underweight children was 32, 45 and 41% for age groups I, II and III, respectively. In contrast, effectively none of the underweight children displayed robust feet. With regard to the short and long feet, there were fewer short feet in the underweight children (between 19 and 3%). However, there were slightly more long feet in this BMI category. In contrast, overweight children displayed fewer long feet, and slightly more short feet compared to normal weight children of the same age group. Differences among the three BMI categories were statistically significant for all age groups on a-level of To illustrate the differences among underweight, normal and overweight children with respect to the age groups of the children, the proportion of the foot types in normal weight children in the specific age groups were set at 100% each. This was used as a reference value to distinguish the effect of over- and underweight conditions. Comparatively higher and lower proportions from normal weight children for these three foot types are depicted in Figure 3. For example, 19% of the normal weight 6- to 9-year-old children showed slender feet, whereas 32% of the underweight and only 5% of the overweight children within this age group displayed this foot type. Given the 19% of slender feet in the normal weight as reference value set at 100%, underweight children had a 65% higher incidence, and overweight a 74% lower incidence of slender feet compared to the normal weight reference. Overall, higher proportion of slender feet (65 7%), as well as lower proportion of flat (4 50%), robust (89 100%) and short feet (21 70%) in underweight children was visible. The differences in the overweight children were even more pronounced. With increasing age, the proportion of robust (69 337%) and flat feet (53 128%) increased. In addition, overweight children displayed a lower incidence of slender (34 74%) and short feet (16 31%) than normal weight children. Discussion Five different foot types could be identified within the population of the children s feet: flat, slender, robust, short and long feet. In contrast to other studies, which mainly obtained single foot measurements or measurements from the plantar surface of the foot based on footprints, 12,22 27 these foot types provide a more comprehensive understanding of morphology of children s feet, including their development. Therefore, the effects of external variables, such as BMI, can be used to distinguish the effects of underweight, normal and overweight on foot morphology and development (Figures 2 and 3). In normal weight children, there was a shift in the occurrence of robust and slender feet linked to increasing age. This complements previous findings by authors who examined single foot measures, mainly the ball-of-foot width normalized to foot length, to control for the proportional changes during growth and found that with increasing foot length, foot width dimensions decreased. 19,28 On the other hand, there was a remarkable decrease in flat feet associated with increasing age. As this foot type is mainly characterized through its lowered MLA, this underlies findings of previous studies which argue that age is the primary predictive factor for flat feet. 23,25,29 There was a diametric development with regard to the long and short feet. For example, as age increased, there was a decrease in short feet and an increase in long feet. That is, younger children displayed feet with

6 Foot Types [%] 100% 80% 60% 40% 20% 0% Underweight Normal Overweight years years years years years years years years years Flat Feet Slender Feet Robust Feet Short Feet Long Feet 1073 AGE groups (overlapping) [years] Figure 2 Distribution of the different foot types for underweight, normal and overweight children in three (overlapping) age groups (I: 6 9 years; II: 8 11 years; III: years; x axis). Children from 8 to 11 years were randomly assigned to age groups I, II or III. The proportion of the foot types in each age group was set to 100%, as there was an uneven distribution of underweight, normal and overweight children across the age groups (y axis). The absolute number of children with the specific foot type is shown in the bars. Deviation from normal weight [%] Underweight Additional % from normal weight Overweight Additional % from normal weight Flat Feet Slender Feet Robust Feet Short Feet Long Feet -100 Minus % from normal weight Minus % from normal weight 6-9 years 8-11 years years 6-9 years 8-11 years years AGE groups (overlapping) [years] Figure 3 Proportion of foot types of underweight and overweight children relative to normal weight children. 0 ¼ Proportion of normal weight set to 100% within the three (overlapping) age groups (see Figure 2); positive values ¼ additional percentage to normal weight; negative values ¼ lesser percentage to normal weight (y axis).

7 1074 larger forefoot lengths and greater volumes than older children. This finding was confirmed by Stracker 30 who found greater values regarding the forefoot length in younger children. The distribution of the different foot types in normal weight children could be validated mainly through findings of single foot measures from previous studies. Moreover, the classified foot types include additional dimensions which allow for a holistic perspective of children s feet, and which are hugely useful for further subgroup analyses, for example, regarding the effects of BMI. Overall, Table 3 shows significant differences among the five foot types and the children s BMI. Significantly higher values for BMI were evident in the robust feet; by contrast, the lowest values occurred in the slender feet. Previous studies, even though mainly verified on small populations, revealed obesity as being one moderating factor of foot shape. 5,9 11 Obese children displayed significantly greater foot dimensions than their non-obese peers. The differences with respect to flat feet were not as obvious as expected, based on knowledge from previous studies. Overall, the BMI value in the flat feet cluster was only slightly increased (Table 3). Some studies found greater plantar pressures for the midfoot area in overweight children, 5,6,8,31 as well as a greater contact area derived from footprints, 10,12,22,32 which could imply a lowered MLA. Therefore, it is questionable whether the flatter feet displayed by overweight children are connected with a thicker plantar fat pad or with a lowering of MLA. Studies which examined the plantar fat pad of the children found no significant differences between normal and overweight children. 10,33,34 This indicates that the flatter feet of overweight children may be caused by structural changes in their foot anatomy, which may affect the functional capacity of MLA. Compared to previous investigations on plantar measures, flat feet in the present study were considered in a more comprehensive way, comprising the additional dimensions, volume and length. This could be one explanation for obtaining results that are not as pronounced regarding the incidence of flat feet in overweight children. To test for a possible interaction effect between age and BMI, and to achieve more differentiated results, differences in the BMI categories were assessed in the different age groups of the children illustrated in Figure 2. Long feet were less represented in overweight children compared to normal or even underweight children. On the other hand, short feet were less frequent or even non-existent in underweight children. Short feet were mainly characterized by a short hind foot proportion, and also by relatively high values in the volume dimension of the foot. Therefore, the higher occurrence of short feet in overweight children could not only be explained through their length proportion, but also through greater width and height foot dimensions. The most obvious differences among the BMI categories were evident for flat, robust and slender feet illustrated in Figure 3. Differences between overweight and normal weight children increased with age such that the older the children, the more robust and flat feet occurred. As mentioned before, the occurrence of robust and flat feet in overweight children can be supported by previous findings of single measures of the foot, mainly plantar and width measures. Moreover, complementary to the rather minor effect of BMI on flat feet in the total sample (Table 3), the effect differentiated in the particular age groups could be clearly recognized. In addition, there were fewer slender feet over all age groups in the overweight children compared to the normal weight children, which was compatible to the findings for the total sample shown in Table 3. Contrary to the results for overweight children, differences were apparent for the underweight children who had remarkably more slender and less robust, flat and short feet compared to the normal weight children. Interestingly, the higher and lower percentages of the normal weight population changed with increasing age. For example, underweight children had a lower incidence of flat feet when they were older than when they were younger. This result could imply that the lower the mass the foot must support, the better the MLA will develop. Limitations that may have influenced the results of the study include the small sample sizes for the younger and older age groups. In response to this, single age groups were summarized into three larger (overlapping) age groups to achieve a more homogenous distribution across the ages and weight groups. It must be acknowledged that this study did not control for other factors such as nutrition, physical activity or the socio-economic status of the children, which often are associated with childhood obesity 35,36 and could possibly confound the results. There is still uncertainty as to what extent these factors possibly influence foot structure. Conclusions In summary, the influence of the children s BMI on 3D foot shape could be established and is represented in five different foot types. In contrast to previous studies, the influence of excess and deficient mass could be verified not only for single foot measures, for example foot width or plantar (pressure) values, but also for the comprehensive foot morphology based on a foot type classification. The latter presents a new perspective on this scope of research. The alarming results about the influence of BMI on foot shape are of great importance, as the incidence of obesity has steadily increased over the past two decades in industrialized societies. 34 It is postulated that changes in foot morphology come along with structural changes in the foot anatomy. In turn, this may affect the functional integrity of the foot, for example, the MLA. If obesity is allowed to continue unchecked until body weight far exceeds recommended levels, numerous chronic disorders may develop, including foot discomfort, various musculoskeletal disorders and physical restrictions in the performance of daily tasks. 4,10 Given that overweight is related to musculoskeletal disorders, physical consequences of constant additional loading

8 on foot structure and the influence of obesity on locomotor abilities need to be determined. Possibly, foot disorders might keep children from being active and therefore reinforce the risk of developing obesity. Therefore, further investigations should be made regarding the relationship of various (interacting) factors, for example, physical activity on the development of the child s foot structure and function. Finally, an understanding of all these factors for predicting foot shape and function can help prevent orthopaedic foot problems and injuries. In turn, this could give more detailed information to parents and orthopaedic and paediatric physicians for diagnosis and therapy by taking the entire foot into consideration. Acknowledgements We thank Reno Fashion & Shoes GmbH, Germany for financially and logistically supporting this project. References 1 WHO. Obesity Prevention and Managing the Global Epidemic. World Health Organisation: Geneva, James WP. What are the health risks? 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