How to measure children’s feet: 3D foot scanning compared with established 2D manual or digital methods

Compared to adult feet, children's feet have characteristic differences in their structure and function [1, 2]. One characteristic is the pediatric fat pad in the midfoot in children, which protects against excessive pressure until the musculoskeletal system has adapted to an upright gait [3]. This initially leads to great flexibility of the child's foot. Foot geometry in children changes rapidly during growth and maturation and depend on the child’s age of beginning to stand and walk [1, 4, 5]. Therefore, a wide heterogeneity of foot shape is typical in infants and young children. However, this is not necessarily associated with pathological deformities [6]. In detail, foot size (length, width) enlarges with increasing age of children [1, 4, 6]. During child development, the foot grows predominantly in length and less in width [1]. Therefore, it has been shown that the relationship between foot width and length evolves towards a narrower foot during growth [1]. Thus, until the age of 8 years, children’s feet have a wider shape compared to older ones. When children become older than 8 years the proportion of their feet becomes more and more similar to those of adults [1]. Furthermore, the longitudinal arch of the feet declines until the age of six and remains on a constant level afterwards [1]. One of the most basic and commonly used parameters, among others, for measuring children's feet is foot length [1, 5]. In particular, foot length serves as a basic quantity to normalize further foot parameters to account for growth and development of the aging child [1, 5]. Due to the great heterogeneity of foot shape as well as rapid growth and maturation, children represent a very special group for foot measurements and the footwear industry.

The importance of foot measurement for footwear fit and design, as well as clinical applications is evident [1, 7‐14]. Although knowledge of the high heterogeneity in foot shape in children is evident [1, 4, 6, 13, 14], the footwear industry still bases the last as well as the shoe development predominantly just on the foot length and ball width [15]. However, it is obvious that children can have the same foot length, but different foot shapes (e.g. wide versus narrow) [4, 6, 15]. This fosters a mismatch between the manifold foot and shoe shapes [1, 4, 15].

Nowadays, manual foot measurements (length/ forefoot width) by use of assistant devices (e.g. german WMS® foot measurement system for children) are the established gold standard in the shoe stores. However, these measurements do not consider the individual and multidimensional foot shape. To enable a more individualized foot analysis, 2D foot scans are only used in special cases, e.g. assignment of individual sport shoes for athletes, and are not available to the general public. Nevertheless, 2D foot scans are limited and cannot measure vertical dimensions (e.g. navicular height or volume/girth) [6]. This issue can be solved by three-dimensional measurements that provide a detailed scanning of the foot shape in all spatial dimensions [4, 7, 15]. Nowadays, there are three-dimensional foot measurement technologies that allow rapid measurement and data evaluation, including recommendation for matching shoes in the shoe store [7]. However, this is limited to selected shoe stores with access to these measurement technologies and is mainly used for adults [7].Therefore, the shoe industry has the need to implement three-dimensional foot data for fit determination already at the stage of shoe-last production [12, 15]. In recent years, the development of technologies (3D scanners, computer-aided design (CAD), computer-aided manufacturing (CAM)) has enabled the production of lasts based on three-dimensional foot data [12, 15].

Nevertheless, information on validity and comparability of the two and three-dimensional measurement methods (manual, 2D, 3D) is not clear yet. Therefore, the purpose of this study was to compare 3D foot scanning with two established measurement methods (2D scanning and 2D manual foot measurements).

Results for all outcomes, including mean ± SD as well as ANOVA-analysis, are reported in Table 3. Significant differences were found for all outcome measures comparing the three methods (p < 0.0001). Differences ([mm]) between the methods are reported in Table 3.

Regarding foot length, differences ranged from 3 mm to 6 mm with 2D scans showing the smallest and 3D scans the largest values.

Foot ball breadth measurements showed a difference of 3 mm between MF and 3D scans.

Foot width measurements in comparison of 3D and 2D scans always showed higher values for 3D measurements with the differences ranging from 1 mm to 3 mm.

The results of bias and limit of agreement analysis as well as heteroscedasticity (person correlation) for comparison of manual, 2D and/or 3D measurements for selected parameters of foot length, width and breadth are detailed in Table 4.

The purpose of this study was to compare 3D foot scanning to the established methods of manual foot measurements (2D) and 2D foot scanning in primary school childrens´ feet. The main result is that there are significant differences for all outcomes of foot length and width comparing the three methods.

The study results show that the different methods somewhat under- and/or overestimate the single outcomes analyzed. This is in accordance to previously reported studies [9]. The presented results show that there is no relevant/significant heteroskedastic error between measurement methods that could have arisen due to variance in foot size in our population. Therefore, smaller feet are not expected to have a smaller measurement difference between the different methods compared to larger feet. Therefore, there is no bias in the average error estimations. Mainly the differences between the measurement approaches are due to the nature of the measurement methods themselves. In detail, all foot dimensions collected with the 3D scanner were greater compared to the 2D scanning as well as the manual foot measurements. One reason for this could be that the 3D scanner detects the outermost points of the superficial boundaries (e.g. metatarsal head) more precise than the manual foot measurements as well as the 2D scan when measuring foot length, forefoot width and heel width [9]. Another reason may be that the examiner presses the soft tissue surrounding the measuring points with the material of the slide during the manual foot measurement [9]. This can lead to a measurement error resulting e.g. in smaller values ​​in foot length and/or width. Moreover, the foot measures collected from the 2D scanning were smaller than those collected using the manual measurement methods as well as the 3D scanning. One reason for this, that needs to be discussed, might be the shape of the human foot: It is curved upwards at the outer (medial, lateral) edges and does not lie completely flat with the entire plantar surface. Because of this, the footprint on the scanner board might be reduced at the edge of the foot, and the foot scan contour captured tends to be smaller than the actual plantar surface contour [9]. Consequently, a standardized measurement procedure is desirable as well as an adequate training for the examiner should be carried out before the use/application of the described measurement methods.

Moreover, the differences between all outcomes of the three analyzed measurement methods are statistically significant but the clinically or ergonomically relevance must also be questioned [7, 9]. Differences of about 0.6 cm are important for the foot length as this is the differences of one complete shoe sizes in accordance with the Parisian point [3]. Furthermore, even half sizes are relevant in some shoe size systems (e.g. US) therefore even the difference between 3D and manual measurements seems of ergonomic relevance. As this is the case for all methods for foot length, a correction procedure (e.g. application of a corrective factor) has to be discussed [20].

Regarding foot width, the differences, presented results ranged between 1 mm to 3 mm, are ergonomically of relevance as the foot width in the german WMS®-system for children’s feet/shoes (wide – middle – small children feet) clusters the foot width by adding 18 mm for each width category.

The 3D foot scanning is the technological gold standard for the assessment of foot morphology [9, 12, 13, 21]. The advantages of using the 3D scanning system to collect foot measures is the high precision and accuracy of the different systems [17]. The 3D foot scanning allows the assessment of volumetric and surface data and provides more detailed information on foot size as well as foot shape in all dimensions compared to the manual measurement as well as the 2D scanning [9]. This is especially important for the growing foot of the children [4]. Nevertheless, the high initial set-up cost as well as the time needed for the processing of the data (about 1 to 2 hours for each scan) are disadvantages to be named. Moreover, the practical suitability largely differs between the different 3D systems. The one used within the presented study needs more time for a scan (up to 5 min) compared to the manual foot measurement (1 to 2 min). Besides, the 3D measurements can be significantly accelerated by using a stationary camera measurement system placed around the foot instead of the hand-held mobile scanner.

Based on the presented results, our study supports the use of 3D foot scanning measurement for collecting foot anthropometric data in school children aged five to ten years of age, especially as a basis for collecting detailed information on foot shape and size in all three dimensions for the last construction of children shoes. A purely individualized shoe production based on 3D scan data in the context of children's feet should not be the goal (high shoe costs), therefore the development of 3D data based shoe lasts for shoe production for these age groups should be aspired. Besides, using different devices/techniques to measure foot measures may produce inconsistent results between studies. Therefore, it is important to consider the measurement method differences when comparing foot data between studies [9, 21].

Certain limitations have to be considered when interpreting these results. In addition, not all outcomes are available for all measurement methods at a time based on the nature of the measurement method (e.g. 2D scanning does not allow measuring the foot girth). In this case, only the two available methods were used for comparison to allow a more detailed comparison of outcome measures that are not only based on foot length and forefoot width. Information on accuracy and reliability of the measuring methods are not analyzed within this study. However, reliability and validity of the measurement methods is described elsewhere and evaluated as good to excellent [1, 9]. The different baseline positions for the foot measurements (2D manual/digital: two-legged parallel stance vs. 3D: slightly lunge) may have influenced the results due to the possible differences in weight distribution on the feet. The slight lunge was technically necessary in order to be able to perform the three-dimensional scan properly, without foreign bodies in the scan area. In order to keep this effect as low as possible, the investigator gave the children specific instructions on how to assume the slight lunge position in order to distribute the weight on both legs as much as possible. However, an influence cannot be completely ruled out.

The presented data may be relevant in the field of ergonomics (shoe industry) as well as clinical practice. For application, it shows the importance, that the measurement method for the feet should be in line with the measurements method of the shoe/last. In addition, the finding of the presented study suggests that when comparing foot data among different studies, it is important to consider the differences caused by the applied measurement methods. Based on the presented (three-dimensional) data, a foot typing might be advantageous for further development of children shoe lasts that account for a higher number of foot shape variability in children.