Providing robust data which characterises the development of the paediatric foot is needed in order to help clinicians and researchers understand the typical trajectory of the foot throughout the complex and variable stages of growth and development. Data which describes the linear anthropometry of the foot throughout childhood has existed for many years [
], but studies reporting anthropometric measures to describe changes in foot shape are limited in what they tell us about three-dimensional shape and development. More recent studies have adopted static clinical measures [
], filming [
] and pressure technology [
] to capture different aspects of foot development but there is no universal agreement on the appropriate measures for determining foot shape (or structure) [
], although there are concerns about the validity of footprint measures [
]. Equally, attempts to describe the typical development of foot shape across childhood using clinical measures such as the Foot Posture Index [
] are limited in that existing measures have not been validated in children, and are based on underlying clinical assumptions about foot characteristics.
Considering the limitations of clinical measures to accurately describe the development of foot shape across childhood, three-dimensional scanning technology shows promise. Research using a 3D stand-in scanner captured variation of children’s feet and resulted in the description of different morphological foot types [
]. Earlier work by the same authors looked at foot morphology of normal, underweight and overweight children [
] demonstrating some real-world clinical impact of this technology, although still limited to 2D measures. Understanding more about the three-dimensional shape and morphological changes through childhood is important to advance the prevailing arguments about the typical development of children’s feet [
] and inform the development of appropriate clinical measures.
Within the existing research in other fields, three-dimensional shape descriptors have already shown capacity to characterise the shape of different body surfaces [
] and bones [
]. This analytical approach produces shape index (a number between − 1 and 1 representing 9 distinct 3D shapes and transitions between shapes) and curvedness for each point (vertex) on the surface of the scanned object, describing the 3D shape and the surface curvature [
]. Despite the application to other areas of the body, there is sparse evidence of advances with this analysis in research relating to the growth and development of the paediatric foot.
Liu, Kim [
] argued that shape-index can capture the local shape, and provide a map of convex (protruding) and concave (indented) areas which will help with the automatic localization of anatomical landmarks of the foot and lower leg. In addition to isolation of anatomical landmarks in adult feet [
], shape index and curvedness can also be used to identify shape changes due to development of the paediatric foot. As the shape and curvedness are represented numerically for each point on the surface, these can be subject to statistical analysis, hence they can be used to quantify the differences or changes in the 3D shape of the paediatric foot surface. Therefore, the aim of this study was to investigate whether 3D shape descriptors derived from 3D scanning data can help identify differences in 3D foot shape between children of different age groups. This study was designed to provide an understanding of how these measures can relate to paediatric foot development and therefore to indicate how the measures can be used in future work with larger cohorts of children.
The purpose of this study was to determine whether 3D shape descriptors derived from 3D scanning can identify differences in foot shape in children of three different age groups. The findings from this study demonstrated the ability of 3D shape descriptors to identify and locate differences between age groups and to provide 3D shape information about the development of anatomical structures.
The changes in the distribution of the curvedness data in the dorsal, medial and lateral surfaces indicate that the overall shape of the foot (excluding the plantar surface and the emerging anatomical landmarks) becomes less rounded with age. This is likely to be related to the changing proportions of the foot as it becomes more slender with age (smaller width and circumference measures along with larger length parameters), as suggested in previous studies [
]. These changes are supported by the shape-index histograms of the same surfaces, showing increasing areas of concavity (indented) among the convex (protruding) areas.
Visual inspection of the curvedness heat maps of the dorsal, medial and lateral surfaces demonstrated the emerging bony landmarks. These are highlighted by higher curvedness at the anatomical sites, with flatter regions surrounding them. The shape-index heat maps of the lateral and medial surfaces show that the increase in concavity is also likely to be related to the emergence of bony landmarks creating “valleys” around them. The increase in concave and saddle ridge shapes on the dorsal surface histograms, along with the changes on the shape-index heat maps, suggest similar changes to the medial and lateral surfaces. This include the changing proportions of the foot and an emergence of bony architecture exposing the developing cuneiform bones.
These findings also confirm previous results where younger children’s feet were found to be more robust (larger width and circumference measures along with smaller length measures) based on linear 2D parameters [
]. However, while the circumference and width measures only give us one measure for the whole foot, the heat maps and 3D shape descriptors used in this study can identify specific regions of the foot where the 3D shape changes (e.g. robust to slender) occur.
When considering the plantar surface, the increase in lower curvedness values (see Fig.
a) suggest an increase in the flat (weight bearing) areas of the plantar surface. The increase in concave areas shown on the shape-index histograms suggests the increase of the area of the MLA. The development of the MLA with age is visible on both the curvedness and shape-index heat maps of the plantar surface. Previous reports have demonstrated increasing area of the MLA during development, however, these studies used 2D foot print measures [
] or contact area [
] and do not allow for the description and the quantification of the surface of the MLA, hence neglecting the 3D properties of a multi-planar structure [
]. A study by Chang, Lin [
] used an indirect measure of the 3D shape of the MLA, the arch volume (the volume under the MLA) and reported an increase with age. In addition to the results of the existing studies, our data using 3D scanning and shape descriptor analysis, has described an increase in concavity of the MLA surface. These data might help us begin to understand more about the structural and functional development of the surrounding soft and bony tissues as a weight bearing, shock absorbing and propulsive unit.
The findings reported in this study demonstrate the potential for greater utilisation of technology and 3D shape descriptors in foot morphology research, as well as in applied environments such as footwear industry and clinical practice. The results reiterate previous recommendations from Telfer and Woodburn [
] which promoted greater utilisation of 3D scanners. This is supported by further work which has explored [
] or recommended [
] the use of 3D shape data in the clinical assessment of the foot shape.
Although there are several current measures which clinicians use to describe foot shape, the relationship between these measures and function is unclear. The measures used in the current study describe shape at a higher resolution with more accuracy and relevance, than current options due to their 3D nature and whole foot approach. We propose that this type of data derived from 3D foot scanning may be able to clarify links between morphology and function. The potential of the approach to provide a clinician with a clear morphological map of the foot and changes between age groups (or between ages within an individual) is an attempt to translate basic research into measures which can be interpreted in an applied clinical setting.
Three-dimensional shape descriptors have shown good potential in isolating changes in foot structure across childhood. Once established, there is the potential that three-dimensional shape data will be beneficial for understanding more about the morphological development of the foot, improving diagnosis techniques, tracking changes over time (for example, pre and post intervention), orthotics and footwear design, understanding disease process and its impact on foot development.
The intention of this work was to demonstrate the potential role of 3D shape descriptors derived from 3D scanning in describing the trajectory of foot shape development and paediatric foot assessment. Although further work is needed to support the translation of the findings into clinical practice, the small sample size enabled a pilot analysis of data, to be used in further research. In this study, 3D shape descriptor heat maps were used, which were based on individual participant foot data, but were checked to be a representative example from that age group. The authors are currently collecting 3D data for a larger cohort of children to further understand the two- and three-dimensional shape development of the paediatric foot. This further work will help to advance understanding of the role of 3D shape descriptors in the quantification of children’s foot shape.
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