Herein, we describe the results of a 2-year longitudinal study investigating the development of the midfoot in elementary school children. In particular, we conducted 3D measurements of the surface structure of the foot, focusing on the height of the navicular bone, a known indicator of flatfoot, in addition to the lateral displacement and misalignment of the foot skeleton. Features of the feet were extracted to predict the navicular bone height 2 years later.
Our analysis clearly showed that the FL and IH increased, while the SHN angle and ABD rate decreased from the overall mean, indicating strong development over the 2 years of follow-up. However, the NH rate, which constitutes the midfoot, decreased. Reportedly, the medial longitudinal arch develops around 10 years of age [1, 2], while the FL increases in boys and girls until 15 and 13 years of age, respectively [19]. In the present study, FL increased, while NH showed a lower rate of increase than FL or a decrease. As shown in Fig. 1, the thickness of the foot is still increasing at 12 years of age as the IH increased significantly, while the NH rate decreased significantly over the 2-year period. In previous studies, NH increased significantly from 6 to 13 years of age; at 9 years of age, it was 2.89 cm for boys and 2.79 cm for girls, increasing to 3.48 cm and 3.39 cm, respectively, at 12 years of age [20]. In the present study, the NH decreased significantly from 3.46 cm for boys and 3.13 cm for girls at 9 and 10 years of age, respectively, to 3.21 cm and 2.99 cm at 11 and 12 years of age, respectively. The NH at the bottom quartile of the SHN angle at 9 and 10 years of age was 3.63 cm for boys and 3.14 cm for girls, while the NH at the lower quartile was 3.30 cm and 3.15 cm, the NH at the upper quartile was 3.14 cm and 2.87 cm, and the NH at the top quartile was 2.79 cm and 2.79 cm, respectively. In the present study, the NH increased slightly or remained the same as that in the 9–10-year-old participants. These values were almost equivalent to those found in previous studies. Although there was a difference in study design (i.e., a longitudinal study vs. the previous cross-sectional design), it is presumed that particular factors influenced the development. During the study period, which coincided with the COVID-19 pandemic, elementary school students’ outings were restricted, and their physical education classes were limited. As extracurricular sports were also not conducted, the reduction in physical activity might have had an impact on their foot measurements.
Generally, the development of children’s feet progresses in terms of both size and height. Even with the exercise restrictions imposed owing to COVID-19, which limited stimuli necessary for muscle and ligament development, the development of the feet could not be halted, potentially leading to deformities. Therefore, we speculate that the axis of midfoot development shifted inward, inducing pronation.
From these results, as shown in Fig. 2, when the SHN angle was large, i.e., when the navicular bone (point e in Fig. 4) was far from the centerline a–b, the ABD and SHN angles became larger, and the NH rate decreased after 2 years. In other words, it was not the increase in FL that decreased the NH ratio but rather the increase in the SHN angle, indicating an NH inversion, resulting in insufficient vertical growth of NH.
Outputs of the three-dimensional (3D) foot scanning system. The positions of the second toe tip (a), heel (midpoint of the width of the calcaneal tuberosity) (b), navicular bone (c), and instep (d) were identified as feature points using the 3D foot scanner. In the present study, five 3D foot skeletal indices were used, including the distance from the heel to the tip of the second toe (foot length [FL: Line a–b]), height of the navicular bone from the floor (navicular height [NH: Line c]), distance from the floor to the highest point of the talus head (instep height [IH: Line d]), second toe–heel–navicular angle (SHN angle: angle made by abc), and the distance from the center line (Line a–b) when the coordinate point of the talus head is projected onto the floor (axis of the bone distance [ABD: Line ab-d]). The derived distance information is influenced by the FL; therefore, normalization was performed with the distance from the heel to the tip of the second toe as a reference (e.g., NH/FL = NH rate)
In addition, as shown in Fig. 3, when the SHN angle is larger at ages 9–10 years, the NH is lower at ages 11–12 years than it was at ages 9–10 years. Previous studies have not investigated the lateral displacement of the navicular bone; however, this study yielded different results in terms of changes in NH [20]. The anatomical reasons for this increase in the SHN angle are believed to include the relaxation of the spring ligament, which includes the calcaneonavicular joint complex, known to be a factor in flatfoot [14]. An increase in the SHN angle indicates an increase in the inversion of the navicular bone.
The navicular bone is connected to the medial and intermediate cuneiform bones, and instability may occur between the first and second metatarsal bones, potentially resulting in greater mobility between the two metatarsal bones [21]. Therefore, flattening of the midfoot may lead to sports injuries, hallux valgus, deformities of the heel bone, pes planus, and other basic musculoskeletal characteristics that could lead to decreased future walking function [3, 22, 23]. The SHN angle from this system is a useful indicator of internal rotation of the navicular bone.
Factors predicting SHN and NH rates were derived from the SHN angle, IH, NH, and ABD 2 years prior. Lateral displacement of the navicular bone, foot thickness, and foot distortion are also related.
As demonstrated above, evaluation indices that assess pronation should be used rather than focusing solely on the height of the navicular bone in relation to flat feet. This is because both the navicular and medial cuneiform bones are involved in the development of the midfoot in children. In other words, this study has revealed that the changes in the midfoot of children can be evaluated using the newly developed 3D measurement indices: the SHN angle, ABD, NH, and IH.
Limitations
The foot 3D scanner developed in the present study measures the surface structure of the foot; therefore, it does not consider the fat layer of the foot, which may influence the measurement results to some degree. The study period included the COVID-19 pandemic, and there may be variability in physical activity. However, we expect our findings to adequately reflect the impact of the suppression of physical activity on NH, providing a predictive factor for growth 2 years later.
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