Objective  The arch of the foot is a key structural component of children's foot biomechanics, and its development relies on natural mechanical stimulation from activities such as walking. In today's environment, many children experience a "high load, low stimulus" growth setting, which can lead to delayed arch development and subsequently cause gait abnormalities, flat feet, and long-term foot pain. Orthotic insoles are an important intervention for improving arch development. Existing studies have confirmed the effectiveness of parameters such as arch support height and material stiffness, but long-term studies on the impact of heel cup encirclement on arch development in young children are still lacking. Therefore, this study aims to investigate the effects of two different heel cup encirclement designs in orthotic insoles on the development of flexible flat feet in children aged 3-6 years, with a one-year follow-up.

Methods This study enrolled 163 children aged 3-6 years with flexible flat feet, who were randomly divided into two groups: Group A, with a moderate encirclement heel cup design, and Group B, with a strong encirclement heel cup design. Follow-ups were conducted every three months, and Clarke's angle, arch ratio index (RI) and valgus angle (VA) were measured using an image processing system. To control for baseline differences, the analysis focused on the change in each parameter relative to the baseline, using generalized estimating equations and linear mixed-effect models for statistical analysis, while including baseline values as covariates.

Results Both types of insoles improved the indicators related to arch development, but the moderate encirclement design in Group A performed better overall, with significant differences between the groups emerging at different times: Clarke's angle showed significant improvement after 12 months, with Group A showing significantly better results than Group B at this time point. Improvements in the RI were mainly observed between 3-9 months, with Group A showing superior results starting at 6 months, and this advantage increasing over time. The VA also showed continuous improvement, with Group A performing better starting at 6 months. The study suggests that Group A's better performance can be attributed to its balanced approach of guiding and stabilizing the arch, while allowing necessary joint movement to correct the alignment. In contrast, the strong encirclement design of Group B may restrict rearfoot movement, which could hinder the development of muscle strength at this stage of childhood. Moreover, Clarke's angle and the RI exhibited different improvement patterns, which reflects the diluting effect of rapid vertical growth in the foot on early changes in angular indicators. These results support the collaborative role of orthotic insoles in children's arch development, suggesting that they can help compensate for the lack of natural mechanical stimulation and guide the arch toward healthy development.

Conclusion Orthotic insoles with a moderate encirclement heel cup design are superior to those with a strong encirclement design in improving the arch development of children aged 3-6 years. The advantages of the moderate encirclement design became increasingly apparent after six months of intervention and continued through to the 12-month mark. This conclusion is supported by the superior performance of Group A in both the RI and VA indicators from 6 to 12 months, and by the fact that significant differences in Clarke's angle were only observed after 12 months. These findings provide a basis for individualized orthotic interventions in this age group.