Data were obtained from patients aged 6–15 years [18,19] who visited our clinic and were diagnosed with idiopathic FFF between January 2003 and February 2022. We included patients with records of at least two plantar pressure exams over a follow-up period exceeding one year. The follow-up period was defined based on the timing of examinations rather than timing of visits. Diagnosis of FFF was based on comprehensive evaluations including physical examinations for medial longitudinal arch collapse and hindfoot valgus, plain radiographs, and plantar pressure assessments. Patients with secondary flatfoot or who were already using corrective shoes or insoles at their first visit were excluded from the study. All patients included in this study were prescribed customized therapeutic insoles with medial longitudinal arch support and a medial heel wedge at our clinic for reasons such as pain, lack of improvement, or parental request.

Plantar pressure examinations were conducted using the emed^® (novel GmbH) to measure both the shape of the footprint on the ground and the pressure applied to each grid. To assess dynamic plantar pressure during midgait, subjects took at least three steps before and after stepping onto the platform. They were instructed to walk at a natural, comfortable pace without specific speed constraints. To ensure consistent results, a minimum of three trials was conducted per subject. Any trials in which the subject altered their speed or stride unnaturally were excluded [20]. We measured the ratio of maximal width of the midfoot (W_MF) to width of the forefoot (W_FF) and the peak pressure of the medial midfoot (MMF) using a pedobarograph. First, the long axis was drawn as the line passing through the origin of the second toe and the peak pressure point of the heel. The distance between the origin of the second toe and the tip of the heel was divided into three equal parts, with the middle section defined as the midfoot. The W_MF was defined as the longest line drawn perpendicular to the long axis, and W_FF was the distance between the two lines connecting the first and fifth metatarsal heads at the intersection of the foot contour. The peak pressure of the MMF was identified as the point of highest pressure medial to the long axis within the midfoot region (Fig. 1).

Plain radiographs were obtained in a weight-bearing state with both lower extremities extended in a neutral position. The LTMA and calcaneal inclination angle (CIA) were measured using the PACS (Picture Archiving and Communication System) program (INFINITT Healthcare; Fig. 2). The LTMA was defined as the angle between the long axis of the talus and that of the first metatarsus. The CIA was measured as the angle between a line drawn along the plantar surface of the calcaneus and the transverse plane [21,22]. The LTMA was negative when the first metatarsal bone was flexed dorsally relative to the talus axis, a finding consistent with FFF. To ensure consistency, a single rehabilitation resident performed all measurements. Previous studies have shown that LTMA and CIA exhibit strong inter-rater and intra-rater measurement reliability [21-23]. Reliability tests were performed by calculating intra-rater correlation coefficients (ICCs) using 20 randomly selected samples, with measurements performed at least two weeks apart to avoid any bias.

The primary outcome was the changes in both the plantar pressure test and the plain radiograph. In the plantar pressure examination, we measured the maximal W_MF-to-W_FF ratio (W_MF:W_FF) and peak pressure in the MMF. In plain radiographs, changes in LTMA and the CIA were evaluated. All indicator values were measured at both the initial and final examinations for comparison. Changes were calculated by subtracting the initial values from the final values. As a secondary outcome, we assessed the correlations between changes in plantar pressure and radiographic findings in patients who had undergone at least two lateral plantar radiographs. Radiographs were considered valid only if obtained within one year of the corresponding plantar pressure test date.

Data from the first and the last examinations were compared for each patient. The normality of each variable was assessed using the Kolmogorov-Smirnov test. The peak pressures of MMF and W_MF:W_FF measured from plantar pressure had non-parametric distributions, whereas the LTMA and CIA from plain radiographs followed parametric distributions. Based on our results, pre-post comparisons were conducted using either the paired t-test for parametric data or the Wilcoxon signed ranks test for non-parametric data. Correlation analysis was performed using Pearson’s correlation for parametric variables and Spearman’s correlation for non-parametric variables. Unless stated otherwise, statistical results are expressed as mean±standard deviation with statistical significance set at p-value<0.05. Statistical analysis was performed using IBM SPSS statistics ver. 22 (IBM Corp.)

The medial concentration of plantar pressure in flatfoot patients was described in a previous study [25]. The decrease in maximum pressure on the MMF was significant in the subject demographics in the present study. With an average age of 10 years at the initial visit and a median follow-up period of 51 months, an anticipated increase in body weight over time underscores the importance of this finding. The observed decrease in pressure at the maximum point despite the expected increase in overall body weight indicates potential structural or functional improvements in foot biomechanics. In a previous study [26], notable changes were not observed in the average midfoot pressure or the ratio of midfoot pressure to total foot pressure over a mean follow-up period of 15 months. However, in the present study, we prioritized the identification of the region with the highest pressure in the MMF rather than assessing average pressure. This distinction is crucial because, even if the average midfoot pressure remains modest, other regions with focused pressure may experience discomfort or symptomatic manifestations. However, further empirical studies are needed to determine pressure thresholds that correlate with symptomatic presentation or prognostic outcomes in flatfoot patients. Furthermore, the Chippaux–Smirak index has low sensitivity for flatfoot [27]. In the present study, we focused on the maximum length of the midfoot instead of the minimum length to enhance sensitivity in identifying changes of test results during follow-up examinations. However, our results showed a degree of change comparable with previous studies utilizing the Chippaux–Smirak index [9]. The use of length ratios offers the advantage of broad clinical applicability because it circumvents the necessity for specialized pressure testing equipment.

In a previous study [15] examining the radiological progression of flatfoot, decrease in the LTMA was noted and the CIA remained unchanged. In another study [6] that included 31 individuals 10–11 years of age and an average follow-up period of 4 years with demographics similar to those in the present study, significant reductions in both the LTMA and the CIA were observed. The amount of change observed was also comparable to that in the present investigation. However, critical evaluation of the clinical significance of a 1°–3° alteration in angle is imperative. The LTMA and CIA are known to have good interobserver and intraobserver reliability [21-23]. However, the minimal clinically important difference (MCID) for these measurements has not yet been established. Additionally, studies remain inconclusive regarding LTMA and CIA cut-off values that reliably differentiate between symptomatic and asymptomatic flatfoot. A study of 28 young male soldiers [28] found mean LTMA values of 13.0 in symptomatic feet and 8.0 in asymptomatic feet, a significant difference. The mean CIA values were 10.8 and 12.5, respectively, also a significant difference. In contrast, another study of 135 flatfoot patients with an average age of 11 years [29], found no significant differences in LTMA and CIA between asymptomatic feet and symptomatic feet when treated conservatively. However, in patients who eventually required surgery, LTMA and CIA showed significant differences compared to those who did not need surgery. When the angle is extreme, symptoms are likely to be severe. However, due to the lack of consensus on a standard cutoff value or an MCID to predict symptom changes, clinicians should approach the subject with caution. In a study of 300 normal young adult Korean men [30], the reference value of LTMA was 0°±6.9°, and that of CIA was 23.9°±5°.

There is limited knowledge about the longitudinal follow-up of flatfoot patients, particularly regarding the correlations between plantar pressure patterns and radiographic changes. In a cross-sectional study [17], associations were reported among MMF pressure, the LTMA, and the CIA, which is intuitively reasonable. However, the correlations between plantar pressure dynamics and radiographic changes over long-term follow-up were not significant in the present study. This indicates that dynamic biomechanical characteristics elucidated through plantar pressure analysis, specifically the distribution of pressure between the foot and the ground, cannot be inferred only from the osseous alignment shown in simple radiographs. The changes in angles measured on plain radiographs, typically 1°–3°, prompt questions regarding clinical significance, contrasting with more significant changes observed in plantar pressure examinations. Consequently, integrating plantar pressure examinations with traditional radiographic assessment offers a more comprehensive understanding of patient condition, providing objective insights into the dynamic functional aspects of flatfoot pathology.

There are several limitations in this study. First, despite leveraging a longitudinal database spanning two decades, data were obtainable for only 50 individuals. However, relative to analogous investigations in the literature, this sample size is comparatively robust. Second, the generalizability of the study findings to the broader population of flatfoot patients may be limited. We included pediatric patients with FFF who had a follow-up duration longer than 12 months and including plantar pressure examination. All patients in this study received therapeutic insoles as part of their treatment, though the specific reasons for these prescriptions were not documented. The extended follow-up period and consistent use of insoles suggest that the study population may represent a more severe cohort of FFF patients compared to the general FFF population. Nonetheless, given the high prevalence of flatfoot among children and the spontaneous resolution observed in many cases without intervention, the findings of this study remain clinically significant. Third, total pressure and body weight were not accounted for when analyzing the pressure of the MMF. Since MMF pressure is influenced by body weight [1,14], it would have been ideal to normalize this value by dividing it by total pressure or body weight. However, due to incomplete data, particularly from older records with poor data storage conditions, these parameters were often unavailable. As an alternative, we focused on the change in pressure over time. A reduction in absolute pressure was observed during follow-up, which is a significant finding considering the age range of the participants, where body weight typically remains stable or increases over time. Last, not all patients underwent plantar pressure examination and radiographic evaluation on the same day. Discrepancies in the timing of these examinations and follow-up durations occurred due to various clinical circumstances including laboratory or patient scheduling conflicts. In some cases, both examinations were completed at early visits, but only plain radiographs were used for long-term follow-up as FFF gradually improved. To minimize these discrepancies, we only considered radiographs obtained within one year of the plantar pressure test. Consequently, the average difference in test dates between the two exams was 2.8 months. For reference, one study [15] reported that LTMA improved by 0.7 degrees per year in patients younger than 15 years with FFF.

Pediatric patients with FFF who used therapeutic insoles showed improvements in both plantar pressure analysis and plain radiographs. However, distinct patterns of change in plantar pressure were found compared with plain radiographs in FFF patients, underscoring the complementary nature of the two diagnostic modalities. To achieve a comprehensive understanding of flatfoot pathology, we advocate for the integration of static assessments of bone alignment through simple radiograph and dynamic evaluations of the forces interacting between the foot and the ground using plantar pressure examination.