Flatfoot is one of the most common conditions encountered in pediatric rehabilitation departments.^1,2 This condition is characterized by a flat loss of the medial longitudinal arch of the foot, causing pain and balance pro-blems. Flatfoot is usually diagnosed at a young age but its incidence can vary with age.³ Although flatfoot is most commonly diagnosed before the age of 6, in many cases, incidence rates tend to decrease after the age of 6, with some studies suggesting that the arch of a child’s foot is formed by the age of 10.^4-6 However, in some children, flatfoot remains uncorrected, causing secondary deformities and various symptoms and abnormalities, including movement disorders and pain, which contribute to poor quality of life.^3,4,7

In clinical practice, flatfoot is generally evaluated through a nondynamic assessment of foot posture to determine the underlying pathology.^8,9 Clinically used anthropometric measurements include medial longitudinal arch angle, navicular drop test, and footprint-based analyses such as the Chippaux–Smirak index or Staheli arch index. In addition, a simple radiological examination, ultrasound, or magnetic resonance image may be used.¹⁰ In line with this, the resting calcaneal stance position (RCSP) angle has been considered an easily measurable and widely used anthropometric measurement, with previous studies showing a high degree of reliability regardless of the child’s age, height, and weight, etc.^11,12 Physiologic risk factors that increase the likelihood for flatfoot include age, gender (male), body mass index (BMI), and joint hypermobility. Nonphysiologic factors include various neuromuscular disorders, hereditary disorders, external injuries, and skeletal abnormalities.^13,14

To correct and treat flatfeet, conservative or surgical interventions may be considered. However, surgical intervention is usually reserved for cases with severe symptoms who fail to respond to conservative treatments. The most common conservative intervention is the customization of a foot insole, the effectiveness of which has been the subject of many previous studies. Biomechanical foot orthoses (BFO) have been shown to promote significant differences in radiometric measurements of flatfoot, as well as in the results of various other methods used to assess flat-foot.^1,15-19 Some studies have found that orthoses had no effect on gait pattern changes, whereas others have revealed no significant results due to group heterogeneity or insufficient follow-up period.^20,21 Furthermore, although most studies have shown that BFO use can ameliorate several symptoms associated with symptomatic flatfoot, its use remains controversial in asymptomatic cases.

Therefore, the current study aimed to determine the quantitative effects of BFO in children diagnosed with flatfoot and prescribed BFO by assessing changes in the RCSP angle with sufficient follow-up interval and analyze various indicators for RCSP angle improvement in patients grouped according to the degree of improvement in hopes of improving the management of flatfoot. We included informa-tion on the age, gender, and BMI, which have been considered as risk factors for flatfoot, and identified the time at which BFO management was first initiated, which is expected to be clinically useful in the future.

1. Participants

Data were retrospectively collected from flatfoot patients visiting the foot clinic of the department of rehabilitation medicine, Daegu Fatima Hospital between January 2017 and December 2022. All patients were prescribed BFO through the outpatient department of rehabilitation at our hospital, and their RCSP angles were followed up periodi-cally. Clinical data such as age, gender, BMI, and age (in months) at the time of the first BFO prescription were collected, as well as data on the first RCSP angle measured, the RCSP angle measured upon follow-up, and the duration (in months) until follow-up. To control for other factors, several inclusion and exclusion criteria were established.

The inclusion criteria were as follows: (1) normal bipedal ambulation; (2) complete RCSP angle measurements; (3) prescription of BFO; and (4) a diagnosis of flatfoot based on a calcaneal inclination angle of less than 18 degrees on the lateral view of the foot radiographic image or an RCSP angle of less than 0 degrees on physical examination.

The exclusion criteria were as follows: (1) age over 20 years; (2) history of ankle or foot surgery, leg length discrepancy, or other clinical conditions that may affect RCSP angles; and (3) weight or foot size deviation of more than −2 to 2 standard deviations from average for the corresponding age group in South Korea.^13,14

2. RCSP angle measurement

RCSP angle measurements were performed as follows. First, the patient was placed in a prone position on the examination table and requested to remain relaxed. Second, the physician bisected the calcaneus using the bimanual technique to mark two points perpendicular to the foot (i.e., top and bottom) and connected these points with a line. Third, the patient was placed in a standing position on their feet and requested to remain relaxed and comfortable. Fourth, the doctor measured the angle between the vertical and the ground and the calcaneal incision (Fig. 1).²²

Figure 1. Measuring the resting calcaneal stance position angle.

3. BFO adjustment

If biomechanical abnormalities were found, a cast was fabricated with a pair of customized BFOs based on the inverted technique defined by a corrective insole (GeoMedi Co., Ltd, Uiwang, South Korea) (Fig. 2). For example, if the initial RCSP angle was −5 to −6, −7 to −8, and −9 to −10 degrees, the inverted orthoses were prescribed at an angle of 20, 25, and 30 degrees, respectively. For patients with more severe conditions (i.e., those with an RCSP angle of −11), we sometimes prescribed orthoses at initial angle of 25 degrees for compliance if the child was 5 years old or younger. Each subject was encouraged to wear the orthotic for at least 2 h per day and walk with the heel strike first followed by the swinging of their arms back and forth to correct their gait pattern. All subjects diagnosed with flatfoot were provided with insoles at an average of 3 months after their first visit.²²

Figure 2. Customized biomechanical foot orthoses manufactured based on the inverted technique (GeoMedi, co., Uiwang, South Korea).

4. Statistical analysis

First, to confirm whether the RCSP angle improved after BFO prescription in all patients, the paired t-test was performed for the right and left foot. The same paired t-test was also performed for each group established according to gender to determine whether RCSP angle improvement differed according to gender. In addition, to investigate whether there was a difference in the degree of improvement of RCSP according to the severity of flatfoot, the severe group and the moderate to mild group were divided into two groups based on the RCSP angle of −10 degrees and compared by independent t-test.

To determine whether the RCSP angle improved in each group, the patients were divided into two groups according to whether the RCSP angle improved by more and less than 4 degrees. We then compared the means of various indicators (age, sex, BMI, and BFO prescription age) between the groups using the independent t-test to determine whether significant differences existed.

1. Patient characteristics

This study included 77 flatfoot patients (50 boys and 27 girls) who fulfilled the inclusion and exclusion criteria. The mean values for age, BMI, age (in months) at which the BFO was first prescribed, and follow-up period (in months) were 11.05 ± 2.49 years, 19.17 ± 4.10, 100 ± 30.89 months, and 27.79 ± 20.97 months respectively. No significant differences in these data were observed between boys and girls.

2. Degree of improvement in the RCSP angle after BFO prescription

In all patients, the mean RCSP angles measured upon initial BFO prescription was −8.68 ± 3.56 and −7.69 ± 3.58 degrees for the right and left foot, whereas the follow-up RCSP angles measured, on average, 28 months later were −5.42 ± 2.96 and −4.90 ± 3.00 for the right and left foot, respectively. A paired t-test was used to determine whether the RCSP angle improved before and after BFO, for which a significant improvement was found (p≤0.001). The average improvement in the RCSP angle was 3.25 ± 2.64 and 2.78 ± 2.64 degrees for the right and left foot, respectively. Similarly, the paired t-test revealed a significant improvement in the groups divided according to gender (p≤0.001; Table 1).

Table 1 . Characteristics of the Normal Group and Flatfoot Group.

Sex	Age	BMI	BFO start (in months)	RCSP angle					RCSP angle difference
				Rt, 1st	Lt, 1st	Rt, 2nd	Lt, 2nd		Right	Left
Boys (N = 50)	10.98 ± 2.52	19.56 ± 4.46	98.72 ± 30.15	−8.96 ± 3.52	−7.60 ± 3.46	−5.56 ± 2.70	−4.92 ± 2.93		3.40 ± 2.69	2.68 ± 2.29
Girls (N = 27)	11.19 ± 2.48	18.46 ± 3.31	102.37 ± 32.65	−8.15 ± 3.64	−7.85 ± 3.86	−5.19 ± 3.43	−4.89 ± 3.19		2.96 ± 2.55	2.96 ± 2.92
Total (N = 77)	11.05 ± 2.49	19.17 ± 4.10	100.00 ± 30.89	−8.68 ± 3.56	−7.69 ± 3.58	−5.43 ± 2.96	−4.91 ± 3.00		3.25 ± 2.64	2.78 ± 2.51

In addition, the group was divided according to the severity of flatfoot to compare the degree of RCSP improvement before and after BFO. The severe group was defined as having an RCSP angle of less than −10 degrees in either foot, and the moderate to mild group was defined as having an RCSP angle of greater than −10 degrees in both feet. Independent t-tests on each mean showed that age, BMI, and age at first prescription for BFO were not significantly different, but the degree of RCSP angle improvement in the severe group was significantly greater in both feet (p≤0.001; Table 2).

Table 2 . In a Comparison between Groups Divided by Severity of Flatfoot, the Improvement in RCSP Angle Was Found to be Greater in the Severe Group.

Group	Age	BMI	BFO start (in months)	RCSP angle					RCSP angle difference
				Rt, 1st	Lt, 1st	Rt, 2nd	Lt, 2nd		Right	Left
Severe group (N = 34)	11.21 ± 2.03	18.86 ± 3.13	87.46 ± 31.90	−11.82 ± 2.25	−10.50 ± 2.56	−7.18 ± 2.97	−6.62 ± 2.96		4.65 ± 2.70	3.88 ± 2.69
Mild to moderate group (N = 43)	10.93 ± 2.82	19.42 ± 4.76	102.37 ± 32.65	−6.19 ± 2.15	−5.47 ± 2.58	−4.05 ± 2.12	−3.56 ± 2.27		2.14 ± 2.01	1.91 ± 1.99
p value	0.633	0.559	0.176	0.000	0.000	0.000	0.000		0.000	0.000

3. Comparison of groups based on RCSP improvement

A total of 40 and 37 patients showed improvements in their RCSP angle by more and less than 4 degrees, respectively, with the paired t-test showing a significant difference in the degree of RCSP angle improvement between each group (p≤0.001). However, independent t-test found that age, age at which BFO was started, and BMI did not significantly differ between the two (Table 3).

Table 3 . Comparison of the Mean Values for Age, Body Mass Index, and Age (in Months) at the Time of First Biomechanical Foot Orthosesprescription between the Groups Whose RCSP Angle Improved by More and Less than 4 Degrees Using the Independent t-test.

Group	Age	BMI	BFO start (in months)
Improvement of more than 4 degrees (N = 40)	11.55 ± 2.48	19.20 ± 3.56	100.43 ± 33.32
Improvement of less than 4 degrees (N = 37)	10.51 ± 2.42	19.14 ± 4.67	99.54 ± 28.47
p value	0.068	0.955	0.901

No significant differences were observed..

Despite the lack of a clear definition of flatfoot and controversial results from studies on its prevalence, epidemiologic studies have shown flexible flatfoot generally occurs in at least 20% of children and adults under the age of 10 and is thought to persist in as many as 3% of the adult population.^23,24 Flatfoot can lead to various painful musculoskeletal problems, movement disorders, and balance abnormalities, especially as patients grow older and engage in more dynamic activities. Flatfoot can cause additional overload to be transferred to the proximal lower extremities due to structural problems of the foot and ankl.³ In fact, researchers have found that patients with moderate to severe flatfoot are twice as likely as controls to have knee pain and intermittent low back pain.^1,25 Hence, if severe foot pain or abnormalities occur, flatfoot may be of such severity that surgical intervention should be considered. However, accurately predicting whether a typical flatfoot would subsequently cause significant disability or pain has remained difficult. Thus, a clear diagnosis of flatfoot, appropriate treatment, and risk factor management are becoming increasingly important.

The evaluation of flatfoot involves a combination of methods. Basically, the morphology of the patient’s foot is observed and confirmed through a physical examination, which can reveal the medial longitudinal arch or footprint. In addition, radiographic investigation can help determine the severity of flatfoot, establish pain treatment, and guide surgical planning, whereas a footscan test can evaluate the patient’s static and dynamic balance to predict the prognosis for gait function (Fig. 3). In this study, the RCSP angle was utilized as a measure to evaluate flatfoot given that it is simple, straightforward, and intuitive to evaluate. Moreover, previous studies have shown a correlation between the RCSP angle and quantitative measures of flatfoot evaluated using different radiographic imaging approaches.²²

Figure 3. Use of the footscan test to evaluate children with flatfoot. The change in the area of the patient’s foot in contact with the ground from a static state is indicative of improvement in flatfoot.

The timing of treatment, observation, and surgery in symptomatic flatfoot patients, especially in children, has long been controversial. Considering that many children’s flatfoots are corrected through conservative treatment alone, it is not clear which and how these treatments are effec-tive.²¹ In particular, treatment with BFO has been contro-versial. Previous studies have found that while BFOs help correct soft tissue deformities in the foot, they do not significantly change important skeletal structures.²⁴ In fact, several studies have even found no significant improvement in radiographic parameters among patients wearing over-the-counter foot orthoses compared to those not treated at all.²⁶ However, a number of studies have provided scientific evidence for prescribing BFOs, particularly in symptomatic flatfoot patients, given their ability to reduce excessive valgus movement and significantly improve step length and width, walking velocity, symmetry, and center of pressure displacement compared to no BFOs.^18,27 Studies using the RCSP angle have also found objective evidence showing a significant improvement in flatfoot patients.^22,28,29

The current study, objectively evaluated 77 children diagnosed with flatfoot and prescribed BFOs to determine the degree of improvement in the RCSP angle. Notably, our results showed a significant improvement in the RCSP angle in both sexes (p≤0.001). To the best of our knowledge, this study was unique in that it compared the indicators between two groups: those whose RCSP angle improved by more and less than 4 degrees. The assumption was that this would allow us to determine which factors should be considered when prescribing BFOs in flatfoot patients. However, our results showed that the improvement in RCSP angle was not associated with age, gender, BMI, or age at which BFOs were first started. While many studies have examined the degree of improvement in flatfoot with BFO use, this study is significant given that it utilized the RCSP angle, which has been shown to be the simplest and most intuitive measure that is also correlated with radiographic evaluation results. In addition, previous studies have shown that BFOs were not associated with a significant improvement in flatfoot with less than 1 year of follow-up; however, the average follow-up period for children in this study was 27.79 months, which is more than 2 years.

This study has some limitations worth noting. First, this was a retrospective study from a single center, which may reduce the generalizability of the data. Second, this is a single-arm study with no control group, suggesting the need for further research on a group of flatfoot children who do not actually wear BFOs. Third, given that all measurements were performed by the same physician, the intra-rater reliability was not confirmed. Also, although the average follow-up period of the patients was more than 2 years, some errors may have been present due to the different follow-up periods of each individual. Finally, the study included a relatively small number of patients; therefore, to increase the reliability of the results, further studies with a larger number of flatfoot patients in a multi-center setting and a control group without BFOs are needed. Furthermore, studies with a longer follow-up period would provide more accurate results.

Some of the patients who presented to our department of rehabilitation had unchanged or worsening RCSP angles after being diagnosed with flat feet and prescribed a BFO. Of the cases identified in this study, 14 children had unchanged or worsening RCSP angles, and each case is tabulated in the Supplementary Material of this article. Their age, BMI, and the number of months they were prescribed their first BFO were not significantly different from the overall group. But their mean initial RCSP angles were −4.86 on the right and −4.36 on the left, suggesting that they may have milder flatfoot than the overall group, and many of them did not wear the prescribed BFOs properly. Therefore, it is thought that not only the prescription of BFOs, but also the compliance of patients and caregivers may affect the outcome. In addition, there may be many cases where flatfoot improves spontaneously without prescribing BFOs. For this reason, a controlled study of the group actually prescribed BFOs with additional data collection is needed.

In conclusion, this study confirmed that wearing BFOs can improve the RCSP angles in children with flat feet. On average, wearing BFOs for more than 2 years improved RCSP angles, regardless of age, gender, BMI, or age at first BFO use. Given the limitations of a single-arm study with no control group, further studies providing more data and including a control group are needed.