Pediatric flexible flatfoot is a common condition seen by orthopedists, pediatricians, podiatrists, and physical therapists. The term “flatfoot” refers to the lowering of the medial longitudinal arch in which the foot comes into near or complete contact with the ground. The arch develops over the first decade of life. Although the majority of children are born flat footed, the prevalence of pes planus was estimated to be 44% to 68% at 3 years of age and reduces to 21% to 24% by 6 years.1–4 The overall incidence of moderate-to-severe pediatric flexible flatfoot, using ink-based footprint measurements, was estimated to be 17% and 18% in the general school age population.5,6 Furthermore, preschool-aged children with gross motor developmental delays have a higher prevalence of flatfeet when compared with peers developing typically.7 Despite the prevalence of pediatric flatfoot, orthopedic treatment guidelines lack strong evidence for nonsurgical treatment, which may be due to the lack of an agreed-upon measurement to define flatfoot.8–11 A survey among physical therapists reported the need for a reliable objective measure to monitor foot posture over time and evaluate treatment effectiveness.12
There is no gold standard for categorizing foot type. Foot type is a clinical concept that aims to simplify the anatomical complexities of the human foot (28 bones, 33 joints, 112 ligaments controlled by 13 extrinsic and 21 intrinsic muscles).13 Current research in this area suggests that foot type is composed of foot structure, function, and flexibility.13–15 Radiographic films are typically used by physicians to evaluate foot alignment. However, there are limitations of x-rays including radiation exposure, a 2-dimensional view of the foot, sensitivity of the x-ray beam angle position, not always tested in a weight-bearing position, and they are not available for physical therapists in the clinic. Foot type can be assessed by a variety of techniques such as clinical examination goniometric measurements, photographs, ink or water footprints, 3D scanner, plantar pressure measuring device, malleolar valgus index, foot posture index, or calipers to measure navicular height.13,16–24 The ink footprint technique has been validated for assessing adult flatfoot by using the Clarke angle, Chippaux-Smirak index, and Staheli index measurements.25 In addition, the Harris mat method, an advancement of the ink technique, was correlated to radiographic measurements in adults.20
Measuring navicular height with calipers has also been validated with radiographic films in adults by Williams and McClay; however, the interrater reliability was inferior to measuring the dorsal arch height.18 The arch height index (AHI) measuring system, which measures dorsal arch height, was developed to improve ease of use and reliability compared with a hand-held caliper-based method. The AHI is defined as the dorsal arch height at one half of the foot length, normalized by the truncated foot length. The AHI has been used by a number of investigators to objectively measure the arch height in both sitting and standing postures to appreciate the flexibility of the arch in adults (JAK Tool, Cranbury, New Jersey).13,14,19,26–29 The AHI also can be used to examine foot structure over time, correlate foot structure with function, and serve as a clinical outcome measure for treatment intervention studies. The AHI measurement and the custom instrument used demonstrated excellent reliability in both sitting and standing postures in adults (intraclass correlation coefficient [ICC] = 0.86-0.99).19,26 In addition, the AHI is able to distinguish rectus, planus, and cavus foot structures in the adult population.13,29
Despite its utility in adults, the AHI is not being used in the pediatric population. One impediment to using this measurement on children is the lack of a pediatric reliability study. Upon literature review, only 1 pediatric study measured navicular height (the distance from the navicular tubercle to the supporting surface) with an intrarater and interrater reliability of 0.91 and 0.77 to 0.80, respectively, in 6- to 11-year-olds.21 Although this study had good reliability results, it is not commonly used in the flatfoot literature as an outcome measure. One possible explanation is that palpation of the navicular, especially in small children, requires very good anatomical knowledge of the foot and clinical experience.
The specific aim of this study was to determine the intrarater and interrater reliability of the AHI in sitting and standing postures in children developing typically. The investigators hypothesized that the AHI would have an ICC 0.7 or more for both intrarater and interrater reliability assessments. The secondary aim was to report the pediatric AHI values and compare the pediatric AHI findings to adult values.
The purpose of this study was to determine the AHI reliability in the pediatric population using an arch height measurement device (Figure 1). Testing and data collection occurred in a single visit at a motion analysis laboratory.
Thirty children developing typically (9.61 ±1.96 years) participated in the study for a total of n = 60 feet. All subjects' parents signed informed consent as part of an Institutional Review Board hospital approved study. Inclusion criteria required ambulatory children developing typically 6 to 12 years of age. Exclusion criteria included pain and any prior surgeries at the foot or ankle.
A basic foot and ankle clinical examination was first performed by the principal investigator to obtain the following foot structure measurements for each subject: resting calcaneal stance position (RCSP) and forefoot to rearfoot position (FF-RF). RCSP and FF-RF were measured with a 1° resolution goniometer. The above foot measurement protocol and its reliability are documented in the Structure and Function of the Foot chapter in the Foot and Ankle Sports Medicine book.30 From these measurements the child's foot was categorized into one of the following 3 foot types: planus: RCSP ≥ 4° valgus or FF-RF ≥ 4° varus; rectus: 0° ≤ RCSP ≤ 2° valgus and 0° ≤ FF-RF ≤ 4° varus; or cavus: RCSP ≥ 0° varus and FF-RF ≥ 1° valgus as defined in adults.13,30
AHI measurements were repeated twice by each investigator from which the AHI intrarater reliability was computed using a custom instrument including 2 measuring devices specific for the right and left foot (Figure 1). To determine the interrater reliability, AHI measurements were taken by each of 2 investigators on all study participants. The first rater, a senior pediatric physical therapist, tested all 30 subjects. The role of the second rater was shared between a pediatric orthopedic surgical resident and a senior pediatric physical therapist. All raters had no experience using the AHI measurement device prior to this research study.
Arch height was measured with subjects positioned on an elevated platform for testers to be at eye level with the foot. Subjects were instructed to remain still and look at the same picture on the wall with arms at their side for both static sitting and standing measurements. Seated AHI measurements were taken on an adjustable bench with subjects positioned at 90° of hip, knee, and ankle flexion. The subject's foot length, truncated foot length (length from the posterior heel to the first metatarsal head), and dorsal height (measured at half the total foot length) were recorded for each subject's right and left foot. Next, subjects stood facing forward with even weight distribution through their feet to obtain the arch height in standing. Measurements for standing arch height were taken in the same manner as seated. The arch height measurement protocol and the AHI equation are outlined in Table 1.
Two or 3 children were tested during a session and had a 20-minute break between replicated measurements to reduce memory bias of the rater. Seated and standing arch height measurements were repeated twice by each investigator, yielding 4 values. The average AHI of the 4 values and the standard deviation was calculated for each subject's right and left foot. This value was used to categorize a planus, rectus, or cavus foot type according to the published adult AHI values for sitting and standing13: in sitting, a planus foot structure was assigned if AHISitting 0.365 or less; a rectus foot structure was assigned if more than 0.365; or a cavus foot structure if AHISitting 0.39 or more. During standing, a planus foot structure was assigned if AHIStanding 0.345 or less; a rectus foot structure if greater than 0.345; or a cavus foot structure if AHIStanding 0.37 or more (see Table 2).
An a priori power analysis study was performed using previously collected adult data13 to determine our sample size for α = 0.05 and β ≤ 0.2 (n = 60 feet). Descriptive statistics including AHI mean and standard deviation for seated and standing were computed. Two-way random effect models were employed to determine the ICC with absolute agreement. Specifically, an ICC (2,1) was used to calculate the intrarater and interrater reliability with 95% confidence interval using SPSS software (version 19).
A total of 30 children (n = 60 feet), 9 girls and 21 boys, with a mean age 9.61 ± 1.96 years participated (Table 3). The children's body weight ranged from 46 to 116 lb with an average of 75.4 lb; height ranged from 45.5 to 63.5 inches with an average of 55.6 inches. Their body mass indices (BMI) ranged from 12.1 to 24.2 with an average BMI of 17.5; 1 subject was above the healthy BMI for his/her age according to the Centers for Disease Control and Prevention BMI for age growth chart.31 Upon clinical examination RCSP and FF-RF foot structure measurements, 55% of the 60 feet were classified a planus foot type, 45% were rectus, and none were a cavus foot type.
ICC Reliability Statistics
AHI intrarater reliability ICC values were 0.80 to 0.82 in sitting and 0.84 to 0.87 in standing. See Table 4 for ICC values and 95% confidence intervals for intrarater reliability. Interrater reliability was 0.78 to 0.79 in sitting and 0.76 to 0.89 in standing. See Table 5 for all ICC values and 95% confidence intervals for interrater reliability.
Minimal Detectable Change (MDC95)
Based upon the lowest ICC values obtained in the pediatric reliability analysis, as a conservative estimate, we computed the minimum detectable change score for a 95% confidence interval (MDC95) of AHI. The following formulas were used:
where SEM is standard error of the mean, ICC is the interclass correlation coefficient, and SD is the standard deviation. The MDC95 for AHIsitting is 0.0368. The MDC95 for AHIstanding is 0.0350.
In this group of 30 children, the average AHISitting was 0.36 ± 0.02 and AHIStanding was 0.32 ± 0.02. See Table 3 for subject age, gender, AHI sitting, and standing mean value and foot structure classification (planus, rectus, cavus) for all 60 feet. Based upon the adult foot structure classification system,13 78% of the children's feet would be classified as a planus foot structure during sitting and 90% planus in standing.
An objective measure to evaluate pediatric foot structure, monitor change over time, guide treatment planning, and assess the effectiveness of intervention is needed.12 This study confirms our hypothesis that the AHI has an ICC 0.7 or more for intrarater and interrater reliability in children developing typically. A clinical utility of the AHI is that it is measured using a simple protocol with a portable device that can be used in the clinic by itself and/or in combination with x-ray findings. The AHI measuring device is noninvasive, radiation free, and can be used to assess foot structure and arch flexibility in sitting versus standing weight-bearing positions. A difference found in the AHI value from sitting to standing can further assist clinicians in evaluating a rigid versus flexible foot type to optimize treatment interventions. The AHI is normalized to each person's truncated foot length, which permits tracking a child through growth and development as well as a comparison to other individuals with different size feet. The AHI has previously demonstrated its utility and reliability for characterizing foot structure in adults.13,14,19,22,26–28 Our study indicates that using the AHI in school-aged children developing typically is reliable as well.
The secondary aim was to report the pediatric AHI values and compare them to the adult foot-type values.13,14 Our study is the first to document AHI values in pediatric subjects. In adults, the AHI average planus foot structure value is 0.35 ± 0.03 in sitting and 0.33 ± 0.03 while standing.13 Our cohort of 30 children developing typically had a mean AHISitting of 0.36 ± 0.02 and AHIStanding of 0.32 ± 0.02, therefore classifying the majority of the children in this study to have a planus foot structure. This is consistent with prior references of flatfoot predominance in school-aged children.5,6
Our foot structure classification result based on the clinical examination measurements of RCSP and FF-RF (55% planus feet and 45% rectus) is not consistent with the foot-type classification obtained using the AHI values (>90% planus in standing). This is not surprising as the static structure of the hindfoot and forefoot positions in supine or prone positions does not necessarily correlate to the arch flexibility in a functional sitting or standing weight-bearing position. In addition, there are limitations in using 1° goniometric static anthropometric measurement for RCSP and FF-RF.32 We recommend both measurements in combination are valuable for clinicians to assist in determining an individual's foot structure and function.
Nonsurgical treatment guidelines for children with flatfeet are lacking.8–11 In addition, children with asymptomatic flatfeet do not typically receive x-rays and observations of the arch in standing and are often subjective and without an objective foot measurement to monitor changes over time. Foot orthoses are a common treatment intervention for flatfeet, but as stated by Kane12 an objective measure to identify children who may benefit from orthotic intervention and its effectiveness is needed. The AHI and/or measuring navicular height21 can assist as an objective measure to fill this gap in the literature.
A limitation of this study is that testing occurred in children developing typically and therefore it is unknown whether it is reproducible in children with neuromuscular disorders or developmental disabilities. The study sample of convenience also resulted in more boys (21) than girls (9); ideally we would have preferred a group of equal gender. Another limitation was that the second rater measurements were taken by 2 individuals: an orthopedic resident (testing 14 of 30 subjects) and a senior physical therapist (16 of 30 subjects) due to clinician availability. Neither discipline reported difficulty performing AHI measurements in the children in our study; nor did previous graduate students, kinesiology faculty, or podiatric physicians in prior AHI adult studies.19,26 The AHI measurement device used in our study (JAK Tool, Cranbury, New Jersey) does have an initial purchase cost, which may be a limitation to clinicians having this specific AHI tool available in the clinic.
Follow-up studies are needed to determine a relationship between AHI values, clinical goniometric foot measurements and the emed-x (Novel, Munich, Germany) plantar pressure values in the pediatric population. Ideally, a longitudinal study design would be beneficial to determine change in AHI throughout growth and development to establish pediatric AHI normative values for each age. In addition, the use of the AHI should be studied in children with painful flexible flatfeet and other conditions such as individuals with cerebral palsy and developmental delay to expand its utility.
Arch height index is a reliable measurement to assess foot structure in children. The AHI has the ability to monitor foot structure over time to identify potential progression of a deformity and assess intervention effectiveness in children with flatfeet in functional positions. Pediatric therapists, orthotists, and physicians should consider using the AHI as an objective measure to assess foot structure, monitor change over time, and assist with treatment planning in the pediatric population.
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Keywords:Copyright © 2017 Wolters Kluwer Health, Inc. and Section on Pediatrics of the American Physical Therapy Association. All rights reserved.
arch height; flatfoot; foot structure