Original article
Differences in and characteristics of four criteria for assessment of flatfoot
2022 年 22 巻 1 号 p. 32-40
詳細
2022 年 22 巻 1 号 p. 32-40
Footprint analysis is one of the easiest methods for assessing flatfoot. However, there is no unified standard for the evaluation criteria because many experts have proposed various criteria for footprint-based assessment of flatfoot. The purpose of this study was to aid in the selection of appropriate criteria by analyzing actual footprint data using the top four criteria frequently used in English-language original papers for detection of flatfoot, with the aim of determining the differences between these criteria. When each of the four criteria was applied to single footprints from 30 participants each, the percentage judged to have flatfoot was 57% with the Arch index, 7% with the Staheli index, 40% with the Chippaux-Smirak index, and 37% with Clarke’s angle. The Staheli index, the results of which are influenced by the reference population, is useful for studies such as prevalence studies and tracking of foot pattern changes. The Arch index and the Chippaux-Smirak index could be useful as a foot health index for promoting at-home care for flatfoot, and Clarke’s angle could be useful as a first screening tool in the clinical setting. The four criteria investigated here have different detection rates of flatfoot, so the interchangeability of these criteria should be explored as an extension of this research.
Flatfoot is a foot deformity in which the medial longitudinal arch of the foot is lowered or lost, and can occur in any person, regardless of age or sex. Compared with other foot deformities, flatfoot often does not cause obvious symptoms such as chronic pain or limping. However, because the arch of the foot is believed to be responsible for shock absorption, weight bearing, and support in heel off during walking, people with flatfoot are at higher risk of problems such as fatigue and injury during activities compared with those with normal feet [1]. Surgical treatment is indicated for severe flatfoot that causes chronic pain, limping, or severe deformity beyond a fallen arch (e.g., protrusion of the navicular bone), whereas insoles are indicated for mild to moderate flatfoot in order to reconstruct and maintain the arch. Proper use of insoles has been shown to reduce fatigue and prevent injury [2]. Today, it is possible to purchase not only prescription orthotic insoles made by a prosthetist, but also inexpensive ready-made insoles at a retail shoe store or online. Some shoe stores even offer a service where they measure customers’ feet and make custom insoles. Proper care for flatfoot starts with creating awareness of the condition of the feet.
There are many methods for assessing flatfoot, including footprint analysis, radiography, visual examination, and measurement of medial longitudinal arch height. The most accurate method is radiographic assessment of bone alignment. However, it is not possible for care providers to take X-rays for every single patient, particularly those without chronic pain. Consequently, footprint analysis has been used in settings such as medical facilities and shoe stores as a non-invasive and simple method for assessing flatfoot. Many experts have proposed various criteria for footprint-based assessment of flatfoot, but there is no uniform standard.
The development history and purpose of the four frequently used criteria are as follows. The Arch index(AI) was proposed as a tool for classifying the arch into high, normal, and flat, based on the footprint data of 107 men and women [3]. The study by Staheli et al. in which the Staheli index(SI) was proposed was conducted to establish a range of normal values for all age groups and to provide the basis for a more rational approach to managing flexible flatfoot [4]. The Chippaux-Smirak index(CSI) was named after Chippaux, a French anthropologist who linked the arch shape to pathological conditions, and Smirak, who effectively utilized Chippaux’s findings to assess flatfoot in children and adolescents 5]. Clark proposed the Clarke’s Angle (CA) to ensure reliability and objectivity in angle-based flatfoot assessment, based on the fact that although conventional angle-based methods were highly reliable, experience was required to calculate angles from footprints [6].
Use of multiple criteria in analysis of a single footprint would increase the accuracy of detecting flatfoot, but such an approach would be inefficient in settings where speed is emphasized in addition to accuracy. The clinical use of footprints for assessing flatfoot has been investigated by many researchers who compared footprint-based and non-footprint-based indexes. Zukauskas et al. showed that Foot Posture Index-6, which is used to assess the entire foot, correlated strongly with the SI and the CSI, both of which are footprint-based indexes [7]. Also, strong associations between assessments using a podoscope and those using SI, CSI, and CA were shown in a study by Fuentes-Venado et al. that examined 367 boys and girls aged 3 to 6 years [8] and a study by Pita-Fernández et al. that examined 1,002 men and women aged 40 years or older [9]. Furthermore, Chuckpaiwong et al. reported that diagnosis by 147 clinicians belonging to the American Orthopaedic Foot and Ankle Society agreed with the results of assessments using the CA, AI, SI, and CSI [10], indicating that footprint-based assessments have a certain degree of reliability. However, no studies have yet investigated how to appropriately select an index according to the purpose of the assessment.
The purpose of this study was to assessed actual footprints, using several criteria used to detect flatfoot in order to determine the differences in assessment outcomes and thereby reveal the characteristics of these criteria.
1. Selection of criteria
Criteria frequently used in original English-language articles on footprint-based assessment of flatfoot were selected for investigation in this study. The articles were found in databases such as Scopus, PubMed, and Google Scholar, using the keywords footprint and flatfoot OR flatfeet OR pes planus OR talipes planus. Four criteria frequently used in these original articles were included in this study.
A total of 44 papers were selected. The criteria used in these papers were AI (14 papers), SI (14 papers), CSI (12 papers), CA (9 papers), the contact index (2 papers), and the footprint index (2 papers). Other papers used a criterion of the authors’ creation, a simple measurement, or unspecified criteria. Some studies used multiple criteria. The top four criteria, namely, AI [3], SI [4], CSI [11], and CA [6], were selected for investigation in this study.
Arch index (AI)
The sole of the foot, excluding the toes, is divided into three parts by drawing two lines perpendicular to the line connecting the tip of the second toe to the center of the heel. The respective areas of the entire sole (A+B+C in Figure 1) and the middle section (B in Figure 1) are calculated using ImageJ. Flatfoot is indicated when the middle section of the sole exceeds 26% of the entire sole [3] (Figure 1).
Staheli index (SI)
The minimum width of the arch region (D in Figure 2) and the maximum width at the heel (E in Figure 2) are measured to obtain the SI (D divided by E). The mean SI of the population is calculated, and flatfoot is defined as an SI greater than the mean plus 2 standard deviations (SD), while high arch is defined as an SI less than the mean minus 2SD [4] (Figure 2).
Chippaux-Smirak index (CSI)
The angle formed between the ball line (F in Figure 3) and the tangent at the lateral margin of the footprint is calculated. A line is drawn at the narrowest point of the arch (G in Figure 3) to make an angle of the same magnitude as this angle, and the respective lengths of F and G are measured. The value of G divided by F is expressed as a percentage, and a value of 45.1% or higher is considered to indicate flatfoot. This index is also used to grade the severity of flatfoot, with 45.1%-50.0% classified as Grade 1, 50.1%-60.0% as Grade 2, and 60.1% or higher as Grade 3 [11] (Figure 3).
Clarke’s angle (CA)
The angle (H in Figure 4) between the line connecting the head of the first metatarsal to the deepest part of the arch and the tangent at the medial margin of the footprint is used for evaluation. An angle <30˚ is considered to indicate flatfoot [6,12] (Figure 4).
2. Participants
Thirty healthy university students of Niigata University of Health and Welfare (mean age ± SD: 19.8 ± 0.9 years, 15 men and 15 women) who did not have any obvious diseases in their feet and were interested in this study participated voluntarily. Candidates for the subjects were recruited by e-mail.
3. Measurement and analysis
Footprints were obtained using a footprint kit (Bauerfeind, Zeulenroda-Triebes, Germany). Participants were instructed to assume a natural standing position with a gap of about 10 cm between the feet with equal weight on both sides of the body while their footprint was being obtained. Only the right foot was analyzed because all the participants were healthy and would presumably have no substantial differences between the left and right sides that would affect results. This was done to reduce the time commitment for participants and to simplify analysis. Footprints were collected several times, and the one with the highest clarity was selected. The areas, lengths, and angles were measured using ImageJ. Three measurements were performed for each parameter and the mean value was calculated. The intraclass correlation coefficient (1, 3) ranged from 0.90 to 0.99.
4. Ethical considerations
This study was conducted with the approval of the Institutional Review Board of Niigata University of Health and Welfare (Approval No. 18774-211105). In addition, participants were fully informed of the purpose and significance of this study and their consent was obtained before measurements were performed.
The footprint measurement data are shown in Table 1. The percentage of participants with flatfoot was determined to be 57% with AI,7% with SI , 40% with CSI, and 37% with CA (Figure 5).
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Assessment using the AI indicated that 57% of the participants had flatfoot. Given that area was used for the AI, whereas lengths or angles were used for other criteria, the AI results might be influenced by soft tissue volume. Moderate flatfoot can be detected using the AI because it is based on area measurements; however, it is not easy to calculate area from a footprint on paper. Among studies using the AI, Galli et al. investigated the effect of flatfoot on walking in 55 children with Down syndrome [13], while Ojukwu et al. investigated the association of flatfoot with pain in the feet, knees, and back in pregnant women [14]. Early detection of flatfoot using the AI can lead to early provision of care, suggesting that the AI may be useful as a foot health indicator.
Use of the SI as the assessment criterion resulted in a considerably lower rate of flatfoot, likely because the SI defines flatfoot as all values two standard deviations above the mean value in the reference population, whereas the other criteria define flatfoot as exceeding a specified standard. For this reason, the mean value of the entire study population was used as a reference for assessment of flatfoot. Given that the SI is strongly influenced by the characteristics of the population, it is considered unsuitable when the reference population consists of many people already complaining of flatfoot symptoms because it will detect only those with relatively severe flatfoot in the population. However, it is considered more likely to produce accurate results in large populations with a wide range of ages and sexes. Among studies using the SI, Ibeabuchi et al. investigated the prevalence of flatfoot in 218 children aged 6-15 years [15], while Ezema et al. investigated the prevalence of flatfoot and related characteristics in children in elementary school aged 6-10 years [16]. Thus, the SI is likely to be suitable for studies investigating the prevalence of flatfoot or changes in footprints over time.
Assessment using the CSI determined that 40% of participants had flatfoot, which was similar to the results using the CA. This might be due to the fact that both indexes are based on the area of the transverse arch. Studies using the CSI have investigated the effects of insoles [17] and the association of obesity with foot structure and plantar pressure [18]. The CSI can classify the foot into 9 classes, comprising 3 grades each for flat, normal, and high arches. Investigating the association of various factors with such detailed foot shapes should improve our understanding of foot condition and enable provision of appropriate care. From this perspective, the CSI is likely to be the most suitable criterion for assessment of flatfoot.
In the present study, the CA had the lowest detection rate among the three types of criteria except SI. This may be due to the fact the CA takes the transverse arch into account, and thus the midfoot, where the longitudinal arch is located, is not directly assessed. Given the good correlation between the CA-based assessment and radiographic diagnosis [19], the detection rate using the CA was considered more reliable than the other types of criteria. Therefore, the CA might be useful as a primary screening tool in the clinical setting. Among studies using the CA, Głowacka-Mrotek et al. investigated the distant adverse effects of cancer therapy on foot posture [20], while Trzcińska et al. used the CA to assess lower limb alignment and foot condition in patients with Turner’s syndrome [21]. The specific angle that determines flatfoot varies among studies because it was not set in the original studies. This makes it difficult to broadly compare results between multiple studies that used CA for assessment of flatfoot, even though the criterion was the same.
This study assessed the same set of footprints but the detection rate of flatfoot varied according to the criterion used. This may be due to differences in the purposes of the studies in which those criteria were originally proposed. Different reference populations were used when these criteria were proposed and there are characteristics of foot shape that are unique to different age groups and ethnicities [22], which might have resulted in different cutoff values.
The detection rate of flatfoot varied according to the criterion used for assessment of footprints collected from the same reference population. Each criterion has its own characteristics, and thus each may be suitable for different purposes.
The SI, which had a low detection rate, is likely to be suited for studies on the prevalence and changes in foot shape over time. The AI, which allows for detection of moderate flatfoot, and the CSI, which enables detailed classification, might be useful as foot health indexes that facilitate provision of appropriate foot care. The CA is likely to be useful in primary screening in the clinical setting.
When comparing existing studies, it is important to note not only whether the studies used footprints to assess flatfoot, but also the criteria they used to define it. As the four criteria investigated in this study are used at similar rates, the interchangeability of these should be explored as an extension of this research.
There are no conflicts of interest to declare.
