Validity and diagnostic accuracy of foot posture Index-6 using radiographic findings as the gold standard to determine paediatric flexible flatfoot between ages of 6–18 years: A cross-sectional study



Diagnosis of flexible flatfoot is usually based upon radiographic or clinical measures. Radiographic measures pose a potential risk of radiation exposure especially in Paediatric population. Clinicians need a valid, accurate, easily used, noninvasive and cost effective measure to evaluate static foot posture clinically. Although, foot posture index-6 (FPI-6) are commonly used in clinical practice, its validity and diagnostic accuracy in evaluation of paediatric flexible flatfoot have not been fully proven yet.


To investigate validity and diagnostic accuracy of FPI-6 to determine Paediatric flexible flatfoot between ages of 6–18 years using radiographic findings as the gold standard measure.

Study design

Cross-sectional study.


A cross-sectional study conducted on 612 participants (1224 feet) with flexible flatfoot aged 6–18 years (mean age ± standard deviation of 12.36 ± 3.39 years). The results of FPI-6 were compared to the gold standard radiographic measures and displayed on the receiver operating characteristic curve. Intra-rater reliability, sensitivity, specificity, predictive values and likelihood ratios were calculated. Posttest probability was calculated from Fagan nomogram.


FPI-6 demonstrate high intra-rater reliability (ICC = 0.96) with p value < 0.001. FPI-6 showed a sensitivity of 83.7%, a specificity of 80.4, a positive predictive value of 64.7, a negative predictive value of 92, a positive likelihood ratio of 4.62 and a negative likelihood ratio of 0.20. FPI-6 shows moderate diagnostic accuracy [AUC = 0.82; 95%CI (0.78–0.85)].


FPI-6 is valid with moderate diagnostic accuracy to determine paediatric flexible flatfoot between ages of 6–18 years.


  • Criterion-related validity of FPI-6 in paediatric was investigated.

  • Sensitivity, specificity, predictive values, likelihood ratios for FPI-6 were detected.

  • ROC curve was used to determine diagnostic accuracy of FPI-6.

  • Fagan nomogram was used to explore posttest probability of flatfoot using FPI-6.


The foot has important impact absorption and ground reaction force transmission functions in bipedal standing position and during gait. The medial longitudinal arch (MLA) is one of the most important foot structures related to these actions ( ), because of its elastic properties along with the plantar aponeurosis, these structures can absorb and spread ground reaction forces over a longer time period, thus reduce the risk of musculoskeletal wear or damage. Theses properties allow the tissue to store the energy of the forces, returning it at the next step leading to reduction of the cost of walking and running particularly ( ).

Flatfoot is a complex foot deformity commonly seen in clinical practice. Flatfoot is characterized by a combination of a collapse of the medial longitudinal arch, foot abduction at the talonavicular joint, and hindfoot valgus (subtalar joint eversion) ( ). There are two types of flatfoot; rigid flatfoot and flexible flatfoot.

Flexible flatfoot is the most common foot abnormality in children characterized by collapse MLA in weight bearing position and returns to normal in non-weight bearing positions. Diagnosis of flatfoot in children clinically is still challenging for the health professionals due to age appropriate skeletal immaturity and ligamentous laxity, increased adipose tissue and immature neuromuscular control ( ; ).

Flatfoot is diagnosed through various measures that have been used to assess static foot posture based on MLA height ( ) including clinical assessment measures [foot posture index-6 (FPI-6), navicular height measurement, anthropometric measurements, and visual observation] ( ; ; ; ; ), radiographic evaluation ( ), ultrasonography, photographic techniques ( ) and footprint analysis ( ; ).

Although radiographic evaluation is considered the gold standard measure in assessment of static foot posture, it has many associated disadvantages such as potential health risks of exposure to irradiation especially for the paediatric population, high cost, and sophisticated methods of interpretation ( ; ).

Moreover, radiographic evaluation for flatfoot may not be suitable in some situations such as screening of large population in epidemiological studies. So, in such situations, valid, safe and noninvasive clinical measures would be helpful ( ). On the other hand, clinical assessment measures are the most applicable methods for evaluating foot posture clinically, but its validity has not yet been fully established in assessment of flatfoot in paediatric population ( ).

FPI-6 is an observational scoring system that takes into account three-dimensional nature of the foot. FPI-6 has good reliability in adults ( ; , ) and children ( ; ; ) FPI-6 is the most commonly used measure in clinical practice ( ; ). FPI-6 has been validated in adults ( ), but these results cannot be considered equally valid or standardized for paediatric population as the foot posture changes with growth and development till adulthood ( ).

FPI-6 has a good inter-rater reliability, concurrent validity ( ), and validated against Rash statistical model ( ), but unfortunately FPI-6 has not been validated against a criterion standard and its use in children is less explored than in adult subjects ( ).

Criterion-related validity is the most practical and objective approach to validity testing. It is the ability of one measure to predict results obtained from the gold standard or criterion standard measure that is assumed to be valid in measurement of the addressed variable, such as comparing FPI-6 to a plain film radiograph as a gold standard measure in diagnosing flatfoot ( ; ).

Up to date, there is no published study that investigated validity and diagnostic accuracy of FPI-6 against a criterion standard measure in paediatric population aged 6–18 years. However, Banwell et al., in 2018 had rated the validity of FPI-6 to be “ with caution ” in paediatric population (3–15years old). In this systematic review the authors based their decision for FPI-6 validity upon two studies ( ; ) that did not clearly investigate validity of FPI-6.

Evans and Karimi in 2015 had investigated the correlation between body mass index (BMI) and foot posture in children aged 3–15 years. The study done by Morrison et al., in 2013 aimed to determine clinical findings of foot posture in children with developmental coordination disorders (DCD) and investigate the impact of foot orthosis on gait characteristics in these children (6–11 years old). So, validity of FPI-6 to determine flatfoot in paediatric population cannot be confirmed because of shortage of the available literature reports.

Thus, to supply the researchers and clinicians with strong evidence about validity of FPI-6 in paediatric population, the current study was conducted to investigate validity and diagnostic accuracy of FPI-6 using the radiographic findings as the gold standard measure to determine flexible flatfoot in paediatric population aged 6–18 years.


Study design

Cross-sectional study.

Sample size

The sample size was calculated with a confidence level of 95% [at 5% type 1 error (p < 0.05)], power level of 80% and a precision of ±5%. The minimum sample size for the current study was 553 participants. A total of 612 participants were included in the study assuming an information loss of 10%–15%.


A convenience sample of 612 consecutive participants (1224 feet) aged 6–18 years [mean age ± SD of 12.36 ± 3.39 years] were recruited from the outpatient foot and ankle care unit, Cairo University teaching Hospitals, Egypt and Ministry of Health.. hospitals Sharjah, UAE from August 2018 to June 2019. The participants were diagnosed as having bilateral flexible flatfoot according to the clinical examination done by an experienced orthopaedic surgeon. Recruitment of the participants was illustrated in the enrollment flow chart ( Fig. 1 ). Examination of patients had been performed from standing. Pes planus (flatfoot) was considered flexible if the MLA height was restored and hindfoot valgus was correctible with standing on toes (tiptoe test), while, inability to restore the MLA height with standing on toes was considered rigid flatfoot and was excluded.

Fig. 1

Enrollment flow chart of the study participants.

The participants had met the following inclusion criteria: (1) Bilateral flexible flatfoot; (2) normal gait pattern without using any assistive devices; and (3) normal BMI. The participants were excluded if they had a history of neurological disorders (e.g. cerebral palsy), neuromuscular diseases, genetic conditions or syndromes affecting gait or posture (e.g. Down’s syndrome), congenital abnormalities of the foot (e.g. tarsal coalition, vertical talus), Structural lower limb deformities, foot shape or size asymmetry between right and left sides, Joint rheumatic pathology, history of foot or ankle fractures or major trauma affecting ankle joint stability, history of major surgeries in the lower limbs, Peripheral vascular disorders.

The current study included participants aged 6–18 years. The age of six years was selected as a minimum cut off at which most children would have developed MLA ( ; ).

The current study was conducted in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki) and was approved by the Human Research Ethics Committee of CairoUniversity and Research Ethics Committee of Minstry of Health nad Prevention, UAE. Before starting the study, written informed consents were obtained from participants and/or their legal guardians.


Demographic data

Each participant was given an identity (ID) number. Age (year), sex, height (cm), weight (kg) and Body mass index (BMI) were obtained from each participant ( Table 1 ). BMI was calculated using the standard Quetelet index (body weight divided by height squared; kg/m 2 ) ( ). Only participants with normal BMI were included in this study according to BMI based classification system of the childhood obesity proposed by International obesity Task Force (IOTF) ( ).

Table 1

Demographic characteristics of the study participants (n = 612).

Demographic characteristics Participants (n = 612)
Age (years) 12.36 (3.39)
Height (m 2 ) 1.61 (0.06)
Weight (kg) 46.66 (4.02)
BMI (kg/m 2 ) 17.30 (0.70)
Male 300 (49%)
Female 312 (51%)
Normal arched foot 153 (25%)
Flat Arched foot 469 (75%)

Continuous data are presented as mean (standard deviation), while categorical data are presented as number (percentage %). Participants were classified as normal or flat arched according to radiographic measure.

Clinical examination of static foot posture using FPI-6

All clinical examinations were performed by an independent examiner who was blinded to the participants’ identity and to the study objectives. The examiner was a senior physiotherapist with 12 years clinical experience in using FPI-6 in evaluation of static foot posture. Before measurement the examiner had attended 2-h training session about using FPI-6 in static foot posture assessment to ensure that clinical measurements would be done properly. Two research assistants [physiotherapy practitioners] were responsible with reception of patients, appointments for clinical measurements and organization of the clinical measurements procedures.

FPI-6 is an observational scoring system of the static foot posture ( ; ; ). The current study followed scoring system, reference values and the measurement protocol described by Redmond in 2005. The participant was asked to stand in a relaxed stance position with double limb support (equal weight on both feet), arms by side and looking straight ahead. The participant was instructed not to swivel to try to check the examiner, as this would affect the foot posture leading to false score of FPI-6 ( ).

Inter-rater reliability had been proven previously for FPI-6 ( ; ; ; ), so the current study did not investigate inter-rater reliability for FPI-6. To investigate intra-rater reliability, all measurements of FPI-6 were repeated one week apart by the same examiner.

Radiographic measurements

Radiography is a highly reliable gold-standard measure for assessment of skeletal alignment of the foot in static weight bearing position ( ; ).

Participants’ feet were radiographed from a relaxed standing position with equal weight bearing on both feet. The participant stood on a wooden platform with their knees extended and their heels and toes at the same level. The X-ray film pack was placed vertically between the feet. Participants were instructed to place equal weight on both feet. The X-ray tube was oriented parallel to the floor with the central beam targeted the midfoot. The tube-film distance was 105 cm and the exposure was adjusted to be 2.8 mA at 45 kV.

Accordingly, the participants’ lateral weight bearing radiographs were viewed on the Phillips Easy Vision viewer (Philips Healthcare, Guildford, UK) and measurements were performed using the Philips Easy Viewer software tools. The images were anonymised by removing identifying data. Lateral radiographic view talus first metatarsal angle (T1MA) or Meary’s angle was measured by an experienced radiologist who was blinded to the study objectives. T1MA was chosen to represent the foot posture based on ease of measurement, good reliability and the degree to which it reflects the static foot posture ( ).

T1MA is the angle between the longitudinal axes of the talus and the first metatarsal bone. In the normal foot, the midline axis of the talus is in line with the midline axis of the first metatarsal ( Fig. 2 ); while in the pronated foot, the talar axis is not in line with midline axis of the first metatarsal bone. Normally Meary’s angle is −4° to +4°; Meary’s angle > +4° (convex upward) indicates pes cavus; Meary’s angle <− 4° (convex downward) indicates pes planus ( ).

Aug 18, 2020 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Validity and diagnostic accuracy of foot posture Index-6 using radiographic findings as the gold standard to determine paediatric flexible flatfoot between ages of 6–18 years: A cross-sectional study
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