Validity and Reliability of Digital Navicular Drop Equipment in Young People with and without Pes Planus

INTRODUCTION

Developing children have soft bones, lax ligaments, increased adipose tissue, and immature neuromuscular control.^1,2 Consequently, “flat” arches are normal for children up to 8 years of age.³ Although various ligaments prevent a decrease in the medial longitudinal arch (MLA) height, such as the plantar fascia and spring ligament,⁴ weakness of the foot intrinsic muscles is also essential.⁵ Adults with a flexible pes planus experience more back and lower extremity pain⁶ and reduced quality of life.⁷ In America, around 5 million people are diagnosed with pes planus⁸ and >40% of young adults are at risk of the condition.⁹ In Germany, 8% of pes planus patients are prescribed foot orthotics, and the related costs reached 466.6 million Euros in 2019.^8,10 A flexible pes planus cannot maintain normal inversion in the terminal stance phase, resulting in potential foot and lower extremity fatigue.¹¹ Evaluation of pes planus is important for avoiding flexible pes planus.

Lateral weight-bearing X-rays are commonly used to assess pes planus, as well as to measure Meary’s talo-first metatarsal angle, the angle of plantar flexion of the talus, also called the talo-horizontal angle, and the talocalcaneal angle.¹¹ These examinations have some restrictions due to radiation exposure, low accessibility, and high costs.¹² Clinical tests such as the navicular drop test (NDT) and arch height index are commonly used in clinical practice because they are convenient, cost-effective, feasible, readily available, and non-invasive, and do not require specific devices.¹²

For this reason, NDT is continuously used to identify the intervention effects related to pes planus, from past study²⁴ to recent study.²⁵ Although the NDT is a widely used clinical test, its intra- and inter-rater reliability is controversial. The intra-rater reliability reportedly ranged from 0.33 to 0.76 and was slightly higher than the inter-rater reliability.¹³ Evans reported that while the intra-rater reliability of the NDT ranged from 0.51 to 0.77, the inter-rater reliability was low, at only 0.46.¹⁴ A manual clinical test may lead to errors due to a lack of training.¹⁵

In addition, placing the body weight unequally on each foot can cause measurement error. Participants should widen their stance more than usual. The criterion for the neutral position of the subtalar joint differs among examiners, and there is a difficulty in accurately positioning it. To solve these problems, Park and Park developed digital navicular drop test equipment (D-NDT), which was shown to have diagnostic validity and reliability for participants with normal feet.¹⁶ However, their data were insufficient for generalization to all people. Therefore, this study investigated the diagnostic validity and inter-rater reliability of D-NDT in people with and without pes planus.

METHODS

Participants

The sample size was determined using the G-Power program (3.1.9.6, Düsseldorf University, Germany) based on the correlation between NDT and D- NDT grades. In a pilot study, the correlation coefficient of the total NDT and D- NDT grades was ≥0.5. Therefore, we needed to enroll 46 people assuming a correlation of 0.50, with a power of 0.95 and significance level of 0.05. Considering the potential for dropouts, 3 participants were added to each group, such that there were 26 people with normal feet and 26 with pes planus (Figure 1). The participants had no foot deformities or diseases, and no foot pain or neurological disorders. Pes planus was defined as ≥10 mm in the NDT,^17,18 the MLA angle of >131 degrees,¹⁹ and the body mass index (BMI) <30.¹⁸ The dominant foot, as determined by kicking a ball, was recorded in all participants.¹⁸ The entire study procedure was approved by the University Institutional Review Board. Table 1 summarizes the general characteristics of the two groups.

Figure 1. Diagram of the study design. Abbreviations: BMI, body mass index; ICC, intraclass correlation coefficient.

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Table 1. Descriptive statistics for participants (n=52)

Variable	Normal group (n=26)	Pes planus group (n=26)	t	p
Age	24.38±4.28	25.88±4.74	–1.198	0.237
Height (cm)	167.27±8.20	166.42±7.28	0.394	0.696
Weight (kg)	66.23±18.63	71.12±9.22	–1.198	0.239
BMI (kg/m2)	23.37±5.01	25.61±2.26	–2.081	0.045*
Gender	Female 15 (57.7%),	Female 11 (42.3%)	-	-
	Male 11 (42.3%)	Male 15 (57.7%)
Dominant foot	Right 26 (100%)	Right 20 (76.9%)	-	-
		Left 6 (23.1%)

BMI, body mass index.

^* p<0.05.

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Digital navicular drop test equipment

After learning how to use the D-NDT,¹⁶ reflective markers were attached to the navicular tuberosities of all participants. Subsequently, they were instructed to place their bare foot on a reflector sensor along a guide line and relax the foot. In a sitting position, the examiner relaxes the subject’s foot and then places it comfortably in the sensor area. Next, the distance between the navicular tuberosity and the ground was measured while both seated and standing (Figure 2). The examiners were experts with sufficient knowledge of the NDT and D-NDT. All assessments were repeated three times in each position.

Figure 2. Digital navicular drop test equipment (A) and operating the equipment in standing position (B).

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RESULTS

Bland-Altman plots

The mean difference between the two tests was 0.50 (95% limits of agreement: –2.83 to 3.83) mm in the normal foot group (Figure 3) and –0.58 (95% limits of agreement: –1.89 to 0.73) mm in the pes planus group (Figure 4).

Figure 3. Bland-Altman plot of result of navicular drop test in normal group.

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Figure 4. Bland-Altman plot of result of navicular drop test in pes planus group.

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DISCUSSION

The NDT is a reliable tool for measuring foot pronation and has an excellent correlation with the foot posture index (r=0.8), another tool for measuring foot pronation.¹⁷ Consequently, the NDT is mainly used to clarify the intervention effect for people with pes planus.¹⁸ The D-NDT was developed to enhance the precision of the NDT, which is clinically valuable, and to clarify the diagnostic validity of the NDT and D-NDT.

In this study, there were excellent correlations between the NDT and D-NDT grades in both the sitting (0.938–0.980) and standing (0.926–0.977) positions. On subtracting the height of the plantar arch in the standing position from that in the sitting position, the total grades had correlations between 0.650 and 0.740. The correlation was higher in the normal foot group than the pes planus group, reflecting the clinical value of the NDT. Furthermore, the Bland–Altman plot showed that all except one case in both groups were within the 95% limits of agreement.

The reported reliability of the NDT varies widely. An ICC value <0.4 is poor, 0.4–0.6 is fair, 0.6–0.75 is good, and >0.75 is excellent.²³ In a previous study, the intra-rater and inter-rater reliability were fair.^13,22 In a recent study, the intra- and inter-rater reliability were >0.9, which is excellent.¹⁷ However, another study reported low (0.46) inter-rater reliability of the NDT.¹⁴ Despite its strengths, quantitative measurement of the MLA still has poor reliability²⁰ and varies depending on the user’s experience.²¹ The D-NDT was developed with ease of use and reliability in mind. In a study of normal subjects, the intra-rater reliability of the D-NDT was excellent, ranging between 0.93 and 0.95 depending on the posture.¹⁶ In this study, the normal group showed reliability values of between 0.616 and 0.978 depending on posture, while the pes planus group had very high reliability values (>0.96) in all postures. In addition, the total grades in the pes planus group had good correlations (>0.75).

The NDT poses a challenge, in that the experimenter needs to shift the legs forward, backward, left, and right to distribute the body weight equally on both feet. With the D-NDT, both feet naturally bear equal weight, increasing measurement accuracy. In particular, this equipment does not require ability to align the neutral position of subtalar joint in a sitting position. This is an equipment that anyone can easily check their pes planus. If the angle of the MLA is then added to the height of the arch, its clinical value increases.

CONCLUSION

The D-NDT showed high diagnostic validity and greater inter-rater reliability than the NDT for subjects with pes planus. Therefore, the D-NDT can be used to assess pes planus in clinical practice, complementing the NDT.