Participants

This cross-sectional study was approved by the Ethics Committee of Fukushima Medical University (approval number: 2021–028) and involved female university students between 18 and 21 years of age. When comparing the two groups (those with and without hallux valgus) for hindfoot morphology and joint mobility, it was calculated that 64 participants per group were required, assuming a significance level of 0.05, power of 0.8, and effect size of 0.5. Since 29.7–41.5% of young women have hallux valgus [2, 3], it was necessary to measure 154–215 women to obtain 64 women with hallux valgus. Exclusion criteria were current outpatient treatment for lower extremity orthopedic disease and a history of knee and ankle surgery. The purpose of the study was explained to the participants both verbally and in writing, and informed consent was obtained.

MeasurementFoot morphology

All measurements were performed barefoot. Foot morphology was measured using a simple, non-invasive 3D automatic footprint measurement apparatus (JMS-3100; Dream GP Inc., Osaka, Japan) (Fig. 1). This foot-scanning system uses a rail-type laser around the entire circumference of one foot to measure approximately 30,000 points on the foot, such as the instep, heel, toe, and sole. First, in a standing position, markers with a diameter of 5 mm were attached to the navicular tuberosity, bottom of the calcaneal tuberosity, and enthesis of the Achilles tendon [11]. Participants placed one foot in the measuring instrument, both feet shoulder-width apart, and stood still with their weight evenly distributed over both feet, and their foot shape was measured twice on each side. Next, the participants were seated in a chair, the hip joints were in adduction/abduction with internal/external rotation in a neutral position, and the knee and ankle joints were at 90°. Footprints were measured twice on each side. The first measurement value was adopted, but if the foot was moved during the measurement and the measurement could not be performed, the second measurement value was adopted.

Fig. 1

Measurement of foot morphology using foot-scanning system. A standing position, B sitting position

The following parameters were measured for foot morphology. The hallux valgus angle was measured as the angle between a line connecting the head of the first metatarsal and the head of the first proximal phalanx and a line connecting the head of the first metatarsal and the posterior medial malleolus. The arch-height ratio was calculated as the ratio of the height of the navicular tuberosity from the floor to foot length (Fig. 2A). It has been reported that the navicular height of the body landmark divided by the foot length is highly correlated with radiographic measurements [15]. Heel-floor angle (HFA) was measured as the angle between the line connecting the bottom of the calcaneal tuberosity and enthesis of the Achilles tendon and the vertical line from the floor surface (Fig. 2B). The line connecting the bottom of the calcaneal tuberosity and enthesis of the Achilles tendon was measured as positive (+) when the line sloped inward from the vertical line from the floor and the sole turned outward (eversion) and negative (−) when the line sloped outward and the sole turned inward (inversion) [8]. In addition, the arch-height ratio and HFA were examined for differences between the values measured in a chair-sitting position and those measured in a standing position.

Change in arch-height ratio = arch-height ratio in a sitting position - arch-height ratio in a standing position.

Change in HFA = HFA in a sitting position - HFA in a standing position.

Fig. 2

Parameters of foot morphology. A arch - height ratio, B heel - floor angle

Ankle range of motion

All ankle joint angles were measured barefoot by a female physical therapist. In this study, ankle dorsiflexion and plantarflexion range of motion (ROM) were measured using a method reported to be more reliable than using a goniometer [16, 17]. The ROM of ankle dorsiflexion was measured using the following method (Fig. 3). First, the participant stood facing the wall, such that their hallux was 10 cm away from the wall. Next, the lower leg was loaded on the side to be measured, the ankle joint was dorsiflexed without lifting the heel from the ground, and the knee joint was flexed, followed by confirmation as to whether the front of the knee could contact the wall. If this position was possible, the foot was moved from the wall in 1-cm increments to identify a position in which the knee touched the wall without lifting the heel. Conversely, if this posture was not possible, the foot position was moved closer to the wall in 1-cm increments. At this time, the position of the lower limbs that were not measured was not specified. The maximum distance from the wall to the tip of the hallux was measured twice as the dorsiflexion range of the ankle joint. The within-session intra-rater reliability (ICC2,3) estimate for the dorsiflexion range of the ankle joint has been reported to be 0.98–0.99 [16].

Fig. 3

Ankle dorsiflexion range of motion

The ROM of the ankle plantarflexion was measured using the functional heel-rise test [17]. The participants stood facing the wall, with the tip of the hallux of both feet 15 cm away from the wall (Fig. 4A). Using a right-angle ruler, the position where the wall and the top of the head formed a right angle (90°) was specified, and the distance from this point to the floor was measured. Next, the participant stood on only the foot being measured, and the heel of the measured foot was maximally raised from the floor (Fig. 4B). The elbow joints on both sides were flexed at 90°, and the participants touched the wall with their fingertips of both hands to maintain balance. In this posture, the distance from the floor to the top of the head was measured. The difference between the maximum and starting height was calculated. This measurement was conducted on the left and right sides. It has been reported that for the plantarflexion range of the ankle joint, the within-session intra-rater reliability (ICC2.1) estimate was 0.99, and the inter-rater reliability (ICC2.k) was 0.79–0.87 [17].

Fig. 4

Ankle plantarflexion range of motion. A starting posture, B posture with the heel raised to the maximum

The angle of inversion and eversion of the ankle joint was measured using the following method (Fig. 5). The participants sat on a chair, and the ankle joint was in a comfortable relaxed position, usually in some plantarflexion, and moved passively. In the center of the medial and lateral malleolus, the angle of the crest of the tibia and the longitudinal midline on the anterior surface of the second metatarsal were measured using a goniometer. The left and right inward and outward inversions were measured twice. It has been reported that for the angle of inversion and eversion of the ankle joint, the within session intra-observer reliability (ICC2.1) ranged 0.82–0.96 and inter-observer reliability ranged 0.62–0.73 [18]. The ROM of all ankle joints was statistically processed using the maximum value.

Fig. 5

Ankle inversion and eversion range of motion. A eversion, B inversion

Statistical analysis

Participants were classified into two groups according to the presence of hallux valgus. According to the Japanese Orthopedic Association Clinical Practice Guidelines on the management of hallux valgus, a hallux valgus angle of ≥ 20° based on radiographic measurement values is judged to have hallux valgus [19]. A footprint-based hallux valgus angle of 16° has been reported to be equivalent to a radiograph-derived hallux valgus angle of 20° [20]. A hallux valgus angle of ≥ 16° on at least one side was judged to have hallux valgus. For statistical analysis, the measured values of either the left or the right foot were used [21]. In cases of bilateral or unilateral hallux valgus, the foot with the larger hallux valgus angle was selected. In cases where the angle of hallux valgus was < 16° bilaterally, a random number table was used to select the same number of left and right feet. We compared the arch-height ratio and HFA in the standing position, the amount of change in the arch-height ratio and HFA from the standing to the sitting position, and ROM of the ankle joint between the two groups with and without hallux valgus. Data normality was analyzed using Shapiro–Wilk test, and data were compared between groups using t-test or Mann–Whitney U test. Spearman’s correlation coefficient was used to analyze the relationship between foot morphology and ROM of the ankle joint in the groups with and without hallux valgus. Statistical analyses were performed using IBM SPSS for Windows (version 28.0; IBM Corp., Armonk, NY, USA), and the significance level was set at p < 0.05.