2.1

This study was designed as a crossover trial to evaluate the effects of different metatarsal pad placements on plantar pressure during gait in patients with hallux valgus. Based on the study design and inclusion of a control condition, this research is classified as Level III evidence. As a result of recruiting from the patients who visited our hospital and were diagnosed as hallux valgus by an orthopedic surgeon who has a specialty with foot and ankle surgery, 22 female patients were diagnosed by an orthopedic foot and ankle surgeons as hallux valgus deformity and were recruited following inclusion and exclusion criteria between May 2021 and February 2022. Inclusion criteria were as follows: (1) with ability to walk independently, (2) with the HV angle of at least 20 degrees according to a diagnostic criterion . Exclusion criteria were as follows: (1) with a history of neurological disease, (2) with hard pain to elicit the unstable gait.

The sample size required for the study was validated using G-power 3 and determined to be at least 22 patients based on effects size f 0.25, α 0.05, power (1-β) 0.80 based on Lehmann’s approach , assuming a 15 % larger sample size for parametric tests , and in addition, an expected drop-out rate of 20 % due to possible drop-out from written informed consent ^,, .

In this crossover study, participants underwent clinical assessment before the gait trial and biomechanical assessment during four gait trials using a differentially located arch pad at the hospital of affiliation. We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed in this research. The study protocol was approved by the Institutional Review Board (IRB) of the affiliated institution, and the study was conducted in accordance with the principles of the Declaration of Helsinki. All participants provided verbal informed consent because the IRB approved the use of verbal consent in accordance with the Japanese guidelines for clinical research. Verbal consent was recorded as an experimental note. No incentive was given to the study participants.

2.2

Based on the anteroposterior and lateral X-ray images in standing, each foot alignment of the affected foot was calculated ( Fig. 1 ). The first and second metatarsal angle (M1-M2 angle) was calculated from the angle formed by the axes of the first and second metatarsals. The first and fifth metatarsal angle (M1-M5 angle) was calculated from the angle formed by the axes of the first and fifth metatarsals to evaluate splayfoot ^,, . Meary’s angle was calculated from the angle formed by the axis of the talus and first metatarsal bone as an indication of the medial longitudinal arch . The calcaneal pitch angle was formed by a line joining the anterior end of the lower surface of the calcaneus and the lowermost end, and a line joining the lowermost end of the calcaneus and the lower surface of the medial sesamoid below the first metatarsal head . All radiographic evaluations were conducted before the analysis of plantar pressure data, ensuring that the assessments were not influenced by the outcome measures. In addition, the hallux scale of the Japanese Society for Surgery of the Foot (JSSF) standard rating system, a clinician-reported outcome measure that evaluates pain, function, alignment, and activity related to the hallux ^, . Subjective foot condition was further assessed using the Self-Administered Foot Evaluation Questionnaire (SAFE-Q), a validated patient-reported outcome measure developed to assess foot-related quality of life across multiple domains, including pain, physical function, social function, shoe-related issues, and general health ^, .

Items measured from X-ray images of the foot. H-V angle and M1-M2 angle were calculated as indicators of the big toe, M1-M5 angle as indicators of the forefoot arch, Calcaneal pitch, and Meary’s angle as indicators of the medial longitudinal arch.

2.3

Before the gait measurement, shoes (JOG 100 2, ASICS Corporation, Kobe, Hyogo, Japan) were prepared for the patient. Disposable insole-type PP sensors (F-scan sensor sheet, Tekscan Inc., South Boston, MA, USA) were inserted into the shoe with confirmation of motionless shoes. After connecting the sensor sheet to the F-scan system, calibration was performed using an alternating one-leg stand based on the manufacturer’s recommendation. Practicing a natural gait, participants were asked to walk back and forth along an eight meter equidistant walking path at a comfortable speed three times, with a two minute break between each test. Walking speed and the number of steps were measured for each trial and gait cadence and stride were calculated. Pressure data during the gait test was recorded in software at 750 Hz by the F-scan system and extracted based on previous reports . This series of measurements were performed under four conditions. Control conditions with no metatarsal pad inserted, and conditions with a metatarsal pad (Transverse arch pad, size: 68 mm (length), 45 mm (width), 15 mm (height), Sanshin Enterprises Co., Ltd, Shinjuku, Tokyo, Japan) inserted at 68 %, 72 %, and 76 % of the foot length on a straight line connecting the second toe and heel of the plantar surface by a prosthetist with at least 15 years experience ( Fig. 2 ). The same prosthetist applied all pads across all participants to ensure consistent placement. During the familiarization trials prior to data collection, the plantar pressure distribution and COP trajectory were visually monitored in real time using the F-scan software to confirm appropriate contact between the foot and insole. The metatarsal pad is an ether-based polyurethane made of polyols and polyurethane called Sorbothane, a material with excellent shock absorption ^, . Before the implementation of each condition, the participants were fully accustomed to walking.

Insertion conditions for the metatarsal pad. The metatarsal pad was inserted at locations corresponding to 68 %, 72 %, and 76 % of the foot length from the heel.

From all PP measurements obtained during walking trials, the first and last steps were excluded. Data collected during walking from one m before and after the change of direction and data collected during directional walking were also excluded. Of the remaining data, five stable steps were selected from the intermediate data during walking per trial. Finally, pressure data for each stance phase of the foot with HV during the 15 gait cycles were extracted and analyzed using F-scan software. The F-scan insole consists of multiple sensor cells, each of which continuously records pressure values (in kPa or N/cm²) during gait. The analysis was based on the peak values of PP (PPP) in the eight regions as follows; the great toe (first toe), second and third toe (2–3rd toes), fourth and fifth toe (4–5th toes), medial metatarsal (Med-mt), central metatarsal (Cent-mt), lateral metatarsal (Lat-mt), midfoot (Mid f) and hindfoot (Hind f) based on previous reports ( Fig. 3 ) . These regions were defined by evenly segmenting the calibrated foot area according to pixel length and width, as obtained during system calibration. This pixel-based division allowed for consistent and reproducible application of regional boundaries across all participants and trials. To compare changes in the concentration of plantar pressure in each foot region, the PPP of each region was calculated as a percentage of the overall PPP (PPP ratio).

PP distribution. The distribution of PP was divided into eight regions based on foot length and foot width length calculated on the sensor.

2.4

The association between radiographic parameters and all PPP ratios at the foot region was examined using Pearson’s product rate and Spearman’s rank correlation coefficients. In addition to the spatiotemporal gait parameter, all PPP ratios at the foot region were compared among the Control, 68 %, 72 %, and 76 % conditions using the Friedman test with the Bonferroni method as a post hoc test. For the state encompassed the significant differences of the PPP ratio at Cent-mt compared to control, a comparison of alignment parameters based on X-ray images, JSSF, and SAFE-Q was conducted between the patients who showed at least a 1 % increase or decrease in PPP ratio using unpaired t -test or Mann-Whitney’s U test. Furthermore, binomial logistic regression analysis to define the effect on the PPP ratio at Cent-mt was performed using the significantly differed factors, age, and BMI as independent variables. The cut-off values based on the obtained regression equations were calculated from ROC curves. Data were analyzed using IBM SPSS Statistics 28.0 (IBM Corp., Armonk, NY, USA), with a significance level of p < 0.05.

3.1

All 22 patients completed all gait tests and were included in the final analysis, showing the results of radiographic parameters and foot function assessment ( Table 1 ). The average of HV angles was 34.5 ± 10.7. They were composed of 15 (68.2 %) moderate and 7 (31.8 %) severe deformities according to Coughlin’s classification . Based on the previously reported criteria with Meary’s angle and Calcaneal pitch values ^, , 18 participants (81.8 %) were defined as having a reduced arch. 19 individuals (86.3 %) responded to the SAFE-Q, and 21 individuals (95.4 %) responded to the JSSF questionnaire.

3.2

Gait speed, cadence, and stride under all conditions are shown in Fig. 4 . While there were no significant differences in cadence and stride, all three located metatarsal pads (68 %, 72 %, and 76 %) enhanced faster gait speed compared to the control condition ( p < 0.01, p < 0.05, and p < 0.05). Nevertheless, faster gait due to all metatarsal pad insertion did not induce pain during and after the gait trials. The PPP ratio at each of the eight sites and in the four conditions was demonstrated in Fig. 5 . In the control condition, a significant positive correlation was found between the PPP ratio in Cent-mt and the M1-M5 angle as shown in Table 2 ( r = 0.50, p = 0.01). Comparing the PPP ratio among the four conditions, the PPP ratio at Cent-mt in the 76 % condition was significantly lower than in the Control ( p < 0.05) and 68 % conditions ( p < 0.01), although there were no significant differences between the other conditions. In the fourth to fifth toes, the PPP ratio in the 68 % condition was significantly lower than the PPP ratio in the Control and 76 % conditions ( p < 0.05) ( Fig. 5 ). No significant differences were found in the other comparisons. Fig. 6 illustrates representative pressure maps for each condition.

Comparisons of walking speed, cadence, and stride across gait conditions were made using the Friedman test. * Indicates significance at p < 0.05. ** Indicates significance at p < 0.01.

Comparisons of PP in each domain across conditions were made using the Friedman test. * Indicates significance at p < 0.05. ** Indicates significance at p < 0.01.

Table 2

The relationship between radiographic images and planter pressure.

	HV angle	M1-M2 angle	M1-M5 angle	Meary’s angle	Calcaneal Pitch
1st Toe	−.157	−.138	−.089	−.290	.028
2–3rd Toe	−.029	.384	−.048	−.243	−.099
4–5th Toe	−.131	.523*	.178	−.033	.171
Medial Metatarsal	.267	.213	.078	.495*	.351
Central Metatarsal	−.223	−.136	.506*	.022	.155
Lateral Metatarsal	.099	−.402	−.119	.133	−.107
Midfoot	−.004	−.227	.027	.172	.044
Hindfoot	−.066	−.126	−.211	−.310	−.219

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