Measurements of Features
In the measurement results, age-based variations in the foot features were observed slightly for men whereas distinctively for women. Particularly, it is surmised for women that the lower NH and increasing flatness of the foot were related to aging. However, aging did not have a great impact on the SHN angle and ABD. The longitudinal arch of the foot is likely to be connected with the tibialis posterior muscle, the muscle strength in the sole and the transformations in the skeletal structure [16,17]. For these reasons, it is surmised that the lowering of the longitudinal arch is caused by severe declines in muscle function such as impairment of the tibialis posterior muscle[18], protrusion of the calcaneus[19] and pronation of the navicular bone[20] as well as the aging process. It is also surmised that the IS angle increases as the thickness of foot skeletal structure decreases with ages.
Correlations among Foot Features
The correlation coefficient between the MPH angle and the TAL is greater than 0.94 in Table 4 for both men and women, indicating a strong correlation, on the other hand, the TAL increases whereas the TAH decreases with age for women in Table 3. Taking into consideration that the MPH angle and TAL are skeletal features related to the width of the forefoot and the TAH is a skeletal feature related to the height of the forefoot, it is surmised that flattening of the forefoot is associated with a broad foot. Particularly, it is surmised for women that the GFH angle increases with age, which indicates the development of HV.
Paying attention to the GFH angle, the skeletal features identified as being correlated with it include the SHN angle and ABD, which are those related to the midfoot. As the SHN angle increases, which means that the navicular bone sticks out more, the ABD also increases with the inside of the midfoot deviating more inward. Therefore, this suggests that foot pronation is associated with the increase of the GFH angle thus HV. This suggestion is supported by the result of a prior study[21] that the risk of HV is significantly increased in persons with pronated foot function.
In addition, it was found that the ABD is correlated with the SNH angle (r2 > 0.40), IH (which denotes the health of the foot’s skeletal structure) (r2 > 0.33) and NH (which denotes the height of the longitudinal arch) (r2 > 0.34) in Table 4. This suggests that misalignment of the skeletal structure of the midfoot is related to the health of feet, particularly, the formation of navicular and cuneiform bones. This misalignment is considered to be influenced by both early childhood development and aging process[22].
Furthermore, correlation between the IH and the TAH was observed for both men (r2 = 0.52) and women (r2 = 0.38) in Table 4. The reason why the correlation is lower for women could be that the transverse arch reflected by the TAH is more strongly influenced by footwear for women.
Finally, as for flatfeet which means the fall of the longitudinal arch, many discussions have utilized footprints[23,24]. From the viewpoint of biomechanics, the main cause of flatfeet may be the influence of the tibialis posterior muscle or the muscles in the sole[25]. Considering pronation, the navicular bone moves directly below these muscles while causing 3D changes in various directions, such as supination of the subtalar joint, pronation of the navicular bone and supination of the forefoot. Therefore, assessing the 3D skeletal structure of the foot is effective for pronation, which is the very benefit of the proposed system.
Multiple Regression Analysis for the GFH Angle
It was revealed from Table 5 that dominant skeletal features determining the GFH angle are the SHN angle and the ABD among those related to the midfoot whereas the TAL and the TAH among those related to the forefoot for both men and women. In addition to these features, it was revealed that one of the dominant features is the BMI for men whereas GSR for women. Regarding the multiple regression analysis, the adjusted coefficient of determination was 0.52 for men versus 0.48 for women, multicollinearity was confirmed in relation to none of the independent features, and β was large for both men and women in the SHN angle and the TAL.
As for other features associated with HV, prior studies pointed out lower BMI and high heel use for women aged 20–64 whereas higher BMI and flat feet for men[2,13]. Our results suggest that the etiologic mechanisms of HV may differ between men and women. Namely, although this study included neither extremely overweight nor underweight participants, it found an association of BMI with HV for men, which was not strong as compared to that in the prior study[2].
Overall Results
A prior study showed that the protrusion of the first metatarsal and the lengths of the first metatarsal and the proximal phalanx of the hallux were increased in both men and women with HV as compared to those in the control group[4], in other words, the first toe is a key part related to HV. On the other hand, this study showed that a longer second toe was associated with a larger GFH angle, in other words, the second toe was a key part related to HV. The reason of this difference may be that the length of the first metatarsal was compared with that of the second metatarsal in the prior study whereas the distance between the tip of the first toe and the heel was compared with that between the tip of the second toe and the heel in this study. The proximal phalanx of the hallux deviates laterally towards the second toe as HV develops, so the distance between the tip of the hallux and the heel gets shortened, naturally resulting in the appearance of longer second toe.
In this study, it has been highlighted that the GFH angle as an index of HV is influenced by the flatness of the forefoot and misalignment of the skeletal structure of the midfoot. In a prior study, the NH was used as an index of flatfeet, which was considered to influence HV[26]. In this study, on the other hand, the independent variable derived from the multiple regression analysis was not the NH but the SHN angle. As the SHN angle and NH were inversely correlated (-0.3– -0.2), it is quite natural that both height and position of the navicular bone can be key features for HV. The SHN angle is not on a horizontal plane, so simple 2D measurement systems cannot calculate it. On the other hand, the developed 3D scanner system can easily calculate the NH as well as the SHN angle, using an automatically constructed 3D foot skeleton model.
By using a 3D foot scanner on smartphone, we have been able to implement a simple quantitative method for assessing the skeletal structure of the foot. This will allow us to clarify the risk of HV.
LIMITATION
This study has several limitations. The measurement is performed from the outside of the foot, so there may be some errors between the GFH angle and the HV angle measured by x-ray. In addition, the measurement is performed manually, so handshake may introduce some errors in the measurement data. Furthermore, the 3D foot skeleton model is constructed by 2D images, so some errors may occur in the construction process.