The main results of the current study showed that in static posture, AIS with cavus feet demonstrated a reduction in contact area and peak pressure on the midfoot and lateral rearfoot when compared to planus and normal feet, as well as the maximum force on the midfoot and rearfoot (medial and lateral). Planus feet showed an increase in peak pressure and maximum force on the midfoot when compared to cavus and normal feet. In addition, planus feet also promoted an increase in peak pressure and maximum force on the medial and lateral rearfoot in relation to cavus feet and a decrease compared to normal feet. The foot is part of the kinetic chain connecting the lower limb to the spine, via the pelvis [26], which is responsible for maintaining support for the static posture and dissipating the plantar load during walking [26–28]. Another issue is that bone maturation and hormonal changes during adolescence can promote substantial alterations in the arch morphology and feet support in patients with AIS [4, 6]. The clinical relevance of the present study was to show that cavus (high medial longitudinal arch) and planus feet (low medial longitudinal arch) have an effect on plantar pressure in different foot areas in patients with AIS.
In a normal, healthy foot, the longitudinal and transverse arches provide the most optimal foot loading and proper force distribution. However, foot arch alterations may lead to structural changes and/or affect load distribution [29, 30]. In the current study, AIS with cavus feet showed a reduction in contact area and peak pressure on the midfoot and lateral rearfoot when compared to planus and normal feet, as well as in the maximum force on the midfoot and rearfoot (medial and lateral).
Cavus feet (increased medial longitudinal arch) can cause lower mobility of the foot12,30 and a weak force absorption mechanism which predisposes to injuries [12–15]. This worse mechanism of distribution of impact forces in the contact of the heel with the floor can also be explained by the lower angle of the calcaneus in relation to the first metatarsal, favoring an elevated longitudinal plantar arch [10]. Another line of reasoning is that with increasing age, healthy female adolescents, interestingly, tended to increase their toe and forefoot plantar pressures compared to males, which may be a possible risk factor for further foot impairments [31]. The differential of this study was to show that adolescents with AIS have lower plantar pressure on the midfoot and rearfoot areas, which can promote greater difficulty in maintaining body balance. This finding can be supported by the association between balance changes due to the reduction in the contact area of the forefoot and rearfoot and alterations in body posture in adolescents with AIS [26, 32].
A key tool in the analysis of foot and lower limb biomechanics is the measurement of the direction and magnitude of force applied to the plantar surface of the foot [4]. A recent systematic review found some evidence of distinctive plantar pressure characteristics in planus and cavus feet [5]. Specifically, when normal and cavus feet were compared, planus feet displayed higher pressure, force, and contact area values in the medial arch, central forefoot, and hallux, while these variables were lower in the lateral and medial forefoot. In contrast, when compared to normal and planus feet, cavus feet displayed higher pressure in the heel and lateral forefoot and lower pressure, force, and contact area in the midfoot and hallux. Another caution in this study was the use of tools for measuring the plantar arch (feet posture), which are reliable and valid for clinical use, and present normative values that been used in previous studies on foot posture [33–35].
Planus feet showed an increase in peak pressure and maximum force on the midfoot when compared to cavus and normal feet. In addition, planus feet also promoted an increase in peak pressure and maximum force on the medial and lateral rearfoot in relation to cavus feet and a decrease compared to normal feet. Studies have reported that planus feet (low medial longitudinal arch) can result in greater plantar loads over the calcaneal medial area, which, in turn, induces greater stretch in the plantar fascia [36–38]. In this study, adolescents with AIS with planus feet presented increased load of plantar pressure on the heel (medial and lateral), as well as on the midfoot, intensifying and worsening the stretching forces on the plantar fascia, which could be a clinical predictor of plantar overload on the heel in adolescents with AIS, as observed in adult runners in a study conducted by Lee and Hertel [39].
The contribution of the present study was to understand the influence of foot posture on plantar pressure during static posture in AIS. The limitation of this study was that it did not evaluate the plantar pressure during gait between different types of foot posture to better understand foot support in AIS. Future studies addressing AI and dynamic variables on plantar load, may further enhance the understanding of the association between these variables in patients with AIS.