ABSTRACT
Objective@#To examine the changes in the cross-sectional area (CSA) ratio of the ankle invertors and evertors following rigid foot orthosis (RFO) application in children with symptomatic flexible flatfoot and to determine the correlation between the degree of change in CSA ratio and pain-severity after RFO application. @*Methods@#We included 24 children with symptomatic flexible flatfoot without comorbidities and measured the CSAs of tibialis anterior (TA), tibialis posterior (TP), and peroneus longus (PL) using ultrasonography, resting calcaneal stance position (RCSP) angle, calcaneal pitch (CP), Meary’s angle, talonavicular coverage angle, and talocalcaneal angle using radiography, and foot function index (FFI) at baseline and 12 months after RFO application. We analyzed 48 data by measuring both feet of 24 children. The CSA ratios, the ratio of CSA of each muscle to the sum of CSA of TA, TP, and PL, were also compared. Correlations between the degree of change in FFI, each muscle’s CSA ratio, RCSP angle, and radiographic measurements were investigated. @*Results@#Following RFO application, significant increase in the PL ratio and CP and significant decrease in the RCSP angle, FFI total, pain, and disability scores were observed. The degree of change in the total score, pain, and disability score of FFI were significantly correlated with the degree of change in the PL ratio and RCSP angle. @*Conclusion@#RFOs applied to children with symptomatic flexible flatfoot might reduce the compensatory activities of the ankle invertors, thereby increasing the PL ratio, and pain decreases as the PL ratio increases.