Is subtle cavus foot a risk factor for chronic ankle instability? Comparison of prevalence of subtle cavus foot between chronic ankle instability and control group on the standing lateral radiograph

•Detecting subtle cavus foot requires experience when judging by standing lateral radiograph.•Subtle cavus foot was not a significant risk factor for chronic ankle instability by judging standing lateral radiograph. Subtle cavus foot (SCF) is an entity characterized by mild cavus. However, few studies have examined whether a SCF may be a risk factor for chronic ankle instability (CAI). This study included 116 patients who underwent lateral ankle ligament repair (modified Broström operation) for CAI and 105 controls. We used the standing lateral radiograph, so compared calcaneal pitch angle, Meary’s angle, heights of the first and fifth metatarsal bases, and fibular positions between groups. Additionally, two observers subjectively rated the standing lateral radiographs for the presence of SCF. There were no significant intergroup differences in any of the radiographic angles. The prevalence of SCF was 20.7% in the CAI group and 18.1% in the control group according to observer 1 versus 21.6% and 28.6% (CAI group and control group, respectively) according to observer 2. There were no significant intergroup differences in the proportion of SCF between the two observers (p=0.105 and 0.211, respectively). SCF was not a significant risk factor for CAI when judging by standing lateral radiograph, and the detection of SCF seems to require considerable experience. Thus, care should be taken when determining whether to perform corrective osteotomies when treating CAI patients with SCF. III, case control