Evaluation of chronic ankle instability and subtalar instability using the angle between the anterior talofibular ligament and calcaneofibular ligament

Abstract Purpose A series of studies have reported a change in the length or thickness of the anterior talofibular (ATFL) and calcaneofibular (CFL) ligaments in patients with chronic ankle instability. However, no study has examined the changes in the angle between the ATFL and CFL in patients diagnosed with chronic ankle instability. Therefore, this study analyzed the change in the angle between the ATFL and CFL in patients diagnosed with chronic ankle instability to confirm its relevance. Methods This retrospective study included 60 patients who had undergone surgery for chronic ankle instability. Stress radiographs comprising the anterior drawer test, varus stress test, Broden’s view stress test, and magnetic resonance imaging (MRI) were performed in all patients. The angle between the ATFL and CFL was measured by indicating the vector at the attachment site, as seen on the sagittal plane. Three groups were classified according to the angle between the two ligaments measured by MRI: group I when the angle was > 90°, Group II when the angle was 71–90°, and Group III when the angle was ≤ 70°. The accompanying injuries to the subtalar joint ligament were analyzed via MRI. Results A comparison of the angles between the ATFL and CFL measured on MRI in Group I, Group II, and Group III with the angles measured in the operating room revealed a significant correlation. Broden’s view stress test revealed a statistically significant difference among the three groups ( p < 0.05). The accompanying subtalar joint ligament injuries differed significantly among the three groups ( p < 0.05). Conclusion The ATFL–CFL angle in patients with ankle instability is smaller than the average angle in ordinary people. Therefore, the ATFL–CFL angle might be a reliable and representative measurement tool to assess chronic ankle instability, and subtalar joint instability should be considered if the ATFL–CFL angle is 70° or less. Level of evidence Level III.