Background: Ankle sprains occur frequently in both athletes and the general population. The social and economic consequences can be significant. In an effort to understand the injury, dynamic and static structures around the ankle have been investigated in detail, but anatomical factors predisposing to lateral ankle instability have not been fully clarified. The aim of this study was to radiologically investigate the relationship between bony variations of the distal tibiofibular joint and arthroscopically proven ankle instability. Methods: Fifty patients with arthroscopically proven ankle instability and 50 patients without instability were included in this study. Measurements were obtained from a magnetic resonance imaging (MRI) section 1 cm proximal to the tibiotalar joint; distal tibiofibular joint anterior facet length (a), posterior facet length (b), angle between the anterior and posterior facets (c), fibular notch depth (d), tibia thickness (e), and fibula thickness (f) was measured. Results: It was found that instability was more frequent when the length of a (P < .001) and e (P < .001) were shorter. In addition, when value of a/b and e/f were evaluated, it was observed that the number of individuals who had instability increased as the ratio became smaller (P < .016-.020, respectively). Pearson correlation analysis indicated strong negative correlation between the values of a-e and instability (r = -0.348, P < .001, and r = -0.328, P = .001; respectively). Conclusion: Lateral ankle sprains are common, and a clear understanding of the relevant structures and clinical function of the ankle complex should extend beyond the talocrural joint. This study demonstrated that the presence of narrow anterior facet (a) and thinner tibia (e) were strongly correlated with lateral ankle instability.