Relative Lateral Malleolar Length on Anteroposterior Radiographs Is Associated With Arthroscopically Confirmed Latent Syndesmotic Diastasis in Chronic Lateral Ankle Instability

Background: The combined latent syndesmotic diastasis (LSD) leads to longer rehabilitation time and poor outcomes for ankle sprains or chronic lateral ankle instability (CLAI) cases. Identifying LSD by imaging is challenging, especially for plain radiographs. This study aimed to determine the osseous ankle characteristics of CLAI combined with LSD on anteroposterior (AP) radiographs, as well as to provide a potential approach to detecting concomitant LSD. Methods: All CLAI patients receiving arthroscopic surgery in our hospital from August 2020 to August 2025 were retrospectively reviewed (CLAI group). Relative lateral malleolar length (RLML), relative medial malleolar length (RMML), and talocrural angle (TCA) were measured on the bilateral ankle weightbearing AP radiographs. The same parameters of 50 healthy ankle joints were measured on the weightbearing AP radiographs and compared with the CLAI group as a control group. The middle width of the syndesmotic space was measured under arthroscopy with the customized probe. With 3 mm as the threshold, the patients were then divided into the arthroscopic normal (AN) subgroup and the arthroscopic diastasis (AD) subgroup. These measurements were then compared between the AD and AN subgroup, as well as the injured and contralateral ankles, to explore the anatomical predisposition to LSD in CLAI patients. Results: A total of 164 CLAI patients were included for analysis. No significant difference was observed in RLML, RMML, or TCA between the CLAI group and the control group. For both the AD and AN subgroups, there was no significant side-to-side difference in all parameters. The AD subgroup showed a significantly greater RLML (0.929 ± 0.052 vs 0.881 ± 0.076, P < .001) and TCA (14.1 ± 2.4 degrees vs 12.6 ± 2.4 degrees, P < .001) compared with the AN subgroup. The optimal cutoff value for RLML detecting LSD was0.911 (sensitivity, 72.9%; specificity, 66.4%). Conclusion: No radiographic osseous differences were observed between CLAI patients and the normal control group, nor were side-to-side differences detected, indicating the anatomy does not predispose to ankle sprain. In CLAI patients, greater RLML is associated with arthroscopically confirmed latent syndesmotic diastasis; furthermore, RLML may help identify patients who warrant closer evaluation for syndesmotic injury, but prospective studies are needed to determine whether RLML-guided arthroscopy improves management or outcomes. Level of Evidence: Level III, retrospective comparative study.