Background:​ Diagnosis of syndesmotic injuries is primarily based upon the assessment of ankle radiographs. The purpose of our study was to redefine the radiographic relationships of the ankle syndesmosis based on a large series of normal ankle radiographs in living subjects.

Methods:​ The study involved 392 patients (218 females, 174 males) with ankle radiographs without known clinical or radiographic evidence of abnormality. Eighty-three of the 392 patients had also had normal contralateral radiographs. Tibiofibular overlap and tibiofibular clear space were measured on anteroposterior (AP) and mortise radiographs. The radiographic measurements were used to calculate means, standard deviations, and intra- and interob-server reliabilities, and compare genders and side-to-side radiographs.

Results:​ The mean overlap was 8.3 mm on the AP and 3.5 mm on the mortise while the mean clear space was 4.6 mm on the AP and 4.3 mm on the mortise view. The least amount of overlap on the AP view was 1.8 mm. On the mortise view, there was a subset of patients that had a complete lack of overlap (less than 0 mm) with the greatest gap noted to be 1.9 mm. The greatest clear space on AP was 8 mm and on the mortise was 7.6 mm. Mortise clear space was the most accurate measure when obtaining contralateral radiographs, with a mean side-to-side difference of 0.7 ± 0.7 mm.

Conclusion:​ Lack of overlap on the mortise view can represent a normal variant, which has not been definitively reported in prior investigations.

Clinical Relevance:​ Our data form the basis for revised radiographic criteria to evaluate the distal tibiofibular syndesmosis which may influence clinical management of these patients.