Introduction: Diagnosis of syndesmotic injuries is primarily based upon the assessment of ankle radiographs. Earlier studies examining normal radiographs are limited by small sample size and methodological issues.

Materials and Methods: One thousand four hundred and fifteen consecutive patients with ankle radiographs were reviewed. 1023 patients were excluded as a result of a history of ankle/hindfoot pain, trauma, or surgery; or radiographic evidence of ankle/hindfoot pathology. 392 patients (218 females, 174 males) with normal ankle radiographs were included. 83 of 392 patients had bilateral normal radiographs. All radiographs were reviewed independently by a fellowship-trained foot and ankle surgeon and a fellowship-trained musculoskeletal radiologist. Tibiofibular overlap and tibiofibular clear space were measured on anteroposterior (AP) and mortise radiographs. These four measurements were analyzed.

Results: Mean AP overlap was 8.3 mm (±2.5). Mean mortise overlap was 3.5 mm (±2.1), 7.7% patients had < 1 mm overlap and 4.9% of patients had < 0 mm overlap. Mean AP clear space was 4.6 mm (±1.1), 7.1% patients had > 6 mm clear space. Mean mortise clear space was 4.3 mm (±1.0), 4.3% patients had > 6 mm clear space. All measurements were significantly different between females and males (p < 0.001). Mortise clear space is the most accurate measure when obtaining contralateral radiographs. Intraobserver and interobserver reliabilities of all measurements were high (intra-class correlation coefficient range 0.820–0.983).

Discussion and Conclusion: Our data unequivocally demonstrates that basing treatment of syndesmotic injuries on previously reported radiographic criteria can lead to unnecessary operative intervention or failure to treat. Lack of overlap on the mortise view can represent a normal variant, which has not been definitively reported in prior investigations. Our data forms the basis for new radiographic criteria to evaluate syndesmotic disruption.