Objectives: The aim of this study was to determine the reliability of a C1:C2 ratio in a cohort of patients with atlas fractures. Second, we aimed to consider the utility of the C1:C2 ratio with regard to diagnosis of transverse ligament (TL) injury. Design: This is a retrospective analysis. Methods: Patients with atlas fractures in the Waikato region between 2008 and 2010 were identified retrospectively through clinical coding and collated radiology trauma database. Main Outcome Measurements: The maximal width of C1 and C2 was measured using the first-taken trauma radiograph series. Combined overhang of lateral masses (△mm) and a C1:C2 ratio was then calculated. Final ratio and atlanto-dens interval (ADI) were measured at the last clinical follow-up. Results: A total of 24 patients with full radiographic records were included. Of these, five patients (21%) had TL injuries confirmed on computed tomography or magnetic resonance imaging. No patient with a ratio 1.15 had an intact TL, whereas a ratio of >1.10 captured 80% of TL injuries. The ratio ( P < 0.001) and delta values ( P < 0.001) were statistically significantly different between TL-injured and TL-intact cohorts. Two patients in the TL injury group demonstrated increased ADI on final follow-up with a ratio of >1.10. Conclusions: A C1:C2 ratio >1.10 on plain radiographs showed a sensitivity of 80% in detecting atlas fractures with associated TL injury. All patients with a ratio of ≥1.15 had TL rupture subsequently confirmed by an advanced modality. A ratio calculation on radiographs is a potentially useful method of describing atlas lateral mass displacement. Level of evidence: Level III
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