Diagnosing Chronic Diastasis of the Syndesmosis: A Novel Measurement Using Computed Tomography

Background: Chronic diastasis of the syndesmosis has been recognized as a cause of persistent pain and dysfunction after a rotational ankle injury. Recently, there has been an increased effort to define the use of computed tomography (CT) imaging in making this diagnosis; however, no clear consensus has been reached on the best way to assess the anatomical integrity of the syndesmosis. In this retrospective case series, we have evaluated the diagnostic capability of 2 novel CT-based measurements. Methods: Fourteen patients with symptomatic syndesmotic instability received a bilateral ankle CT scan. Two measurements were performed. The first measurement was the angle subtended by 2 lines drawn tangent to the anterior and posterior surfaces of the distal tibia and lateral malleolus 1 cm above the talar dome. The second measurement was the area bound by these lines, the lateral tibia and the medial aspect of the lateral malleolus. The injured and contralateral sides were compared using a Wilcoxon rank sum test with a significance set at P <= .05. Results: When comparing the injured to the contralateral ankle, we found a significant decrease in the angular measurement of the syndesmosis (63.4 +/- 6.1 degrees vs 68.4 +/- 6.6 degrees; P = .018) and a significant increase in the area measurement (1.71 +/- 0.44 cm(2) vs 1.21 +/- 0.25 cm(2); P = .00003). Eleven of 14 patients had a smaller angular measurement, and all 14 had a larger area measurement in the injured ankle. All patients were confirmed to have instability via manual testing intraoperatively. Conclusions: This study suggests that a comparison of angular and area measurements will help identify a diastasis of the syndesmosis in patients with persistent pain after a rotational ankle fracture. These measurements are straightforward to perform and rely on landmarks that are easy to identify. Level of Evidence: Level IV, diagnostic.