Using 3D Volume Measurements on Weightbearing Computed Tomography Scan to Diagnose Syndesmotic Instability

CATEGORY: Ankle INTRODUCTION/PURPOSE: Diagnosing syndesmotic instability, especially when subtle, remains challenging. Weight bearing computed tomography (WBCT) offers a unique opportunity to evaluate the distal syndesmosis under physiologic load while simultaneously comparing the injured and uninjured side. We hypothesized that WBCT volumetric measurements of the distal syndesmosis were increased on the injured side as compared to the contralateral uninjured side among patients with syndesmotic instability. Our secondary hypothesis was that these 3-dimensional calculations were an even more sensitive determinant of instability as compared to 2-dimensional methodology. METHODS: Twelve patients with unilateral syndesmotic instability requiring operative fixation who underwent preoperative bilateral foot and ankle WBCT were included in the study group. The control group comprised of 24 patients without ankle injury who underwent similar imaging. For each WBCT scan, 2-dimensional measurements of the interspace between the distal fibula and tibia were measured 1cm above the joint line in the axial plane, namely the syndesmosis area and the direct anterior, middle and posterior difference. Furthermore, three volumetric measurements of the interspace between the distal fibula and tibia were evaluated: 1) from the tibial plafond extending until 3cm proximally, 2) 5cm proximally, and 3) 10cm proximally from the joint line. RESULTS: In patients with unilateral syndesmotic instability, all weightbearing volumetric measurements were significantly larger on the injured side as compared to the contralateral, uninjured side (p<0.001). In the control group, there was no difference between syndesmotic volumes at any level. Of the three anatomic reference points, the volumetric measurement spanning from the tibial plafond to a level 5cm proximally had the highest relative volumetric ratio between the injured and uninjured side, suggesting it is more sensitive in distinguishing stable and unstable syndesmotic injuries (median ratios (IQR) 1.3(1.2-1.4), 1.8(1.6-2.1), 1.4(1.3-1.5), respectively; p<0.001). Additionally, this relative volumetric ratio was also more sensitive than 2-dimensional measurements (p=0.001). CONCLUSION: Volumetric measurement of the distal tibiofibular interspace using WBCT appears to be the most effective way to diagnose syndesmotic instability. The measurement from the tibial plafond extending until 5cm proximally is more sensitive to detect syndesmotic instability than using either more traditional 2D WBCT syndesmosis measurements or using more distal (3cm) or proximal (10cm) 3D volumetric measurements. This does not seem surprising given the overall spectrum and 3D nature of the syndesmotic injury across the injured population. TABLES: