Analysis of the uninjured tibiofibular syndesmosis using conventional CT-imaging and axial force in different foot positions.

Background: Syndesmosis measurments and indices have been controversial and showed interindividual variability. The purpose of this study was to analyze, by conventional axial computed tomography images and a simulated load device, the uninjured tibiofibular syndesmosis under axial force and forced foot positions.

Methods: A total of 15 healthy patients (30 ankles) were studied using adjustable simulated load device (ASLD). This device allowed to perform bilateral ankle CT scans in two forced foot and ankle positions (30° of plantar flexion, 15° of inversion, 20° of internal rotation and 15° of dorsal flexion, 15° of eversion, 30° of external rotation). Axial load was applied simultaneously in a controlled manner (70% body weight). Measurements on the axial image of computed tomography were: syndesmotic area (SA), fibular rotation (FR), position of the fibula in the sagittal plane (FPS), depth of the incisura (ID) and direct anterior difference (ADD), direct middle difference (MDD) and direct posterior difference (PDD).

Results: In patients without injury to the tibiofibular syndesmosis, the application of axial load and forced foot and ankle positions showed statistically significant differences on the distal tibiofibular measurements between the stressed and the relaxed position, it also showed interindividual variability : SA (median = 4.12 [IQR = 2.42, 6.63]) (p < 0.001), ADD (0.67 [0.14, 0.67]) (p < 0.001), MDD(0.45, [0.05, 0.9]) (p < 0.001), PDD (0.73 [-0.05, 0.73]) (p < 0.002). However, it did not detect statistically significant differences when the tibiofibular differences between the stressed and the relaxed position in one ankle were compared with the contralateral side: SA (-0.14, SD = 4.33 [95% CI = -2.53, 2.26]), ADD (-0.42, 1.08 [-1.02, 0.18]), MDD (0.29, 0.54 [-0.01, 0.59]), PDD (-0.1, 1.42 [-0.89, 0.68]). Interobserver reliability showed an Intraclass correlation coefficient of 0.990 [95% CI = 0.972, 0.997].

Conclusions: Wide interindividual variability was observed in all syndesmotic measurements, but no statistically significant differences were found when comparing one ankle to the contralateral side. Measuring syndesmosis alignment parameters, may only be of value, if those are compared to the contralateral ankle.