Syndesmotic injuries occur in up to one-quarter of all ankle fractures. Despite mitigating efforts, malreduction of the syndesmosis is common after both rigid and flexible fixation methods, causing inferior patient function. Conventional assessments of syndesmotic reduction do not account for normal syndesmotic motion with ankle range-of-motion (ROM). The aims of this thesis were to use fourdimensional computed tomography (4DCT) to determine normal syndesmotic motion and to investigate the impact of rigid and flexible fixation on postoperative syndesmotic kinematics.

Fifty-eight uninjured ankles were imaged to quantify normal syndesmotic kinematics. Thirteen patients after rigid or flexible fixation underwent bilateral ankle 4DCT to evaluate postoperative syndesmotic kinematics. Measures of syndesmotic width including anterior, middle, and posterior syndesmosis distances as well as tibiofibular clear space and tibiofibular overlap were automatically extracted from 4DCT data. Sagittal translation and fibular rotation were also recorded. Linear mixed effects models were used to determine the position of the syndesmosis at neutral dorsiflexion as well as syndesmotic motion, defined as the change in syndesmotic measurements with ankle ROM.

In uninjured ankles, various measures of syndesmotic width decreased by 0.7-1.1 mm as ankles moved from dorsiflexion to plantarflexion (p < 0.001). The fibula externally rotated by 1.2° with plantarflexion (p < 0.001). There was no significant motion in the sagittal plane (p = 0.43). Rigid fixation increased syndesmotic width compared to uninjured ankles when measured by middle syndesmotic distance and tibiofibular clear space only (p = 0.039 and 0.032 respectively). Rigid fixation demonstrated reduced motion compared to uninjured ankles in middle and posterior syndesmotic distance, tibiofibular clear space, and tibiofibular overlap (p < 0.01). There were no differences in syndesmotic position or motion between flexible fixation and uninjured ankles.

Ankle plantarflexion leads to decreased syndesmotic width and fibular external rotation in uninjured ankles, indicating ankle position must be accounted for when performing syndesmotic imaging and fixation. Flexible fixation better restores syndesmotic position and motion compared to rigid fixation. These findings may be used to decrease the rate of syndesmotic malreduction and, consequently, improve post-surgical outcomes.