Defining Reference Values for the Anatomical Axis of Syndesmosis and Landmark for the Clamp Placement to Minimize Malreduction

CATEGORY: Ankle; Sports; Trauma INTRODUCTION/PURPOSE: Malreduction of the fibula within the incisura is often caused by an eccentric clamp or screw placement and short fibular length. Weightbearing computed tomography (WBCT) has proven to be a reliable method to diagnose syndesmotic instability and can be used as a template to determine the syndesmotic axis and optimal position to place the clamp. We aimed to determine the anatomic axis of the syndesmosis, or the trans-syndesmotic angle (TSA) in healthy individuals on WBCT, and to see if side-to-side and gender based variations exist. We also aimed to determine the clamp's medial tine placement along the trans-syndesmotic axis. METHODS: The study group was made up of patient population without ankle injury who underwent bilateral foot and ankle WBCT imaging (n = 100; 200 ankles). Measurements on bilateral WBCT images included: 1) TSA at 1cm, 2cm, and 3cm proximal to tibial plafond, respectively; 2) Medial tine of the clamp positioning at 1cm and 2 cm along the syndesmotic axis. The medial tine clamp position was described in terms of the percentage of anterior to posterior tibial diameter from the anterior cortical boundary. In addition, the aforementioned TSA measurements were compared to historically defined 30 degrees of syndesmotic axis. Paired t-test was used to compare side to side and gender based differences. A p-value < 0.05 was considered statistically significant. RESULTS: In the uninjured healthy population, the mean trans-syndesmotic angles were 17.60, 21.60, and 24.10 at 1cm, 2cm, and 3cm proximal to the tibial plafond respectively. The clamp's medial tine should be positioned 24.7% and 21.3% of the AP tibial cortical distance, posterior to the anterior tibial cortex at 1cm and 2 cm proximal to the tibial plafond. There was no significant side to side or gender based differences for any of the measurements. Additionally, all three weightbearing TSA measurements were significantly larger than the historically defined syndesmotic angle of 30 degrees (p<0.001). CONCLUSION: Preoperative WBCT imaging provides a reliable template to determine TSA and to plan optimal clamp tine positioning along the syndesmotic axis. Our study has established normal ranges for cross-sectional syndesmotic axis measurements during weight-bearing and established that no differences exist between laterality and gender in patients without syndesmotic injury.