Can Subtle Syndesmotic Injury Be Assessed Using Weightbearing CT Scans?

Abstract

Category: Sports Introduction/Purpose: Between 1-18% of all ankle sprains and 23% of all ankle fractures involve injury to the distal tibio-fibular syndesmosis. Syndesmotic injuries can create a substantial diagnostic and therapeutic challenge for orthopaedic surgeons. While acute injuries can be assessed using conventional radiographs, subtle syndesmotic injuries may be misdiagnosed using X-rays. Misdiagnoses may result in chronic ankle instability, pain and post-traumatic osteoarthritis of the tibio-talar joint. Recently, weight-bearing computed tomography (CT) scans gained popularity with foot and ankle surgeons. This method is advantageous in that the distal tibio-fibular syndesmosis can be assessed in greater detail and under weight-bearing conditions. However, there are no studies investigating weight-bearing CT scans for assessment of subtle syndesmotic injury. Methods: Five pairs of cadavers (tibia plateau to toe-tip, mean 61 years, range 52-70 years) were scanned with weight-bearing CT (170 lb) including external rotational torque (10 Nm). The following conditions were tested: First, intact ankles (Native) were tested. Second, one specimen from each pair underwent AITFL resection, while the contralateral underwent deltoid resection (Condition 1). Third, the remaining intact deltoid ligament or AITFL was resected in each ankle (Condition 2). Finally, the interosseous membrane (IOM) was resected in all ankles (Condition 3). Using coronal CT images, the width between the anterior tibia and fibula (A), distance between the anterior tibial incisura and anterior fibula (F), the tibio-fibular overlap (TFO), and the angle between the medial malleolus and the longitudinal axis of the fibula were assessed (a). Statistical analysis was performed using paired (comparison within groups) and unpaired (comparison between groups) t-test where p=0.05 was considered significant. Results: Regarding measurement A, a significant difference (p=0.046) was observed between Condition 2 and 3 vs. Native, independent of which ligament was dissected first. Measurement F was significantly different between Condition 2 and 3 vs. Native (p=0.011) if the AITFL was dissected first, but only reached significance for Condition 3 vs. Native if the deltoid ligament was dissected first (p=0.007). The TFO and a were significant in Conditions 1, 2, and 3 vs. Native if the deltoid ligament was dissected first (p=0.050). When the AITFL was dissected first, significance was reached for the same conditions (p=0.046) with the exception of the TFO for Condition 2 vs. Native. No differences were found when comparing the conditions of the AITFL with the same conditions of the deltoid group. Conclusion: With weight-bearing CT scans and applied external rotation torque, the TFO and angle between the tibia and fibula (a) may be used to assess subtle syndesmotic injury to either the AITFL or the deltoid ligament. When both ligaments (AITFL and deltoid) were injured, the tibio-fibular width (A) and distance between the anterior part of the tibia and fibula (F) could also be used for assessment. Weight-bearing CT scans cannot be used to distinguish between injuries to the AITFL or deltoid ligament. Further studies are needed to assess weight-bearing CT scans in the clinical setting.