Bilateral External Rotation Torque CT Remains Reliable in Syndesmotic Assessment Despite Lateral Ligament Injury, but Reveals Increased Tibiotalar Rotational Instability

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Abstract
Background: Understanding syndesmotic injury in conjunction with lateral ligament disruption is essential for accurate diagnosis and treatment. Although the influence of deltoid ligament injury on syndesmotic evaluation during bilateral external torque computed tomography (BET-CT) has been examined, the effect of concurrent lateral ligament lesion remains unclear and is rarely examined in the literature. This leads to the question of whether a lateral ligament lesion influences the previously established measurement parameters of syndesmosis instability in BET-CT and whether it results in a quantifiable increase in tibiotalar instability. Methods: Seven paired cadaveric lower limbs (n = 14; mean age 78.6 years) with pre-existing iatrogenic syndesmotic transection underwent sequential lateral ligament sectioning: anterior talofibular (ATFL), calcaneofibular (CFL), and posterior talofibular ligaments (PTFL). BET-CT imaging was performed under applied external rotation torques of 0, 2.5, 5.0, and 7.5 Nm. Outcome measurements included the anterior tibiofibular distance, tibiofibular clear space, posterior tibiofibular distance, medial tibiotalar gutter angle, and lateral fibulotalar gutter angle for each ligament condition and torque level. Results: The anterior tibiofibular distance remained stable despite progressive lateral ligament dissection and increasing torque ( P > .05). Tibiofibular clear space and posterior distance measurements showed inconsistent trends and did not reach statistical significance. In contrast, the medial tibiotalar gutter angle increased significantly from 22.9° in isolated syndesmotic injury to 27.6° (±6.3°) following complete disruption of the lateral complex ( P < .001). Conclusion: Concurrent lateral ligament injury does not impair the assessment of syndesmotic widening in BET-CT, as tibiofibular alignment remains preserved. However, disruption of the lateral complex exacerbates rotational tibiotalar instability. Clinical Relevance: In this cadaveric model, BET-CT remains reliable for assessing syndesmotic lesions even in the presence of lateral ligament injury. However, the increase in rotational tibiotalar instability with combined injuries highlights the need for comprehensive evaluation of ankle ligament injuries, pending clinical validation.

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