Diagnostic Accuracy of Conventional Ankle CT Scan With External Rotation and Dorsiflexion in Patients With Acute Isolated Syndesmotic Instability

Background: Syndesmotic injury in an athletic population is associated with a prolonged ankle disability after an ankle sprain and often requires a longer recovery than a lateral collateral ligament injury. Although several imaging tests are available, diagnosing syndesmotic instability remains challenging. Purpose: To determine the diagnostic accuracy of conventional ankle computed tomography (CT) scans with the joint in external rotation and dorsiflexion and compare it with that of conventional ankle CT scans in a neutral position. Study Design: Cohort study (Diagnosis); Level of evidence, 2. Methods: Between September 2018 and April 2021, this prospective study consecutively included adults visiting the foot and ankle outpatient clinic with a positive orthopaedic examination for acute syndesmotic injury. Participants underwent 3 CT scan tests. First, ankles were scanned in a neutral position. Second, ankles were scanned with 45 degrees of external rotation, dorsiflexion, and extended knees. Third, ankles were scanned with 45 degrees of external rotation, dorsiflexion, and flexed knees. Three measurements, comprising rotation (measurement a ), lateral translation (measurement c ), and anteroposterior translation (measurement f ) of the fibula concerning the tibia, were used to diagnose syndesmotic instability in the 3 CT scans. Magnetic resonance imaging was used as a reference standard. The area under the curve (AUC) was used to compare the diagnostic accuracy, and Youden's J index was calculated to determine the ideal cutoff point. Results: Images obtained in 68 participants (mean age, 36.5 years; range, 18-69 years) were analyzed, comprising 36 syndesmotic injuries and 32 lateral collateral ligament injuries. The best diagnostic accuracy occurred with the rotational measurement a , in which the second and third CT scans with stress maneuvers presented greater AUCs (0.97 and 0.99) than did the first CT scan in a neutral position (0.62). The ideal cutoff point for the stress maneuvers was 1.0 mm in the rotational measurement a and reached a sensitivity and specificity of 83% and 97% for the second CT scan with extended knees and 86% and 100% for the third CT scan with flexed knees, respectively. The ideal cutoff point for the first CT scan with a neutral position was 0.7 mm in the rotational measurement a , with a sensitivity of 25% and specificity of 97%. Conclusion: Conventional ankle CT with stress maneuvers has excellent performance for diagnosing subtle syndesmotic rotational instability, as it shows a greater AUC and enhanced sensitivity at the ideal cutoff point compared with ankle CT in the neutral position.