2024 J. Leonard Goldner Award Winner: Semi-Automated 3D Distance Mapping of Conventional Non-Weightbearing CT Scan with External Rotation Stress Demonstrates High Diagnostic Accuracy for Subtle Syndesmotic Instability

Category: Ankle; Sports Introduction/Purpose: Subtle syndesmotic instabilities are often under-diagnosed and pose long-term implications for ankle joint health. Previous studies have assessed the diagnostic accuracy of conventional Computed Tomography (CT) in patients with syndesmotic instability. However, these measurements were obtained manually and only at 1 cm proximal to the tibiotalar joint. To fully leverage the three-dimensional (3D) advantages of CT imaging, automated 3D distance mapping algorithms can provide a more comprehensive analysis. Further, while previous studies have demonstrated the sensitivity of these algorithms when analyzing syndesmotic instability using weightbearing CT (WBCT), many clinics do not have access to WBCT. Therefore, the aim of this study was to develop a distance mapping algorithm and scan protocol that can detect subtle syndesmotic instability using the more accessible imaging modality, conventional CT. Methods: Forty-five patients with rotational ankle injuries (21 syndesmotic injuries and 24 lateral collateral ligament injuries) diagnosed via radiologist MRI read, were analyzed. Each patient underwent three ankle CT protocols: neutral position; 45° external rotation with dorsiflexion and extended knees (Stress B); and 45° external rotation with dorsiflexion and flexed knees (Stress C). Semi-automatic CT scan segmentation was performed using a commercially available software, and syndesmosis volume and distances were calculated using MATLAB (Figure 1A). Distance was defined as the normal distance between the tibial subchondral surface and its opposing surface on the fibula or talus. Medial and lateral talar gutters were also studied. The syndesmosis was sectioned at 1 cm, 3 cm, and 5 cm proximal to the tibiotalar joint, and into anterior and posterior regions (Figure 1B). Receiver-operating characteristic (ROC) curves and area under the curve (AUC) were calculated to compare measurements and assess diagnostic accuracy (α = 0.05). Results: Significant differences were observed between syndesmotic and lateral collateral ligament injuries at 1 cm, 3 cm, and 5 cm proximal to the tibiotalar joint under stressed positions. The highest diagnostic accuracy was noted at 1 cm and 3 cm within the syndesmotic incisura, with AUC values of 0.91 for Stress B at 1 cm (Figure 1C), and 0.92 for Stress B and C at 3 cm (Figure 1D). In the anterior syndesmotic incisura, AUCs were 0.83 for Stress B and 0.89 for Stress C at 1 cm (Figure 1E), and 0.85 for Stress B and 0.93 for Stress C at 3 cm (Figure 1F). AUCs of 0.90 for Stress B and 0.83 for Stress C were observed in the posterior gutter medial to the talus. Conclusion: This proposed automated 3D distance mapping CT algorithm, enhanced by external rotational ankle stress maneuvers, demonstrates high diagnostic accuracy in detecting subtle syndesmotic instability, distinguishing well between syndesmotic and lateral collateral ligament injuries. A significant posterior widening at the medial gutter in the syndesmotic injury group may implicate concomitant deltoid ligament injury. Most importantly, however, this study suggests that weightbearing CT may not be essential for diagnosing subtle syndesmotic instabilities and that conventional CT imaging and external rotational stress can allow for high diagnostic accuracy of these injuries. Further validation of this algorithm is warranted to confirm its clinical utility.