Diagnostic Accuracy of Conventional Ankle CT Scan With External Rotation and Dorsiflexion: Letter to the Editor

Dear Editor: We have read with interest the article published by Rodrigues et al, ‘‘Diagnostic Accuracy of Conventional Ankle CT Scan With External Rotation and Dorsiflexion in Patients With Acute Isolated Syndesmotic Instability,’’ in which the authors analyze the accuracy of computed tomography (CT) scans of the foot and ankle in forced positioning to diagnose subtle tibiofibular syndesmosis instability without an associated fracture. In keeping with our research, which shares similarities with this methodology, we would like to comment on a number of connotations. First, performing an accuracy diagnostic study in the context of a pathology that cannot be confirmed without invasive methods may affect the results. In the published study, patients were classified as stable or unstable according to the ligamentous involvement shown on magnetic resonance image (MRI) and the classification of Sikka et al. However, the relationship between injured ligaments and instability is not proven. That is, there is no consensus on what is the minimum ligament incompetence that triggers instability. Some authors claim that isolated injury to the anteroinferior tibiofibular ligament (AiTFL) is sufficient to produce kinematic changes in the distal tibiofibular joint. Others, however, suggest that incompetence of several ligaments is necessary to trigger syndesmotic instability, which may be any of the following combinations: AiTFL and deltoid ligament; AiTFL and interosseous ligament (IOL) in addition to deltoid ligament; or AiTFL, IOL, posteroinferior tibiofibular ligament (PiTFL) and deltoid ligament. There are even studies claiming that isolated involvement of the deltoid ligament can trigger distal tibiofibular instability. In this context, the initial classification of patients as stable or unstable is based on an unproven theoretical inference and could impact the research results. Second, we believe that a device that maintains the position of the foot in external rotation during the scan, such as the adjustable simulated loading device (ASLD), will increase the reproducibility of the position of the ankles under study in the same patient and between patients. In the context of patients with suspected acute or subacute syndesmotic injury, relying on voluntary muscle contraction exerted by the patient under study to maintain the required position of the ankle joint may cause discrepancies in the position of both ankles of the same patient by an antalgic mechanism. In our study, in addition to using ASLD to maintain external rotation, we offered the possibility of performing a local anesthetic block in case of discomfort generated by the forced position of the affected ankle. This made it possible to maximize the stress applied to the syndesmosis during the test. Third, we believe that the application of an axial force that simulates weightbearing during the test could increase the diagnostic power. Our belief is based on studies showing differences in the diagnostic ability of this pathology when comparing weightbearing cone-beam CT with unloaded CT. Furthermore, in order to stress the tibiofibular ligaments, we believe that applying eversion to the foot could create a greater lateral displacement force on the fibula by everting the talus in the ankle mortise and enhancing the contact of the talus with the fibula. This method could enhance the lateral translation of the fibula and improve the diagnostic sensitivity of the ‘‘c’’ measurement in subtle instability of the syndesmosis. Fourth, we believe that it is convenient to highlight the need to standardize the cuts on which the measurements are taken. That is, the axial slice on which the measurement is taken should always be located at the same distance from the tibial plateau, and furthermore, the axial slice plane should be adjusted to be parallel to the tibial plafond. Otherwise, in an investigation such as the one presented here, in which the knee was flexed during the scan, the default shear plane given in the axial image will differ from that applied with the legs in extension. This could affect the accuracy of the measurement. Finally, although advanced visualization systems allow very accurate measurements to be made between bony structures analyzed by CT, the 1-mm threshold as a cutoff point for classification into stable and unstable could be challenging for evaluators untrained in making such measurements. In our opinion, the measurement of the syndesmotic area is more reliable when comparing subtle injuries to the ankle joint, as it has higher accuracy and reproducibility with less interobserver variability.