Paper 94: Diagnostic Accuracy of Weightbearing CT in Detecting Chronic Subtle Syndesmotic Instability: A Prospective Comparative Study

Abstract

Objectives:Subtle syndesmotic instability (SSI) is a difficult albeit important diagnosis to make, as undiagnosed instability frequently leads to posttraumatic ankle arthritis. Stressed conventional radiographs, ultrasonography, bilateral comparative conventional computed tomography (CT) and magnetic resonance imaging (MRI) serve as important diagnostic tools, however, the current diagnostic gold-standard is arthroscopic assessment, an invasive surgical method. The advent of weight-bearing computed tomography (WBCT) is considered to be an important additional diagnostic tool for improved non-invasive SSI diagnosis, particularly by utilizing distal tibiofibular syndesmotic (DTFS) area and volume measurements. However, to date, no studies have assessed WBCT diagnostic accuracy for chronic SSI. The purpose of this study was to prospectively evaluate the diagnostic accuracy of WBCT area and volumetric measurements in patients with suspected chronic SSI, when compared to the gold-standard arthroscopic assessment.Methods:In this IRB-approved prospective comparative study, 11 patients with suspected SSI were enrolled from July 2019 to December 2020. Patients were assessed preoperatively by bilateral standing WBCT. Raw 3D WBCT was automatically segmented by dedicated software. WBCT measurements included semi-automatic DTFS area (1cm proximally to tibial dome apex) and DTFS volumes (1, 3 and 5cm proximally to tibial dome apex). Threshold values for WBCT abnormality were defined based on currently available data (area>105mm2 and volumes>796mm3, >3062 mm3, and >6733 mm3 for 1, 3 and 5cm, respectively). Subjects underwent surgical treatment including DTFS instability arthroscopic assessment, defined as positive when a 3mm diameter sphere could enter the syndesmotic incisura. Confirmed unstable cases were treated with open reduction/internal fixation. WBCT measurements sensitivity, specificity, positive and negative predictive values (PPV/NPV) and accuracy were calculated using confirmed arthroscopic instability as diagnostic gold standard. Paired t-tests/Wilcoxon analysis was used to compare measurements. P-values<0.05 were considered significant.Results:When compared to non-injured sides, DTFS area and volumes were significantly higher in injured ankles at 1cm (667 vs 554mm3) and 3cm (2331 vs 2038mm3). Medial gutter volumes were also increased in injured sides (398 vs 370 mm3). DTFS volumes at 5cm and lateral gutter volumes were not different. Nine of eleven patients had confirmed arthroscopic DTFS instability. Considering WBCT area measurements, 4/11 patients were found to be positive (>105mm2), including 3 true positives (+WBCT/+Arthroscopy), 1 false positive (+WBCT/-Arthroscopy), 6 false negatives (-WBCT/+Arthroscopy), and 1 true negative (-WBCT/-Arthroscopy), leading to a 33.3% sensitivity, 50%, specificity, 75% PPV 75%, 14.3% NPV and 36% accuracy. When analysing WBCT DTFS volumes (1cm), 3/11 patients were found positive (>796mm3), depicting 3 true positives, 0 false positives, 2 true negatives and 6 false negatives, with resultant diagnostic accuracy of: 33.3% sensitivity, 100% specificity, 100% PPV, 25% NPV 25%, and 45% accuracy.Conclusions:This is the first study to prospectively assess the diagnostic accuracy of WBCT area and volume measurements in detecting chronic SSI and to compare it to the arthroscopic diagnostic gold standard. When compared to uninjured side, DTFS area and volumetric measurements were significantly increased in injured sides of patients with suspected SSI, including medial gutter volumes, consistent with associated deltoid ligament instability. However, interestingly, we observed a diagnostic accuracy for WBCT area and volumetric measurements to be lower than initially expected. Further incorporation of additional patients, as well as introduction of an external rotational stress can potentially optimize the WBCT diagnostic accuracy for chronic SSI.