Cadaveric Diagnostic Study of Subtle Syndesmotic Instability Using a 3-Dimensional Weight-Bearing CT Distance Mapping Algorithm.

Background: The diagnosis of syndesmotic instability is challenging, and chronically unstable injuries can potentially lead to ankle arthritic degeneration. The objective of this cadaveric study was to utilize a 3-dimensional (3D) weight-bearing computed tomography (WBCT) distance mapping algorithm for the detection of subtle syndesmotic instability, induced by complete syndesmotic ligament sectioning and stressed by isolated axial load. We hypothesized that this algorithm would accurately detect subtle syndesmotic instability.

Methods: Nineteen matched pairs of through-the-knee cadaveric specimens (38 legs) were utilized. Specimens were mounted in a frame that allowed simulated axial weight-bearing (356 N). Specimens were scanned using cone-beam WBCT in the normal pre-injury state and after complete syndesmotic ligament sectioning. The deltoid ligament was kept intact, and no external rotational stress was applied. Syndesmotic incisura and lateral gutter distances were assessed and compared between pre-injury ipsilateral, contralateral, and injured states using a 3D WBCT distance mapping algorithm. The receiver operating characteristic (ROC) curve and area under the curve (AUC) were calculated for the comparison of syndesmotic distance measurements between injured specimens and controls. P values of <0.05 were considered significant.

Results: Overall, significantly increased distances were observed in injured specimens when compared with controls, with average relative syndesmotic widening in injured specimens of 16.9% (p = 0.0003), 11.3% (p = 0.0015), 6.4% (p = 0.0027), and 2.9% (p = 0.037) at the first 1, 3, 5, and 10 cm (proximal to the apex of the distal tibial articular surface), respectively. Widening was more pronounced in the anterior aspect of the syndesmosis, where the diagnostic accuracy was found to be highest at the first 1 and 3 cm of the syndesmotic incisura, with AUC values ranging from 80.9% to 83.0% (p < 0.0001) and with threshold diagnostic values of relative syndesmotic widening as low as 0.43 mm.

Conclusions: The newly proposed 3D WBCT distance mapping algorithm was able to accurately detect subtle syndesmotic instability in a cadaveric model of complete syndesmotic sectioning under isolated axial weight-bearing load. This algorithm needs to be further validated in patients with suspected traumatic syndesmotic instability.

Clinical Relevance: This cadaveric study demonstrated high diagnostic accuracy of a 3D WBCT distance mapping algorithm to detect subtle syndesmotic instability when stressed with isolated axial loading and in the absence of deltoid injury. The future use of this algorithm in patients with suspected unilateral traumatic syndesmotic instability could hopefully optimize the diagnosis and treatment decision-making.