| Summary: | Abstract Background Chronic ankle instability (CAI) arises from the two etiological factors of functional (FAI) and mechanical ankle instability (MAI). To distinguish the contributions of the two etiologies, it is necessary to quantitively assess functional and mechanical deficits. Validated and reproducible assessment of mechanical instability remains a challenge in current research and practice. Physical examination, stress sonography and a novel 3D stress MRI have been used, while stress radiography has been called into question and arthrometry is limited to research purposes. The interaction of these primarily mechanical measurements with the functional and subjective components of CAI are subject to debate. The aim of this study was the evaluation of the clinical and biomechanical preferences of the three different methods in the diagnosis of MAI. Methods In this cross-sectional diagnostic study, we compared three different diagnostic approaches to mechanical ankle instability: (1) manual stress testing (anterior drawer test [ADT] and talar tilt test [TTT]), (2) stress sonography and (3) 3D stress MRI (3SAM) The latter includes quantification of 3D cartilage contact area (CCA) in plantarflexion-supination compared to neutral-null position. We applied these measurements to a cohort of patients suffering from chronic mechanical ankle instability (n = 25) to a matched cohort of healthy controls (n = 25). Perceived instability was assessed using the Cumberland Ankle Instability Tool (CAIT) and Forgotten Joint Score (FJS). Functional deficits were measured using postural sway and the y-Balance test. Results Significant differences between the two groups (single-factor “group” ANOVA, p < 0.05) were found in all of the mechanical assessments with strong effect sizes. Spearman’s correlations were strong for CAIT and manual stress testing (TTT rho = − 0.83, ADT rho = − 0.81), 3D stress MRI (rho = − 0.53) and stress sonography (TTT rho = − 0.48, ADT rho = − 0.44). Furthermore, the correlation between manual stress testing and CCA in the fibulotalar articulation (CCAFT) was strong (rho = 0.54) and the correlations to stress sonography were moderate (ADT rho = 0.47 and TTT rho = 0.43). The calculation of cutoff values revealed a distance of > 5.4 mm increase in ligament length during stress sonography (sensitivity 0.92, specificity 0.6) and > 43% loss of articulating surface in the fibulotalar joint (CCAFT in supination-plantarflexion using 3SAM, sensitivity 0.71, specificity 0.8) as potential cutoff values for diagnosing MAI. Conclusions Manual stress testing showed to be a valuable method of identifying mechanical ankle instability. However, due to is subjective character it may overvalue patient-reported instability as a factor which explains the high correlation to the CAIT-score, but this may also reduce its value in diagnosing the isolated mechanical quality of the joint. Thus, there is a persisting need for objective and reproducible alternatives focusing on MAI. According to our results, 3D stress MRI and stress sonography represent valuable alternatives and may be used to quantitively assess mechanical ankle instability in research and practice. Trial registration German Registry of Clinical Trials # DRKS00016356 , registered on 05/11/2019.
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