Differences between mechanically stable and unstable chronic ankle instability subgroups when examined by arthrometer and FAAM-G

Background: The objective measurement of the mechanical component and its role in chronic ankle instability is still a matter of scientific debate. We analyzed known group and diagnostic validity of our ankle arthrometer. Additionally, functional aspects of chronic ankle instability were evaluated in relation to anterior talar drawer. Methods: By manual stress testing, 41 functionally unstable ankles were divided as mechanically stable (n = 15) or mechanically unstable (n = 26). Ankle laxity was quantified using an ankle arthrometer. Stiffness values from the load displacement curves were calculated between 40 and 60 N. Known group validity and eta(2) were established by comparing manual and arthrometer testing results. Diagnostic validity for the ankle arthrometer was determined by a 2 x 2 contingency table. The functional ankle instability severity was quantified by the German version of the Foot and Ankle Ability Measure (FAAM-G). Stiffness (40-60 N) and FAAM-G values were correlated. Results: Mechanically unstable ankles had lower 40-60 N stiffness values than mechanically stable ankles (p = 0.006 and <0.001). Eta for the relation between manual and arthrometer anterior talar drawer testing was 0.628. With 5.1 N/mm as cut-off value, accuracy, sensitivity, and specificity were 85%, 81%, and 93%, respectively. The correlation between individual 40-60 N arthrometer stiffness values and FAAM-G scores was r = 0.286 and 0.316 (p = 0.07 and 0.04). Conclusions: In this investigation, the ankle arthrometer demonstrated a high diagnostic validity for the determination of mechanical ankle instability. A clear interaction between mechanical (ankle arthrometer) and functional (FAAM-G) measures could not be demonstrated.