Plantar Pressure Analysis and the Ankle Instability Index: Quantifying “Giving Way” in Functional Ankle Instability

Background: Individuals with functional ankle instability (FAI) typically present with abnormal plantar pressure distribution, while “giving way” is the most significant symptom. This study aims to explore the relationship between ankle instability and the deviation of the center of pressure (COP) trajectory during stance, which could potentially serve as an objective parameter for quantifying the giving way and identifying FAI. Methods: A total of 243 participants (20.3±1.1 years) were categorized into FAI group and the Coper group based on ankle stability status and the presence of giving way. Plantar pressure analysis was conducted to measure the maximum medial-lateral deviation of the COP during the forefoot contact phase and foot flat phase, which was defined as the Ankle Instability Index (AII). The difference in AII between the 2 groups was assessed using an independent-sample t test. The relationship between AII and self-reported ankle instability was explored, and a discriminant function analysis was performed to determine the optimal cut-off value of AII for identifying FAI, subsequently the diagnostic accuracy was explored. Results: A significant difference in AII was observed between the 2 groups (FAI: 18.06±4.82, Coper: 9.13±3.82, P < .001), and a significant correlation was found between AII and the scores of the Cumberland Ankle Instability Tool (CAIT) and Identification of Functional Ankle Instability (IdFAI) ( r = −0.927 and r = 0.976, respectively, P < .001). AII exhibited a robust diagnostic value for FAI, with an area under the receiver operating characteristic curve of 0.931. The optimal threshold for AII in identifying FAI was 11.4, yielding an overall diagnostic accuracy of 91.99%. Conclusion: The findings revealed a robust correlation between the severity of ankle instability and AII, which is an effective parameter for quantifying giving way and ankle stability status. Level of Evidence: Level III, retrospective case-control.