Reverse deviation of the talus relative to the fibular malleolus under anterior ankle drawer stress

This study investigated talar displacement patterns and their relevance to chronic ankle instability (CAI). Forty-two participants (24 males, 18 females; mean age 20.6±1.2 years) with a history of ankle sprains were enrolled using the International Ankle Consortium criteria. An ultrasound imaging system was used to measure the distance between the lateral malleolus and talus under 130 N of anterior drawer stress. Muscle activity was monitored via electromyography to exclude reflexive contractions. Measurement reliability was high (intraclass correlation coefficient=0.94). Under stress, talar displacement ranged from 30.51 to -17.30 mm and from 15.65 to -10.44 mm (mean: 0.78±8.41 mm; 1.83±6.09 mm) in the anterior-posterior and superior-inferior directions, respectively. A subset of participants (21 in anterior-posterior direction, 15 in superior-inferior direction) exhibited talar displacement opposite to the applied stress. Among participants with CAI (9/42), similar paradoxical movement patterns were observed. In conclusion, talar displacement in the opposite direction of the applied stress suggests that CAI involves complex biomechanical interactions beyond simple anterior talofibular ligament dysfunction. Specifically, the interplay between ligament integrity and anterior stress transmission to the talus via the subtalar joint may produce three-dimensional talar motion. These findings highlight the importance of three-dimensional talar motion analysis in CAI assessment.