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Category: Ankle, Hindfoot Introduction/Purpose: Restoration of bilateral symmetry is used clinically to evaluate surgical and conservative treatment outcomes. However, the degree of symmetry and differences between sexes in ankle kinematics in healthy individuals remain unknown. Because relative motion between the tibia, talus and calcaneus cannot be accurately measured using conventional skin- mounted motion capture systems, biplane radiography is emerging as the preferred technique to measure in vivo ankle kinematics during functional activities. Therefore, the aims of the present study were to use biplane radiography to determine the degree of bilateral symmetry in ankle kinematics in healthy individuals and to identify sex-dependent differences in kinematics during the support phase of gait. It was hypothesized that rotational ankle range of motion (ROM) during gait is not different between males and females. Methods: Twenty healthy individuals (10 male, 10 female, age 30.7 ± 6.3years) with no history of ankle injury provided consent to participate in this IRB-approved study. Each participant walked through a biplane radiography system 6 times at a self-selected pace (1.3±0.2 m/s). Synchronized radiographs of the ankle were collected at 100 images/second for 3 trials of each ankle (90 kV, 125 mA, 1 ms exposure/image). Motion of the tibia, talus and calcaneus was tracked using a validated model-based tracking process that matches 3D bone models to the radiographs. Anatomic coordinate systems were created and used to calculate ankle kinematics. All kinematics were converted to percent stance phase and averaged over all trials for each ankle. Bilateral symmetry was determined by calculating the average absolute difference between right and left ankle joint kinematics over the full support phase of gait. Differences between male and female rotational ROM were identified using unpaired t-tests. Results: The average absolute side-to-side difference in tibio-talar joint rotations was 3.3° or less, while the average absolute side-to-side difference in subtalar joint rotations was 3.0° or less (Table 1A). For males and females, at the tibio-talar joint, the largest ROM was plantar-dorsiflexion, followed by internal/external rotation and then inversion/eversion (Table 1B). At the subalar joint, the largest ROM was inversion/eversion, with similar amounts of dorsiflexion/plantarflexion and internal/external rotation, on the order of 2° to 3°. Males demonstrated significantly less ROM in subtalar dorsiflexion/plantarflexion and tibio-talar internal/external rotation (Table 1B). Conclusion: The average side-to-side differences in healthy ankle ROM during gait are small, suggesting that the contralateral ankle may serve as a reference standard to assess kinematic outcomes after conservative or surgical treatments. The difference between male and female subtalar ROM (0.6°) may be too small to be functionally significant, however, sex differences in tibio- talar ROM appear large enough to merit consideration when assessing functional outcomes and designing ankle joint replacements. The results are limited to over-ground gait performed by relatively young and healthy adults and may not be applicable to other activities or older adults.