Three-dimensional in vivo kinematics of the subtalar joint during weight-bearing standing on coronal-wedge surfaces using upright computed tomography

BACKGROUND: The human subtalar joint is crucial for bipedal posture and locomotion, providing stability, adaptability, and force dissipation. While its kinematics have been studied, most research has been under non-weight-bearing conditions, leaving its function under physiological loading unclear. RESEARCH QUESTION: What is the three-dimensional kinematics of the human subtalar joint during weight-bearing postures, as quantified by the helical axis? METHODS: This study analyzed three-dimensional in vivo subtalar joint kinematics during weight-bearing using upright CT. Fifteen healthy elderly adults (11 females, 4 males; mean age: 64.9 ± 5.0 years) underwent CT scans in neutral, inverted, and everted postures using 10° coronal wedges. Kinematics were quantified via helical axis representation relative to two talar coordinate systems: the foot longitudinal axis and the talus principal axis. RESULTS: The helical axis ran obliquely from anterior-medial-dorsal to posterior-lateral-plantar, with inclination and deviation angles of 22.4° ± 9.6° and 35.2° ± 5.6° in the former system, and 36.5° ± 10.7° and 12.0° ± 7.1° in the latter. Rotation along the axis was 9.9° ± 3.8°, with minimal translation (0.5 ± 0.5 mm). SIGNIFICANCE: These findings suggest subtalar joint movement is more constrained under weight-bearing conditions than in non-weight-bearing studies. This study underscores the importance of weight-bearing analyses for clinical applications and understanding human foot adaptations for bipedalism.