Kinematics
The kinematics of the subtalar joint involve intricate gliding movements that occur between three pairs of articular surfaces. These complex interactions produce a characteristic curved path of motion between the calcaneus and the talus. Although individual anatomical variations exist, the primary axis of rotation for this joint follows a unique trajectory that begins in the lateral-posterior region of the calcaneus and travels anteriorly, medially, and superiorly, passing through the subtalar joint. This axis forms specific angles with anatomical reference planes, measured at approximately 42 degrees to the horizontal plane and 16 degrees to the sagittal plane.
The inversion and eversion movements of the subtalar joint occur along an arc perpendicular to this axis of rotation. These are composite motions in which the calcaneus moves relative to the talus, or in weight-bearing scenarios, the talus moves relative to the calcaneus.
Due to the distinctive inclination of the axis of rotation, inversion and eversion motions consist of three primary components, with medial/lateral tilt and abduction/adduction being the most prominent. Consequently, inversion is primarily a combination of medial tilt and adduction, while eversion is mainly composed of lateral tilt and abduction.
Although the calcaneus can perform minor dorsiflexion and plantarflexion relative to the talus, the magnitude of these movements is minimal and is therefore typically not clinically significant. Overall, the kinematic properties of the subtalar joint are far more complex and extensive compared to those of the talocrural joint.
To facilitate understanding, the osteokinematics of the subtalar joint are often described in terms of calcaneal motion relative to a fixed talus. However, during actual gait, when weight-bearing restricts calcaneal movement, inversion and eversion of the subtalar joint are achieved through horizontal plane rotational movements of the talus and fibula.
The intrinsic stability and structural congruence provided by the subtalar joint’s interlocking surfaces enable horizontal plane rotation of the talus to be mechanically linked with tibial rotation. Additionally, the small range of accessory horizontal plane motions at the talocrural joint effectively accommodates these subtle rotational components.
Range of Motion
In a comprehensive study by Grimston and colleagues, the active range of motion (ROM) for medial and lateral tilt at the subtalar joint was analyzed in 120 individuals aged 9 to 79 years. The study reported that the ROM for medial tilt measured 22.6 degrees, nearly double the ROM for lateral tilt, which was 12.5 degrees.
Although this measurement included minor rotational movements occurring at the talocrural joint, the tendency for greater medial tilt compared to lateral tilt aligns with the findings of other studies that exclusively focused on the subtalar joint.
Furthermore, studies measuring passive ROM found that the ratio of medial to lateral tilt could reach as high as 3:1, indicating an even greater disparity in motion ranges. Despite these differences between active and passive ROM, the lateral tilt motion is naturally limited due to the distal position of the lateral malleolus and the thicker structure of the deltoid ligament.