The influence of talus size and shape on in vivo talocrural hopping kinematics

Talus implants often come in standard sizes and shapes; however, humans vary in their bone size and shape. Consequently, patient-specific implants are becoming available. Understanding how shape changes alter function in a healthy cohort may help designers determine how much specificity is required in talocrural implants. Nine participants (5 females) hopped on one leg while biplanar video radiography and force plate data were collected. 3D bone models were created from computed tomography scans. Helical axes of motion were calculated for the talus relative to the tibia (rotation axes) and a cylinder was fit through the talar dome (morphological axis). Bland-Altman plots and spatial angles tested whether the rotation and morphological axes agree. A shape model of 36 (15 females) participants was established and a cylinder fit was morphed through the range of {+/-}3 standard deviations. The rotation and morphological axes largely agree regarding their orientation and location during hopping. The morphological axis consistently overestimates the orientation-component in anterior-posterior direction. Some shape components affect talar dome orientation and curvature independent of size. This suggests that besides size, the shape of the talar dome might affect the movement pattern during locomotion. Our findings are important to inform talocrural joint arthroplasty design.