A Passive Three Degree of Freedom Transtibial Prosthesis With Adjustable Coronal Compliance and Independent Toe Joint

The effects of including lateral compliance or a toe joint in transtibial prostheses have been studied independently, showing the potential to improve the gait biomechanics in terms of stability, walking speed, and metabolic cost. However, both of these features are not commonly found in commercial prostheses despite their importance in human gait. In this work, we present a multi-axis passive transtibial prosthesis with three degrees of freedom (DOF). The prosthesis includes a compliant and adjustable coronal articulation using beam springs, an independent 3D-printed flexible forefoot as a toe joint, and sagittal dorsiflexion-plantarflexion stiffness using a helicoidal spring. We mechanically characterize each degree of freedom in terms of the provided stiffness. The measured stiffness values were 3.26Nm/deg, 4.94, or 5.63 Nm/deg in the sagittal plane (with different springs), and 2.54 Nm/deg, 2.79 Nm/deg, 2.94 Nm/deg, or 3.72 Nm/deg in the coronal plane (by adjusting the mechanism). Finally, the effect of different types of infill and infill levels for the 3D printed toes were explored, showing stiffness varying from 2.05 N/mm to 350 N/mm. The obtained sagittal stiffness is beneath the ones found in able-bodied persons; in contrast, the lateral stiffness values are comparatively higher than that of the able-bodied persons. However, the current design is simple to rearrange and modify the stiffness values. Lastly, the wide range of stiffness achievable in the 3D printed toes can be useful to achieve proper torque requirements in the forefoot for a broad range of users.