Design of an Ankle Prosthesis for Two-Degree-of-Freedom Motion in the Sagittal and Coronal Planes

The ability to get up and walk from one place to another under one’s own power epitomizes a level of independence and the perception of a good quality of life. Loss of this ability can directly affect one’s level of physical fitness and thus critically affect health and well-being. This ability to independently walk can be lost due to many causes including lower leg amputation. This leads to the need for an ankle prosthesis to aid the user in performing gait. Most current commercial ankle prostheses available for below-knee amputees allow for (limited) dorsiflexion and plantarflexion at the ankle. Therefore, the primary objective of this project was to develop an ankle prosthesis which can passively accommodate two degrees of freedom: motion in the sagittal plane (dorsiflexion and plantarflexion) and motion in the coronal plane (inversion and eversion). Two design iterations were done to accomplish this goal. The second design or Design 2 utilizes three torsion springs to passively allow for bi-planar motion in the sagittal and coronal planes. The range of motion for the ankle prosthesis was based on the range of motion of the ankle during normal gait and also accounts for the maximum grade and cross-slope of rural local streets. The Design 2 ankle prosthesis offers slight variability for motion in the sagittal plane by varying the stiffness of the ankle prosthesis for dorsiflexion and plantarflexion. With the exception of the latter part of the pre-swing phase, the theoretical behavior of the ankle prosthesis qualitatively replicates that of natural ankle motion in the sagittal plane found in literature. It also provides sufficient lateral stability during normal gait. The ankle prosthesis which was designed for this project can passively accommodate motion in both the sagittal and coronal planes as well as provide some degree of “push-off” during normal gait. Advisor: Carl A. Nelso