Characterizing the mechanical properties of low-profile prosthetic feet

Passive-elastic prosthetic feet are manufactured with different numerical stiffness categories that are prescribed based on the body mass and activity level of the user, but the mechanical properties, such as the stiffness values and hysteresis are not typically reported by the manufacturer. Since the mechanical properties of passive-elastic prosthetic feet can affect the walking biomechanics of people with transtibial or transfemoral amputation, characterizing these properties would provide objective values for comparison and aid the prescription of prosthetic feet. Therefore, we characterized the axial stiffness values, torsional stiffness values, and hysteresis of 33 different categories and sizes of a commercially available passive-elastic prosthetic foot model, the Ossur low-profile (LP) Vari-flex with and without a shoe. We measured axial stiffness from compression and torsional stiffness from dorsiflexing and plantarflexing the prostheses. In general, a greater numerical prosthetic foot stiffness category resulted in increased heel, midfoot, and forefoot axial stiffness values, increased plantarflexion and dorsiflexion torsional stiffness values, and decreased heel, midfoot, and forefoot hysteresis. Moreover, a greater prosthetic foot size resulted in decreased heel, midfoot, and forefoot axial stiffness values, increased plantarflexion and dorsiflexion torsional stiffness values, and decreased heel and midfoot hysteresis. Finally, adding a shoe to the LP Vari-flex prosthetic foot resulted in decreased heel and midfoot axial stiffness values, decreased plantarflexion torsional stiffness values, and increased heel, midfoot, and forefoot hysteresis. In addition, we found that the force-displacement and torque-angle relationships were better described by curvilinear than linear equations. Ultimately, our results can be used to objectively compare LP Vari-flex prosthetic feet to other prosthetic feet in order to inform their prescription and design and use by people with a transtibial or transfemoral amputation.