Design of a prosthetic dynamic response foot with adjustable stiffness

Abstract

Lower limb loss due to diabetes or vehicle accidents is a harsh reality for many South Africans. Most lower limb amputations require a prosthetic foot to assist the individual in returning to their lifestyle, yet prosthetics are expensive and not affordable for the average South African. A prosthetist fits an amputee with a prosthetic foot that accommodates their weight and activity level. Prosthetic feet that are too flexible or too stiff can have a noticeable limp effect and discomfort when the user walks. The aim of this study, therefore, was to design a prosthetic foot with an adjustable stiffness, assisting with different activities and body weights. Multiple design options were evaluated based on their complexity and manufacturing cost, with the chosen design incorporating three composite blades and gas springs to adjust the foot stiffness. Detailed design and calculations were completed and verified through finite element analysis, for a standard adult weight of 80 kg (800 N) with a specified deflection of 23.6 mm of the forefoot and 13.6 mm of the heel. An MTS Universal testing machine was used for experimental testing to validate the modelling and simulation methodology. The prosthetic foot was attached to a jig that would allow for forefoot testing (with and without gas springs), as well as heel testing. The experiments were found to be repeatable, and in general the results of the measured deflections were comparable with the calculated estimations. The largest deviations in the forefoot blade deflection was 2.4% at 16 N, and for the heel blade it was 14.6% at 45.56 N. The addition of the gas springs resulted in an increase in the reaction force to affect foot displacement. When two 150 N gas springs were attached, deflection initiated at a force of approximately 200 N, two 350 N springs required 450 N, and the last set of 500 N springs required 650 N. The measurements with the gas springs attached saw an increase in deviation between the measurements and calculations, from 3% without the gas springs and up to 8.5% when the gas springs were attached. The results indicated that adjustment of the stiffness of the foot performed as intended. However, because the adjustments are made with discrete levels in gas force resistance, the approach might be better suited to adapting to different body weights rather than different activities. A high level cost evaluation using realistic manufacturing and certification costs was done to compare the estimated cost of the prosthetic foot to that of commercial feet and estimated a cost of R17 000 for the prosthetic foot, with the possibility of lowering the cost when mass producing. This cost was about R4 000 lower than the lowest available compensation option from government sources.