Design and evaluation of a cantilever beam-type prosthetic foot for Indian persons with amputations

Abstract

The goal of this work is to design a low cost, high performance prosthetic foot in collaboration with Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS), in Jaipur, India. In order to be adopted, the foot must cost less than $10 USD, be mass-manufacturable, and meet or exceed the performance of the Jaipur Foot, BMVSS' current prosthetic foot. This thesis investigates different metrics that are used to design and evaluate prosthetic feet and presents an analysis and evaluation of a solid ankle, cantilever beam - type prosthetic foot. Methods of comparing prosthetic feet in industry and in academia are discussed using a review of literature. These comparisons can be categorized into mechanical, metabolic, subjective, and gait analysis comparisons. The mechanical parameters are the most useful for designing a new prosthetic foot, as they are readily translated into engineering design requirements; however, these are the furthest removed from the performance of the foot. On the other end of the spectrum are metabolic and subjective parameters, which are useful in evaluating prosthetic feet because the objectives of minimizing energy expenditure and earning user approval are clear. Somewhere between these is gait analysis. The literature review reveals that not enough information is available to bridge these categories, that is, there is no consensus on how any particular mechanical parameter affects the subjective ranking of a prosthetic foot. Two mechanical parameters emerge as necessary, but not sufficient: the rollover shape and the energy storage and return capacity of a prosthetic foot. A simple model of a solid ankle, cantilever beam - type prosthetic foot is analyzed in the context of these two parameters. By applying beam bending theory and published gait analysis data, it is found that an unconstrained cantilever beam maximizes energy storage and return, but does not replicate a physiological roll-over shape well regardless of bending stiffness. Finite element analysis is used to find the roll-over shape and energy storage capacity from the same model when a mechanical constraint is added to prevent over deflection. The results show that for very compliant beams, the roll-over shape is nearly identical to the physiological rollover shape, but the energy storage capacity is low. For stiff beams, the opposite is true. Thus there is a trade-off between roll-over shape and energy storage capacity for cantilever beam type feet that fit this model. Further information is required to determine the relative importance of each of these parameters before an optimal bending stiffness can be found. A proof-of-concept prototype was built according to this model and tested in India at BMVSS. It was found that another parameter - perception of stability, which is perhaps dependent on the rate of forward progression of the center of pressure is equally important as, if not more than, the other parameters investigated here. Perception of stability increased with bending stiffness. The prototype foot received mixed feedback and has potential to be further refined. However, the solid ankle model is inappropriate for persons living in India, as it does not allow enough true dorsiflexion to permit squatting, an important activity that is done many times a day in the target demographic. Future work will use a similar method to design and optimize a prosthetic foot with a rotational ankle joint to allow this motion.