Design optimisation and development of a pneumatic prosthetic foot

Abstract

Lower-limb amputation is a life-saving and life-changing surgery that significantly impacts mobility and quality of life, particularly in South Africa, where access to advanced prosthetic technology is hindered by socio-economic factors and infrastructure challenges. Prosthetic feet are classified into three distinguishable categories: conventional feet, which include solid ankle cushion heel and articulated prosthetic feet, energy storage and release feet and bionic feet. Conventional passive prosthetics, such as the SACH foot, often fall short in replicating the normal walking dynamics, leading to asymmetries when walking and increased energy cost of walking. This study piloted a pneumatic prosthetic foot to investigate the biomechanical benefits of using this innovation while walking at self-selected walking speed over flat surfaces. The study utilized a quantitative (experimental) research method, commencing with the Finite Element Analysis (FEA), using the ANSYS software to simulate axial structural loads during standing positions on titanium and aluminum alloy shank segments. A prototype was developed featuring a crank-slider mechanism and a pneumatic cylinder to modulate ankle stiffness. Clinical evaluation involved a case study of two transtibial participants. Walking gait was analysed using the Templo markerless motion capture system (Theia3D) across three conditions: the prescribed passive prosthetic foot, an unpressurized version of the pneumatic prosthetic foot, and a pressurized version of the pneumatic prosthetic foot (4 bars). Spatiotemporal parameters, kinetics, and kinematics, including stride length, cadence, and vertical ground reaction forces (vGRF), were systematically recorded and analyzed across varying conditions. A stark contrast between participants was revealed by the study findings, participant 1 demonstrating improvements in walking symmetry (spatiotemporal parameters and kinetics), while participant 2 demonstrated minimal benefit when using the pneumatic prosthetic foot. The study findings suggest that device performance, one way or another, was influenced by the user adaptation and biomechanical conditions of the participant. The preliminary findings align with the broader body of literature, suggesting that semi-active prosthetic devices can bridge the gap between expensive powered devices and passive prosthetics. On the contrary, the pneumatic prosthetic foot was not practically lighter than other powered prosthetic devices. This research developed a functional pneumatic prosthetic prototype that can withstand the axial loading of the human body, and can be used for mobility. Though the pneumatic prosthetic prototype has demonstrated potential, the findings need to be interpreted with caution due to the small sample size (n=2), which limits the generalizability of the findings. The current pneumatic prosthetic foot prototype requires further refinements to reduce both the mass and the height of this prosthetic foot. Also, improvements in the control system are required to modulate the ankle stiffness during walking. Additionally, the system faced challenges in replicating passive shock absorption during the load acceptance phase in the early stance. Future research should include large and diverse participant cohorts, and longitudinal studies to monitor neuromuscular adaptation and changes in the walking dynamics.