In an intact ankle, tendons crossing the joint store energy during the stance phase of walking prior to push-off and release it during push-off, providing forward propulsion. Most prosthetic feet currently on the market – both conventional and energy storage and return (ESR) feet – fail to replicate this energy-recycling behaviour . Specifically, they cannot plantarflex beyond their neutral ankle angle (i.e. a 90° angle between the foot and shank) while generating the plantarflexion moment required for normal push-off.
Various research prototypes have been developed that mimic the energy storage and return seen in anatomically intact subjects. Many are unpowered clutch-and-spring devices that cannot provide biomimetic control of prosthetic ankle torque. Another solution is to add a battery and electric motor(s), but this increases size, weight and cost. Interestingly, miniature hydraulics is commonly used in commercial prostheses, but not for energy storage purposes. Therefore, the aim of our work has been to design a novel prosthetic joint based on miniature hydraulics, including an accumulator for ESR, to imitate the behaviour of an intact ankle. Furthermore, hydraulic energy transfer between joints could be easily achieved using pipes.
A series of designs have been developed and their performance assessed through virtual prototyping. The most sophisticated design is based on a hydraulic variable displacement actuator (VDA) . This provides continuous biomimetic control of ankle torque throughout the gait cycle, mimicking the intact ankle, while storing the eccentric work done from heel strike to maximum dorsiflexion, which is then returned to power push-off. The simulation results were promising, but a new VDA would be required that has half the displacement of the smallest commercially available device. Unfortunately, a VDA is a specialised and complex component and it would not be appropriate to develop a new VDA just for the prosthetics application. Therefore, in our most recent work a simpler design has been investigated , which is less flexible but performs well and is suitable for physical prototyping.
Team: Salford team: Prof David Howard, Prof Laurence Kenney, Dr Martin Twiste, Dr Anna Pace, Dr James Gardiner, Dr Zeeshan Bari. External partners: Dr Lei Ren (Manchester University), Dr David Moser (Blatchford Ltd).