Unilateral trans-femoral amputee gait consumes up to 60% more energy than able-bodied gait. For higher level amputees, research suggests that energy efficiency drops by well over 80%. Furthermore, it has been shown that energy consumption increases significantly when walking on slopes, suggesting studies in level walking may underestimate the extent of the problem. The negative effects of high energy consumption are compounded by reductions in walking speed of typically 40% for trans-femoral amputees with associated low activity levels, particularly in elderly amputees. These deficits are even greater in bilateral amputees. This has a tremendous impact on what amputees can achieve and the consequences for their quality of life.
The energy storage and return (ESR) capabilities of prostheses are crucial to improving the situation and yet modern prostheses only store and return significant energy below the knee, and energy is not returned in a controlled manner. For example, stored energy is not available for plantar-flexion (push-off) at the end of stance. Furthermore, modern prosthetic systems don't transfer energy between joints, which is a lost opportunity as, for example, the excess of eccentric work at the knee could be stored and used in a controlled manner at the ankle joint. For these reasons, there is an opportunity for truly transformative research leading to a step change in the performance of lower limb prostheses.
We have been undertaking simulation studies to establish the potential for hydraulic technology to enable controlled storage, transfer between joints, and return of energy in lower limb prostheses. In an EPSRC funded project (2013-2017), we used design optimisation and simulation to investigate the feasibility of a hydraulic variable displacement actuator (VDA) based prosthetic ankle (Gardiner et al, 2017). The proposed device stores the eccentric ankle work done from heel strike to maximum dorsiflexion in a hydraulic accumulator and then returns the stored energy to power push-off. Optimisation was used to establish the best spring characteristic and gear ratio between ankle and VDA. The corresponding simulations show that, in level walking, normal push-off can be achieved with an additional 22% of surplus energy stored in the accumulator, which is then available to aid short periods of uphill walking. Although the results are promising, for this approach to be a success, a new miniature, low-losses, lightweight VDA would be required that is half the size of the smallest commercially available device.
Currently, an Italian PhD student is investigating the possibility of using cam-driven hydraulic rams to achieve the required ankle torque profiles in a novel ESR prosthetic ankle. This could lead to a simpler design than that of the VDA based one and also lower energy losses. After establishing the best approach, we plan to move on to trans-femoral prostheses where the potential gains are greater because hydraulics is ideally suited to transferring energy between knee and ankle joints.