Veterans Affairs

Slope adaptive ankle prosthetic

Safe and efficient walking on sloped terrain, with improved stability for standing and swaying tasks

Medical & Biotechnology

Scientists at the U.S. Department of Veterans Affairs (VA) have recently developed an ankle-foot prosthesis which automatically adapts to level, as well as sloped walking surfaces. The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.

Most currently available prosthetic ankles are spring-like structures that operate around one equilibrium point (i.e., one resting angle). These systems can work effectively on level terrain but cause instabilities when users walk on sloped surfaces.

There have been many attempts to address this gap in the market. Systems have been put forward that use hydraulic dampers or variations of damping to adjust the properties of the prosthesis including the use of microprocessor-control to adjust damping properties. The inherent problem with damping control of the ankle is the associated loss of energy that occurs. Another device uses a motor to change the equilibrium point of a spring-like prosthetic foot. However, this system requires multiple steps on a new terrain before it is able to adapt to the new slope. Lastly, powered ankle-foot systems are being developed. These systems actively push the prosthesis user with a motor during various times in the gait cycle and require large power sources – heavy batteries and motors. Such examples are expensive to the point of being impractical for the majority of users. In addition, the high power requirements necessitate carrying additional batteries and frequent charging of batteries.

Researchers at the VA have addressed the above issues with a new ankle-foot prosthesis which utilizes microprocessor controlled damping without the associated energy loss. The simple and compact design includes a foot plate (34), ankle frame (22), yoke (12), hydraulic damper (18), stiffness member, and a fluid circuit for controlling extension and compression of the damper. The anterior portion of the ankle frame (36) includes a curved surface inclined upwardly relative to the foot plate and forwardly toward the deflectable portion of the foot plate. The curved surface (38) forms a roll-over surface for limiting a dorsiflexion deflection of the deflectable portion of the foot plate.

For a normal gait cycle, the heel will strike and the foot will find the surface under a low stiffness in the neutralizing spring. The compression damping is low so the ankle reacts primarily in this phase of gait to the neutralizing spring. The compression damping could be altered for different users but would be static throughout the gait cycle once set by a prosthetist or the user. After foot flat, the ankle is at a maximally plantarflexed position and would normally start to dorsiflex. In this device, the extension damping is high, essentially locking the ankle in a plantarflexed position. As the person rolls over the device, the flexible footplate flexes up to the follower, producing a biomimetic ankle-foot roll-over shape. After the opposite foot contacts the ground, energy is returned from the flexible footplate and the ankle goes into late stance plantarflexion (the angle at which it was set to at foot flat). As the device leaves the walking surface (toe-off), the extension damping switches to a low level, allowing the neutralizing spring to bring the ankle back to a neutral position, which allows for toe clearance during the swing phase. After the ankle has returned to neutral and before the prosthesis gets to foot flat on the next cycle, the extension damping shifts to a high level for the next cycle. The damper is designed to have different values for compression and extension damping controlled by a microprocessor.

For uphill walking, the foot finds the surface in a more dorsiflexed position compared with that for level walking and thus the equilibrium point of the ankle is set in more dorsiflexion. For downhill walking, the foot finds the surface in a more plantarflexed position compared with that for level walking. In this way, the ankle-foot device automatically adapts to different terrain on each and every step of walking.

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