Design Pro Technology in Poland has created the first ankle joint prosthesis, D-Ankle, that actively moves the foot with a motor when walking and holds the foot in an anatomically natural position during the course of each step. Here, dorsal flexion – the bending of the foot toward the tibia – is of crucial importance during the swing phase. “Increasing the distance between the tip of the toe and the ground decreases the risk of tripping,” explains Marcin Dziemianowicz, founder of Design Pro Technology. “With a passive prosthesis, the wearer achieves this by making a circular movement with their hip or by lifting their leg higher. These compensatory movements are unnecessary with D-Ankle; walking becomes more natural and less tiring.”
When the prosthetic foot is set down on the ground, its mechanics will perform the natural change in angle during the support phase. D-Ankle is the only prosthesis with active heel-to-toe recoil functionality, including pushing off from the ground for the next step. Here, the motor-driven plantar flexion, or stretching in the joint, is activated. It also contributes to a harmonious gait and saves energy. Although the artificial hinge joint is unable to perform the lateral movements that a natural ankle joint allows, they are made possible as passive deformation through the elastic material of the prosthetic foot, carbon fibre. As a result, the foot achieves full sole contact even on uneven surfaces.
The controller detects the gait rhythm
The integrated controller of the prosthesis receives signals from several sensors in order to distinguish between the different phases of a step cycle. A potentiometer measures the angle between the foot and lower leg; a bilateral pressure sensor measures loading at initial contact of the foot and unloading in the transfer phase. The accelerometer unit detects the overall movement, including speed, foot inclination, and path gradient.
“The algorithm merges the signals from the few most recent steps and evaluates them,” says Dziemianowicz. “From this data, it derives the gait rhythm and the optimum foot position for each step phase. For example, the ankle joint is bent more when walking uphill than when walking on a level surface, and the take-off force is also increased to make walking uphill easier. On downward slopes, it’s the other way around, so that the best possible contact between sole and ground can be achieved. Furthermore, a smartphone app can be used to adjust parameters such as take-off force, the sensitivity of the pressure sensor, or the length of a step cycle phase.”
Sporty drive with great endurance
The integrated drive ensures that the control signals are converted into the appropriate movement. At its heart is a brushless motor of the BP4 series from Faulhaber, the power of which is transferred to a lead screw. The motor and lead screw rotate in both directions, and thus achieve active dorsal and plantar flexion of the foot. The high energy efficiency of the drive enables an operating time of 12 hours on one battery charge. The motor also tolerates the considerable heat emission that can occur in everyday operation.
“Our objectives were altogether quite sporty,” recalls Dziemianowicz. “The motor was able to emulate a jogging motion – with three steps per second, or three complete cycles of dorsal and plantar flexion. Furthermore, rapid changes in pace and direction were possible. For this application, you need very high speed and high torque in the smallest possible volume and with the lowest possible weight. We tried out various drive solutions from leading motor manufacturers. With Faulhaber, we not only found the most suitable product, but also received outstanding technical support.”
After extensive and successful trials with test amputees, the foot prosthesis was introduced onto the market at the end of 2023. Its standard adaptor allows it to be attached to any modular prosthesis stem. Individual prosthesis adjustment is carried out by an orthopedic technician. Should the battery charge not be enough after a very long day, the wearer can continue to walk as if with a passive prosthesis.
“With active movement of the foot, we are literally making huge steps both toward natural movement anatomy and toward improved support of amputees,” says Dziemianowicz. “After the experiences with this product and the great cooperation with Faulhaber, we have a number of ideas on how to utilise the compact motor power for other prostheses.”
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