Natural gait via artificial foot

Conventional prostheses can hardly be compared with the modern versions, as these have joints, controlling algorithms, and spring-loaded elements made of high-tech materials. With their help, the gait pattern becomes much more natural. Some are even designed for maximum performance: athletes with amputations at the lower leg level who use carbon prostheses achieve outstanding times running short distances.

Instead of the usual passive suspension at the foot components, the D-Ankle prosthesis from Design Pro Technology assists its user with motor power. As in a natural gait, the artificial foot is actively bent up and down with each step. The gait becomes more harmonious, more stable, and less tiring. An intelligent controller finds that correct gait rhythm and a brushless motor from FAULHABER provides the necessary propulsion.

Sport prostheses are designed for fast running but standing still and performing normal activities while wearing them is difficult, even impossible. Therefore, ankle joint prostheses intended for everyday use have a completely different design compared to the arched skids used in competitive sports. Usually mirroring natural anatomy, they consist of a lower leg component and a foot component, connected by a joint. The passive artificial ankle joint ensures the prosthesis always remains in a predictable position. However, it allows a very limited range of movement during locomotion.

As the foot recoils – during forward movement – it’s pressed toward the lower leg; once the foot is kicked out, the elastic force returns the foot to a near perpendicular fixed starting position. “However, this fixed position doesn’t correspond to the natural position of the foot during the transfer phase. There’s a risk of the tip of the prosthetic foot getting caught on the ground or on small obstacles,” says Marcin Dziemianowicz, the engineer, whose focus is on biomechanics. Dziemianowicz founded Design Pro Technology in Białystok, Poland in 2016.

Active dorsal flexion to reduce the risk of tripping

With its D-Ankle, Design Pro Technology has created an ankle joint prosthesis that actively moves the foot with a motor when walking and holds the foot in an anatomically natural position during each step. Dorsal flexion – the bending of the foot toward the tibia – during the swing phase is of crucial importance.

“Increasing the distance between the tip of the toe and the ground decreases the risk of tripping,” Dziemianowicz explains. “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 has active heel-to-toe recoil functionality, including pushing off from the ground for the next step. Here, the motor-driven plantar flexion is activated, i.e., stretching in the joint. 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’re made possible as passive deformation through the elastic material of the prosthetic foot – carbon fiber. As a result, the foot achieves full sole contact even on uneven surfaces.

Controller detects the gait rhythm, drive ensures appropriate movement

The integrated controller of the prosthesis receives signals from several sensors 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 as well as unloading in the transfer phase. The accelerometer unit detects the overall movement including speed, foot inclination, and path gradient.

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. Motor and lead screw rotate in both directions achieving 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,” Dziemianowicz says. “The motor was 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 to be 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 to the market at the end of 2023. Its standard adapter allows it to be attached to any modular prosthesis stem. Individual prosthesis adjustment is carried out by an orthopedic technician. The height of the heel can be varied so D-Ankle can also be worn in women’s heeled shoes. Should the battery charge not be enough after a very long day, the wearer can continue to walk as they would with a passive prosthesis.

“With active movement of the foot, we are quite literally making huge steps both toward natural movement anatomy and toward improved support of amputees,” Dziemianowicz says. “After the experiences with this product and the great cooperation with FAULHABER, we have a number of ideas on how to utilize the compact motor power for other prostheses.”

Design Pro Technology:
https://designprotechnology.com

FAULHABER:
https://www.faulhaber.com