New prosthesis aims to restore natural gait to amputees

Mo Rastgaar, an assistant professor of mechanical engineering–engineering mechanics, and PhD student Evandro Ficanha are working on a microprocessor-controlled ankle-foot prosthesis that is claimed to come close to achieving the innate range of motion of this highly complex joint.

These computerised artificial legs have pressure-sensitive sensors on the bottom of the foot that detect how the amputee is walking. The sensors instantaneously send signals to a microprocessor, which in turn adjusts the prosthesis to make walking more natural.

The microprocessor-controlled prostheses on the market can move an artificial foot in only one direction, toe up and toe down, which is fine if you are marking time on a treadmill, Rastgaar said in a statement.

‘But in reality, we never walk in a straight line for any length of time,’ he said. ‘When you walk and reach an obstacle, you have to turn, and there’s always something in our way.’

Rastgaar and Ficanha designed an ankle-foot that can move on two axes, incorporating a side-to-side roll as well as raising the toe up and down. They moved the power and control mechanism up and away from the leg using a cable-driven mechanism, thereby lightening the prosthesis and making it more comfortable and easy to use.

The cable that moves the prosthetic ankle-foot runs from the control box to the ankle mechanism and can turn the foot in almost any direction.

As part of their study, the team designed and built a large circular treadmill on which the robotic foot “walks” in circles. In tests, the prosthetic was able to copy the angles of a human ankle walking in a straight line and turning.

Kenton R. Kaufman, director of the Biomechanics/Motion Analysis Laboratory at the Mayo Clinic in Rochester, Minnesota, is collaborating in the effort to refine the prosthesis and make it available to amputees - especially wounded service personnel - with a primary focus on improving safety.

‘Amputees have lots of problems with falling; 64 per cent of above-the-knee amputees fall every year, compared to 33 per cent of older adults, said Kaufman.

The researchers expect to begin refining their design at the Mayo Clinic in summer 2014. In the interim, they will present a paper on their work, Ankle Angles during Step Turn and Straight Walk: Implications for the Design of a Steerable Ankle-Foot Prosthetic Robot, at the 2013 ASME Dynamic Systems and Control Conference, to be held October 21-23 at Stanford University.