Putting a Spring in Their Step

Having lost a foot and part of his leg to a land mine in Iraq, Garth Stewart had become much too familiar with the tenderness in his back at the end of each day. The retired U.S. Army specialist also was familiar with the source of that pain: the artificial leg he received upon his return to the United States.

“Your hip ends up doing so much work, because it has to draw the foot forward,” Stewart told The Boston Globe. “At the end of the day, you have soreness in your back.”

Imagine Stewart’s surprise, then, when his back pain disappeared. The catalyst for that turnaround was not a new therapy or a change in his busy schedule (Stewart boxes and practices juijitsu). Rather, his vanishing back pain can be traced to a new motor-powered prosthetic foot developed by researchers at the Massachusetts Institute of Technology (MIT), Brown University and the Providence Veterans Affairs Medical Center in Providence, R.I. The computer-controlled appendage—which is still being tweaked—mimics its biological counterpart by propelling the wearer forward more quickly and adjusting its stiffness on rough terrain.

“This prosthesis can go up stairs easily, go down an incline or climb up an incline. You really can’t tell that someone has this prosthesis,” Alena Grabowski, Ph.D., of MIT’s Biomechatronics Group, told an audience of suppliers and contract manufacturers on the opening day of Orthopedic Design & Technology’s Conference and Exhibition in Fort Wayne, Ind.

“People with a prosthetic have to expend 20 to 30 percent more metabolic energy to walk at the same speeds as those who don’t have one,” she continued. “They [amputees] fatigue sooner than everyone else. The prostheses that are available today are not serving amputees well. They do not have an ankle joint, and that doesn’t give them much power. Conventional prostheses release less than half the mechanical energy normally released by the ankle.”

Clinical studies show that amputees experience asymmetrical gait patterns, which can include a higher than normal hip extension, knee flexion and ankle dorsiflexion on the unaffected side. On the side with the artificial limb, amputees have less than normal hip and knee flexion, according to MIT data.

Hugh Herr, MIT’s Biomechatronics Group Director, claims the motor-powered ankle “mimics the elegance of nature” to propel the body upward and forward in walking. The prosthesis impersonates Mother Nature through multiple springs and a small, battery-powered motor (the battery lasts for about 24 hours, according to Grabowski). Energy produced from the forward motion of the person wearing the prosthesis is stored in a tendon-like spring, and then is released as the foot pushes off. Extra mechanical energy is added to boost momentum.

“This is the first ankle prosthesis to restore near-normal biological walking ability in leg amputees,” Grabowski said. “In the future, we hope that this foot can help the elderly and the ill and restore the ability to walk for those who cannot do so. We believe this project would carry over well in the orthopedic industry.”