On the right foot

An army veteran who lost part of his leg in Iraq walked with more spring in his step today as he unveiled the world's first robotic ankle - an important advance for lower-limb amputees that was developed by a team at MIT.

Garth Stewart, 24, who lost his left leg below the knee in an explosion in Iraq, demonstrated the new powered ankle-foot prosthesis during a ceremony at the Providence, Rhode Island, Veterans Affairs Medical Center.

Stewart walked in the device, which, unlike any other, propels users forward using tendon-like springs and an electric motor. The prototype device reduces fatigue, improves balance and provides amputees with a more fluid gait. It could become commercially available as early as the summer of 2008.

MIT Media Lab Professor Hugh Herr and his team of researchers developed the ankle-foot. Herr, NEC Career Development Professor and head of the biomechatronics research group at the Media Lab, is a VA research investigator. He is also a double amputee who tested his own invention.

‘This design releases three times the power of a conventional prosthesis to propel you forward and, for the first time, provides amputees with a truly humanlike gait,’ Herr said.

Because conventional prostheses only provide a passive spring response during walking, they force the amputee to have an unnatural gait and typically to expend some 30 percent more energy on walking than a non-amputee. The new ankle is light, flexible, and - most importantly - generates energy for walking beyond that which can be released from a spring alone.

This is accomplished through a device equipped with multiple springs and a small battery-powered motor. The energy produced from the forward motion of the person wearing the prosthesis is stored in the power-assisted spring, and then released as the foot pushes off. Additional mechanical energy is also added to help momentum.

Within the next year, Herr hopes to get small, wireless implants into his muscles near the neuromuscular juncture, so that when the muscle contracts, the electrical impulse will send precise information to control the artificial limb.

Looking longer term, Herr is working to determine if an implant could be used to mechanically attach the artificial limb directly to the amputee's residual limb bone. The goal: to transmit loads directly to the bone structure of the amputee, eliminating the discomfort now experienced with conventional prostheses.