Exciting Benefits of Powered Prosthetic Systems

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By Lt. Col. Joseph K. Hitt, PhD; Kevin Hollander, PhD; and Thomas Sugar, PhD

Powered prosthetics, bionic systems, and robotic devices being developed in the research laboratory are becoming quite sophisticated. These robotic systems are made possible thanks to advancements in microprocessors, batteries, sensors, and intelligent controllers, and will increase the offering of prosthetic systems in the commercial market. We are often asked the question, "Why should I choose a powered ankle device over the recent advancements in energy-storing carbon-fiber keels?" By highlighting some of the ongoing research and some of the advantages we have observed and reported on, we hope to answer that question.

No single keel device can be used for a wide variety of tasks. In contrast, a powered, articulated ankle can assist in walking, jogging, ascending and descending stairs, walking up and down inclines, jumping, and carrying loads. Such devices even allow one to walk forward and backward. We have demonstrated that if the spring or compliance in the system is tuned properly, these devices can make it easier for wearers to carry loads of different weights and walk over different surfaces. There are also some unseen, less obvious benefits. First, a powered ankle can allow a lower K-level walker to improve dramatically. One of our subjects, a gentleman with a transtibial amputation secondary to diabetes, was not a powerful walker and had poor balance. However, when he wore a robotic ankle, he was able to walk at four miles per hour on a treadmill. He reported feeling great and losing weight. The robotic ankle improved this user's walking ability.

When a second subject—a high-level transtibial amputee—wore a robotic ankle, he was able to walk briskly, jog, jump, and carry weight. He commented that for the last 20 years, he had not felt like he was really walking although he had been wearing an advanced carbon-fiber keel.

We believe these less obvious advantages might be even more important. Subjects tell us that their back does not hurt, and we believe this is because the ankle articulates and they do not have to vault over a passive foot. Gait symmetry is improved allowing for smoother motion and possibly less hip pain and osteoarthritis. Metabolic costs have been shown to return to able-body levels or sometimes even lower. Finally, one subject said that he just loved to stand using a powered ankle because his socket did not "dig into his shank." Many keels are pitched a bit forward to allow rollover, but this angle does not allow for comfortable standing.

When fully powered robotic ankle devices are commercialized (anticipated within the next year), we believe they will provide users more comfort in walking and the ability to perform new tasks. These devices will certainly be more expensive because of the addition of a motorized ankle joint to a sophisticated carbon-fiber keel. Such devices will also need more maintenance compared to a passive keel. There will be other challenges as well, such as building a mechanical device to last 3.6 million cycles per year. To put this in context, an automobile wheel rotates approximately 112 million cycles in 100,000 miles of driving.

The future holds many other exciting advances such as peripheral nerve interfaces and osseointegration to the robotic ankle. Sensors will allow one to feel the surface, and simple biofeedback sensors will aid in tuning power and motion. New surgical techniques and biomaterials will allow the removal of the problematic limb/socket interface. As robotic devices improve, so will these bionic devices.

In the future, research from powered ankles will morph into powered exoskeletons. These powered exoskeletons (powered orthoses) will assist people who are weak or need help on that once in a lifetime hiking trip. These new orthoses will greatly enhance the quality of life of elderly individuals by providing them assistance in many of the physical tasks of everyday life. They will help to return many of our wheelchair bound, disabled, and obese people to their feet.

Lt. Col. Joseph K. Hitt, PhD, is an assistant professor of civil and mechanical engineering at the U.S. Military Academy at West Point, New York. Kevin Hollander, PhD, is a co-founder of and director of product development at SpringActive Inc., Tempe, Arizona. Thomas Sugar, PhD, is an associate professor at Arizona State University, Tempe, and co-founder of SpringActive Inc.