Marilynn Wyatt, Naval Medical Center San Diego

Carbon-fiber and plastic polymers

are making artificial limbs stronger and lighter. They can be controlled using just a thought or a muscle twitch. They can even be re-engineered for rock-climbing or Olympic sprinting . But a continuing problem is that most prosthetic limbs don't provide sensory feedback to the user.

As you walk, muscles and neurons constantly send information to your brain about where your legs are, where your feet hit the ground, and how hard they push off. Without that feedback, it can be hard to coordinate movement. As a result, amputees who wear prosthetic legs commonly develop gait abnormalities such as shorter strides, slower walking speeds, and standing on tip-toe to swing the prosthetic leg.

"The lack of sensation can affect mobility and quality of life," says Zachary McKinney, a graduate student in biomedical engineering at UCLA. McKinney and his colleagues have been working on a simple feedback system that can be incorporated with almost any below-the-knee prosthetic leg. "Our goal is to improve sensory awareness of the prosthetic," he said at a meeting of the Biomedical Engineering Society in late October.

The system uses tactile feedback on the thigh to tell the amputee whether he's stepping on the correct parts of his prosthetic foot with the proper amount of force. The device won't make the prosthetic leg "feel" like a living leg, but it can provide instantaneous feedback to help the amputee correct his gait.

The setup consists of three parts: a shoe insole that collects information from the foot; a data-relay center worn on the torso; and a cuff on the thigh that uses pressure to communicate with the user. The shoe insole has four sensors that pick up how the amputee is stepping on the prosthetic foot. Located at the heel, the big toe, and on the left and right sides of the ball of the foot, the sensors collect pressure information and send it to a small data processor that's strapped around the user's torso. The processor uses the information to pneumatically inflate several dime-sized silicone balloons on the thigh cuff.

The cuff holds 12 balloons arranged in four sets of three, and each set corresponds to each of the four sensors on the insole. As each sensor receives pressure, the amputee can feel a corresponding set of balloons inflating. Between the foot and the cuff, the feedback arrives within less than one-tenth of a second, "which is perceptually instantaneous," McKinney says.

Amputees often fail to shift their full weight onto the prosthetic leg because they don't fully trust it. As a result, the other leg has to compensate, leading to asymmetry. That's why each sensor has three corresponding balloons in the cuff. If the amputee steps with only a small force on his heel, only one balloon in that set will inflate; if he steps with the proper amount of force, all three will inflate. The amputee's goal is to make all 12 balloons inflate with each step, indicating that the leg is hitting the ground in all the right places and with the correct amount of force.

"I think this is a really cool idea," says Stacy Bamberg, a mechanical engineer at the University of Utah who has also invented a smart insole to help correct gait problems. (Bamberg's insole uses visual and audio feedback.) "What I like about what [McKinney's] group is doing is, when we walk we don't think about the pressure distribution in our feet. By building it in with tactile feedback, they've got the potential that patients will stop thinking about it. Over time they'll just incorporate it into their daily routine."

McKinney tested the system at San Diego's Naval Medical Center. The nine test subjects were former or current military members who'd lost a leg in service. For testing, the team began by recording how each veteran normally walks on his or her prosthetic leg across a 30-foot space. Then they let the test subjects wander around the hospital for 10 minutes to get familiar with the sensorized insole and thigh cuff. Then they recorded each person's 30-foot walk again, this time with feedback, and compared the before and after.

The results were very promising. Seven gait variables improved significantly, including the weight the subjects shifted onto the prosthetic while stepping, the hip rotation angle, and the force with which they pushed off with the prosthetic. Some subjects showed vast improvements in their walking rhythm and speed, indicating that they felt more confident while walking with the feedback device, McKinney says. "And remember, that was the first time they'd tried it on. Using the skin on your thigh as a way of perceiving forces on bottom of your foot is a form of sensory substitution. That's the kind of thing you'd expect people to have to learn over time. The fact that we see an immediate effect on gait shows that the system has a very strong potential impact as a rehab device."

Still, this system is very much a work in progress, and two variables were slightly worsened with the use of the feedback system. The trunk of the body tilted more toward the prosthetic side, throwing off the center of gravity, and the vets were more likely to swing their leg out to the side while stepping.

Overall, however, the response from the test subjects was positive. "One patient said he really loved playing hockey, and that the system could help him balance his weight over the skate," McKinney recalls. "They like to joke that it makes them feel like a bionic man or Robocop."

A medical device company has shown interest in developing it, and McKinney hopes that it will become available on the market within the next two years. Now the team is looking forward to taking the device outside of the lab and into the real world, giving test subjects more time to become familiar with the tactile feedback system. "We really want to know how [it] effects the course of recovery of the patients," McKinney says.

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