A robotic hip exoskeleton for amputees

Researchers from the Department of Mechanical Engineering and Robotics Center at the University of Utah, US, have engineered a combined exoskeleton with a passive prosthesis that significantly improved walking for above-knee amputees compared to a passive prosthesis alone.

Results from the phase 1 trial were reported on Tuesday in Nature Medicine.

The researchers noted that recent technological advances have seen the development of powered knee and ankle joints that can theoretically replicate the biomechanical functions of the missing biological leg.

“These emerging powered prostheses aim to restore natural walking efficiency by providing positive energy at the prosthetic joints,” they wrote in the paper. “However, the required batteries and servomotors increase the mass of the prosthesis.”

They decided to take another approach: rather than putting the energy assistance in the place of the missing leg, they employed a powered hip exoskeleton to assist the remaining portion of the limb.

“Powered hip exoskeletons can be very lightweight because they only need to provide a fraction of the hip’s biological torque to assist the residual limb,” they wrote, and further noted that powered hip exoskeletons have been shown to reduce the metabolic cost of movement in young non-amputees.

The powered hip exoskeleton is comprised of a 3D-printed device that’s designed and fitted to the body to control the biomechanics of the limbs. This device, called an orthosis, is strapped to the pelvis. The robotic system connects the orthosis to a flexible cuff that attaches to the residual limb by wrapping around the socket of the (passive – unpowered) prosthetic lower limb.

The robotic actuation system, which links the orthosis and the cuff, includes a brushless DC (direct current) motor, helical gears, a ball screw and a composite spring.

The researchers tested the efficacy of the powered hip exoskeleton in six people with an above-knee amputation. Each was measured walking both on a treadmill, at a speed of 1m/s; and at a “self-selected” speed, with and without the powered hip exoskeleton, on a 12m walkway.

Results suggested that the robot leg reduced the energy needed to walk by 15.6% compared with using a standard prosthesis – the equivalent of removing a 12kg backpack from someone with both legs.

“All participants were able to walk overground [on the walkway] with the exoskeleton, including starting and stopping, without noticeable changes in gait balance or stability,” the authors wrote. “By significantly reducing the metabolic cost of walking, the proposed hip exoskeleton may have a considerable positive impact on mobility, improving the quality of life of individuals with above-knee amputations.”

Exoskeletons are often used in people with neuromotor deficits, while people who have had one of their legs amputated above the knee generally use passive prostheses. However, these are highly inefficient and severely reduce the mobility and quality of life of amputees.

Also, in many amputees, the residual thigh muscles that originally functioned around the hip joint lose tone following transfemoral amputations. This weakness causes an imbalance that amputees tend to compensate for, relying on the intact limb and trunk. “Biomechanics studies have shown that individuals with above-knee amputation overexert both the residual and contralateral hip,” wrote the authors.

Over time, this compensation mechanism leads to gait asymmetry, reduced walking stability and secondary physical conditions, such as osteoarthritis and back pain.

The authors indicated the need for future clinical studies to optimise the robot leg and assess its efficacy in people with different amputations and ambulation needs.

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