A New Artificial Joint RESTORES WRIST-LIKE MOVEMENTS to Forearm Amputees

Most of us probably take it for granted, but the simple turn of the wrist is an essential movement that we use every day. Consider using a door handle, a screwdriver, a knob on a cooker, or even turning a page. For those missing their hand, these are much more awkward and uncomfortable tasks, and current prosthetic technologies offer only limited relief to this problem.

“A person with forearm amputation can use a motorized wrist rotator controlled by electric signals from the remaining muscles,” explains Max Ortiz Catalan, associate professor at the Department for Electrical Engineering at Chalmers University of Technology in Gothenburg, Sweden. “However, those same signals are also used to control the prosthetic hand. This results in a very cumbersome and unnatural control scheme, in which patients can only activate either the prosthetic wrist or the hand at one time and have to switch back and forth. Furthermore, patients get no sensory feedback, so they have no sensation of the hand’s position or movement.”

The new artificial joint works instead with an osseointegrated implant system developed by the Sweden- based company Integrum AB, one of the partners in this project. An implant is placed into each of the two bones of the forearm – the ulna and radius – then, a wrist-like artificial joint acts as an interface between these two implants and the prosthetic hand. Together, this allows for much more naturalistic movements, with intuitive natural control and sensory feedback.

Patients who have lost their hand and wrist often still preserve enough musculature to allow them to rotate the radius over the ulna – the crucial movement in wrist rotation. A conventional socket prosthesis, which is attached to the body by compressing the residual limb, locks the bones in place, preventing any potential wrist rotation, and thus wastes this useful movement.

“Depending on the level of amputation, you could still have most of the biological actuators and sensors left for wrist rotation,” Ortiz Catalan says. “These allow you to feel, for example, when you are turning a key to start a car. You don’t look behind the wheel to see how far to turn – you just feel it. Our new innovation means you don’t have to sacrifice this useful movement because of a poor technological solution, such as a socket prosthesis. You can continue to do it in a natural way.”

Biomedical engineers Irene Boni and Jason Millenaar were at Chalmers as visiting international students.
Boni was a visiting student from the Sant’Anna School of Advanced Studies in Pisa, Italy, and Millenaar from Delft University of Technology in the Netherlands. They worked with Dr. Ortiz Catalan at his Biomechatronics and Neurorehabilitation Lab at Chalmers, and with Integrum AB on this project. Dr. Marco Controzzi at the Biorobotics Institute, Sant’Anna School of Advanced Studies, also participated in the research.

“In tests designed to measure manual dexterity, we have shown that a patient fitted with our artificial joint scored far higher compared to when using conventional socket technology,” explains Millenaar. “Our new device offers a much more natural range of movement, minimizing the need for compensatory movements of the shoulder or torso, which could dramatically improve the day-to-day lives of many forearm amputees,” adds Boni.

The researchers found that restoring the full range of movement to all degrees of freedom in which the forearm bones can move was not necessary – the key parameter for returning a naturalistic wrist motion is the “axial,” or circular, motion of the ulna and radius bones.
“The wrist is a rather complicated joint,” says Ortiz Catalan. “Although it is possible to restore full freedom of movement in the ulnar and radial bones, this could result in discomfort for the patient at times. We found that axial rotation is the most important factor to allow for naturalistic wrist movement without this uncomfortable feeling.”

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