Researchers from the Chalmers University of Technology have developed a new artificial joint that can restore wrist-like movements for those with amputated forearms.

In the new system, an implant is placed into both the ulna and radius—the two forearm bones—with an artificial joint that acts as an interface between the two implants and the prosthetic hand. The entire set-up enables more naturalistic movements with intuitive natural controls and sensory feedback.

“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,” biomedical engineer Irene Boni, who worked on the project as an international student visiting Chalmers, said in a statement.   

One of the most challenging things for those missing a hand is the inability to rotate their wrist for everyday tasks like turning a door handle or simply turning over an item like a piece of paper.

“A person with forearm amputation can use a motorized wrist rotator controlled by electric signals from the remaining muscles,” Max Ortiz Catalan, an associate professor at the Department for Electrical Engineering at Chalmers, said in a statement. “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,” he added. “Furthermore, patients get no sensory feedback, so they have no sensation of the hand’s position or movement.”

Patients who have lost both their hand and wrist often preserve enough musculature to enable them to rotate the radius over the ulnar. A conventional socket prosthesis, which is attached to the body by compressing the stump, locks the bones in place and prevents any possible wrist rotation.

“Depending on the level of amputation, you could still have most of the biological actuators and sensors left for wrist rotation,” Ortiz Catalan said. “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,” he added. “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.”

The artificial joint works with an osseointegrated implant system developed by Integrum AB, a Sweden-based company that has partnered with Chalmers on the project.

The study was published in IEEE Transactions on Neural Systems & Rehabilitation Engineering.