Researchers have developed a prosthetic joint that connects directly to the forearm bones of an amputee, allowing them to use a prosthesis with a wrist-like rotary movement. Use of the new joint could dramatically improve the quality of life for amputees, making everyday tasks simpler and more intuitive.

For most of us, the ability to simply rotate our wrists is taken for granted, yet without this simple movement, countless mundane yet vital everyday tasks would be far more challenging to achieve. For example, we would struggle to open doors, manipulate tools, or even to turn over a piece of paper.

According to the international team of researchers behind the newly-developed synthetic joint, currently available forearm prostheses fail to address this problem.

Some motorized prostheses provide wrist rotation controlled by electric signals sent from the remaining muscles in a patient's forearm. However, these prostheses have a significant drawback, in that the user cannot control the movement of both wrist and artificial hand at the same time. They also fail to provide any sensory feedback, making it difficult for users to be aware of the position of their hand.

The newly-developed joint offers a different solution to providing wrist movement by interfacing directly with the remaining bones in a patient's forearm – the ulnar and radius. Osseointegrated implants placed into the bones attach to the artificial joint, which in turn interfaces with a prosthesis. This allows a user to shift the "wrist" and attached hand by rotating the ulnar over the radius, as is the case with a natural wrist.

"Depending on the level of amputation, you could still have most of the biological actuators and sensors left for wrist rotation," comments Max Ortiz Catalan, Associate Professor at Chalmers University of Technology in Sweden, and lead researcher on the project. "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."

The joint can only be used by patients who have enough musculature remaining in their forearm to rotate the radius over the ulnar.

Individuals fitted with the new system were found to achieve higher scores in tests designed to test dexterity than those fitted with more traditional prostheses.

"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," says Irene Boni, a biomedical engineering student from the Sant'Anna School of Advanced Studies in Italy, and one of the co-authors of the new study.

The paper detailing the development of the joint has been published in the journal IEEE Transactions on Neural Systems & Rehabilitation Engineering.

The video below shows the implant and wrist joints in action, coupled with a prosthetic hand.

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