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Prosthetic Joint Provides More Natural Wrist Movements

Wrist joint

Wrist joint connects to transradial implant and prosthetic hand (Biomechatronics and Neurorehabilitation Laboratory/Chalmers University of Technology)

29 Nov. 2018. An academic-industry engineering team in Sweden developed and tested a prosthetic wrist joint that provides more natural movement and control than most current devices. A team from Chalmers University of Technology and the company Integrum AB, both in Gothenburg, describe the prosthetic joint and test results with an amputee in the 13 November issue of the journal IEEE Transactions on Neural Systems and Rehabilitation Engineering.

Researchers led by Chalmers biomedical engineering professor Max Ortiz Catalán are seeking to provide better wrist and hand control for people with transradial amputations, where the arm below the elbow is severed or removed. Among the biggest challenges facing amputees, even those with today’s advanced prosthetic devices, is the ability to rotate the wrist, so a prosthetic hand can move from facing up to facing down, and still hold and control objects in its grip. Ordinary movements like turning a door handle or a screwdriver become awkward and difficult.

As Ortiz Catalán describes in a Chalmers University statement, “A person with forearm amputation can use a motorized wrist rotator controlled by electric signals from the remaining muscles. However, those same signals are also used to control the prosthetic hand.” He notes, “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.”

The prosthetic device designed in the Ortiz Catalán lab aims to preserve natural forearm rotation. Their solution is based on a prosthetics technology offered by Integrum that anchors the devices directly to the bone, a process called osseiointegration. The prosthetic device attaches to implants on the two forearm bones, ulnar and radius, and the new wrist joint connects these bone implants to the prosthetic hand. The new wrist joint also takes advantage of any remaining muscles and nerves remaining in the individual’s forearm to provide sensory feedback.

To test the wrist joint, the Chalmers/Integrum team fitted the device on a transradial amputee with an osseiointegrated forearm implant and a prosthetic hand. The researchers tested the individual’s ability to perform wrist movements with a conventional socket attachment and the new wrist joint, using standard indices for prosthetic devices and manual dexterity in rehabilitation, respectively the Southampton Hand Assessment Procedure and Minnesota Manual Dexterity test.

The team found the new wrist joint distributed its actions about equally between the two forearm bones, 60 percent on the radius and 40 percent on the ulna, with no unpleasant feelings reported by the participant. In addition, the participant was able to perform near natural forearm rotation with the new wrist joint, returning improved performance scores on both standard indices, compared to the conventional socket connection.

Irene Boni — an engineer at Integrum, former student in Ortiz Catalán’s lab, and first author of the paper — adds, “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.”

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