A new artificial joint from the Chalmers University of Technology in Sweden offers forearm amputees a bodily function many would imagine difficult to live without—wrist mobility.
High levels of wrist mobility allow for the hand to carry out many day-to-day functions seamlessly–using a screwdriver, opening a door, turning a page in a book. Current prosthetics offer limited solutions for handling these complex maneuvers.
“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,” says Max Ortiz Catalan, Associate Professor at the Department for Electrical Engineering at Chalmers, in a press statement explaining the problem.
“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 wrist system uses a process known as osseointegration. Taken from the Latin ossum “bone” and integrare “to make whole,” osseointergration is the process of connecting living bone and artificial implants. The Swedish system, developed with prosthetic company Integrum AB, connects to two bones in the forearm—the ulna and the radius.
These two bones are crucial to wrist movement. As a wrist turns, the radius moves over the ulna. Many amputees still retain enough musculature to make this movement, but traditional prosthetics typically cant make use of this.
There are two implants within the system. These are each connected to the ulna and the radius. Then a third wrist-like artificial joint acts as a connection between the ulna implant, the radius implant, and the prosthetic hand at the end of the system.
“Depending on the level of amputation, you could still have most of the biological actuators and sensors left for wrist rotation. 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,” says Ortiz Catalan.
Although the researchers found that they offer total freedom of movement in the ulnar and radial bones, patients noted discomfort. This full freedom of movement, it turns out, was not necessary—what mattered most was the axial, or circular, motion of the ulna and radius bones.
Prosthetics have come a long way—in the Middle Ages, it was recently discovered, a man used a knife and leather straps as a replacement limb.