While scientists have made enormous advances in limb prostheses, these devices still lack a sense of touch. Now, scientists from Northwestern University, in Chicago, have shown that transplanting the nerves from an amputated hand to the chest allows patients to feel hand sensation there. The findings are the first step toward prosthetic arms with sensors on the fingers–now under development–that will transfer tactile information from the device to the chest, making the wearer feel as though he or she has a real hand.
Currently, patients operate their prostheses through visual feedback: they know that they’ve touched a cup when they see the arm hit it. Without sensory information, it’s difficult for patients to determine if they are grasping the cup strongly enough to hold it without breaking it. “Sensation is a big missing piece of current prosthetics,” says Robert Kirsch, associate director of the Functional Electrical Stimulation Center at Louis Stokes Veterans Affairs Medical Center, in Cleveland. “If this can provide a path to do that, it’s a big step.”
Earlier this year, the Northwestern researchers, Todd Kuiken and his colleagues at the Rehabilitation Institute of Chicago, showed that a similar nerve-transplant approach could be used to intuitively control movement of a prosthetic arm. (See “A Prosthetic Arm That Acts Like a Real One.”) Motor nerves, which relay motor signals from the brain to the muscles, were transplanted from the stump of the lost arm to the chest. When the patient thought about moving his hand, his chest muscle twitched. Those muscle contractions were used to control movement of a motorized elbow, wrist, and hand.
In the new study, the researchers took the nerves that would normally carry sensory messages from the hand to the brain and implanted them into a patch of skin on the patient’s chest. After allowing the nerves to grow for several months, Kuiken and his colleagues tested the sensory abilities of two amputees. “They can feel very light touches and can feel hot and cold, just like in the missing hand,” says Kuiken, who led the new work. The findings were published today in the Proceedings of the National Academy of Sciences.
Both patients could tell the difference between different grades of sandpaper rubbed against their skin. But they each developed very different senses of touch. For one patient, the sense was very broad: touching a large patch of skin on the chest sparked the perception of sensation in three fingers at the same time. The second patient had a more refined sensory map. She felt sensation in different fingers linked to different spots on her chest, as well as other odd sensations, such as the feeling of skin being stretched or a finger being pushed back.
In both cases, the new sensory maps on the chest appear to be randomly organized, rather than reflective of the topography of the hand. For example, the patch of skin linked to the middle finger is not located next to the patch of skin linked to the ring finger. “It will take time to figure out the mechanisms that guide reinnervation, as well as if we can direct it to get more-refined results for patients,” says Kuiken. “The brain may reorganize itself to take advantage of the information given to it.”
The researchers, along with collaborators from different institutions, are now developing new components to add to prosthetic arms that will allow them to sense the environment and transfer those signals to the wearer’s chest. That task is likely to be difficult: the device will need to be wearable with the prosthesis and precisely stimulate different parts of the chest.
“Our hands are incredible instruments that can feel things with exquisitely light touch and incredible resolution; to emulate that through a device is incredibly challenging,” says Kuiken. “All we’re giving our patients is a rough approximation, but something is better than nothing.”