Can Prosthetics Sense Pain?
By Kasha Patel
BU News Service
Over 1,500 soldiers return from Iraq and Afghanistan with missing limbs. Many amputees are looking to prosthetic limbs, but will the prosthetic limb be able to feel pain, softness, or coldness like the real thing?
Yes, but not as you would expect.
Amputees who receive a mind-controlled prosthetic limb through a technique called targeted muscle reinnervation (TMR) might feel sensation, but not in the expected body part. For example, if you touch the prosthetic arm of a TMR patient, he or she will not feel anything, but touch you touch the chest, the patient might think you’re poking the missing hand.
Annie Simon, a biomedical engineer who focuses on the control of artificial limbs, explains that when a person loses an arm, the severed motor and sensory arm nerves are still connected to the brain and are functional. The brain can tell the nerves to fire but because the nerves are not attached to anything, there will be no effect. However, if you touch the nerves, the brain will sense it.
In TMR, the unattached end of the nerves is rerouted and connected to the chest. Even though the nerves are connected to the chest, they still have the same function as before, which was to control the arm.
“The brain doesn’t even know that nerve has been connected to a different muscle. It’s sending out the same signal as if it was controlling a hand muscle,” said Annie Simon who works on targeted muscle reinnervation at the Chicago Rehabilitation Institute.
From the arm nerves on the chest, electrodes are connected to a device that produces an electromyogram (EMG) signal that eventually leads to prosthetic arm movement. While the message from the motor nerves is relayed to the prosthetic arm, the signal of the sensory neurons is not. Therefore, the person only feels sensation as far as the sensory neurons reach— and sometimes that means feeling hand sensations in the chest.
However at the University of Pennsylvania, Neurosurgeons Douglas Smith and D. Kacy Cullen are developing a technique in which amputees can control their prosthetic arm with their brain and feel objects and sensations with the prosthetic arm. In this technique, the amputee’s arm nerves are intertwined with graft neurons to extend down and connect to the bionic arm. The graft neurons are connected to an electrode in the bionic arm.
Incorporating sensory feedback into the prosthetic arm though has an inherent complication though. An amputee can relearn to pick up an object because the brain remaps the motor neurons fairly easily. The brain has a much harder time remapping sensory neurons and coordinating a sensory neuron with a specific sensation, as that is a much more difficult task.
To overcome that problem, the prosthetic arm has receptors called transducers that are programmed for certain sensations. For example, if a person says a particular electrode feels like the first finger on the left hand, then a pressure transducer would be connected and the brain would not have to remap any sensory neurons.
“If you have a prosthetic and you already programmed the sensory feedback, there is no training,” said Smith. With this brain-prosthetic interface, messages can now be sent back and forth between the brain and transducers on the bionic arm.
While the technology is promising, it is still in the experimental stage.
“Our [approach] is harder to do now because you can imagine how many connections you have to worry about, [but that number of connections] will dictate the ultimate function of that hand,” said Smith.