Bionic Prosthetics Using "Smart" Signals

Limb amputations can be extremely debilitating, especially to those who lose an arm above the shoulder. If you think about it, the majority of tasks you complete each and every day revolve around the use of your arm and hands. Without this vital limb, most jobs become overly arduous and some become impossible. This was the case for Les Baugh, who lost both arms at the shoulder due to an electrical accident when he was younger. Despite his devastating injury, Baugh might now able to perform daily life tasks thanks to a new prosthetic arm that is controlled by his thoughts.

In a presentation on Targeted Muscle Reinnervation (TMR), Dr. Gregory Dumanian explained the use of TMR in creating new prosthetics for amputee patients. The big problems of previous models of prosthetics for upper limbs is that those prosthetics can only move one joint at a time and the prosthetic is controlled by the wrong muscle signals.  These prosthetics lack adequate control methods because the proximal muscles used to generate motions of the prosthetic hand and wrist are not normally used to direct those movements.

In order to solve that problem, Dumanian integrated TMR into a new prosthetic limb. Even if an arm is lost, the main nerves in the arm above the injury are still attached to the brain. What he did not yet know was how to utilize those signals to create a “smarter” prosthetic. With knowledge that muscle signals are 100 times stronger than nerve signals, Dumanian was able to create a technique to both utilize the nerves in the arm and the muscle signal generated by the innervation of the arm nerves.

Motor nerves are the nerves that either contract or relax our muscles during coordinated movement controlled by the brain. When a motor nerve is cut, the nerve will search for a receptor or a muscle to innervate. To take advantage of that property, Dumanian took those nerves in the amputated arm and connected them to another muscle that is not normally used, such as the pectoral chest muscle. Connecting these nerves to the new muscle provides a new muscle to be contracted by the signal starting at the brain. From there, he placed electrodes over the newly innervated muscle to detect the “correct” signal. In order to move the hand or wrist, the brain sends the same signals as if the arm were still attached. The signal is instead sent to the newly innervated chest muscle, which contracts. When the muscle contracts, the computerized prosthetic detects the signal pattern and initiates movement of the whole limb, hand, or wrist. These movements are synchronized and executed with as much conscious effort as a person without an amputation. Through TMR and pattern recognition of the signals generated by the chest muscle, this new prosthetic can intuitively “read minds” of those with these inventions. 

With this new procedure and a robotic arm with 26 joints, Mr. Baugh can use his new Modular Prosthetic Limbs (MLPs) to perform daily tasks that do not require proximal muscle use. This new technique and prosthetic now allows for intuitive motion using “smart” signals to generate movement, broadening the potential for dexterity of prosthetic arms and hands. It also points out the greater need for “neural-machine interfaces” to provide sensory feedback to a person with an amputation and allow that person to interact with their environment. However, there is one issue with the MLP: price of the prosthetic. Currently, the Johns Hopkins University Applied Physics Lab is in contact with industry partners to investigate commercial options to make it more affordable for those with upper limb amputations. 

Another interesting feature of Dumanian’s presentation was the discussion about phantom limb pain. When a sensory nerve is cut, neuromas are likely to ensue. A neuroma is the swelling of the nerves or growth of the nerve tissue, which can become very painful. As such, neuromas are very common in patients with amputations. Dumanian found that when TMR and targeted sensory reinnervation (TSR) were implemented to amputee patients, their phantom limb pain prevalence decreased. TSR is similar to TMR because the afferent nerves are connected to a different skin surface. After noticing this correlation, Dumanian began implementing TSR to patients without amputations, but with painful neuromas. He found significant improvements in pain for those patients with Morton’s neuromas and neuromas located in other parts of the body.

Sources

Kuiken, Todd A., Laura A. Miller, Robert D. Lipschutz, Blair A. Lock, Kathy Stubblefield, Paul D. Marasco, Ping Zhou, and Gregory A. Dumanian. "Targeted Reinnervation for Enhanced Prosthetic Arm Function in a Woman with a Proximal Amputation: A Case Study." The Lancet 369.9559 (2007): 371-80. Web

https://www.nytimes.com/2015/05/21/technology/a-bionic-approach-to-prosthetics-controlled-by-thought.html

http://orthoinfo.aaos.org/topic.cfm?topic=a00158