The Bionic Arm

    Bionics: the study of mechanical systems that function like living organisms or parts of living organisms. This is the topic of recent and exciting scientific endeavors aimed at returning relatively normal functioning to patients recovering from amputations of limbs.  This research is finding ways to restore neural connections to, and thus neural control of, the prosthetic replacement for the amputated limb.  For Amanda Kitts, who was in an accident that left her with only the uppermost part of her left arm, a “stump” in the vernacular, bionics is providing a prosthetic arm that not too long ago would have existed only in science fiction.  Ordinarily Amanda would have been fitted with a prosthetic limb very limited in its ability to replace the functions of her natural arm.  Now, thanks to the scientists at the Rehabilitation Institute of Chicago (RIC) and a technique known as targeted muscle reinnervation, Amanda is experimenting with a neurally controlled, motorized, bioengineered arm that can approximate most of the functions of a normal arm (1).

    Most amputees receive body-powered prosthetics, such as those that operate the hand via remaining shoulder motion that is captured with a harness and transferred through a cable (2). These devices, however, do not effectively restore function and are only capable of operating one joint at a time. Tasks such as buttoning a shirt or making a sandwich require much more coordination of the joints and muscles than is allowed by body-powered prostheses.  Amanda, who can now fold her shirts, was fortunate to be chosen to test-drive the latest prosthetic technology.  Dr. Todd A. Kuiken, a biomedical engineer and physician at the RIC, is credited with developing what is now referred to as the “bionic arm,” and he is in the forefront of research into targeted muscle reinnervation (1). Targeted muscle reinnervation, or TMR, is a surgical technique that transfers “residual arm nerves to alternative muscle sites. After reinnervation, these target muscles produce electromyogram (EMG) signals on the surface of the skin that can be measured and used to control prosthetic arms” (2).

     TMR depends on the ability of the nerves in the stump to conduct neural signals to and from the brain.  The phenomenon of the “phantom limb” shows that these nerves continue to transmit signals. In an amputated arm, for example, in which only the upper part of the limb remains, the muscles and sensory input for the lower portion are missing. However, the whole arm still exists as a part of a stored motor representation known as the central pattern generator (CPG).  The CPG for the limb is found in the nervous system, not in the limb itself.  The sensation of movement of the amputated limb is still detected by the brain because the central pattern generator for that limb is still functioning and sending signals to the brain. Corollary discharge signals are activated with movement of the arm (stump) and carry these input signals back across the nervous system to the sensory side, initiating the amputee’s feeling that the lower part of the arm is moving.  Amanda has an image left in her brain of her arm, now a phantom, and when she feels her elbow flex, it is the existence of this phantom that allows this sensation.  The phantom limb phenomenon is the basis for targeted muscle reinnervation as a means for controlling a bionic prosthetic.  When the nerves that once extended down the arm to the hand are relocated, the patient will now be able to use these nerves, in conjunction with the central pattern generator, to direct the movement of artificial limbs via impulses from the brain.

     TMR surgery relocates nerves remaining after amputation to new sites to improve EMG signals that can be used to control a motorized prosthesis.  The muscle sites that will provide nerves for relocation are adjacent to the site of amputation and are less functionally relevant areas, due to the loss of the limb (3, 2). Nerves may be relocated to the chest wall or to other muscles in the upper arm.  In one patient, who underwent a bilateral shoulder disarticulation amputation, the nerves serving the limb (i.e. the radial, ulnar, and median nerves) were redirected to the chest muscles.  Additionally, “the subcutaneous tissue and fat between the skin and the target muscles was removed to allow the skin to directly appose the muscle surface…to provide clear EMG signal transmission for better prosthesis control” (3).  Amanda had a transhumeral amputation following a car accident.  Her radial nerve and median nerve were relocated to her triceps and biceps, respectively.

     Before relocation, the amputated nerves were connected to nothing.  The relocated nerves now innervate muscles, and signals to those muscles are detectable.  The labs at the RIC utilize electrodes (connected to a computer) to find the signals coming from her brain that direct the movement of her (phantom) limb. When these signals are detected, a virtual arm on a computer screen moves, a visual projection of her phantom arm. Many different muscle movements are performed, such as upward extension of the wrist or inward rotation of the hand. Once these muscle movements are performed, the signals associated with them are identified and stored in a computer in the prosthetic arm and then the arm is programmed to look for and then respond to them by activating the right motor (1). The updated prosthesis is fitted to Amanda and now it is time to practice using it.

     In a simulated apartment in the lab, Kitts uses her new arm to perform everyday tasks, such as making a sandwich. Her movements are choppy at first, but with practice they become more fluid and lifelike. However, sensation is still lacking, evident in her inability to ascertain how hard she is gripping a paper cup (she might grip it until the top pops off).  Bioengineers at the Johns Hopkins University Applied Physics Laboratory are also working on a new prototype, one that has “pressure-sensing pads on the fingertips” (1).  Other research is using the nerves relocated by TMR.  The nerves contain both motor neurons and sensory (afferent) neurons.  “The sensory afferents of the redirected nerves reinnervate the skin overlying the transfer site. This creates a sensory expression of the limb in the amputee’s reinnervated skin” (3), and it is this occurrence that demonstrates that sensory function is returned to the denervated skin and is likely returned via the original sensory pathways that once served the now missing limb. This suggests that tactile perception could be improved through tactile interface devices that would return “physiologically appropriate sensory feedback from the prosthetic devices” (3). When these individuals are touched in these areas, they feel as though they are being touched on their missing limb, an experience described as a phantom limb.

     Bionics, and targeted muscle reinnervation, is a rapidly developing area of research and is not limited to replacement of limbs. Cochlear implants and “bionic eyes” are among the small group of devices that are replacing ruined body parts. These devices are embedded in the nervous system and respond to commands from the brain. The cochlear implant involves quite a large number of electrodes that are passed into the cochlea. A microphone detects sound and the signals are sent to the electrodes and then on to the nerves. The bionic eye follows a similar pattern of information transmission. Patients wear dark glasses fitted with a tiny video camera and a radio transmitter. Video signals are directed from the camera to a computer on a belt, where they are translated to electrical impulse patterns that can be understood by ganglion cells. From here, the signals are sent to a receiver behind the ear, and then through a wire to an array of electrodes that are gently attached to the retinal surface. These impulses trigger the electrodes, which in turn trigger the cells, and finally the brain completes the process. The result: patients are able to see edges and coarse shapes (1).

     The phantom limb phenomenon has, in the past, been solely a painful and distressing reminder of what was lost. Here the phenomenon becomes the basis for a remarkable solution that will restore a degree of functioning to a limb amputee that they could never have realized before. Pushing the envelope even more, scientists are trying to use this technology to give those with damaged body parts the ability to regain some of their function, even going so far as to directly wire the motor cortex of completely immobilized patients to a computer in an attempt to allow them to move objects with their minds. Most of this research seems to have jumped from the pages of science fiction novels and into the laboratory. But what is so fascinating is that it is quickly becoming more of a reality every day. Amanda Kitts, Dr. Kuiken, and the RIC are at the leading edge of the technological advances in bionics. Bioengineering and neuromedicine are brought together to interface the living with the mechanical. Who knows what’s next.






1. Fischman, Josh. "Bionics." National Geographic Jan. 2010: 1-8. Web. 22 Mar. 2010.    <http://ngm.nationalgeographic.com/2010/01/bionics/fischman-text/1>.

2. Kuiken, Todd A, M.D., Ph.D, et al. "Targeted Muscle Reinnervation for Real-time
    Myoelectric Control of Multifunction Artificial Arms." Journal of the American    Medical Association 301 (Feb. 2009): 619-628. JAMA. Web. 26 Mar. 2010.    <http://jama.ama-assn.org/cgi/reprint/301/6/619>.

3. Marasco, Paul D, Aimee E Schultz, and Todd A Kuiken. "Sensory capacity of    reinnervated skin after redirection of amputated upper limb nerves to the    chest." Brain 132 (June 2009): 1441-1448. Pub Med. Web. 26 Mar. 2010.    <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2685921/?tool=pubmed>.