Last year, ex-marine Claudia Mitchell, who lost her left arm in a motorcycle accident when she was 24 years old, became the world’s second recipient of a “bionic arm” after she had a pioneering surgical procedure performed on her by surgeons at the Rehabilitation Institute of Chicago.
Mitchell is one of only four people on whom the technique, called targeted muscle reinnervation, has been performed. In today’s issue of The Lancet, the surgical team describe the procedure. They report that Mitchell has become proficient in controlling her prosthesis, and that she can experience sensations of her missing hand being touched.
When Mitchell had her arm amputated, the nerves in the limb – called the radial, ulnar, medial and musculocutaneous nerves - remained intact down to the point of amputation. The targeted muscle reinnervation procedure is a nerve-muscle graft, which involved transplanting the intact residual nerves from the arm to the pectoral and serratus muscles, which are located just above the left breast. Sensory components of the nerves were also cut, and surgically fused to the motor components that were rerouted. This is the first time that the surgical team attempted to redirect sensory nerves to the chest; the idea occurred to them when, after performing the procedure for the first time, some sensory nerves spontaneously innervated the chest muscles. (Listen to a Lancet podcast about the procedure here.)
The procedure is possible for a number of reasons. First, on their path from the spinal cord to the arm, the radial, ulnar, medial and musculocutaneous nerves form the brachial plexus, a network of nerve fibres located under the collar bone. Following amputation, the information regarding the movements of, and sensations from, the upper limbs is propagated from the brain to the brachial plexus, but remains there as it can be carried no further. Thus, that information can transmitted to another muscle group if the nerves are transplanted. Furthermore, these nerve fibres are very large, and contain many motor neurons; it is therefore relatively easy to reroute them to a small number of chest muscles.
Some three months after the surgery was performed, and the grafted nerves had innervated the muscles in the chest, Mitchell noticed that her chest muscles began twitching when she thought about moving her missing limb, indicating that there was some recovery of nerve function. After another three months had passed, surgeons fitted Mitchell with a prosthetic arm manufactured by the Holliston, Massachusetts-based company Liberating Technologies.
Attached to the prosthetic arm are myoelectric sensors, which are contained within a harness worn over the shoulder. Thus, the electrical signals generated when Mitchell thinks about moving her arm are sent to the chest, where they are detected by the sensors and relayed to the prosthetic arm. After a few days of intensive training, Ms. Mitchell was able to move the hand, and she can now control the hand, wrist and elbow of the prosthesis simultaneously. She reports that controlling the artificial limb is intuitive – when she thinks about performing a movement, the prosthesis moves accordingly. And when the area of her chest overlying the nerve-muscle graft is pressed, she feels as if the fingers of her missing limb are being touched.
This shows that there is a possibility of providing amputees with real sensory feedback from their prosthetic limbs. Professor Todd Kuiken, who led the surgical team and is lead author of the Lancet paper, is expanding the technique in the hope that it may work for patients with amputated legs. He believes that, at some point this year, he will be able to provide such prostheses to U. S. troops who have been injured in Iraq and Afghanistan.
Kuiken and his colleagues are also trying to develop pressure and temperature sensors that would be fitted into the fingertips of the next generation of prosthetic arms. The aim would be to transmit the information from these sensors to a device on the skin in the chest, which in turn would relay the information to the sensory nerves. This would make the prostheses more complicated, and the additional wiring and connections needed would make the devices heavier than they are now. It is likely, however, that these drawbacks would eventually be overcome as the technologies for making prostheses became more advanced.
Kuiken, T. A., et al. (2007). Targeted reinnervation for enhanced prosthetic arm function in a woman with a proximal amputation: a case study. The Lancet 369: 371-380.
Kuiken, T. A. et al. (2004). The use of targeted muscle reinnervation for improved myoelectric prosthesis control in a bilateral shoulder disarticulation amputee. Prosthet. Orthot. Int. 28: 245-53.