The main reason for Kuiken and his partners to develop the TMR was to enable patients to have a more intuitive and natural movement of their prosthetic. The current available prosthetic technology makes use of different muscles to command a bionic limb. As an example for the current system, if a patient has an amputation passed the elbow, he/she will have to be trained with the prosthetic to learn how to control it. The required contractions to move a wrist, for example, would be to flex the biceps. This causes the patient’s interaction with the prosthetic to be slow, jerky, and unnatural. The following short video is part of the TED talk mentioned in the previous post. Here you can see that the patient, Jesse Sullivan, who had both arms amputated, moves almost twice as fast after the TMR surgery. It is also worthwhile to mention that he had been wearing the original prosthesis for 40 months, while the other one was only worn for 2 months, showing the extreme difference between the two.
In Sullivan’s case, he had a shoulder disarticulation on both arms due to trauma, which resulted in a surgical amputation. This means that his entire arm was separated from the shoulder joint. In more technical terms, the humerus bone was removed from the scapula and the clavicle, which are respectively the shoulder blade and the collar bone. The target muscles for Sullivan’s reinnervation were the ones in the pectoral area. As seen on the drawing below, different nerves are assigned different muscle areas, so when rooted, the nerves will control just that area.
If you recall what I wrote a few lines ago, the objective of this technique is to amplify the signal for improving a myoelectric prosthesis. Now, the unexpected and exciting discovery they made changed the course of the research. If you stop to think about the process that goes on with reinnervated muscles, you will realize that it doesn’t act differently than any other muscle in your body. Your brain simply sends a message through your nerves to your muscles saying “contract” or “extend”, and that’s exactly what was done… so what? So if it works on the path brain-nerve-muscle, shouldn’t it work the same way around? YES, it should and it does! Three months after the patients were submitted under the TMR surgery, they started a series of tests. And when they were touched in their reinnervated muscles, they could feel as if they were being touch in their lost limbs! Not only that, but they could also feel temperature, pressure, and texture as well. This is very exciting because it means that when researchers have the means for it, a fully functional limb that “feels” could be created. An immense drawback to the development of this “perfect” limb would be the absurd cost of such a device. Michael McLoughlin, the lead designer of bionic arms at Johns Hopkins, estimated that the price for a device like these would fall under the cost range of $50,000-$60,000. Although bionics with such a high pricing would be economically unviable for many people, it is usually easier for engineers and researchers to cut down the cost of something that has everything it needs to offer.
The information and pictures on this post were based off the academic paper published by Kuiken and his partners. For further understanding and direct access to the paper, click here.