Targeted Muscle Re-Innervation Pushes Prosthetic Performance
Ben - When somebody loses a limb, although it's possible to replace the missing part with a prosthesis, making it move is another matter entirely. But a technique being pioneered at the University of Chicago could change that.
Todd Kuiken has been experimenting with targeted muscle re-innervation surgery, and what he does is to take the nerves that used to supply the severed body part and reroute them into a piece of muscle further up the limb. Now, when the patient thinks about moving the missing body part, the rewired muscle will change its activity instead. By using electrodes to eavesdrop on that activity, it can be used to control the motors of a prosthetic arm.
Chris Smith met with Todd Kuiken and his patient Sgt. Glen Lehman, who's actually undergone this procedure, at the AAAS Conference in Washington DC...
Todd - Our big challenge is how to control an artificial limb. You lose your arm and we can make robotic limbs, but how do you tell it what to do? So we've developed a technique that we call targeted muscle re-innervation where we've developed a neural interface to capture what the person wants to do with their limb. Essentially, the way it works is we take the major nerves that used to go to the amputated arm, and they're still functional. They send motor commands, and if you'll stimulate them, you'll feel the missing arm. So we take those nerves and we transfer them to some spare muscles in the residual limb. Those nerves will then grow into those muscles and when they - Glenn for example - thinks to close his hand, now his medial biceps contracts, and we can detect a signal from that muscle contraction and tell his artificial hand to close. In this way, we can get much more function and its intuitive. He thinks to close his hand. His hand closes.
Chris - Glenn, can you give us a demonstration?
Glen - I can.
Chris - First of all, talk us through what actually happened to you and how long have you been using the prosthesis you've got?
Glen - November 1st, 2008, I lost my arm. I was on a combat patrol in Iraq, they threw an RKG-3 hand grenade at my truck, it penetrated the armour and separated or amputated my arm. After that, I was evacuated and sent through Walter Reed where I received treatment. Dr. Baechler and Dr. Kuiken came to me and asked me if I would be a candidate for the targeted muscle re-innervation surgery, and then just this last week, I received this arm or went out to IRC in Chicago, and trained with them with this arm.
Chris - Can you show us what it can do?
Glen - I can raise them lower the elbow, I can rotate the hand, so it's in our out. I can open and close the hand, and I can flex the wrist either in or out. Those movements were all controlled by me, thinking about my phantom limb.
Chris - So you're thinking about moving fingers that you no longer have, but are present on the prosthesis obviously, and those thoughts are being translated into what the prosthesis does.
Glen - Yes, that's correct.
Chris - Is it easy to learn to do?
Glen - I've only used this arm for two weeks, so it was very easy.
Chris - What sort of resolution of movements can you manage? If I gave you some peas to pick up, could you do that?
Glen - I believe I could, yes. The larger the item, the easier it is to actually grasp. I mean, a bottle of water or something like that is easier. It's very hard to pinch things off the table.
Chris - If you didn't have this, what would you have instead and in what way has this enriched your life?
Glen - I would just have a conventional arm. I would be able to operate the elbow and the hand, but it wouldn't be simultaneous. I would only be able to cycle through each thing by switching co-contracting muscles. So it's like the comparison between a minivan and a sports car. It's different categories.
Chris - Martin [Baechler], you had to do some of the surgery to make this feasible. What's actually involved in implementing a prosthesis like this in terms of actually rerouting the nerves to muscles and so on?
Martin - Well the performance of the surgery is actually very simple. The anatomy is predictable and the procedures of transferring the severed nerve to a healthy piece of muscle is quite simple.
Chris - How long does it take patients to actually begin to use and work one of these prostheses?
Martin - Well since we put the severed nerve so close to the new muscle, it only takes a couple of months before we start getting some contraction of the muscle. It may take six months or maybe even a year before it fully matures and the connection between the brain and that newly innervated muscle is plateaued and it's what's going to be.
Chris - Just coming back to you Todd, in terms of actually how the process works, so there are actually, physically, electrodes which are listening to what the muscle is doing when Glen thinks what he wants to do.
Todd - That's correct. We have sets of little electrodes that are like antennas, listening to his muscles.
Chris - If he had had his injury 20 years ago compared to very, very recently - you got to him when it was a fresh injury, would that have made a difference to whether this could be done?
Todd - It may have. The answer isn't really known. We're comfortable doing the surgery for 5 or 10 years after injury in a younger patient, but there may be a limit someday that we'll find, that we don't want to cross over. We know that the nerves are still viable for decades after the injury, but how good they are at regenerating is a question.
Chris - And what about sensation because at the moment, he obviously can see what he's doing, but he can't feel what he's doing. What about taking it into that domain next?
Todd - We have been able to provide hand sensation for some of our amputees. What we do is cut the nerve to the skin over these nerve transfers and then the hand sensation nerves will grow into this residual limb skin. So when you touch it, it feels like your touching the missing hand. It's very exciting for us because it gives us potential of putting sensors in the prosthesis to see when you touch something or how hard you're squeezing and feed that information back, so that the patient feels that they're getting that touch or pressure on their missing hand.
Ben - And if this prosthetic is like a sports car compared to the traditional minivan, then just imagine what it would be like when you can feel what your prosthetic is touching as well. That was Todd Kuiken, Martin Baechler and Sgt. Glen Lehman, speaking to Chris Smith at the AAAS Conference in Washington this week.