Modern prostheses have come a long way from their hand hook and pegleg predecessors. Although they look a lot more like the real thing, they are still lacking in the natural feel department. To address this, researchers at MIT are working on a new surgical technique that uses existing nerves and muscle grafts to provide the wearer with a better sense of where the prosthetic limb is in space and how much force is put on it.
When a limb is amputated, the surgeon's first priority is to remove the injured section and configure the remaining muscles, bones, blood vessels, and nerves into a stable package of a stump that will heal properly. It's to this stump that a prosthetic is attached and, though there's been significant progress in things like myoelectrics, machine/brain interfaces, and robotics as a way to control artificial limbs and even to give them something resembling a sense of touch, they still don't feel like a natural limb.
According to the MIT team, what's lacking is proprioception. That is, the ability to sense the relative position of a limb in space and how much force is being applied to it. In a living limb, there are muscle spindles in the muscles and tendons that act a bit like strain gauges that allow the brain to sense how muscles work against one another. But amputate the limb and those muscle spindles go with it.
However, most limb muscles don't work individually, but in pairs. For example, using the flexor muscles to bend your leg will also cause extensor muscles that straighten them out to stretch. The balance between these allow limbs to move smoothly and precisely, while the spindles send signals to the brain, so it's possible to know what position a limb is in and to balance things without looking. This is called the agonist-antagonist muscle relationship.
But in an amputee, the stump muscles are cut off from this ability and the wearer of a prosthetic must use their eyes to see where the limb is and what it is doing. As to balancing, there is no way to gauge forces and compensate without some sort of robotic system.
The MIT approach is to recreate the agonist-antagonist muscle relationship by using intact existing nerves and connecting them to muscle tissue measuring 4 x 1.5 cm (1.6 x 0.6 in). This muscle tissue has been removed from other parts of the body and grafted into place on the stump to create agonist-antagonist pairs.
The brain is able to send signals to the pair and when one muscle contracts, the other stretches. This produces feedback, which allows the patient to sense how the limb moves and how hard. As a refinement, the team also developed a control system to translate nerve signals using a microprocessor to provide better control of the artificial limb to the point where it's even possible to sense the torque in an artificial wrist.
"Using this framework, the patient will not have to think about how to control their artificial limb," says Hugh Herr, lead author of the study. "When a patient imagines moving their phantom limb, signals will be sent through nerves to the surgically constructed muscle pairs. Implanted muscle electrodes will then sense these signals for the control of synthetic motors in the external prosthesis. We think that because the brain is so good at remapping and it's so plastic, it will quickly adapt to knowing how much it has to contract each muscle graft for natural prosthetic control."
So far, the concept has been tested on rats, where sensory information was transmitted to the brain. The team sees the technology as working on nearly any amputee – both those who lost their limbs recently and years ago.
"For almost any amputation scenario, as long as we have a little bit of the healthy nerve left, we can take that and put it into regenerative muscle grafts," says graduate student Shriya Srinivasan, "We can harvest these muscle grafts from almost anywhere in the body, making this applicable to a large number of cases ranging from trauma to chronic pain."
The study was published in Science Robotics.