Our new print edition includes a brief introduction to AMI surgery, the new amputation procedure that’s been generating lots of media buzz lately. In addition to its potential to support improved bionic technology, AMI seems to mitigate phantom limb pain. While early tests of the surgery have involved first-time amputations, the technique is said to be viable as a modification to an existing amputation.

For a deeper dive into how AMI surgery works, we turned to Bionics for Everyone, the authoritative, highly accessible encyclopedia of advanced prosthetic technology. Our thanks to Wayne Williams for sharing this article. Read the original at BFE.

by Wayne Williams

All human skeletal muscles work in pairs, which allows us to both flex and extend our joints. The mechanical benefit of this is obvious. What isn’t so obvious is the critical role these pairings play in our brain’s ability to perceive the position and movement of the body in space, otherwise known as proprioception.

Hugh Herr, one of the pioneers
of AMI surgery

The brain retains a dynamic mental map of the body. When muscles move, this information is passed through nerves to the brain, which automatically adjusts its map. This is the natural feedback mechanism that allows you to close your eyes yet still touch your finger to your nose.

When you move a limb, the contraction of one muscle (the agonist) results in the extension of another (the antagonist), and the corresponding nerve signals enable the brain to map the limb’s new spatial coordinates. Unfortunately, agonist-antagonistic pairings are often severed in traditional amputations, so the brain loses much of the feedback it needs to track a severed limb’s position. This loss also seems to contribute to phantom pain.

Enter AMI Surgery

AMI (or agonist-antagonist myoneural interface) surgery is a method to provide proprioception from a synthetic device to the human nervous system. Invented by Hugh Herr’s Center for Extreme Bionics at the MIT Media Lab, AMI gives amputees better control and positional awareness of a bionic limb.

First, agonist-antagonistic muscle pairings are surgically recreated, one for each bionic joint that is to be controlled. Even without the attachment of a bionic device, this improves the connection between the brain and its residual limb. If you close your eyes and attempt to flex your missing foot upward after this surgery, the associated muscle pairs will contract/extend in response, and the brain will receive information about this movement. In other words, the AMI procedure itself restores a degree of natural proprioception.

However, most amputees don’t want to just move their phantom limbs. They want to interact with the real world via prosthetic limbs. And, here, the role of AMI is somewhat more complex. To explain this, we’ll use the example of a bionic ankle/foot.

If you have read our article on Bionic Leg & Foot Control Systems, you know that most current bionic lower limbs do not share any kind of connection to the user’s brain. Instead, local sensors monitor the position, movement, and speed of the bionic joint many times per second and transmit this information to a microprocessor. The microprocessor interprets this information and adjusts the bionic joint accordingly.

To take a specific example, here is how Blatchford’s Elan bionic foot/ankle adjusts when you walk briskly or go up a slope. The plantarflexion resistance increases as you accelerate or ascend, allowing for more optimal energy storage/return. Dorsiflexion resistance softens at the same time, aiding forward momentum and body position while minimizing the effort required to walk quickly or uphill.

This is a wonderful innovation, but it is entirely reactive. The foot must first experience the terrain before adjusting to it. Scientists have been looking for ways to use myoelectric control to make this process more proactive, following a sequence like this:

1) The user sees an approaching slope.
2) The brain sends signals to the muscles in the residual limb to adjust for the slope.
3) Myoelectric sensors embedded in the muscles detect these signals and send them to a microprocessor.
4) The microprocessor translates the signals into commands for the bionic limb.
5) The bionic limb adjusts for the approaching slope, perhaps by raising the front of the foot while in Swing Phase (i.e. when the foot is in the air) and adjusting the foot’s hydraulic resistance while in Stand Phase (i.e., bearing the body’s weight).

This solution is obviously an improvement over a purely reactive system, but it still suffers from a major flaw: The brain has poor awareness of the bionic limb’s position. As a result, the user must visually guide key aspects of the limb’s movements, which is not only tiring but can also impair multi-tasking.

AMI eliminates this major flaw. It still uses myoelectric sensors to detect muscle movements, and these signals are still translated into commands for the bionic limb. The difference is that the control system for the bionic joint is specifically calibrated to the muscle actions of the restored agonist-antagonist muscle pair. Because of this, the brain can sense the position of that joint by interpreting the natural feedback from that muscle pair.

Agonist-antagonist pairings are
surgically rejoined in AMI surgery

This restored proprioceptive sense makes it possible to control the bionic limb without having to look at it. For example, an AMI user can approach a set of stairs the same way that a person with natural limbs does: by glancing at the stairs, letting the brain intuitively calculate the geometry of the situation, and then simply relying on the brain’s proprioceptive sense of the bionic limb and the improved control system to carry out the required movements.

As an added benefit, it appears that AMI reduces phantom pain (likely by restoring the brain’s connection to its missing limb) and also reduces muscle atrophy in the residual limb.

All in all, this is a truly beautiful solution.

Adding in Sensory Feedback

The one thing missing from an AMI solution is a sense of touch, in which information from artificial sensors on a bionic limb is communicated to the user’s brain via nerve stimulation. This capability helps the user connect to the outside world and may further reduce phantom pain. But it requires surgery to implant electrodes that stimulate the nerves, and surgery always carries significant drawbacks such as high costs, the pain of recovery, potential nerve damage, the risk of infection, and the risk of scarring, which may eventually inhibit the ability to stimulate the nerve.

There is also a problem with precision: It is notoriously difficult to precisely stimulate a nerve with an electrode. If the calibrations are slightly off, touching an object with a bionic forefinger may be misinterpreted as touching it with the pinky instead, which could lead to a certain amount of clumsiness. To make matters worse, recent research indicates that the brain is not capable of adjusting its sensory map to overcome such errors.

Given the obvious benefits of AMI and the risks/limitations of current sensory feedback technologies, it’s reasonable to ask if an AMI solution can satisfy most user needs without sensory feedback. The answer to this question may differ for lower-limb versus upper-limb bionics.

Lower Limbs

When it comes to lower-limb bionic devices, the most desirable type of sensory feedback is simple contact. Knowing that the toe of your bionic foot hit an obstacle that you were trying to step over is an early indicator that you might stumble or fall. The same might be said of stepping onto a hard versus soft surface, as this would allow the user to adjust their stance for optimal shock absorption. But there are other sensory clues to these events, such as sound or vibration.

Knowing that you’ve stepped onto a slippery surface or uneven terrain might be helpful, but the first consequence of such situations is always sudden movement, which might be communicated to the brain via AMI. It could also be dealt with to some extent by the bionic limb’s local sensor and microprocessor system.

Being able to sense temperature cannot be reproduced by other means, but this ability has very few use cases, and bionic limb designers always have the option of making the user aware of unsafe situations via audible alarms or stimulating the skin under the socket using small vibrators or transcutaneous electrical nerve stimulation (TENS).

The fact is, once you create a working proprioceptive sense for a bionic lower limb and dramatically improve user control, adding advanced sensory feedback seems to offer only limited functional value.

Upper Limbs

It should be noted that AMI has not yet been attempted for upper-limb amputees, though scientists believe it should work in principle. If this proves to be the case, sensory feedback will be very important to upper-limb amputees using bionic limbs after AMI surgery.

To understand why, imagine a scenario where you are headed out your front door. You glance at the house key sitting on a hallway table and reach for it with your natural hand as you pass. You don’t attempt to pick up the key precisely because your gaze has already moved on before you pick it up. Instead, you grab it in whatever way is convenient and then rely on a combination of sensory feedback and nimble finger adjustments to correctly position the key by the time you are ready to insert it in the lock.

If you have undergone AMI surgery in your forearm, you will likely be able to reach for the key in much the same way you would with your natural hand, but here is where a lack of sensory feedback would inhibit you. Without it, you have very little chance of grasping the key and no chance at all of manipulating it into the right position. You would be forced to visually manage both of these tasks.

The same could be said for most upper-limb tasks. AMI might be part of a package that eliminates the need for advanced sensory feedback in lower-limb bionics—and we emphasize the word “might” here—but it has no hope of doing the same for upper-limb bionics. People need to feel with their hands, period. For more information on this subject, see Sensory Feedback for Bionic Hands.

Using AMI to Modify an Existing Amputation

AMI has only been around since 2016 and has only been used with about a dozen patients. In every case, AMI has been implemented as part of the amputation surgery itself, which is the ideal time to incorporate this new approach.

But what if you have already had your amputation? The good news is that it appears possible to leverage the regenerative capabilities of nerve and muscle tissues to build an AMI in a followup surgery. Scientists are expecting to run trials on this technique in the near future.

One of Bionics For Everyone’s main missions is to help those with disabilities to understand the technology options available to them, while we remain free of commercial influence. AMI is an important one of those options. We believe it is so groundbreaking that anyone facing amputation from this point forward should explore whether they can incorporate AMI into that surgery, even if the goal might be just to preserve the possibility of better control and restored proprioception for a bionic device at some later point.