Targeted Muscle Reinnervation Surgery

The science of reusing nerves.

Being able to move your arms is often something we take for granted. There are so many small details that our bodies take into consideration when preforming simple actions with our arms, like grasping a cup. To hold a cup, your brain first tells your arm to get into the right position and extend to get near the cup. Next, you need to be able to rotate your hand and open it to the specific “C” shape needed to grab the object.

Because you’ve preformed the task so many times before, your brain knows the exact force needed to hold the cup without the grip being too loose or too tight. On top of that, we don’t even need to think about this motion. Everything happens almost instantly once we make the decision to hold the cup.

With all of these calculations that prove essential to controlling a limb, how can prostheses work? Traditionally, prosthetic limbs were not very functional because of this reason. They had very limited ability and did not support many essential actions that we need to be independent. But thanks to a field called bionics and great advances in surgery, in the last few years surgeons are able to reuse nerves from an amputee to create more functional prostheses.

Can Nerves Regenerate and be Reused?

In the central nervous system (CNS), once neurons are damaged they permanently stay that way. This is why injury to the spinal cord and brain stem can cause permanent paralysis. However, nerves in the peripheral nervous system (PNS), such as those controlling your limbs do have the potential to regenerate even though this growth is slow (a few millimeters a day). Because of this, nerves in the PNS have the ability to be reused and reattached to other muscles in the same pathway if the length of the nerve allows for it.

In targeted muscle reinnervation surgery (TMR), existing nerves from amputees are reattached to surrounding muscles to create more distinct movements for electrodes to pick up on. After turning these precise signals/movements to actions, prostheses can become much more capable.

This surgery is usually done on those with amputated arms and with two surgical plans: one for the ventral and dorsal side of the arm.

An adipofascial flap (fat and fashia tissue) is used in both plans to physically separate muscles from each other. This is to make sure that signals that are picked up by the prostheses are further differentiated and are not influenced by surrounding muscle movement.

Relocations of Nerves

In total, there are around five relocations to be preformed if the length of the nerves allow for it:

  1. The musculocutaneous nerve to the lateral biceps.

The musculocutaneous nerve controls the biceps and skin on the side of the forearm. This nerve needs to be rerouted to serve only the lateral biceps, which is a part of the first surgical plan. The end neuromas formed on all nerves because of the amputation must be resected before reattached to the respective muscle for better control. Neuromas are large benign growths usually found at the end of the nerve with severed communication because of amputation. The attachment to the lateral biceps with the musculocutaneous nerve allows for elbow flexion on the prostheses, or contracting the elbow.

2. The median nerve to the median biceps.

The median nerve is another nerve that originates from the brachial plexus. The median nerve is the only nerve that travels through the carpal tunnel in the wrist, and is the nerve responsible for carpal tunnel syndrome. In an amputee, this nerve can be used and reattached to the median biceps for control of the hand in prostheses. More specifically, the median nerve produces the “hand close” signal which is turned into the action by the prostheses.

3. The distal radial nerve to the lateral triceps.

The radial nerve is used to control the thumb, index, and middle finger in the hand. It also controls the triceps muscle and provides sensory information about part of the hand. In TMR surgery, the distal radial nerve is attached to the lateral triceps to produce the “hand open” signal, which is then translated into action by the prostheses.

4. The proximal radial nerve to the long triceps.

The proximal radial nerve is attached to the long triceps. The image to the left shows the difference between the muscles controlled by the proximal and distal radial nerves. This rerouting of the proximal distal nerve allows for elbow extension to occur through the prostheses. This is extremely important for elbow functionality since it distinguishes between advanced prostheses and more limited kinds.

5. The ulnar nerve to the brachialis.

The ulnar nerve (visible in previous diagrams) controls part of the hand and sends sensory information along the skin of it’s pathway through the arm to the brain. This nerve must be attached to the brachialis muscle, which is then responsible for controlling the hand and wrist in prostheses, which includes rotating the wrist.

After all nerves have been reassigned to individual surrounding muscle and separated by adipofascial flaps, nerve signals can be easily picked up on by prostheses. This means that no special sensors need to be implanted in the brain to produce mind controlled prostheses and that movement comes naturally.

The recovery time for this surgery is a few months, and after that, more procedures need to be done towards fitting of the prostheses to make sure nerve signals are accurately being turned into electrical signals for the device, then turned into precise movements. The results, however, can be extraordinary with patients often feeling as if their prosthetic is their real arm. This is also due to advances in machine learning that is used to compute angles and positioning of the prosthetic at all times.

Accidents in which limbs are lost are very common, with over 2.2 million people living with limb loss in the US in 2019. With advances made in TMR surgery and the field of bionics (basically biological robotics), the world is seeing more and more introductions of these advanced prostheses.