Hi guys, it's Wasim from Curious Doc.
Today we're going to talk about this really fascinating technique which allows amputees
to control their prosthetic limbs with just their thoughts.
It's called targeted muscle reinnervation or TMR for short.
So let's do a quick rundown.
When there's a patient with an amputation, a lot of their muscles become useless because
they're no longer attached to a limb and a lot of the nerves become useless too because
they're no longer innervating muscles in that limb.
So not too long ago doctors and scientists had a bright idea to redirect the useless
nerves to a useless muscle which is called the target muscle.
When the target muscle contracts the signal could then be read using skin electrodes and
sent to motors on a prosthetic limb to move in the way that the amputee wants.
So imagine a patient wants to grab a cup of coffee.
The nerves responsible for that movement are redirected to a part of a target muscle.
That part of the target muscle contracts and this contraction is picked up by an electrode
which tells the prosthetic limb to close the hand.
It's really exciting technology because it has a lot of benefits over traditional ways
of moving prosthetic limbs which we'll explore in a bit more detail.
So the first case of targeted muscle reinnervation was developed by two doctors at Northwestern
University.
They had a patient who had bilateral arm amputations all the way up to the shoulder.
The two pectoralis muscles, pectoral major and pectoralis minor, were no longer functional
in this patient because they usually attach the humerus which was no longer there.
So the goal was to reinnervate the pec muscles with the nerves that usually innervate the
rest of the arm.
There are 4 main nerves which innervate the arm.
The musculocutaneous nerve is responsible for flexion of the elbow.
The radial nerve is responsible for extension of the wrist and digits.
And the ulnar and median nerve together are responsible for flexion of the wrist and digits.
This is a super simplified view of the nerves but enough to understand how TMR works.
So what the doctors from Northwestern did was they first cut the native nerves that
innervate the pectoralis muscles.
They then transferred the musculocutaneous nerve to the upper portion of the pec major,
the median nerve to the middle part of the pec major and the radial nerve to the lower
part of the pec major.
The last nerve, the ulnar nerve, was transferred to the pec minor.
However, since the pec minor is underneath the pec major, it would interfere with the
signals from the pec major, so it was transferred to the lateral chest wall.
After the surgery was done, the patient took a few months to heal up and for the nerves
to grow into the respective muscles.
They then played a video to the patient of different arm movements, such as opening and
closing the hand, flexing and extending the elbow or abducting the shoulder.
When the patient tried to do these movements, the researchers found that the corresponding
part of the pectoralis muscle contracted successfully.
These contractions were read by 115 electrodes placed on the skin above the pectoralis muscles
and signal was relayed to motors on a prosthetic limb.
So, for example, when the patient wants to bring a spoon up their mouth, their brain
would naturally activate the musculocutaneous nerve to flex the elbow.
But since the nerve has been redirected, the upper pec major will contract instead.
This contraction is read by skin electrodes as a spike in electrical activity, which will
relay that signal to a motor on a prosthetic limb that flexes the elbow.
The outcome of this first case was quite good.
This patient had bilateral arm amputations, so a special TMR prosthesis could be used
on the left side and a traditional prosthesis could be fitted on the right side for comparison.
They gave the patient a ball and block test where he had 2 minutes to move one-inch cubes
from one box to another over a short wall.
What they found was that the side with the TMR surgery moved 2.46 times more blocks in
2 minutes than the other side with the traditional prosthesis.
Since that first case in 2006, numerous patients have had TMR surgery with great outcomes and
an improved quality of life.
There are lots of benefits to using TMR over traditional limb prosthetics.
Firstly, you don't have to retrain your brain to use the nerves in different ways.
The idea is to use the nerves in the most natural way as possible.
In traditional myoelectric prosthesis, you'll have electrodes attached to pre-existing muscles
and you have to retrain those muscles to contract when you want to do a specific movement.
For example, you'd have to contract your trapezius muscle to move your hand, which you can imagine
would be a very weird sensation.
In TMR, you're using the nerves for their native function, so theoretically, less rehab
would be needed.
Secondly, you can perform multiple movements at a time.
Instead of concentrating on contracting your trapezius to move your hand then concentrating
on contracting your deltoid to move your elbow joint just so you can eat food, TMR can move
all the joints at the same time in a natural way.
So in the future, once this technology is further developed, the idea is that you could
ask an amputee to throw a ball, and they would be able to do so without even thinking twice.
Thirdly, it's non-invasive it doesn't require planting electrodes in the brain or in the
muscles.
We spoke about brain-computer interfaces like Elon Musk's neuralink in another video so
I won't go into it in too much detail here but basically putting anything foreign inside
the body causes the body to wall it off with scar tissue.
Over time this scar tissue prevents the implant from working properly and reading signals.
Fourthly, it significantly reduces pain for amputees.
I'm sure you've heard of phantom limb pain where amputees feel pain from the missing
limb.
The other thing that happens is that amputated nerves often grow back abnormally and form
tumours called neuromas.
Both these things, phantom limb pain and neuromas, are extremely painful but, fortunately, TMR
makes it much less likely these things happen.
Lastly, the beauty in TMR lies in the fact that the muscle amplifies the signals from
the nerves naturally.
If we try to read the signal directly off the nerves, it's very difficult because the
electrical signals produced are very weak and have a lot of noise which is hard to filter
out.
Instead if we innervate muscles with the nerves, the muscle will automatically filter out the
noise and amplify the signal so it's easy to read with simple electrodes.
Since it's still the early days for TMR, it's hard to get the exact control of the prosthetic
limbs as you'd hope.
At the moment, a maximum of 5 different signals can be achieved for shoulder amputees so it
wouldn't be possible to gain control of individual fingers.
One reason is because we're using big nerves that usually innervate multiple muscles and
we don't have the ability yet to split up the nerves into its constituents.
The other issue is that even if we were able to split the nerves up, skin electrodes are
not a very precise way of measuring muscle contractions.
There's a lot of crosstalk and interference from adjacent muscles and even your own heartbeat.
To minimise this, researchers have developed algorithms to filter out the interference
and during surgery all the subcutaneous fat is removed from above the muscles to ensure
the skin electrodes are as close to the muscles as possible.
Another big problem with TMR is that it involves major surgery.
This carries its own risks and sometimes the nerve transfer fails.
For the first patient we talked about, unfortunately the ulnar nerve to the pectoralis minor transfer
was unsuccessful due to damage to blood vessels and, as a result, the patient only had three
movements left from the other nerves.
Despite all these challenges, the future of TMR is exciting.
There's ongoing research into splitting the nerves into further muscle groups so that
amputees can gain even finer control over their prosthetic arms like moving individual
fingers.
And if we can find a way around the scar tissue problems, we might even be able to use implantable
electrodes in the muscles to detect muscle contractions better with less interference.
In August 2020, TMR was used in an osseointegrated implant for the first time ever.
I made a video already about osseointegration but basically the prosthetic limb is fused
directly to the bone, instead of a traditional socket prosthesis which can cause skin problems
long-term.
Currently, TMR is being tested in different types of amputations like those with mid arm
amputations or leg amputations.
While TMR prosthetics are miles ahead of traditional prosthetic limbs, there's still a way to go
before they are as accurate or effective as a normal limb.
I don't know about you but I'm looking forward to a future where we are all super strong
half-human, half-machine cyborgs.
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