While life is all about feelings, adding the extra value of targeted sensory reinnervation (TSR) brings more complexities to the osseointegration (OI) upper-limb mix than one might imagine.
The Role of Sensation
“Nerves are a two-way street that send motor information from your brain and also carry sensory information back—like touch, hot, and cold,” explains Levi Hargrove, PhD, P. Eng., Shirley Ryan AbilityLab. “The first time they did targeted muscle reinnervation (TMR), they were expecting to get motor response—and they got sensory as well, by a lucky accident. So they started doing it on purpose and began calling the technique targeted sensory reinnervation (TSR).
“Since then, Diane Atkins and Dr. Alex Gardetto have continued to improve on that surgical technique and are applying it in a more precise fashion, largely for pain reduction. In the future, that might allow the sense of touch to be returned through the skin.
“We can convey the sense of touch by stimulating nerves directly, or by stimulating the skin that has been reinnervated. They’re both interesting approaches, and lots of ripples of those techniques remain to be explored.
“TSR is interesting for pain, because there are five to ten times more sensory fibers in the nerves in your arm than there are motor fibers in those nerves,” Hargrove says. “The ratio is more than 100,000 sensory nerves to less than 20,000 motor nerves, depending on the specific nerve and person. If the pain is from sensation, and you’re doing TSR on purpose, maybe you’re going to reduce the pain a bit more than TMR; nobody knows yet. But it makes sense and is worth evaluating. I think someone will do a study on that—and should. The field will figure it out.”
Eric Earley, PhD, explains how various control methods can tap into this sensory area. “The e-OPRA relies on a direct connection to the electrodes implanted inside of the limb, directly on the muscle; these electrodes read the muscle contraction and use that to steer the prosthesis,” says Earley, assistant research professor, Bone Anchored Limb Research Group, University of Colorado Anschutz Medical Campus.
“[In our study], we primarily used the nerve cuffs for sensory feedback, electrically stimulating the nerve to elicit a sense that is somatotopically mapped to the appropriate dermatome. The sensory stimulation will elicit a feeling somewhere on that map, in the area of the hand that used to be innervated. In addition to the sensory feedback, it is also possible to use the nerves signals to control the prosthesis just as we do with the muscle contractions.
“Instead of relying on the implantable electrodes, the Myo-Band uses surface electromyography, with electrodes on the outside—similar to what you would have with a socket. Those electrodes do not provide sensory feedback. If you wanted to provide feedback with targeted sensory reinnervation, you would need a small vibrotactile motor or a small electrical stimulator over the targeted sensory reinnervation site,” he says.
“Essentially you need three things: You need a way to measure what you’re trying to communicate; you need a computer that can interpret that and turn it into a sensory signal for you; and then you need an actuator—an output that actually provides the feedback back to the person. The first and the third elements are modular and allow you to do a lot of different things. Classically, when people talk about sensory feedback, they’re talking about grip force, but you could provide feedback about object contact, you could provide feedback about slip, you could provide feedback about proprioception or kinesthetics; so the position, the aperture of the hand, the speed of the movement, are all things that you could measure.
“And then when it comes to the output, vibrotactile is common, electrotactile is fairly common; you could even use an auditory signal, if you chose. By using a little actuator that creates skin tension, people have used skin stretch as a sensory output;” explains Earley.
“Those are all customizable responses, and I see a future in sensory feedback where those types of options are customized to the patient the same way that the patient has the ability to choose what grasping functions they have on a multifunction hand—whether a lateral grip to hold a credit card or key, or a pinch grip or a cylindrical grip.
“We were working with four e-OPRA patients in 2024, providing sensory feedback about object slip rather than gripping force. One person thought it wasn’t useful, even though his performance was better with the feedback, and one person thought it was better than the grip force feedback he was using daily.”
Depending on the person, depending on the task, these are things that should be swapped in and out, Earley says.
Osseoperception, he notes, is an area of sensory perception that is not thoroughly understood. “Like legs, the osseointegrated arms also deliver osseoperception, that in upper limbs communicates the motor vibrations of the hand opening and closing, or the wrist rotating—an additional source of incidental feedback that needs to be considered along with the rest.”
Those incidental signals, received as a result of the osseointegrated limb’s bone-anchored connection to their skeletal frame, make a profound impression on patients—who often regard their limb as a part of them rather than an add-on, Earley reports.
The level of sensory perception a patient is likely to experience as a result of TMR or TSR is a subject of investigation according to the experts we spoke with. “Those are things I expect to see studied more closely in the future,” says Haris Kafedzic, CPO, director of prosthetics, Eschen Prosthetic & Orthotic Labs, New York. “The osseointegrated upper limbs we’ve done have all received TMR, which essentially reroutes nerves to nearby muscles and provides stronger signals for control. This does help produce clearer movement patterns when patients are trying to operate their elbow, wrist, or hand. However, I haven’t received much feedback regarding the sensory side.
“That said, certain prosthetic components incorporate vibration feedback—when the user touches something, the device vibrates slightly to close the feedback loop and give them a sense that they’ve made contact,” he notes.
Brooke O’Steen, OTR, upper extremity clinical and educational specialist, ForMotion Clinic (formerly SRT Prosthetics and Orthotics), Indiana, hasn’t worked with a patient who has received TSR, but is aware of research in that area underway in the United Kingdom.
“One of the benefits of OI is that rod up the medullary canal of the bone,” she points out. “Because it’s integrated into the bone, do they get true sensation? No. But the kinesthetic or proprioceptive feedback that they get from that being in the bone is significant.”
Earley observes that the osseoperception phenomenon is frequently reported in lower-limb OI patients. “It boils down to kind of an improved sensation of their environment through their prosthetic foot in a way they couldn’t do beforehand with the socket.
“So people report having a better sense of what type of ground they’re walking on, or an ability to feel if a sidewalk is uneven, things like that. That may allow the person to have a better understanding of how they are walking, to make small, minor corrections in their walking, in their gait, in their balance—which may have benefits in, for example, reducing fall risk, although that’s very much an area of active research, and I don’t know that we have the data on that yet.”
“In an upper-limb osseointegration patient,” O’Steen says, “I think it would pare as something similar to that sensory nerve sense, or TNS (trigeminal nerve stimulation) or TSR. So, with a patient wearing their OI device at the transhumeral level, we’ve had them move a body-powered hook along the wall and they can ‘feel.’ It’s more vibratory and kinesthetic feedback, but they can feel the difference in the striation of paint, like the texture of paint strokes.”
Earley observes that for body-powered hooks especially, it’s possible to feel whether the hook is open or closed because of the tension on the cable and the harness the patient is wearing. “That’s part of the reason why, even with all the world’s most advanced myoelectric hands and arms, some people still prefer the split hook that has been around for over 100 years. It’s simple, effective, and it works 100 percent of the time.”
O’Steen reports on blind testing done recently with a patient with an OI thumb. “Even through the abutment of his thumb, just through the thumb and a device that we’re beta testing, he could sense hot and cold because of the embedded abutment.”
Can Osseoperception Be Achieved Without TMR or TSR?
“[Sensory feedback] is a big area of research,” Earley says. “My focus throughout my PhD and my post-doctoral work was on sensory feedback in arms mostly, but a lot of the fundamentals still apply. Over the past ten years in the field of sensory feedback, especially in upper-limb prosthetics, there has been an acceptance and a shift in view of how the sensory feedback is working.
“It was previously thought that people have only a limited sense of feedback because they have a prosthetic hand. You’ll provide feedback and their lives will improve, their function will be better, they’ll have better control of their prosthesis, etc., etc.
“But what that fails to account for is the incidental sources of feedback that exist within our bodies and within the environment—sources that your brain recognizes and knows how to use without added help. Examples would be vision, for one, especially for prosthetic hands, but also things like the sounds and the vibrations of the motors. I can get a sense for how fast or slow my prosthetic hand is opening or closing just by listening to the sound of the motors and feeling the type of vibration. Even with the socket, some research from 2018 showed that people actually have pretty good feedback just through these incidental sources; they can control their prosthesis pretty well without adding anything else. So it raises the fundamental question of ‘what is the point of this sensory feedback?’
“So sensory feedback, including targeted sensory reinnervation is increasingly being viewed differently; i.e., the sensory feedback it provides for the upper limb is here for more than just functional purposes—it’s also an experiential benefit, delving into the phenomenon of prosthesis embodiment, and helping to address the philosophical question of ‘how do I know that my prosthesis is a part of me?’”
Even if this type of sensory feedback provides only marginal functional benefit like the number of boxes stacked or the number of eggs picked up in a minute, which research likes to record, Earley defends the value of recording instances when patients report a greater sense of connection with their prosthesis. “Every action has a reaction that is sensible, even if it doesn’t feel 100 percent natural.
“Working in Sweden with the e-OPRA system, we were stimulating people’s nerves, but it still feels electric to some extent. But it is aligned with people’s recognition of a predictable reflexive response to a recognized stimulus: e.g., I grab an object, I feel a pulse.’ And for some people that is enough to alleviate some of that feeling of disconnection with their prosthesis.”
He points to a paper that was published in Science Robotics in 2023, studying a woman with a transradial amputation. “Through a combination of the sensory feedback, the motor control over the prosthesis, and the osseointegration and osseoperception and everything else involved, she reported a reduction in her phantom limb pain—which is suspected to be due in large part to the increased psychological connection with her prosthesis.”
Does TMR Offer Value for Lower-Limb OI Patients?
Earley recalls an early conversation with Giacomo Valle, PhD, a researcher now occupied at Chalmers University in Sweden. “Valle told me that there may be more benefit to sensory feedback in the lower limb because of things like foot placement on a staircase. For example, if I’m walking up and down a flight of stairs, I don’t have to pay too much attention to exactly where I place my foot. I’ve got enough sensory information and control over my legs that I can do that. But with prosthetic legs, it’s a much more attention-intensive activity. Having some sensory feedback as to the stability of the foot placement, how much of my foot is on the stair, things like that, would be a large benefit. The tricky thing, when it comes to osseointegration, is that the osseointegration and the osseoperception are providing a lot of incidental feedback—where people are feeling the dynamics of their foot, of the knee, of the inertia of their leg as they’re walking, the stability of their foot on the ground, so it is possible that to get additional benefit from an artificial sensory feedback system requires way more fidelity than what you would require for a socket prosthesis.
“That’s where some of that challenge comes in,” Earley concludes. “But the benefit of embodiment may still be there.”
The adventure and the mystery of research and discovery continue.
Judith Philipps Otto is a freelance writer who has assisted with marketing and public relations for various clients in the O&P profession. She has been a newspaper writer and editor and has won national and international awards as a broadcast writer-producer.
