Artificial joint restores wrist-like movements to forearm amputees
A new artificial joint restores important wrist-like movements to forearm amputees, something which could dramatically improve their quality of life. A group of researchers led by Max Ortiz Catalan, Associate Professor at Chalmers University of Technology, Sweden, have published their research in the journal IEEE Transactions on Neural Systems & Rehabilitation Engineering.
For patients missing a hand, one of the biggest challenges to regaining a high level of function is the inability to rotate one’s wrist, or to ‘pronate’ and ‘supinate’. When you lay your hand flat on a table, palm down, it is fully pronated. Turn your wrist 180 degrees, so the hand is palm up, and it is fully supinated.
Most of us probably take it for granted, but this is an essential movement that we use every day. Consider using a door handle, a screwdriver, a knob on a cooker, or simply turning over a piece of paper. For those missing their hand, these are much more awkward and uncomfortable tasks, and current prosthetic technologies offer only limited relief to this problem.
“A person with forearm amputation can use a motorised wrist rotator controlled by electric signals from the remaining muscles. However, those same signals are also used to control the prosthetic hand,” explains Max Ortiz Catalan, Associate Professor at the Department for Electrical Engineering at Chalmers. “This results in a very cumbersome and unnatural control scheme, in which patients can only activate either the prosthetic wrist or the hand at one time and have to switch back and forth. Furthermore, patients get no sensory feedback, so they have no sensation of the hand’s position or movement.” The new artificial joint works instead with an osseointegrated implant system developed by the Sweden-based company, Integrum AB – one of the partners in this project. An implant is placed into each of the two bones of the forearm – the ulnar and radius – and then a wrist-like artificial joint acts as an interface between these two implants and the prosthetic hand. Together, this allows for much more naturalistic movements, with intuitive natural control and sensory feedback.
Patients who have lost their hand and wrist often still preserve enough musculature to allow them to rotate the radius over the ulnar – the crucial movement in wrist rotation. A conventional socket prosthesis, which is attached to the body by compressing the stump, locks the bones in place, preventing any potential wrist rotation, and thus wastes this useful movement.
“Depending on the level of amputation, you could still have most of the biological actuators and sensors left for wrist rotation. These allow you to feel, for example, when you are turning a key to start a car. You don’t look behind the wheel to see how far to turn – you just feel it. Our new innovation means you don’t have to sacrifice this useful movement because of a poor technological solution, such as a socket prosthesis. You can continue to do it in a natural way,” says Max Ortiz Catalan.
New ‘e-skin’ brings sense of touch, pain to prosthetic hands
A team of engineers at the Johns Hopkins University, including one of an Indian-origin, has developed a novel e-dermis that will enable amputees to perceive a real sense of touch through the fingertips of their prosthetics.
Made of fabric and rubber laced with sensors to mimic nerve endings, e-dermis recreates a sense of touch as well as pain by sensing stimuli and relaying the impulses back to the peripheral nerves.
“We’ve made a sensor that goes over the fingertips of a prosthetic hand and acts like your own skin would,” says Luke Osborn, a graduate student in biomedical engineering. “It’s inspired by what is happening in human biology, with receptors for both touch and pain.
“This is interesting and new,” Osborn said, “because now we can have a prosthetic hand that is already on the market and fit it with an e-dermis that can tell the wearer whether he or she is picking up something that is round or whether it has sharp points.”
Bringing a more human touch to modern prosthetic designs is critical, especially when it comes to incorporating the ability to feel pain, Osborn says.
“Pain is, of course, unpleasant, but it’s also an essential, protective sense of touch that is lacking in the prostheses that are currently available to amputees,” he says. “Advances in prosthesis designs and control mechanisms can aid an amputee’s ability to regain lost function, but they often lack meaningful, tactile feedback or perception.”
That is where the e-dermis comes in, conveying information to the amputee by stimulating peripheral nerves in the arm, making the so-called phantom limb come to life. The e-dermis device does this by electrically stimulating the amputee’s nerves in a non-invasive way, through the skin, says the paper’s senior author, Nitish Thakor, a professor of biomedical engineering and director of the Biomedical Instrumentation and Neuroengineering Laboratory at Johns Hopkins.
“For the first time, a prosthesis can provide a range of perceptions, from fine touch to noxious to an amputee, making it more like a human hand,” says Thakor, co-founder of Infinite Biomedical Technologies, the Baltimore-based company that provided the prosthetic hardware used in the study.
Inspired by human biology, the e-dermis enables its user to sense a continuous spectrum of tactile perceptions, from light touch to noxious or painful stimulus. The team created a “neuromorphic model” mimicking the touch and pain receptors of the human nervous system, allowing the e-dermis to electronically encode sensations just as the receptors in the skin would. Tracking brain activity via electroencephalography, or EEG, the team determined that the test subject was able to perceive these sensations in his phantom hand.
The e-dermis is not sensitive to temperature—for this study, the team focused on detecting object curvature (for touch and shape perception) and sharpness (for pain perception). The e-dermis technology could be used to make robotic systems more human, and it could also be used to expand or extend to astronaut gloves and space suits, Osborn says.
Amputees feel as though their prosthetic limb belongs to their body
The famous idiom “seeing is believing” is not enough to help amputees with the use of their prosthetic limb. Many amputees opt out of prolonged use of their prosthetic limb because their missing limb simply does not fit their prosthesis. In other words, their own perception of the missing limb, or the brain’s representation of it, does not match-up with what they see of the prosthesis.
The underlying problem is twofold. Amputees still feel their missing limb, even if it is physically gone, and this ghost limb aka phantom limb is perceived as much smaller that the lost limb. Next, the commercially available prosthetic limb does not yet provide sensory feedback other than what the patient sees, meaning that the patient has no sense of touch from the prosthetic limb and must constantly watch it for correct use.
Now, in a scientific collaboration led by EPFL (Ecole polytechnique fédérale de Lausanne), scientists show that amputees can actually be convinced that the prosthetic hand belongs to their own body. They do this by going beyond the “seeing is believing” idiom based on established research on how the brain identifies what belongs to its own body. Instead of using the sense of sight alone, they used an astute combination of two senses: sight and touch. The results are published today in the Journal of Neurology, Neurosurgery & Psychiatry.
“The brain regularly uses its senses to evaluate what belongs to the body and what is external to the body. We showed exactly how vision and touch can be combined to trick the amputee’s brain into feeling what it sees, inducing embodiment of the prosthetic hand with an additional effect that the phantom limb grows into the prosthetic one,” explains Giulio Rognini of EPFL’s Laboratory of Cognitive Neuroprosthetics led by Olaf Blanke, in a collaboration with Silvestro Micera of EPFL and Scuola Superiore Sant’Anna in Italy. “The setup is portable and could one day be turned into a therapy to help patients embody their prosthetic limb permanently.”
In two hand amputees, the scientists provided artificial tactile sensations at the tip of the index finger – of the phantom limb – by stimulating the patient’s nerve in the stump. At the same time, the patient wore virtual reality goggles which showed the index finger of the prosthetic limb glowing in synchrony with the administered touch sensations. This combination of virtual reality with artificial tactile sensations takes the rubber-hand illusion to another level.
Cranking up the power setting may help some who use prosthetics
Amputees who use powered prosthetic ankles may be able to avoid the energetic costs typically associated with prosthetics by cranking up the power provided by their devices.
A UCF engineering professor recently published a study in Scientific Reports that shows that people with transtibial amputations—the loss of a limb below the knee—may improve their walking ability if they change the power-setting on their devices. Hwan Choi, who received his doctorate in engineering from the University of Washington, is an assistant professor in the UCF department of Mechanical and Aerospace Engineering.
According to a study conducted by the National Institutes of Health, approximately 185,000 amputations occur in the United States every year and 49-95 per cent of lower-limb amputees reportedly use a prosthesis. Most of those on the market are passive prosthetics. On average, amputees spend up to 30 per cent more energy than unimpaired individuals when performing tasks such as walking. This could be due to the fact that most ankle prostheses are passive-elastic, meaning that they can store and release energy when they come in contact with the ground but are unable to perform positive net ankle work that allows for muscle shortening contractions to occur. In fact, these prostheses are only able to provide one eighth of the power of the intact gastrocnemius and soleus muscles, the key muscles that support and propel the body during walking.
As passive prostheses increase the energetic demand on the user, individuals may have to compensate by increasing muscular effort in the residual or intact limb. Powered ankle prostheses, on the other hand, use actuators to reduce the increased metabolic costs placed on amputees by delivering positive work. BiOM (now known as EMPOWER), the only commercially available powered ankle prosthetic, uses a visual display that allows the wearer to tune the power setting on the device. Ideally, they would select a power setting between 0 per cent and 100 per cent that best approximates that of a healthy ankle at the user’s preferred walking speed. But the question remains: how much power should the prosthesis provide?
Too little power and they may experience the same metabolic costs of those using passive prostheses, but too much and they may experience problems such as knee hyperflexion and increased energy absorption in the knee that can raise the metabolic costs of the user.
Choi, along with co-authors Kimberly Ingrahm, David Remy, Emily Gardinier, and Deanna Gates from the University of Michigan, tested ten individuals with transtibial amputations. They measured the metabolic cost of transport (COT) and the BiOM’s net ankle work at different power settings, while the amputees walked on a treadmill with the BiOM ankle.
Choi said that they discovered that the ideal power that reduced metabolic cost was actually greater than biological norms. In other words, the best tested setting actually decreased the amount of excess energy used by the subject more than the prosthetist-chosen power setting.
“The key finding of this study was that none of the subjects had the minimum metabolic cost when they walk with unimpaired individuals work or power. When they had greater power, then the impaired individuals actually reduced metabolic cost.”
Smart seat cushion is adaptable for prosthetics
The University of Texas at Arlington has patented a smart seat cushion that uses changes in air pressure to redistribute body weight and help prevent the painful ulcers caused by sitting for long periods of time in a wheelchair.
The same technology can be used to create prosthetic liners that adapt their shape to accommodate changes in body volume during the day and maintain a comfortable fit for the prosthesis. Poor prosthetic fit can cause skin damage and create sores in the residual limb of the wearer.
“Pressure ulcers caused by long periods of sitting without relieving pressure at boney regions such as the tailbone, frequently occur in people who spend significant amount of time on wheelchairs. In the case of prosthesis users, poor fitting of the prosthesis leads to pressure injuries for amputees that can severely affect their daily life,” said Muthu Wijesundara, co-inventor of the technology and chief research scientist at UTA’s Research Institute or UTARI.
“Our technology improves on existing solutions by including real-time pressure monitoring and automated pressure modulation capabilities to help combat the formation of pressure ulcers or sores.”
The researchers recently presented the results of their studies on a full-sized seat cushion prototype at the ASME 2018 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference held August 26-29, 2018 in Quebec City, Canada.
When a person sits on the cushion, a network of sensors generates a pressure map and identifies vulnerable areas where pressure relief is needed. Automated pressure modulation uses this data to reconfigure the seat cushion surface to offload and redistribute pressure from sensitive areas. Additionally, the seat cushion periodically changes the pressure profile to eliminate pressure build-up over time.
The researchers demonstrated the effectiveness of the technology using healthy volunteers with different weights who assumed different positions: leaning forward, backward, to the left or right. In all cases, the seat cushion measured the pressure immediately and automatically performed an effective pressure redistribution to offload pressure from sensitive areas.
“This technology has multitude of applications in biomedical fields,” Wijesundara said. “We really feel that it shows great promise in helping patients and their caregivers avoid the pain of stress ulcers and sores.” Wijesundara added.
