What’s State-of-the-Art With Upper-limb Prostheses?

By its nature, upper-limb prosthetic technology has an immediate impact on the user’s social circle because the devices are more visible and are actively used to manipulate objects; thus prosthetic goals range from more tool-like functional use to mimicking the appearance and movement of the human hand. The clinical prosthetic goals are to provide protection and balance, encourage bimanual activity, allow incorporation into body image, minimize harnessing, maximize suspension, and maximize the number of grip options. This is provided by more pragmatic design specifications to mimic physiological hand speed and provide functional grip strength, precise and consistent positioning and control, sufficient component rigidity and durability, and maximum grip options and positions. Each of these goals provides unique challenges being addressed with new technologies and processes but may introduce their own design paradoxes. Advanced TMR Surgical Techniques [caption id="attachment_253758" align="alignright" width="221"] Partial hand prosthesis with i-Digits’ individual finger external power control.[/caption] As a surgical procedure, targeted muscle reinnervation (TMR) has existed for almost 20 years. Classically it is a nerve-transfer process that reimplants residual nerves from the amputated limb to reinnervate muscles at new target sites. A major delineating factor is that the native nerves are detached so the targeted nerves can reinnervate the muscle groups. In this way the muscle can function as a targeted amplifier where other types of control such as pattern recognition can be used that consider even smaller signals that create patterns of signal when the patient considers various movements. TMR has shown significant improvement in neuroma pain and with the advent of a current procedural terminology code became even more popular. However, in many surgeries called TMR, the original innervation is not detached. The issue this causes is that the resulting signal pattern becomes even more complex because the new nerves may or may not innervate the targeted muscle group. Also, the controls would not necessarily be from a signal level such as the hand and wrist but may incorporate the existing innervation. This makes pattern recognition more challenging when the patient is coached for the movement. One of the areas where TMR has been applied for greater control in partial hand amputation is by Glen Gaston, MD, chief of Hand Surgery for the Orthopedic Surgery Residency at Carolinas Medical Center, where the intrinsic muscles of the hand can be targets for nerve reinnervation. Formerly, the myosites were placed on the thenar and hypothenar areas for standard two-site control. This limited the function to all fingers opening and closing and in turn contributed to a relatively high rejection rate for partial hand prostheses. By innervating the intrinsic dorsal interossi muscles, pattern recognition can be used to operate individual fingers for much greater function. Osseointegration After Upper-limb Amputation Like transfemoral-level amputations, some of the biggest advantages for osseointegration are present for the transhumeral level due to increased range of motion and decreased infection rates. Unlike lower limbs, there is more axial distraction than loading with upper limbs, so the design of the implant needs to be a bit different. Also, the attachments may not exist for the upper-limb componentry and may have to be created. [caption id="attachment_253760" align="alignright" width="237"] Osseointegrated upper-limb prostheses provide distal attachment.[/caption] For example, Eric Schatz, CPO, president and owner, International Prosthetics, New York, says, “The true benefit of utilizing 3D- printing technology for upper-extremity osseointegration is to be able to make something that otherwise was not possible with traditional fabrication techniques. As an engineer and prosthetist, I have had to develop new tools that don’t exist for osseointegration. I recently had to make a connector to an arm for a patient where the manufacturer of the implant didn’t have one. The CAD-designed and 3D-printed connector enables universal connections to any prosthesis. Due to the enclosed nature of a traditional socket, the patient was continually getting infections at the stoma, so we created a flexible 3D-printed socket with an open lattice structure that still supported the tissue, but provided adequate aeration.” Sensing and Control Options Beyond Myoelectrodes Percutaneous titanium electrodes are nothing new and were successfully used to find more accurate myo signals. Implantable myoelectric sensors (IMES) can be placed in the muscle belly for stronger signals. The issue is powering these implanted devices beneath the skin. Two EMG devices are implanted, and they are powered with an induction coil. Normally this coil would be circular to achieve accurate charging, but with a prosthesis, the coil is irregular so each one needs to be calibrated separately. [caption id="attachment_253761" align="aligncenter" width="439"] Implantable electrodes powered by an induction coil.[/caption] The prosthetist can choose between myoelectric, pull/push switches, and force-sensing resistors that must be physically actuated. Other types of sensors are being examined that can also aid patients to increase their modes of control. Sonogram sensors are one of them that “listen” to muscle contractions. Other sensors use small magnets that can measure small amounts of muscle movement in terms of positioning. Pattern Recognition and More Pattern recognition, or the ability to build a probabilistic control model, was developed at the University of New Brunswick in the late 60s. It was originally utilized with TMR surgery when it was apparent that a “weather map” of signals was being created. It uses ten parameters from each electrode pair, so 80 factors are measured, filtered, and processed in real time, and it builds a probabilistic mathematical model of movement to control the device intuitively. This allows the prosthesis user to apply control even with crosstalk or cocontraction. In fact, those artifacts, considered unfavorable for standard two-site control, are actually helpful for pattern recognition since it considers all consistent signals, even small ones, as a system. This means the patient can intuitively learn more control options that they did not have access to, such as more finger presentations and wrist and elbow movement. The key is training the patient to be consistent in control utilization rather than using isolated single muscle groups. Pattern recognition really represents machine learning that is adapting to the patient, rather than the patient adapting to the control scheme. One limitation of myoelectric control has been the physical barrier when using a liner. WillowWood now offers electrodes that are embedded in the liner with a single distal attachment. In this configuration, pattern recognition can adapt to the patient during calibration no matter how the liner is donned. With pattern recognition, we may have reached the limit of control from surface electrodes, which are prone to have a degree of artifact or errors of signal as the socket is loaded and shifted. This is why the general loading of the liner works so well. The solution has been to look at direct neural control that can use nerve cuffs that lock into the remaining peripheral nerves. Pat Prigge, CP, clinical affairs manager, Fasikl, says, “In the future I don’t see pattern recognition being limited to one type of sensor, but may consist of multiple types providing varying signals. They will all be integrated like our own intricate physiologic systems. I look for a time that the device communicates beyond just grasp but includes loading, positioning, resistance, texture, and more. We are really getting to a point of a truly integrated, closed loop system.” Upper-limb Controls Training Upper-limb prosthetic care is not just about components; it is also about training the patient with respect to the controls. The key is to develop and strengthen the intuitive connection between the somatosensory and motor areas of the brain. This can be developed with video games and virtual reality. Many of us remember training patients with the racecar and maze game, but other methods involve hitting levels and pulses similar to GuitarHero, Dance-DanceRevolution, or BeatSaber games. As the patient uses the game, they develop more precision and intuitive control. Haptic Feedback One of the many challenges to upper-limb prostheses is that the user can only rely on a limited number of feedback loops from the interface, such as visualization and sense of feeling (haptic sense). This is why a good general loading of the interface with compliant materials such as rolled silicone is so essential. Some designs provide tactile haptic feedback with vibration to provide a degree of touch that can be adapted by the patient for grip strength or positioning. It can be used with sensory reinnervation with the residual limb to increase the sensitivity for feeling, and the user’s mind can adapt to its use, similar to a cochlear implant. Other approaches involve connecting directly to patients’ nerves so there is communication for prosthetic control and feedback from the prosthetic device to the patient. The goal is greater phenomenological osmosis or kinesthetic projection, termed as embodiment when the patient begins to feel they are one with the device. Battery Advancements: The Power Source What restricts prosthetic devices from increased strength is the tight space within them for the actuators. For improvements we can look to the future of battery technology that has used lithium-ion for 30 years. Lithium-sulfur batteries, for example, have four times the energy density of lithium-ion batteries, combined with a solid-state architecture that creates a safer, and more stable, less volatile battery. Flexible batteries that can fit in confined spaces between the socket and frame have been well received, but often suffer durability issues if bent excessively. Printed Zn-MnO 2 and Li-MnO 2 batteries have shown promise in smart cards, microelectronics, and greeting cards but still do not have enough output for prosthetic hands that can draw as much as 5 amps when they begin movement. Innovative Motors and Actuators: The Workhorses Much emphasis is placed on grip strength, but it is the routine 5 lb. of grip applied 1,500 times a day that may present the greater challenge. For highly active users, this can be a challenge, especially when combined with the need for water resistance. Bigger bearings, seals, and fittings can be used, but size is a consideration. While new designers assume miniaturization can easily take place in production, it can prove challenging without sacrificing durability. In the past, the 12-24v motors used by the automotive industry to raise and lower car windows were used. While very durable and accessible, these motors were fairly large, with a considerable amount of current draw. Brushless motors that were much more powerful and could be miniaturized were also options. With pulse-width modulation, the motors could increase grip strength with “impulse popping” to increase grip strength. New motors such as flux pancake motors or ultrasonic nonmagnetic wavelling Piezo motors provide very accurate movement in small areas like surgical robots. They can reduce components to increase durability and speed and are very energy efficient. Jeremy Farley, CPO/L, FAAOP, clinical support specialist, Össur, a biomedical engineer from Rensselaer Polytechnic Institute says, “In combination with rolled silicone and 3D-printed technology in its various form, the near future promise of batteries and motors from drones and RC tech could greatly change upper-limb partial hand prosthetics. However, upper-limb prosthetics requires even more durability and testing for practical use. I agree that improvements in surgical techniques offer huge potential for better control, but we have to make sure that the hardware is up to the task.” Microfluidics and Nonmotorized Actuators Since small motors are limited in their ability to hold objects, other developments seek to eliminate the actuator motor. It is well known that hydraulic systems can provide substantial loading, and this applies at the microscopic level as well. Hydraulic systems can be created using high-precision machining or metal printing to create a small chip that has a functioning hydraulic system in miniature. A motor would pulse the individual hydraulic system to operate the finger and the hydraulics would provide the necessary passive grip strength. Another option is to use an artificial tendon that shortens. In the past this was a pneumatic device known as the McKibben muscle that shortened when it was pressurized. Another was a metal tendon that used a shape-memory titanium alloy or a dielectric elastomer. One of the newer developments is a liquid crystal elastomer that provides actuation. Heating of the elastomer with electric current transforms the tendon from a monodomain to a polydomain state, which causes contraction. Currently the transformation from lengthening to contracting takes about 20 seconds, which is inadequate, but improvements are being made. Precision and control of movement is not only created by componentry, but also with the integration of the patient’s gadget tolerance, surgical presentation, haptic/visual feedback, and cognitive engagement. In many cases, the sensors, motors, and processing algorithms are outpacing the prosthetic user’s physiologic ability to recognize and respond to changing contexts. Finger and Partial Hand Options [caption id="attachment_253762" align="alignright" width="300"] M-Finger and Point Digits for a partial hand.[/caption] Even though the population of finger and partial hand amputation is seven times greater than all other upper-limb amputations combined, the number of options for partial hand prostheses were slowly dwindling due to component obsolescence and low acceptance rates of 35 percent. The devices available simply did not provide the functional advantage and appearance that the users required. However, the Didrick X-finger and Naked Prosthetics fingers use the remaining residual limb to drive finger movement at the PIP joint, which allows users much more functionally than the previous passive silicone devices. For more proximal amputations, wrist driven, nonarticulated devices could be used in combination with rolled silicone intimate fits. In combination with the precision of digital technology and additive construction, partial hand patients were provided with additional functional options. Next Generation Digital Technology 3D printing also changes socket designs with respect to how interfaces are created. Rather than mimicking previous processes, additive printing can now take advantage of semisolid materials and reticulated construction that can be breathable and offer varying stiffnesses. This has been a huge benefit to partial hand and pediatric users who were inherently restricted by the lack of componentry since smaller devices can be fabricated using 3D printing. While durability remains an issue, new designs are being created and modified. However, in a recent survey I conducted, 82 percent of upper-limb impressions were still hand casted, with only 15 percent scanned digitally, and 2 percent using prefabricated brims. Self-suspension for transradial and transhumeral fitting remains a challenge for many clinicians because of casting and modification techniques, but new algorithms promise to provide a shape from a scan, and measurements that can be input into the computer then adjusted for a better starting point. Shane Grubbs, CPO/L, FAAOP, regional clinical manager, Ottobock Patient Care, Georgia, says, “In the past I think we were using 3D printing as an adapted form of the original process. You could say it was just replacing lamination or thermoforming, but now we are refining it and truly taking advantage of some of the innovations such as varying lattice and honeycomb structures, modification, and greater precision. We are able to keep the strength and lower weight and even make the interface more open. This allows me even more of what I wanted to do as a clinician, not necessarily as a disciple of digitization, but as a prosthetist. [Recently] we created a more dynamic flexible inner design to increase comfort and function. Then we used the outer topographic digital file of the assembly to get a carving. This approach allowed us to thermoform a diagnostic socket, align, and transfer the prosthesis for lamination. In this way we got the best of both worlds by hybridizing analog and digital solutions. Body-powered Developments [caption id="attachment_253764" align="alignright" width="300"] More efficient and adjustable body-powered options have also increased.[/caption] Body-powered devices have been greatly enhanced recently with high-tensile strength, low-friction, prestretched, die-extruded methods originally used for mountain bikes and aircraft. Jagwire and Shimano make low-friction “slick” cables and housings that can be adapted for upper-limb use, and the Python cable can be adapted from ultralight airplanes for friction reduction and strength. New types of mold-making utilizing 3D printing technology allows functionally adaptive components to be created less expensively. High-tensile componentry is also routinely used for adjustable volume interfaces that can incorporate breathable liners. New urethanes and multidurometer construction allow adaptive devices to be created that allow specialized gripping action. Innovation Continues Because of its prominence and sociologic desire for increased function availability, innovation in upper-limb prosthetics will continue to evolve. It must be remembered that not all the advancements will be high-tech but will be appropriate technology that users need. No matter the complexity or expense, the one thing that will remain necessary is the well-informed prosthetist to manage the implementation of devices and materials while considering the functional needs and expectations of the patient. Grubbs says, “We can’t become so enamored with how we are doing something that we forget why and who we are doing it for. I think that’s really how I feel about clinical care and technology.” Gerald Stark, PhD, MSEM, CPO/L, FAAOP(D), is the director of clinical affairs for Ottobock Patient Care, Texas. Photograph credits in order of appearance: Courtesy Össur. Courtesy Gerald Stark. Image reprinted with permission: Weir, R. F., P. R. Troyk, G. A. DeMichele, D. A. Kerns, J. F. Schorsch, and H. Maas H. 2009. Implantable myoelectric sensors (IMES) for prosthesis control. IEEE 56(1)159-71. Top: Courtesy M-Finger; Bottom: Photograph by Heather Swanson. References and Suggested Readings Li-S batteries: https://saft.com/media-resources/our-stories/three-battery-technologies-could-power-future Flexible Batteries: https://onlinelibrary.wiley.com/doi/full/10.1002/smm2.1007 New Motors: https://www.nidec.com/en/technology/casestudy/flexwave/ Artificial Elastomeric Tendon: https://www.mdpi.com/2072-666X/12/7/736 Overview of Developments: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7340716/ History of TMR: Cheesborough JE, Smith LH, Kuiken TA, Dumanian GA. Targeted muscle reinnervation and advanced prosthetic arms. Semin Plast Surg. 2015 Feb;29(1):62-72. doi: 10.1055/s-0035-1544166. PMID: 25685105; PMCID: PMC4317279. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4317279/ Starfish Procedure: Denduluri SK, Rees A, Nord KM, Loeffler BJ, Gaston RG. The Starfish Procedure for Independent Digital Control of a Myoelectric Prosthesis. Tech Hand Up Extrem Surg. 2023 Mar 1;27(1):61-67. doi: 10.1097/BTH.0000000000000412. PMID: 36189486.https://pubmed.ncbi.nlm.nih.gov/36189486/#:~:text=The%20Starfish%20Procedure%20was%20developed,muscles%20to%20a%20subcutaneous%20location. New thoughts on socket designs and management Augmented Reality: https://jneuroengrehab.biomedcentral.com/articles/10.1186/s12984-021-00822-6 Stark, G., (2024). Clinical use of hand cast, scanned, direct formed, and prefab sockets. FreePaper/Poster. AAOP Annual Meeting, Chicago, Illinois.

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