Upper Extremity Prosthetic Advances
by Zach Harvey, CPO
I’ve been in the field of prosthetics and orthotics for 20 years now and have seen a lot of change. In recent years, there has been a rapid advancement in prosthetics for the upper extremity. With more options comes more decisions, and our old way of doing things needs to be re-evaluated. Some more significant game changers include: options for partial hands and digits (fingers/thumb), more comfortable material interfaces, multi-articulating joints, and better ways to control electrically powered systems. Thanks to the Defense Advanced Research Projects Agency (DARPA) funding and universities/laboratories around the world, some very smart people are working on emerging technology, some of which has already come to fruition. After all, there’s a big need for improvement as we’ve struggled to provide highly comfortable and functional designs. Surgical advancements are also an important factor in this revolution. Before I start, I want to point out that this is coming from my perspective and may not exhaust every advancement happening. I won’t be talking much about 3D printing, even though it’s a hot topic, because I’m currently not utilizing it in my practice.
I’ll start with the increasing commercially available options for the partial hand and digit level. This is significant because the hand and digits represent the highest number of upper limb amputations in the US. The human hand is very complex, and a focus on functional goals is very important with this population. A good occupational therapist can be instrumental in the planning stages and working with the individual after receiving the device.
1) High definition silicone restorations aren’t new, per se, but are worth mentioning because they are quite common and highly functional in some cases. These custom devices are shape- and color-matched by artists with details as small as freckles and fingernails.
2) Metal ratcheting digits are a robust option for those requiring full finger(s) and/or thumb prostheses. These can be flexed into a number of locking positions and spring back into extension with a push of a button or when fully flexed. This allows the ability to hold or carry objects, and provides something to press against.
3) Body powered digits use an anatomically intact higher joint to power a prosthesis. For example, a person with an index finger amputated at the middle joint could flex and extend to move a mechanically fixed finger tip. This would allow the finger tip to curl into the palm of the hand or to pinch the thumb tip.
4) Electrically powered digits are especially beneficial when there is the absence of an entire thumb and/or finger(s). These utilize myoelectric sensors, carefully placed over muscles of the hand to control movement.
Whether high definition, ratcheting, body powered, or electrically powered, the custom interface is critical. Comfort and protection against skin irritation are some of the primary goals with any prosthetic device. A material called HTV silicone is well suited for these goals and works for other amputation levels as well. Fabrication requires a unique skill set, but specialty labs across the country are making this technology widely available for prosthetists to utilize.
Multi-articulating hands mimick the human hand, making motions more natural than conventional hands because motors in each finger allow nearly endless programmable grip selection. Conventional hands are still used and are admittedly more durable, but have limitations because they simply open/close. The advantage of multi-articulating hands is that less grip pressure is needed while holding an object because of the way the fingers conform around it. They also reduce awkward arm motions to position the hand which can cause overuse syndromes over time. Thanks to 3D printing and open source platforms, these hands are becoming more affordable and accessible worldwide. Many people prefer a hand that looks and moves more naturally and, in my practice, I’ve seen a large acceptance rate of multi-articulating hand use over conventional hands. Many people even embrace the “bionic look.” However, powered prosthetic devices have been limited in the number of joints which actively move. Thanks to DARPA funding, the Luke arm (previously called Deka arm) was developed under the Revolutionizing Prosthetics Program with the goal of restoring near natural hand and arm control. After years of development, the Luke arm is now commercially available and has up to 10 joint motions that can happen simultaneously.
The more movements that hands and arms are able to make, the greater the need to control these movements by the user. Hence, the advancements in pattern recognition and targeted muscle re-innervation (TMR) surgery. Traditional myoelectric prostheses use one or two signals on the surface of the skin to detect electricity when the muscle is fired to power a hand, wrist, or elbow. It gets more complicated the higher up the amputation is because of the increased number of joints requiring myoelectric control. Switching from elbow to wrist to hand is slow and cumbersome to the user. Pattern recognition, in contrast, uses an array of electrodes surrounding the remaining limb in the prosthesis. Instead of specific muscles being used, the software algorithm makes sense of the data from all the signals and interprets the desired movement of the user. This can result in faster, more intuitive arm movement. TMR surgery can also improve results because this surgery takes nerves which previously controlled hand and arm motion and attaches them to smaller nerves and muscles higher up the arm or chest. Control of the prosthesis may not only improve with this procedure, but pain and phantom pain has been shown to reduce as well. The Luke arm uses something called “end point control,” which allows up to 10 degrees of freedom to operate simultaneously. Inertial motion units (IMU’s) are one way to control the Luke arm. IMU’s are clipped to the user’s shoelaces and one foot controls hand placement in space while the other operates the hand. I got to demo the arm, and within minutes realized how much easier it was than traditional means of sequencing through each joint motion. Check out this video to see it in action:
Emerging technology that may improve the outcomes for upper extremity amputees include: haptic feedback, osseointegration, implantable electrodes, and arm/hand transplant.
Haptic feedback was perhaps underestimated in importance until being studied in a laboratory setting. Prosthetic hands equipped with pressure sensors in turn stimulate vibration devices similar to those in cell phones or even the sensory nerves themselves. Initial feedback has been encouraging with less reliance of the visual system, less tendency to drop or crush objects, and an overall sense of embodiment of the prosthesis. Participants in these studies have gone as far to say that once the study was over “was like losing my hand all over again.”
Osseointegration is the direct skeletal attachment to bone with a protruding metal abutment which attaches to the rest of the prosthesis. There are many advantages for the upper extremity amputee in comparison to traditional prosthetic sockets which include: reduced perception of weight, perfect translation of bone movement to the prosthesis, positive suspension at all times, and no heat build up/sweat. The FDA is becoming more accepting of the procedure and this will likely become an option for some upper extremity amputees who struggle with traditional prosthetic sockets.
Implantable electrodes in the e-OPRA system from Integrum are not far from becoming a reality. This system uses wire leads which run through the osseointegration implant into various muscle bellies and nerves of the remaining arm. It is currently under development and in clinical trials. Unlike traditional electrodes that can move around and lose contact with the skin, the wires maintain contact with the muscle in an unrestricted range of motion with a potential for sensory feedback, and reduction in phantom pain.
More than 85 hand/arm transplants have been performed around the world with the longest survivor going on 11 years. Outcomes have been mixed, and the procedure requires a lifetime of immunosuppressants. It is not clear yet if this should be the standardized treatment for selective arm amputees. Evidence has not yet proven that hand/arm transplants improves functional outcomes and quality of life. Also, there is considerable financial burden and potential immunosuppressive complications. This may change as more surgeries are performed and longer term data is acquired.
As more options become available, certain prosthetic devices may fall by the wayside. When the first cable controlled split hook was developed around 1860, it made the “Captain Hook” design virtually obsolete. However, I’m having a hard time thinking about anything in the last 20 years in upper extremity prosthetics that has become obsolete due to a technological advance. Maybe this is because the function of the human hand is so hard to replicate. That said, I wonder if we will reach a time in which either regenerative medicine will allow arms to grow back or if a superior mechanical prosthetic design will make all previous designs look like “Captain Hook” arms. Until then, I’m happy to have an increasing number of options to work with!
References and additional resources