A step in the right direction
IEs cannot replace life before amputation, but improving prosthetic design and manufacturing can better the lives of today’s amputees
By Chun (Chuck) Zhang and Ben Wang
French surgeon, biologist and Nobel Prize Winner Alexis Carrel once said, "The quality of life is more important than life itself." For many, quality of life comes from creature comforts or exotic vacations, but for others, quality of life is determined at a more basic level. The loss of a limb can be emotionally and physically devastating. Even after many amputees come to terms with the loss emotionally, the discomfort they feel each day from current prosthetic limb design can be a painful reminder and a detriment to a happy and comfortable life.
Just like a shoe that is too large or too small, a prosthetic socket, the portion that attaches to the limb, can cause pinching, sores, blisters and more. In the heat of summer, humidity creates pools of perspiration within the hard plastic casing of current prosthetic design. For some, pain can be felt with every step – not an ideal quality of life.
Unfortunately, this is an everyday experience among amputees who can afford prosthetic limbs. Today, more than 2 million people in the United States live with limb loss, according to the Amputee Coalition. With an aging society and ongoing military activities around the world, this number is expected to double by 2050. As of 2008, approximately 185,000 amputations occur in the United States each year. The primary causes are vascular disease (54 percent), trauma (45 percent) and cancer (less than 2 percent).
Veterans are commonly associated with prosthetics, and the Department of Veterans Affairs reports that almost 45,000 veterans are living with leg amputations. This is expected to grow as nearly 5,000 veterans lose legs to disease or accidents each year.
Beyond the wooden leg
Despite the discomfort with current design, prosthetics have come a long way since their humble beginnings. Many will recall Captain Ahab, who had a leg made of whale bone, in Herman Melville’s well-known novel saying, "It was Moby Dick that dismasted me; Moby Dick that brought me to this dead stump I stand on now." But prosthetics have been around since well before the 1851 publication of Moby Dick.
Prostheses, or any artificial device that replaces a missing body part such as a limb, tooth, eye or heart valve lost through trauma, disease or congenital defect, were first developed in the early civilizations of Egypt, Greece and Rome. With the birth of these civilizations, the scientific approach toward medicine and subsequently prosthetic science arrived. The earliest known prosthesis device, a wooden toe, was found on an Egyptian mummy that dates back to between 1,000 to 600 B.C.
Prostheses were used during the Dark Ages to treat battle wounds or to hide deformity. The Renaissance revitalized scientific development, and subsequent refinements in medicine, surgery and prosthetic science improved amputation surgery and the function of prostheses.
In recent years, advancements such as computer-controlled prosthetic knees, multiarticulated feet and polycentric knees have improved the functionality of prosthetic limbs, offering greater mobility and flexibility than their predecessors. Unfortunately, prosthetic design and manufacturing are still not where they need to be.
Deconstructing current design and manufacturing
Ill-fitted prosthetics continue to cause discomfort, and the manufacturing process is cumbersome at best. In lower extremity prosthetics, for example, the socket is completely custom fit, totally handmade and designed around the patient’s anatomy. This complex, lengthy process has multiple labor-intensive and tedious steps and depends heavily on operator skill. Often, the device needs multiple adjustments depending on a practitioner’s experience. The need to make multiple molds is expensive and time-consuming. In addition, it involves significant subjectivity, making it difficult for evidence-based improvements.
Compounding this problem is the fact that there are not enough trained personnel for producing prosthetic limbs. Studies by the World Health Organization indicate that while the current supply of technicians falls short by approximately 40,000, it will take about 50 years to train 18,000 more skilled professionals.
This lack of trained technicians and the tedious manufacturing method for prostheses cannot meet the growing global demand. Even if manufacturers could meet demand, conventional manufacturing processes make it difficult, if not impossible, to create a perfect fit or offer aesthetic appeal for highly customized prostheses.
Besides, current prosthesis design poses many challenges. For lower extremity prosthetics, the most difficult portion is the socket. The rigid shell is custom-made to fit the residual limb. The socket acts as an interface between the residuum and the rest of the prosthesis. Ideally, it should provide comfortable weight-bearing movement control and proprioception, or an awareness of the position of the body and limbs. Proper fitting of the socket is imperative to its comfort level and to avoid painful pressure points. It also needs to be pliable yet sturdy to allow normal gait movement without bending under pressure.
To make a difficult design even trickier, a soft protective liner is placed between the socket and the residual limb. This membrane, usually made of flexible material such as silicone, is rolled over the limb when the amputee wears the socket. The liner acts as a "second skin" between the movable soft tissue of the residual limb (muscles, tissue, skin) and the hard shell of the socket. The purpose of the liner is to minimize the movement and friction between the skin and prosthetic socket, providing comfort.
The hard outer case of the socket offers another design challenge. Although the socket must be sturdy enough to withstand a human’s body weight, it should accommodate normal body fluctuations. In the first few months to a year after amputation surgery, human limbs will change shape and volume during the recovery period. They also swell and contract as vascular dilation and contraction takes place each day. The rigid case of current prosthetic socket design does not allow for this fluctuation and can constrict a swollen limb or cause friction from "pistoning" of a contracted limb.
Finally, thermal management inside the socket is another key issue. Studies conducted by the VA Center of Excellence for Limb Loss Prevention and Prosthetic Engineering and Prosthetic and Orthotic Associates found that activities like walking and running can increase skin temperature by 5 degrees Fahrenheit. The situation is worse for amputees who are athletically active, where practitioners have observed the local temperature at above the knee dermal sites rising to 118 degrees Fahrenheit, approximately 20 degrees higher than average skin temperature in a standard temperature environment.
This elevated temperature produces more sweat, increasing the skin’s moisture level. Elevated moisture and temperature in an enclosed space like a prosthetic socket allows for microbial growth, which can cause skin infections or, if the skin is broken, blood poisoning.
Breaking the mold: Developing the ideal prosthesis
Understanding these challenges can help develop new designs and manufacturing processes for custom-fit prostheses. New prosthetic components/devices and manufacturing processes have been incorporated in recent years. For example, some new prostheses have pressure sensors attached, although most have proven ineffective. Conventional sensors are limited in their ability to measure pressure points in numerous locations, and their bulkiness doesn't allow them to be integrated properly into the design.
An ideal prosthesis should allow the patient to perform normal daily activities, provide needed comfort and fit, and ultimately become an integral part of a patient’s body. It should tackle current design flaws to accommodate volume change within residual limb(s), offer an engagement mechanism between the socket and the limb, and provide sensing and control/compensation for whole field pressure for enhanced comfort and fit.
Through the use of evolving technologies and materials, orthotic and prosthetic (O&P) researchers and manufacturers are developing a more holistic approach to design and production of these intricate, custom-made products. For example, 3-D printing could create prosthetic devices that are more aesthetically pleasing and fit the specific individual. Using the 3-D geometry of a residual limb obtained from an optical scanner, 3-D printing can in a short period of time produce a prosthetic socket that perfectly fits the limb.
Another emerging manufacturing technique with great potential for O&P application is printed electronics, which incorporates tiny flexible sensors or electronic components into the socket and liner in precise locations. This provides sensing capabilities with little or no added weight and size while intruding minimally into the socket and liner structures. These sensors and electronics can track and/or adjust temperature, moisture and pressure inside the socket to provide enhanced comfort and fit. The information that comes from measuring such parameters can help O&P practitioners adjust prostheses and provide necessary treatment for amputees.
In terms of materials, prosthetics re-searchers, practitioners and manufacturers are taking ideas from other industries. New materials, such as those used in aerospace, are being explored in prostheses design, including lightweight, high-strength composite and nanocomposite materials for socket structure, adaptive shape-changing materials for prosthesis volume change management, and phase-change materials for interior thermal management of the socket.
Many advanced prosthetic components and systems are designed using new and emerging technologies, including microprocessor knees (C-Leg) and bionic ankles (iWalk), the Defense Advanced Research Projects Agency’s smart ARM, targeted muscle reinnervation and volitional control of prosthesis using electromyography (EMG) signals from residual limb muscles. These new prosthetic devices/systems or technologies have played key roles in improving amputees’ quality of life.
The C-Leg prosthetic controls the flexing of the knee through the use of sensors, microprocessors and hydraulic cylinders, allowing the amputee to walk at varying speeds and navigate different terrains. By using robotics to replicate the calf muscles and Achilles tendon, the iWalk BiOM feels and functions like a natural leg. With the iWalk BiOM, users experience natural walking mechanics and increased stability, mobility and confidence. The Revolutionizing Prosthetics program launched by DARPA in 2006 has created two high-tech arms, the Gen-3 Arm System and the Modular Prosthetic Limb, which can offer unprecedented range of motion and control.
In terms of amputation surgery, targeted muscle reinnervation (TMR) is a recent technique developed by Drs. Gregory Dumanian and Todd Kuiken of the Rehabilitation Institute of Chicago. TMR enables mechanical arm movement by taking nerves that once carried motor signals from the brain to the missing arm and connecting those nerves to others located near a large muscle, such as the biceps or pectorals.
When TMR patients think about moving their hand, the resulting command is routed to their chest muscle. This muscle serves as a biological loudspeaker, booming an electromyography signal to external electrodes placed on the skin. When the electrodes pick up that signal, processors translate the EMG into useful information, which drives the myoelectric arm.
Progress toward the volitional control of prosthetic devices has been made using the EMG signals from the amputees’ residual limb. EMG signals are a rich source of neural information. When an amputee generates a muscular contraction, it generates some myoelectric signals. These myoelectric signals are extracted to actuate the appropriate degree of freedom.
The enhanced comfort of SOCAT
Clearly the O&P industry is at a turning point. The Georgia Tech Manufacturing Institute, in collaboration with Florida State University, Advanced Materials Professional Services, Prosthetics and Orthotics Associates, Quantum Motion Medical and St. Petersburg College, is tackling the hard-to-conquer challenge of socket design and manufacturing. As part of the $4.4 million U.S. Department of Veterans Affairs’ VA Innovation Initiative, the research team is addressing prosthetic socket design on behalf of military amputees.
Known as SOCAT (socket optimized for comfort with advanced technologies), the advanced socket research and development program seeks to develop a design that offers prolonged comfort and built-in real-time monitoring and control capabilities for various socket conditions using advanced materials, sensing and manufacturing technologies. Figure 2 shows a schematic of the design concept of the multifunctional SOCAT system.
The SOCAT design attempts to address most of the existing challenges by using innovative materials and advanced manufacturing technologies. It uses lightweight, multifunctional materials to make the prosthetic socket less cumbersome, while incorporating adaptive materials inside the socket to accommodate volume change. Biomimetric materials, also inside the socket, offer pistoning control and antimicrobial protection. The team has incorporated solid state active cooling, in conjunction with novel nanomaterials and phase-changing materials, to provide temperature and perspiration control. Whole-field pressure monitoring is achieved through the use of lightweight piezoelectric nanofoam sensors, while embedded printed electronics reduce the number and dimensions of "parasitic devices" for sensing, wireless communication and data storage.
This design offers a wide array of benefits for practitioners, manufacturers and patients. For the practitioner, the data retrieved and analyzed by embedded printed electronic sensors will help clear the pathways of communication with patients and appropriately address each issue that arises within the socket. Additionally, practitioners and insurance providers can use this data to ensure proper treatment plans and minimize insurance ambiguities. Innovative materials will help the manufacturing community provide advanced and affordable sockets that offer improved customer satisfaction, and the insurance industry can use the consistent real-life data provided by the SOCAT system to facilitate the evidence-based approval process and expedited delivery of product to the patient.
From the patient’s perspective, as noted, the new SOCAT design will reduce treatment timelines drastically. Figure 3 details the differences between the old process, which can take four to six weeks while requiring a temporary prosthetic and multiple fittings, and the SOCAT process, which has a cycle time of a few days. Furthermore, by gaining control of volume change, socket temperature, pressures, pistoning and skin integrity, the SOCAT design will improve comfort, gait, mobility and functional capacity. No longer will patients depend on multiple sockets to cope with volume fluctuations.
For new amputees, the limb changes and limb-liner contact will be detected by the pressure sensors in the temperature control, anti-pistoning/biocide and sensing (TABS) liner. Information and data provided by temperature and pressure monitoring will improve treatment and rehabilitation. The use of auxetic foam helps the prosthetic automatically accommodate the residual limb’s volume changes throughout the day.
SOCAT technologies can be scaled and integrated with components in all aspects of lower extremity prosthetics. The integrated system can adapt easily to advancements, including microprocessor knees, C-Leg, iWalk and bionic ankles. Overall, this system will provide a new outlook on life for practitioners, manufacturers and the insurance industry. But most importantly, it takes a step in the right direction by greatly improving the quality of life for prostheses wearers.
Chuck Zhang is a professor in the H. Milton Stewart School of Industrial and Systems Engineering at the Georgia Institute of Technology and an affiliated faculty member of the Georgia Tech Manufacturing Institute. He has conducted and managed more than 40 research projects sponsored by a number of organizations, including the National Science Foundation and industrial companies such as Cummins and General Dynamics. Zhang has published more than 130 refereed journal articles and 180 conference papers, and he holds 11 U.S. patents.
Ben Wang is executive director of the Georgia Tech Manufacturing Institute, professor and Gwaltney Chair in Manufacturing Systems in the School of Industrial and Systems Engineering, and professor in the School of Materials Science and Engineering. He serves on the U.S. National Materials and Manufacturing Board of the National Research Council and is a fellow of the Institute of Industrial Engineers, the Society of Manufacturing Engineers, and the Society for the Advancement of Material and Process Engineering. He has authored or co-authored more than 220 refereed journal papers and 150 conference articles. He has a portfolio of more than 25 issued and applied-for patents.