The Future of Prosthetic Legs is Now
Research Paper by Travis Pollen
A common thread between the ancient warriors and chivalrous knights of the past and today’s war veterans and land mine victims is that of the need for prosthetic devices in order to regain ambulation. In fact, artificial legs, or prostheses, have been in use throughout history to replace limbs lost through disease, trauma, and birth defects. From simple peg legs and hinge knees to the revolutionary C-legs and flex feet, prosthetists have made tremendous strides in recent years, and they show no signs of slowing down. The processes of tomorrow for fabricating, testing, and controlling prostheses are quickly coming into use today. The previously thought to be mere science fiction future of artificial legs indistinguishable in look and function from their real counterparts is actually not too far off. Rehabilitation, biomechanical, computer, and materials engineers have played no small role in these advances.
Designing a device to replace the complex human leg, with its various joint articulations and musculature, is a daunting but essential task, indeed. Says Joseph Bronzino, “It is possible to function fairly well with one arm, but try walking with one leg” (2058). The core issue is whether to model the leg after human anatomy or to simplify the design with the hopes of improving performance. After all, the artificial leg must support half the weight of the upper body when standing, walking, and even running. Additionally, with the absence of a human foot on which to bear weight, the load must be distributed elsewhere (2055). The prosthesis must therefore be strong and sturdy to support the body in stance yet light and easily maneuvered to facilitate ambulation (Hin 10-4). Materials coming into use in recent years such as carbon fiber and polypropylene help to provide this delicate balance (Bronzino 2059).
The most crucial component of a prosthesis is the socket. No matter the type of amputee – trans-femoral (above-knee) or trans-tibial (below-knee) – the socket serves as the interface between the user and the leg. Oftentimes, an amputee dons the prosthesis right up against the skin for all of his or her waking hours, so comfort is the top priority. To provide such a perfect fit, every socket is meticulously crafted either by hand via plaster casting or, more recently, with advanced computer technology.
Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) have dramatically increased the consistency, quality, and productivity of socket design. The first step in the process is to measure the contours of the residual limb using lasers. These measurements are then converted to reusable digital data in order to generate the shape on the computer (Hin 10-6). Manipulation and rectification are then carried out on the computer to relieve areas of skin with low load tolerance and provide an optimal fit (Bronzino 2055). This step is paramount, for the forces and pressure must be distributed along the entire limb – not just onto the bottom – either using computer algorithms (2060) or by manually and systematically altering volume along the socket. A positive model can thus be carved with an automated milling machine, and from this the physical socket can be realized by draping and vacuum forming heated thermoplastics over it (Hin 10-6). The entire procedure can be completed in a matter of hours, as compared to days, making it preferable to casting by hand. New technology can even calculate ideal pressure distribution for minimal soft tissue displacement. In combination with CAD and CAM, an iterative loop is formed whereby a socket is created and pressure distribution is calculated. The socket is recreated repeatedly until distribution is satisfactory (10-7).
In general, there is an intermediate step between casting and the finished socket called the check socket. In this stage, the socket is made of a transparent thermoplastic sheet that can be cut down and heated for reforming (Schlager 16). Additionally, the alignment and angle between the socket and the artificial knee can be adjusted only now and not after the socket has been laminated. One further consideration is the coupling of the leg to the person. The two contemporary alternatives are belts that wrap around the body and suction, although direct fixation to the bone (and full internal prostheses) is surely forthcoming. Biocompatibility and manner of affixation remain problematic (Bronzino 2055).
Sockets fabrication is not the only area of prosthetics that has received a makeover in recent years. Pneumatically, hydraulically, and battery powered prosthetic knees made of titanium and even graphite are the latest commodities (Perkowitz 91). Some knees, most notably the C-leg, contain computers that provide control over the leg when the foot is lifted off the ground. During this so-called swing phase, the computer adds resistance when bending at toe-off, swings the leg through quickly to assure its full extension at heel strike, and again adds resistance before complete extension to dampen internal forces. Accordingly, the artificial knee intelligently matches the sound leg and makes for smooth and natural transitioning all around. Amputees are in this way better able to walk at various speeds, as parameters are constantly being readjusted (Bronzino 2065). In fact, with these legs, amputees can even ascend and descend stairs normally.
A new wave of artificial feet has accompanied the innovative knees and methods of molding sockets. The generic term for this pylon, ankle, and foot combination is the flex foot, with the company Ossur at the forefront of their design. The J-shape of the flex foot perfectly illustrates the dismissal of human anatomy for superior performance (2059). Made of carbon fiber and Kevlar (Hin 10-14), the feet essentially behave as springs, compressing as the foot strikes the ground and storing energy then extending to release the energy and propel the user forward (Perkowitz 91). Using Ossur’s Cheetah model, South African sprinter and double-amputee Oscar Pistorius nearly qualified for 2008 Olympics. Shocks and stiffness tuning add an additional dimension of adjustability (Stark). Yet another astonishing feat is that of pressure sensitive artificial feet. The idea is that pressure transducers in the feet send signals to electrodes in the residual limb, nerves receive the signals, and amputees are effectively able to feel the ground (Schlager 18). While the technology is still in its early stages, the possibilities appear endless.
Despite the fact that a prosthetist’s empirical observation usually suffices, newfangled qualitative methods to measure amputees’ gait restoration have been devised. The gait analysis consists of temporal-distance measurements (for instance, speed, stride length, and cadence), kinematics, kinetics, energy, and muscle activity. Studies can be done using different types of knees and feet to determine which provide the individual with the most natural looking and least strenuous walk. As with the rest of the field of prosthetics, these tests come at a high cost at present but in the coming years could become more accessible (Hin 10-4).
Besides advanced gait measurements, there are now methods of testing the ever-important properties of durability, longevity, and safety of the very prostheses themselves. Static load application can be used to determine the strength of entire legs or just individual components. Key values are applied force until deformation occurs and, of course, weight of load at failure of the limb. Also significant is the mode of failure, as breakage of the artificial limb must not result in additional damage to the rest of the body. Cyclic loads (one load per second a few million times, for instance) can demonstrate limb life. Further tests can be performed to determine the materials’ resistance to friction, analogous to that which results from wearing the leg (Schlager 18). Only twelve years have passed since official standards were set for these tests (Hin 10-4).
An equally recent modernization is that of combining cosmetics with function. Using computers, full artificial leg polyurethane foam covers can be designed to perfectly replicate their real counterparts, with stockings on top to match skin pigmentation. Rubber feet with veins and painted toenails often fit over the top of the metallic feet. As the appearance of fake legs comes ever closer to matching the human figure, so too does their manner of control. Myoelectricity makes use of electric signals from patients’ remaining muscles to move the legs (Schlager 14), and the field of neuroprosthetics is linking neural pathways with electronic ones, allowing for control of external devices from the mind (Perkowitz 87). As a matter of fact, implanted electrodes in monkeys’ brains have already allowed them to use their thoughts to control robotic arms in order to eat. One hitch is that bulky computers are currently needed to process the brain signals (Barry). Nevertheless, portable nueroprosthetic technology for humans and their artificial legs is surely coming soon.
The primary obstacle standing in the way of all this progress is, as could be expected, money. As Bronzino points out, prostheses “tax the skills of most engineers, both to design the product at reasonable up-front costs and to manufacture it economically in low volume.” Prostheses obviously cannot be mass-produced because each one is built uniquely for one individual, and no one leg will work for two people (2068). Yet with flexible plastics, computer-controlled knees, carbon-fiber pylons, and energy storing feet already in full use, it is not difficult to believe that artificial legs could soon become seamless extensions of the person, impossible to tell apart from human flesh and bone.
Barry, Patrick. Monkey think, robotic monkey arm do. Science News Web edition. Society for Science & the Public. 28 May 2008. <accessscience.com/content.aspx?id=SN13783>.
Bronzino, Joseph D. The Biomedical Engineering Handbook. Orthopedic Prosthetics and Orthotics in Rehabilitation. Hartford: CRC Press, 1995.
Hin, Teoh Swee. Engineering Materials for Biomedical Applications. London: World Scientific, 2004.
Perkowitz, Sidney. We Have Always Been Bionic. Digital People: From Bionic Humans to Androids. Washington, D.C.: Joseph Henry Press, 2004.
Schlager, Neil. Artificial Limb. How Products are Made: an Illustrated Guide to Product Manufacturing. Detroit: Gale Research Inc., 1994.
Stark, Gerald. Perspectives on How and Why Feet are Prescribed. Journal of Prosthetics and Orthotics. 2005 Vol. 17, Num. 4S. pp. 18-22.
Motivation for Project
I myself wear an artificial leg and was very interested in learning about the future of prosthetics as it pertains to myself as well as bioengineering. The situation over the summer involving the sprinter Oscar Pistorius really piqued my curiosity. Could artificial limbs really work better than human ones? Taking into consideration that I’m also missing my knee, could my artificial leg (with the flex foot and hydraulic knee) eventually be as good or better than my human leg? At what price? Also, after seeing I, Robot, I want to know if the technology exists now or is in the making for prosthetic limbs to look like human ones.