"High Tech" became a part of the vernacular of prosthetics about the time we started to see prosthetic feet that looked like the tip of a snow ski bent at an angle and attached to a socket for a prosthesis. The black material of the "new" foot was carbon fiber a "space age material." We were introduced to other components made from carbon and "titanium", the metal used in supersonic military aircraft. We began to hear about "CAD/CAM" (Computer Assisted Design/ Computer Aided Manufacturing) in which computers are used to design and carve models for prosthetic sockets.
A full discussion of all the new high-tech components in lower extremity is beyond the scope of this article; however we are going to look briefly at two very exciting advances in microprocessor controlled knees.
In September 1993, Chas. A. Blatchford & Sons, Ltd. of Great Britain introduced to the world the first microprocessor controlled "Intelligent Prosthesis" and in 1995 an improved version called the "Intelligent Prosthesis Plus" They recognized then that for this new idea to be accepted they would need to "convert the culture of prosthetics to accept more advanced microprocessor control systems." The "Intelligent Prosthesis" was the first generation of these new "High-Tech" prostheses and the new generation now is the "Adaptive Prosthesis."
The Adaptive Prosthesis is a unique marriage of hydraulic control, pneumatic control, and the microprocessor to provide the amputee with a gait that is energy efficient, stable, and responsive.
These new microprocessor controlled knee joints are in some ways similar to the current military jet aircraft of the United States Air Force. The F-16 was designed to be unstable to give it high performance, making it a more effective weapons platform. Without computers the aircraft is very unstable and would be impossible for the pilot to fly.
What this means to prosthetics is that it is possible to fit a knee joint to an amputee that is not as inherently mechanically stable as traditional knees, making for a more energy efficient gait, while at the same time , through the use of the microprocessor, it can be made as stable as needed by responding to the amputee’s gait. The "onboard" computer samples the amputee’s gait through strain gauges and the position sensors build into the systems.
In the Adaptive Prosthesis from Blatchford, a hybrid hydraulic and pneumatic system is controlled
by two stepper motor valves operated by the microprocessor. The hydraulic part of the system controls stance, flexion, and terminal impact. The pneumatic part of the system controls swing phase and extension assistance.
These systems allow the amputee to have a more stable gait on even terrain, ramps and inclines, and on stairs. This gait is more symmetrical and physiological. A wider spectrum of above knee amputees can be fit with these systems and residual limb length is not as important a factor as with other knee control systems.
Other features of the Adaptive Prosthesis are voluntary locking mechanism for extended standing and a "stumble control" that has immediate response for a misstep to prevent knee buckling.
A radio-linked programmer permits basic, standard, and advanced levels of control. The prosthetist is guided through a series of operations that sample the amputee’s gait as he or she walks, setting the parameters of the microprocessor for the amputee’s specific gait. The system is powered by one set of batteries that will last for several months, and when replacement is necessary, the software is designed so that there is no loss in programming.
Although over 3,000 amputees have been fitted with the first generation of microprocessor prostheses over seven years, there are less then one dozen amputees wearing the second generation Adaptive Prosthesis.
Another system is the Otto Bock 3C100 C-Leg which should be available in the United States by the time this article is published. It was released for sale in Canada about a year ago. The 3C100 is a carbon fiber frame that holds a hydraulic cylinder, microchip, and rechargeable battery. After a 3-hour charge, the battery lasts 25-30 hours before it needs recharging or the knee joint goes into a safety mode operation. The C-Leg system includes a strain pylon that senses weight and position proving information about the amputee’s gait to the microprocessor. The system has a high sample rate (every 20 milliseconds) for data to control the functions of the hydraulic cylinder. This allows a more stable gait without extra mechanical stability needed.
In researching this article, I interviewed an experienced amputee who, given the nature of his job, has had an opportunity to try most of the Otto Bock knees as well as knee joints from other manufacturers. I believe that the feedback and answers to my questions were useful, sophisticated information because I have known this individual for a number of years as a client and co-worker. He has always been very objective and honestly critical of the different knee systems he has worn over the years.
His experience with the C-Leg for about six months has been very positive. One of my concerns was how well an electronic lower extremity prosthesis would hold up in everyday use. I was worried about the need to recharge the battery everyday and how fragile this kind of system must be. This did not appear to be a problem in the real world of everyday use. In the past he has used the swing and stance control knee systems and he prefers the C-Leg to these systems for inclines, ramps and stairs. With the C-Leg, he says this is no longer a concern.
The C-Leg is the first electronic knee system that has both hydraulic stance and swing phase control. The unit comes with software and cables so that the amputee’s prosthetist can make adjustments to the system. For the time being, Otto Bock will work with each prosthetist and amputee individually to assure the maximum benefit is derived from the system.
For maximum accuracy, Otto Bock does recommend using the L.A.S.A.R Posture, L.A.S.A.R. Assembly, and L.A.S.A.R. Alignment systems as well as the new Balance Apparatus to determine socket alignment parameters.
Just as the F-16 is not for every pilot, the microprocessor prosthesis is not for every amputee but it does represent a major breakthrough in the marriage of modern technology to an art and science (prosthetics) that dates back thousands of years.
This article only gives a short overview to two of the new microprocessor system knee joints that are being researched and designed by the manufacturers of components for prosthetics. There are others, and in the future you will be reading, seeing or hearing about them.
Published May/June 1999 Volume 9 Issue 3
InMotion
Magazine.