Microprocessors Usher In New Era In Prosthetic Knee Technology
By Harry Newman, CP
October 2005
Until recently, the options available to the transfemoral amputee had serious drawbacks. The traditional single axis hinges allowed the knee to flex and extend, but overly restricted many patients from engaging in a range of activities. Knees built to maximize stance phase stability tended to interfere with knee flexion, while those designed to enhance swing control tended to weigh more heavily and have complicated mechanical designs.
That all changed in the mid-1990s, when prosthetics engineers succeeded in adapting the burgeoning technology of microprocessors to the mechanics of knee motion. Among the first companies to develop an electronic knee was England’s Blatchford & Sons (known stateside as Endolite), close followed by Germany’s Otto Bock and Ossur. Today’s transfemoral amputee has several remarkable options to choose among, and is likely to find his or her quality of life enhanced considerably as a result.
All of the knees discussed in this are similar in that they make walking more natural, effortless and comfortable for the transfemoral amputee.
Fig. 1.—Endolite’s Adaptive knee builds on the technology in their pioneering Intelligent Prosthesis.
Endolite’s Pioneering Knees
First introduced in 1993 and enhanced in 1995, Endolite’s Intelligent Prosthesis Plus (IP+) was the first artificial knee to incorporate microprocessor technology to automatically adjust the swing to match the individual amputee’s walking speeds.
Working with a hand-held programming device, the prosthetist is able to calibrate the knee to the patient’s normal, fast and slow gait.
More recently, Endolite has introduced The Adaptive Knee, which combines microtechnology with the power and stability of hydraulics and the natural comfort of pneumatics.
Inside the knee is a computer that responds to the wearer’s body movement and terrain for a smoother, more confident performance.
The Adaptive (see Fig. 1) provides hydraulic stance control when needed and constantly monitors for changes in speed. The computer analyzes real-time data 62.5 times per second and adjusts control based on actual gait analysis. The company claims that The Adaptive can be used with any foot and that it encourages residual limb strength.
Fig. 2.—The electronics of the C-Leg are integrated within the shell of the appliance.
Otto Bock’s C-Leg
First introduced in 1997, the C-Leg came to market in 1999 and almost immediately gained wide and largely positive attention in the field.
It is not difficult to understand why the C-Leg has caught on so strongly in the community of amputees and those who treat them (see Fig. 2).
The C-Leg features a microprocessor-controlled hydraulic knee with multiple sensors that read and adapt to movement by measuring the load on the foot and recording knee angles and motion more than 50 times a second.
The C-Leg features both swing and hydraulic stance phase control, customized settings and real time adjustments. Microprocessor-controlled flexion and extension motors are continuously adjusted to create optimal stability during stance phase and optimal response during swing phase.
The C-leg also has force sensors in the shin that use heel, toe and axial loading data to determine optimal stance phase stability. A knee angle sensor provides data for control of swing phase, angle, velocity and direction of the moment created by the knee.
The result of these electronic calibrations and automatic adjustments is to relieve the transfemoral amputee from having to concentrate on his or her stance and gait, and to permit the smooth execution of a gait that is almost indistinguishable from a natural one.
The ability to modulate speed, ascend and descend slopes, traverse rough terrain, and walk up and down stairs is dramatically enhanced.
The makers of the C-Leg have recently introduced a new model of microprocessor knee called The Compact, which the company says provides enhanced stability.
Fig. 3.- Ossur’s Rheo Knee
Ossur’s Rheo Knee
Introduced earlier this year, Ossur’s Rheo Knee (RK) has already received a large amount of positive notice in such high-profile publications as The Wall Street Journal and Fortune.
Building on the microprocessor technology of the IP+ and the C-Leg, the RK is the first artificially intelligent knee system with the ability to learn and adapt to its user’s movements, resulting in a continually improved and optimized performance, according to Ossur. Through the use of a microprocessor, integrated sensors, and an innovative magnetorheologic fluid actuator, the Rheo lets the user walk more naturally, with an improved mechanical gait (see Fig. 3).
Unlike the IP+ and C-Leg, which adapt according to preprogrammed algorithms, according to the manufacturer, the RK requires no initial programming and is able to respond more dynamically to the individual’s walking style and conditions encountered by the wearer.
Shared Features
All of these knees claim to enhance the wearer’s ability to transition from level ground to ramp, stairs and uneven terrain without fear of inadvertent knee collapse.
Additionally, disturbances in the walking path are recognized automatically and stance support instantly activated to protect the wearer from potential stumbles and falls.
All of the knees discussed above are powered by rechargeable lithium-ion batteries. Some or all come with built-in safety features. For example, the C-Leg’s warning vibration is activated in the knee when the battery begins to run low on power. If the battery runs out of power, the knee will automatically default to a straight-leg position, ensuring that it will be stable enough to hold up the wearer. Batteries can typically be fully charged in two hours, and can last for up to 24 hours when fully charged.
Choosing among the Models
The growing popularity and prevalence of electronic knees mean that the transfemoral amputee has a new range of life-enhancing options to choose among. The decision as to which knee is most appropriate will depend on a number of factors, including the patient’s age, weight, and lifestyle.
More than ever, it is important for those considering an artificial knee to discuss the relative pros and cons of the various models with their prosthetist, who can help guide the patient to the best decision.
Once a tentative decision has been reached, transfemoral patients are carefully evaluated to determine if they are good candidates for electronic knees. This evaluation is performed by a prosthetist who has been trained and certified by the manufacturer.
Price and Insurance Coverage
One barrier to initial acceptance has been the high price tag, which can reach up to many thousands of dollars. Recently, however, several factors have contributed to the growing success of these devices.
Perhaps most importantly, a series of high-profile articles have spurred interest in electronic knees and created pressure on both private and governmental payors to cover the device. In 2001, for example, The New York Times featured an article about how an amputee was able to walk down 70 flights of stairs at the same speed as most others fleeing the World Trade Center during September 11’s terrorist attack. And in 2002, The Washington Post called attention to Air Force Lt. Col. Andrew Lourake, a C-Leg recipient who became the first transfemoral amputee in the history of the US Defense Dept. to return to flying status after losing a limb.
Recently, articles about soldiers returning without limbs from the Iraq war have emphasized the new generation of high-tech prostheses available to help them resume their lives.
In January 2002, Medicare issued new codes specifically for the C-Leg and its componentry. Five years after its introduction, in 2005, most patients’ insurance plans cover the C-Leg if medical necessity can be demonstrated and the proper documentation is provided.
Most O&P companies, including Ballert, work with amputee patients and insurance companies to expedite the authorization process.
Fig. 1.—Endolite’s Adaptive knee builds on the technology in their pioneering Intelligent Prosthesis.
Fig. 2.—The electronics of the C-Leg are integrated within the shell of the appliance.
Fig. 3.- Ossur’s Rheo Knee
