Prosthetic Knees: Whatís Currently New and Impressive?

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Everyone agrees that there is no perfect prosthetic knee--if there were, the knee researchers and design engineers could all quit and go home. What makes the continuing search for that perfect knee so interesting is the myriad widely divergent approaches designers are now exploring--hydraulic, mechanical, and computerized--as reflected in the variety of available knee designs currently regarded as state-of-the-art.

Space and other limitations make it impossible to showcase every recently marketed prosthetic knee, but here's a representative sample:

Otto Bock C-Leg

Otto Bock C-Leg
Otto Bock C-Leg

The Otto Bock C-Legģ tends to jump to the forefront in any listing of state-of-the-art knees, largely because it appeared first on the US scene (about four years ago) with such dramatically different technology, and its successes achieved widespread coverage--any marketer's dream! Former knee designs traditionally locked between swings in order to support the patient's weight--which necessarily slowed them down and made them hazardous for walking on anything but a level surface. The C-Leg's microprocessors adapt and compensate for stairs, slopes, and irregular terrain, allowing the wearer to walk naturally without thinking about it or compensating.

The C-Leg's multiple sensors, which relay data at a rate of 50 times per second, provide the feedback used to operate its mechanical and hydraulic systems. Two strain gauges measure pressures on the leg and note how often the heel strikes; magnetic sensors report changes in knee angle. Greg Schneider, CP, clinical specialist in prosthetics for Otto Bock Health Care, Minneapolis, Minnesota, estimates that more than 5,000 C-Legs have been sold worldwide in over 20 countries since 1997. "As C-Leg technology becomes more and more accepted as a standard' type of prosthesis, then it becomes more accepted in the field as far as payment goes, too," he points out. "Sometimes insurance companies declare that they will only pay for a standard' prosthesis. When they talk about "standard" prostheses, they might be talking about technology that was developed and patented back in the 60's!

Schneider continues, "Definitely microprocessors in every aspect of our lives are becoming more prevalent--more standardized. Forty years ago you'd use a typewriter, engineers would use a slide rule, and if we wanted to communicate with someone we would do so by mail rather than e-mail. Times change. Microprocessors are obviously becoming the standard in everything--prosthetics is no different in that respect."

Ohio Willow Wood GeoLite Knee

Ohio Willow Wood GeoLite Knee
Ohio Willow Wood GeoLite Knee

Ohio Willow Wood's GeoLite" Knee, an evolution of its earlier GeoFlex" Knee, was introduced in March 2002, and offers significant advances, according to Mark Ford, director of marketing for the company, based in Mount Sterling, Ohio. "The new GeoLite knee has taken the benefits of the polycentric, variable-friction design and made it shorter, lighter, and less expensive." The knee still provides outstanding stability to amputees, but now takes up less room on the prosthesis.

Both the GeoLite and GeoFlex Knee enable low-activity amputees to manage all types of terrain with confidence that the knee will support them in the event of a stumble, with up to 20 degrees of flexion adding stability. No locking mechanism is required; a sliding crank design and variable friction controls provide a stable knee without manually locking and unlocking it. The design also helps amputees to better manage hip flexion contracture by providing stability despite the contracted limb. The GeoLite is available with two different proximal connection options; the GeoFlex offers four different connection combinations.

Fillauer Swing Phase Lock (SPL)

Fillauer Stance Control Lock with remote
Fillauer Stance Control Lock with remote

The Swing Phase Lock (SPL) stance control knee is Fillauer's current star, says Gerry Stark, BSME, CP, FAAOP, director of education and technical support for Fillauer Inc., Chattanooga, Tennessee. "It's position-dependent rather than load-dependent," Stark explains. "In orthotics, that's a great application because there's not a lot of axial load that passes through the knee itself. In prosthetics, we can utilize that load to engage the knee."

Benefits listed on the Fillauer website note that it automatically locks prior to stance phase, closely mimics normal gait patterns, allows free flexion during swing phase, minimizes energy consumption, eliminates circumduction vaulting, allows toe clearance during swing phase, offers three modes of control, eliminates cables and special heel connections, fabricates easily, and lets the patient feel secure. †

Endolite Adaptive Knee

Endolite Adaptive Knee
Endolite Adaptive Knee

The Adaptive Knee, the newest offering from UK-based Chas. A. Blatchford & Sons Ltd, is described by Alan Kercher, technical service and education manager for Endolite North America, Centerville, Ohio, Blatchford's US company. The knee is the third generation of microprocessor control in Endolite's history. "It's a hydraulic and pneumatic hybrid cylinder which is controlled by microprocessors through time, force, and swing sensors," Kercher explains. "It detects various things happening in the gait cycle at 62.5 times a second and adjusts accordingly. It creates an electronic image of what's being used in the terrain the patient is walking on, and adjusts to the necessary support modes. The end result is a lot more natural gait than most knees offer--and it also gives the wearer the freedom to control a prosthesis that will work with them rather than for them."

"The C-Leg is a great invention, but is a different concept than the Adaptive," Kercher continues. "Voluntary control is what we advocate, using the muscles to control the prosthesis. The Adaptive walks with the wearer and they're in control of it. With the Adaptive, we're also after symmetry: muscles in the socket are encouraged to work as they would on the sound side, resulting in amputees having a healthier residual limb. In walking a ramp, for example, the leg detects extra force and increases the resistance to provide the extra needed support--resulting in a more natural walk, operating just as the sound side does." The improved symmetry and more natural style of walking have encouraged patients to free up their shoulders and swing both arms as they walk, Kercher adds.

Ossur Hydraulic Knee
Ossur Hydraulic Knee

The Adaptive's rechargeable batteries last from three to ten days, dependent on settings. An optional shock and torsion pylon is also available to take away much of the shear and stress forces through the residual limb, providing a more natural function. The warranty is for three years, and every anniversary of the fitting date, Endolite provides an exchange unit updated with any newer software and improvements.

About 50-60 patients have been fitted with an Adaptive Knee since its introduction late last year, Kercher believes. "We rolled it out slowly--didn't want to get caught with hundreds of people phoning up, because we haven't got hundreds of staff to go and fit them!"

Kercher points out that one novel aspect of the Adaptive Knee is that experts from Endolite attend the first fitting when a prosthetist purchases one of these knees. The next day, Endolite representatives are again there to follow up as needed: to fine-tune the knee, to make sure everything is functioning properly and that the prosthetist is aware of how the knee is performing.

Ossur Improved Total Knee 2000

Ossur's improved "Total Knee 2000"" was recently reengineered (May 2003) by adding needle bearings, retaining rings, and an improved extension assist, all of which contribute to enhanced durability, according to Ossur North America, Aliso, Viejo, California. A two-year warranty is standard with the Total Knee 2000, which also includes geometric locking to provide for maximum security and natural walking motion; three-phase hydraulics, which help the user to alter walking speed smoothly and swiftly; stance flex, which acts as a shock absorber to simulate the natural flexing action of the normal knee; and a unique seven-axis design, which imitates true knee motion and facilitates a more natural gait, the company notes.

Ossur Mauch XG

Ossur Mauch Knee
Ossur Mauch Knee

Ossur's Mauch XG" carbon frame knee, introduced in 2002, combines high performance with a sleek new design. The frame is made of ultra-lightweight carbon fiber wound with Kevlarģ, a fabric used in bulletproof vests. "The Mauch XG" package is targeted to the high-activity above-the-knee amputee requiring strength and performance, in order to pursue an extreme lifestyle during both work and play," says Michelle Hamilton, director of marketing and public relations for Ossur. "This is the strongest carbon frame on the market and has a weight limit of 285 lbs."

Prolite Smart Magnetix

The Prolite" Smart Magnetix" AK Prosthetic is a product of German-based Biedermann OT Vertrieb, which utilized Rheonetic" (magnetorheological) technology from the Lord Corporation, Cary, North Carolina, to create the device.

Lord Corporation's website describes it: "The prosthesis, which uses a modified Lord RD-1005 MR fluid damper, enables above-the-knee amputees to walk with a more natural gait and makes climbing up and down stairs and inclines much easier than before--at a fraction of the cost of other high-end knee products. The unique characteristics of Lord Rheonetic MR fluid dampers--high controllability, millisecond response time, and velocity-independent force--make this product possible."

Lord Corp. - MR Damper Knee 2003 Lord Corp - All rights reserved.
Lord Corp. - MR Damper Knee 2003 Lord Corp - All rights reserved.

"In addition to the great improvement in function over passive damper systems, the Prolite Smart Magnetix Knee&is less costly, less complex, and more dependable than state-of-the-art motor-controlled active damper systems. The Prolite Knee includes a powerful two-day battery derived from cell phone battery technology and includes sensor and control technology developed by Biedermann. The system senses and adapts to all possible movement changes in milliseconds, allowing the wearer to walk naturally up and down stairs and inclines, ride a bicycle, carry heavy objects, and walk or run with varying gaits. Prosthetics specialists are now being trained to fit amputees in Germany with the new knee."

Lord's Lynn Yanyo, PhD, sales and marketing manager, reports a continuing increase in interest regarding the MR process and methods of adapting it for other prosthetic uses, such as ankles and elbows; a resistance element for exercise during rehabilitation; calibrating how well a joint is doing during therapy by allowing comparison with measurements from the sound joint, etc.

We've considered just some examples of what's happening now. What will the future see in high-tech prosthetic knees to benefit amputees? Stay tuned.

Judith Otto is a freelance writer based in Holly Springs, Mississippi.

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