Feature Story for Igus on Century XXII Innovations
written by Richard Stewart
Acclaimed Artificial Knee
Drawn From Labor of Love
Unique Prosthesis Features Patented Hydraulic System,
Aluminum Components, and Self-lubricating Plastic Bearings
An award-winning prosthetic knee, light enough for a child amputee
to wear, with all the flex and gait controls needed for normal activity,
began as a labor of love seven years ago in Sweden. It was then Finn
Gramnas realized that no suitable prosthesis was available for his
daughter, Lisa, who was only six when her leg was amputated above
the knee. Today, the active 13-year-old leads a normal life, thanks to
the unique prosthesis her dad designed in his home workshop.
Marketed as the Total Knee by Century XXII Innovations, Inc. of
Jackson, Mich., the prosthesis weighs less than 620 grams, about half
as much as comparable devices on the market, and it requires less
effort on the wearer's part, according to Bill Donahue, vice president.
Key to the weight savings is Gramnas' patented rotary hydraulic
system, a compact, encapsulated unit, providing multi-phase swing
control for a smooth and natural gait. Light-weight materials are used
throughout, including high-tensile aluminum alloy forgings and self-
lubricating thermoplastic iglide bearings from igus, inc. of East
Providence, R.I. The bearings also play a critical role in the smooth,
friction-free operation of the prosthesis.
Moving Into Production
The Total Knee is the showcase product of Century XXII
Innovations, a start-up company formed when Gramnas came to the
U.S. looking for partners to further develop the prosthesis and
market it commercially. Donahue, who had spent 20 years in orthotic
and prosthetic manufacturing, and John Stonecipher, company
president and a certified prosthetist with 35 years in the field, formed
the company soon after evaluating the Total Knee.
"We immediately recognized that it was an incredible breakthrough in
knee prosthetics," recalled Donahue. "We were very impressed with
the light weight of the system and the smoothness of the swing. With
the Total Knee, the wearer doesn't exhibit the hip-hiking you typically
see as the amputee ambulates, loading and unloading weight and
braking the prosthesis to control the swing," he explained.
Protection Against Falls
Based on a polycentric design concept that imitates the motion of a
natural knee, the device features a geometric locking system that
prevents the extended leg from collapsing when the heel comes in
contact with the ground. Designer Gramnas decided to incorporate
this locking moment into the device when he noticed his daughter
having difficulty slowing down from a fast walk, especially when
"With the prosthesis she was wearing, she couldn't brake her speed;
the leg would just collapse when she tried to do what you do normally,
using your muscles to slow down," he recounted. "There were
prostheses available that could partly take care of this problem, but
only for adults. They were too heavy for children to use."
So Gramnas, who works as a human resources manager in Sweden,
set up shop in his small garage and began a six-year process of
designing and making prototype knees that a child could wear. "I
made my own solutions, based on the body weight line. When the
weight is on the toe, the knee has to be free to bend; when the
weight is on the heel, the knee is always locked," he said. "It's a
plain geometric solution, but there was nothing on the market that
In operation, when the heel strikes the ground, the device securely
locks, and a polyurethane bumper with adjustable durometer
based on the weight and activity level of the wearer cushions the
impact while allowing a limited knee flex. As the weight line shifts
forward toward the toe, the knee unlocks, allowing free flexion at
toe-off. Gramnas' three-phase hydraulic dampening system provides
resistance to control the swing of the knee. It can be adjusted to
the proper gait for each wearer.
Multitude of Parts
Roughly 60 parts go into the Total Knee, including seven major
components: the main hydraulic head with rack-and-pinion gearing,
the front, back and bottom links that provide the unique geometry,
a balancing unit that operates like a teeter-totter and locks and
unlocks the mechanism, and a clevis-shaped chassis. All the major
components are machined from forgings of 7075-T651 aluminum,
except the chassis, which starts as an aluminum casting.
Plastic bearings are used on the knee's seven pivot points. They
play a critical part in the design of the Total Knee, because it is
imperative that the prosthesis operates as friction-free as possible,
according to Century XXII's Donahue. "The bearings have been
a major breakdown point in prosthetics over the years," he
commented. "With the bearings we're using, we simply are not
experiencing problems," he added.
Gramnas specified self-lubricating iglide® bearings produced by
igus, inc. after testing bearings from half a dozen other
manufacturers in his prototypes. Since the Total Knee is very
compact and machined to very close tolerances, the bearings also
had to be precision-made and very durable and resistant to
compression. Manufactured from a tough, wear-resistant
thermoplastic material known as T500, the iglide bearings provide
excellent anti-friction properties and have exhibited no signs of
wear after millions of cycles of operation, both in the test lab and
in the field, Gramnas acknowledged.
"It's a very special application," the designer noted, "and I needed
a bearing which could take those high loads and have a long life.
Most of the bearings I tried tended to compress to a such level that
they couldn't spring back to their shape. We operate with very low
tolerances and need to have this application absolutely free from
all kind of play," he said. "The iglide bearings are quite easy to
work with and very durable."
The iglide bearing material consists of a thermoplastic alloy base,
a network of long composite fibers, and a mix of solid lubricant,
which homogeneously impregnates the material, explained igus
spokesman Carsten Blase from the company's East Providence,
R.I. headquarters. The company manufactures and stocks a wide
range of sizes and grades for prototype, pre-production and
production use. The advantages of iglide bearings in this
application are their high load-bearing capabilities, light weight,
and the fact that the bearings are not affected by environmental
conditions, Blase related.
"They can get wet, hot, cold and the performance of these
bearings is not affected. Plus, they're quiet. With metal bearings,
which have to be lubricated, you can get a clicking sound or
scraping and squeaking. That kind of annoying noise would be
unacceptable in this application," he noted, "You don't get that
with the iglide bearings."
Variations on Total Knee
Century XXII is preparing to market a non-hydraulic, friction-
controlled version of the Total Knee as well as a pediatric version
that small children can wear. An athletic version has also been
developed and is being tested by a mountain climber amputee.
Says Donahue, "Once we've proven the Total Knee system with
this high-activity type of amputee, we'll special-manufacture it for
people who require it." The primary differences in the athletic
version are in the hydraulic unit. Expansion chambers are provided
to accommodate the hydraulic fluid when it heats up during high
activity. Radiator-type vents assist in cooling the fluid.
"We've torture tested this product, both the architecture and the
materials we're using, and they've survived millions and millions
of cycles 450 pounds coming down on them, six times a second.
That kind of testing validates the componentry and the design,"
The editors of R&D Magazine announced in the September 1994
issue that Century XXII Innovations had been selected as a winner
in the R&D Top 100 Awards competition for the development of
the prosthetic knee. Each year since 1963, the editors, along with
75 scientific experts in a variety of disciplines, have selected the
100 most significant new products and processes of the year. The
magazine cited the outstanding technological advancement of the
Total Knee's locking system.
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