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Prosthetic Knees: What’s on the Way?
By Judith Otto  It has been said that there's nothing new under the
sun, but somehow there always seems to be a new modification,
approach, or philosophy that, when applied to an "old" concept,
certainly appears to create something new. And why worry about
semantics when the result is something that works better, faster,
or easier to improve the lives of lower-limb
amputees?
What's waiting to debut now--and what could be coming in the
more distant future?
Otto Bock's C-Leg has already undergone numerous
changes since its debut six years ago, said Greg Schneider, CP,
clinical specialist in prosthetics, Otto Bock Health Care,
Minneapolis, Minnesota. "Some changes have been fairly major, such
as adding a second mode into the C-Leg for use in sports and other
kinds of activities; others have been somewhat invisible tweaks to
make things work better and more efficiently. It will continue to
evolve in a similar fashion--we'll just keep making it a better and
better product."
What's in R&D for the C-Leg can't be discussed, Schneider
said, but microprocessors continue to be an improving technology in
O&P. "There will be lots of exciting things coming up;
prospects are virtually limitless as batteries keep getting
better," he exclaimed. "I can envision every prosthesis having a
microprocessor someday--but that's just my personal vision." Also
in the design stages at Otto Bock: new approaches to the
20-year-old weight-activated stance mechanism for the safety knee.
With the old weight-activated stance brakes, the user had to take
the weight off to get the knee to flex for the swing phase. Some of
the new developments will have weight-activated stance mechanisms
that don't need to be unweighted to flex the knee during stop for
pre-swing and swing phase. "Those types of knees will be out soon,"
said Schneider.
Looking toward the future, Fillauer Inc., Chattanooga,
Tennessee, is examining other ways of providing stance control in
knees, such as the application of other kinematic, mechanical, or
position-dependent locks, said Gerry Stark, BSME, CP, FAAOP,
director of education and technical support. "Besides locks which
represent better involuntary controls, more should be done to
enhance voluntary control." He hinted that new products can be
expected in these areas within a year or so.
Ohio Willow Wood, Mount Sterling, Ohio, is also cautious about
revealing too much, but Mark Ford, director of marketing, noted
that the company "will continue to develop more new versions of the
GeoLite and GeoFlex knees based on the same technology. Such
versions should be introduced within the next 12 months."
Regarding its new knee, Ossur North America, with offices in
Aliso Viejo, California, stated the company did not want to comment
at this time and hopes to keep the knee under wraps until its
introduction. Also in the wings on Ossur's stage is a motorized
prosthetic knee soon to be ready for market. The active powered
knee is in final development stages at Victhom Human Bionics in
Quebec, Canada, Ossur's new partner. Ossur will act as manufacturer
and distributor of Victhom's prosthetic products, and will
co-develop subsequent O&P products using Victhom's cutting edge
bionic technology.
Unlike passive lower limb components, Victhom's knee is capable
of active flexion and extension, which should make it more
energy-efficient and more natural in motion than currently
available knees. A computer will control the powered knee's
movement, selecting from a range of normal walking parameters which
were decoded and computerized by Victhom's founder and chief
scientific officer, Stephane Bedard. Victhom reportedly anticipates
its commercial availability within a few months.
Endolite North America, Centerville, Ohio, was also wary about
revealing new product directions. "At this moment, it's early
days," said Alan Kercher, technical service and education manager.
"I don't really want to comment on some things that are in
progress. Blatchford is a very innovative company that likes to
push the boundaries; we're always looking at new possibilities,"
Kercher hinted.
Sandia National Laboratories, Albuquerque, New Mexico, is still
hard at work on its Smart Integrated Lower Limb. Its microprocessor
will not only control the knee, but will manage, process, and
coordinate input from the ankle, foot, and socket. Reportedly,
technical requirements for the limb will be set by Seattle Systems,
Poulsbo, Washington, while materials work and testing will be
performed by Sandia's peacetime partners in the Russian nuclear
weapons laboratory, Chelyabinsk 70.
The leg is intended to simulate a human gait regardless of
irregular or steep uphill/downhill terrain. A
microprocessor-controlled module implanted in the leg will respond
to sensor input from multiple sources; the microprocessor will
control hydraulic joints and electric motors that power the ankle,
knee, and socket. The leg is thus "smart" enough to behave
responsively, reacting to current conditions.
"This is about making a leg that is more like a missing limb
than a collection of components ever can be," said a Sandia
spokesperson. "This limb will have a digital control system to make
it smart."'
 Also coming soon: A very unusual prosthesis,
perhaps best labeled a "false prosthetic," was described by Jack
Engsberg, PhD, director of the Human Performance Laboratory,
Department of Neurological Surgery, Washington University, St.
Louis, Missouri:
"Our objective is to create a training aid that will help teach
physicians and clinicians to assess strength and spasticity. This
will take the form of a mannequin fitted with artificial joints at
the knee, elbow, ankle, and hip. The joints can be set for
different levels that mimic strength and spasticity levels
encountered in human patients."
Patient spasticity and strength can vary according to time of
day, anxiety level, motivation, fatigue, age, gender, size, and
strength. Current methods for determining spasticity require
manually moving a patient's passive limb through a range of motion
and assessing the resistive characteristics (spasticity) based upon
six grade descriptions that are largely subjective. Methods for
determining strength rely on the patient isometrically resisting
applied loads; the evaluator senses, then grades the resistive
isometric force of the patient. Again, the determination is
somewhat vague. "So where first responders can practice their
skills on a CPR dummy, our students will practice assessing
spasticity and strength correctly on our training mannequin--which
can also be used for testing and certification of their assessment
skills," Engsberg explained.
Lord Corporation, Cary, North Carolina, is working with Engsberg
to develop an appropriate resistive brake for the mannequin's
artificial knee joint. The prosthetic knee is the first joint to be
developed using magnetorheological (MR) fluids that can instantly
change viscosity characteristics to provide specific resistive
torque at a fixed angle or a torque profile over an entire angular
range of motion. Engsberg anticipates that the spasticity dummy
will be available in about three years.
The Future: What's Possible?
In what direction do you anticipate prosthetic knee design to
evolve in the future?
Gerry Stark, Fillauer: "We are all looking at different ways of
triggering the stance control mechanism. In the past, most of the
triggers have been load-related. "Right now, what people are
looking at are different linkages, braking systems, positional
control, and electronic triggers. I believe that in the future
we'll be looking at even more electromechanical solutions--a good
example is Otto Bock's C-Leg, which combines technology that
monitors both position and prosthetic loading."
Stark foresees active movement of the lower-extremity
prosthesis, including knees and ankles, rather than just passive
compliance. "Virtually every company is looking at different ways
of controlling the stance triggers they have--or looking at
different types of control fluids. Recent examples are
Biedermann-Motech's rheonetic fluid, and Ossur's stance control
knee, TKO1500, which utilizes a cylinder type of braking system.
And Ossur-Victhom is trying to provide active motion of the knee as
opposed to passive resistance."
The lower-extremity prosthesis is primarily a passive device,
Stark explained, so it is defined more by the movement it allows
rather than the movement it initiates. By nature, in normal human
locomotion, that's what walking is, anyway: a controlled fall.
"So," said Stark, "most of the motions we're using in normal
human locomotion are eccentrically lengthening the muscle and just
controlling the fall. A prosthesis is really just an extension of
that--to aid in a controlled fall. The trouble begins when we
encounter obstacles such as a hill or stairs. Now we've got
problems, because now we require active movement of the knee to
concentrically contract our muscles.
"One characteristic of lower-extremity prostheses is that we've
never really had active movement as we do in the upper-extremity
prostheses," Stark continued. "The tremendous load demands on the
prosthesis and the weight constraints have always been prohibitive
to adding extra gadgets and active movement."
More active motion is likely to be built into future legs, Stark
believes. "Even the best prosthetic leg is a far cry from the
anatomic leg and what it can do and how it adapts. And because we
really don't have any active movement of the device in any way, I
think we're going to be looking hard at things like that: how to
pick up the toe, even using that motion to help generate some
potential energy with a spring or as a trigger for electronic
devices."
Looking far in the future, there will be a knee that moves as
the result of a signal from the patient, Stark predicts. "In the
near future, however, I think people are really looking at low-cost
alternatives--trying to work with different mechanisms, fluids, and
triggers to develop a better way to lock and unlock the knee when
the patient desires it.
"When we look to the future, a lot of it is limited by the
weight of the limb; and anything we make with motors and batteries
has to be all self-contained," Stark continued. "We could do all
kinds of wonderful things if we could pull behind them a wagonload
of hydraulic controls--we could even make toes wiggle! But it's not
going to happen in the near future because it has to be all
self-contained.
"From a development perspective that's why we're all waiting to
see what happens with osseointegration," Stark said. "With that
attachment, the prosthesis could weigh a little bit more and we
could add more gadgets and movement. The human leg weighs about
25-35 lbs. But we could never make a prosthesis that weighed that
much-- the skin interface can't tolerate that much load, so we're
shooting for seven to ten lbs."
Engineers could develop a lot more intricate devices if the
patient could tolerate a little bit more weight, Stark explained.
The problem has always been with the boundary layer, or the
infection rate, he added, but with successful osseointegration,
engineers can be more novel in their approach.
People did pretty well with prostheses in the past, Stark
pointed out. "I think sometimes it's modern hubris that we assume
that our stuff is so much better. In the 40s there are films of
highly active patients playing basketball in quad sockets and
exoskeletal legs. The componentry was relatively simply, but with
effective use of alignment and interface design, high activity was
possible."
Stark foresees greater control of microprocessors and perhaps
even active movement, but added, "I think there will still be room
for improved mechanical apparatus that are controlled in a more
conventional manner."
Some prostheses from the past have been revisited with new
technology, Stark noted. "An old hydrocadence knee was utilized in
the 40s and 50s--and that was one of the few knees that actually
allowed for toe pickup. I've heard developers talk about how
significant it was, although the hydraulic system leaked and it was
bulky. As such systems evolve; there might be ways to improve from
mechanical solutions to hydraulic solutions, and now to electronic
solutions!
"So basically, a lot of the new things aren't really that new,"
Stark continued. "Many of the concepts existed a long time ago, you
can see in the literature of the time. The materials or techniques
simply weren't present to make the design objectives possible or
the devices consistently successful. When you look to new designs,
I think we all try to revisit those products because of the
advantages associated with them and attempt to improve on them with
modern materials and design technique."
Charles W. Radcliffe, MS, ME, professor of mechanical
engineering (emeritus), University of California at Berkeley:
"There will be a continued effort to incorporate microprocessors
into the control systems for artificial knees. Current designs are
extremely simple devices compared to the possibilities of the
engineering field of mechatronics.' Engineers in school today are
eager to design components that will benefit amputees, and I am
sure they will create designs far better and less expensive than
what is available today."
Radcliffe added a caveat: "Unfortunately, a less expensive
device may not be greeted warmly by some prosthetists. There is
often a tendency to fit a more expensive device, whether or not the
amputee can use it effectively. This is due to many factors, one
being the amputee who wants the best' and believes the best must
obviously be the most expensive."
The first application of microprocessors was to control the
needle valves in a pneumatic swing control used with a single-axis
knee, Radcliffe noted. "Current pneumatic swing controls with
simple mechanical needle valves, properly designed and adjusted,
will do a good job in controlling the speed of walking without a
change in valve position after the valves have been adjusted for a
particular amputee." Microprocessor control of the extension and
flexion resistance valves may improve the response of the
system, but the improvement may not be significant, he said,
adding, "Current pneumatic and hydraulic swing control devices are
more than adequate for all but very active amputees.
"The major benefit for microprocessor control is stance phase
control using hydraulics or electromagnetic fluids," Radcliffe
continued. "The control system components can be quite
inexpensive." The system is typically assembled
from components easily available and used by engineering students,
he explained. The more challenging problems are in the design of
special transducers to provide input of forces, moments, position,
velocity, and acceleration. Then, with the possibility of many
inputs, which set of inputs is optimal and what functions actually
need to be controlled must be considered. "At present, the
angular position and angular velocity of a single-axis knee are
being controlled by the microprocessor system to provide both swing
and stance control with a variety of sensors measuring control
inputs. In the future there will be many improvements in the inputs
and control strategies used."
In what direction would you PREFER to see future prosthetic
knees evolve--and why?
"There is a tendency in prescribing and fitting transfemoral
amputees with specified devices that provide knee security as the
number one priority," Radcliffe pointed out. "As a result, there is
widespread use of so-called safety' knees that provide automatic
knee braking action at heel contact. As a result the amputee
becomes a lazy' walker who depends on the device more than his own
residual musculature.
"This may provide good security when walking on level surfaces,
but may actually introduce a dangerous situation under some
conditions, such as negotiating a down slope or coming down stairs
where it may be difficult to unlock the knee under
load. The amputee thus adopts a safe but slow one-step-at-a-time
style of gait whenever a dangerous situation is encountered."
Hydraulic "swing and stance" control can provide stumble
recovery by allowing the knee to flex slowly with hydraulic
resistance, Radcliffe noted. This function is also
theoretically useful for stair descent, but usually requires an
athletic amputee to master the technique.
"I am a great believer in using the residual hip musculature on
the amputated side as an excellent source of power and control for
prosthetic knees," Radcliffe said. "This is particularly true for
an active amputee using a four-bar knee. A properly designed and
fitted four-bar prosthetic knee can provide excellent control of
knee stability throughout the stance phase of the walking
cycle.
"A four-bar knee, because of the kinematics of an elevated
instant center, allows an effortless and energy-saving transition
from stance to swing with control provided by hip muscles. The only
missing function is stumble recovery. However, another feature of a
well-designed four-bar knee is a definite increase in toe clearance
during swing-through, hence the amputee seldom, if ever, stubs the
toe and stumbles. Better voluntary control of knee stability also
improves safety on rough ground and slopes. Unfortunately, few
prosthetists really understand the function of four-bar knees and
believe their only advantage is improved stability at heel
contact. As a result, they are often aligned such that they
function as a safety knee. This is a holdover from very early units
that were specifically designed to provide a very safe knee."
Radcliffe would like to see a prosthetic knee that combines the
flexibility of microprocessor control of a hydraulic device for the
swing phase with the advantages of a polycentric knee in the stance
phase. Such a device could be much simpler than current designs
since the microprocessor would be used primarily for control of the
resistance pattern in the swing phase, he said. A major cost item
in current passive hydraulic swing control devices is the need for
a mechanical system of adjustment of the flow through a series of
very small holes. Microprocessor control of the resistance pattern
is a simple problem of programming the resistance pattern of a
single valve as a function of angular position and angular velocity
of the knee, he explained, adding, "The program is easily varied to
suit the needs of each individual amputee wearer. Sensor design and
programming for stumble recovery is a much more challenging
problem, and adds considerably to the cost of a
microprocessor-controlled swing and stance device."
What advantages might mechanically enhanced knees offer that
microprocessor-controlled knees could not?
"The primary advantage of mechanically adjusted devices is
simplicity and low cost. These attributes do not necessarily
mean poor function," Radcliffe said. Passive pneumatic devices have
fewer leakage problems, he commented. "Hydraulic devices, with
either mechanical or microprocessor control, do tend to leak
eventually and require more frequent maintenance."
What Might Someday Be Possible?
Jules Verne taught us not to scoff at visionaries; what one man
can dream, another can do. Many "science fiction" possibilities are
being explored for prostheses that may be closer to market than we
suspect.
Avenues being pursued include using animal tissue to create a
"living knee" fed by nutrients rather than fueled by batteries.
Some researchers are developing special actuators for knees that
contain reactive gel. Joel Burdick, deputy director of the
California Institute of Technology's Center for Neuromorphic
Systems Engineering, Pasadena, is applying embedded chip technology
that may allow the brain to directly control permanently attached
prosthetic limbs. Reggie Edgerton, Life Sciences Department,
University of California-Los Angeles (UCLA), is applying a similar
philosophy to develop solutions for patients with severe spinal
disabilities, who thus have difficulty walking.
And who knows what yet unimagined possibilities may suggest
themselves--far sooner than we think? Judith Otto is a freelance writer based in Holly Springs, Mississippi. 
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Prosthetic Knees: What’s Currently New and Impressive?
- October 2003
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.
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Table Of Contents - October 2003
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