The SYMBIONIC® LEG:
A synergy of intelligent knee and ankle functions
Scott B. Elliott, C.P. (ABC)
Össur Asia-Pacific
Introduction
Throughout history, individuals have attempted to better the function of prosthetic
devices by replicating nature. Our human physiology is designed to function in a
synchronized and coordinated manner so that gross and fine motor tasks are
intuitive, complementary, safe and efficient. Replicating the myriad functions of
individual joints has itself been a design challenge which has typically led to very
specific and limited functional output in any one particular knee or ankle joint design.
However, a few designers have approached the replication of knee and ankle
functions from a synergistic perspective, designing functions to be complementary
and thus allowing for a potentially greater functional output. Approaching the design
of lower extremity prosthetic systems from a synergistic perspective has shown
potential to be very beneficial for trans-femoral amputees, and the addition of
microprocessor-controlled elements only further advances mobility functions, safety,
symmetry and efficiency.
Historical work on knee-ankle prosthetic systems
During the early 1950’s, a few prosthetic devices started to emerge with coordinated
knee ankle function. The first commercially available unit, the Stewart-Vickers
Hydraulic Leg, which later became marketed as the Hydracadence (initially released
by USMC and presently by Proteor of France) is a hydraulically controlled knee-shin
and ankle system that allows for coordinated knee and ankle motion. It allows for
dorsiflexion during swing for improved ground clearance, early plantarflexion during
loading response for increased knee stability, and heel height adjustability1. A study
by Sapin et al in 2008 examined functional gait analysis (3-D kinetic and kinematic)
between individuals using the Hydracadence system vs. non-linked knee-ankle
prosthetic systems vs. non-amputee volunteers. The following functional benefits
were found with respect to the Hydracadence system over the non-linked knee-ankle
prosthetic devices: Controlled plantar-flexion during loading response led to a
greater knee extension moment in early stance thus contributing to increased knee
stability. The Hydracadence swing phase characteristics in the sagittal plane were
equal to that of the Mauch SNS. However, the greatest difference and potential
advantage was the swing phase clearance between the prosthetic foot and floor.
The minimum height of the sole of the foot during swing phase was found to be
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36.9mm for the Hydracadence, 18.2mm for the other knee-ankle prosthetic systems,
and 40mm for the normal control group. Even considering the greater swing phase
clearance for the Hydracadence during level ground walking, no significant
differences were found with respect to pelvic motion between the three test groups2.
But what would occur if these same tests were completed on uneven surfaces or
during ramp ascent? With double the ground clearance during swing phase, isn’t it
possible that coordinated motion of the ankle is an excellent functional safeguard in
preventing stumbles during swing and limiting compensatory motion on alternative
walking surfaces? If so, then this is a design approach that inherently betters safety
and walking ease for the trans-femoral amputee.
Another lesser-known historical attempt at
developing mechanical synergistic kneeankle systems was attempted by Hans
Mauch at Mauch Inc. in Dayton, Ohio in the
1950’s. In 1956, Hans Mauch had developed
a hydraulic ankle design that allowed transfemoral amputees to walk up and down
ramps while maintaining good knee stability.
Priority was given to the Mauch SNS control
cylinder for further development between
1956 and 1963. In 1963, work resumed on a
foot/ankle design which included an
adjustable hydraulic dorsiflexion stop,
hydraulic plantar-flexion, transverse rotation,
inversion/eversion, and active dorsiflexion
during swing. Production units were made
by 1970 and a more refined prototype design
was created by 1974 compatible with
Engineering drawing of Mauch knee-ankle system.
endoskeletal and exoskeletal knee/shin
Mauch Inc. Dayton Ohio.
systems with Mauch SNS control cylinder. A
preliminary functional evaluation of the Mauch knee-ankle system was carried out by
the V.A. in New York on 2 trans-femoral amputees with results showing that the
system simulated anatomical motions during uneven ground walking, stair descent
(step over step), ramp ascent and descent, and running. Unfortunately, the ankle
unit had many issues with hydraulic leakage and never became proliferated within
the Mauch product range3.
Finally, the WLP-7R or ‘Waseda Leg’ was another interesting synergistic knee-ankle
prototype design created by researchers at Meisei University (Fukushima, Japan)
and Waseda University (Tokyo, Japan) in 1987. This design hydraulically connected
the knee and ankle joint through the shin of the device which was the hydraulic
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cylinder itself. The aluminium and carbon fibre reinforced plastic device reportedly
weighed only 2.4 kg. Walking experiments at Kanagawa Sougou Rehabilitation
Centre showed that the synergistic effect of balancing knee flexion and dorsiflexion
of the ankle was beneficial for level ground walking, descending AND ascending
stairs in a foot-over-foot manner4. The design was never brought to market but
highlights the potential mobility advantages offered by coordinating knee and ankle
motion.
Clinical Evidence for the Symbionic Leg
With the advancement of
microprocessor, sensor and actuator
technologies, more advanced knee and
ankle systems have been developed
that can increase the number of
functional features and benefits
available in one device compared to
purely mechanical devices. The Rheo
Knee (a microprocessor-controlled
swing and stance knee system initially
released by Össur in 2004) was
developed using a rotary
magnetorheologic actuator design that
was chosen for its extremely low fluid
drag characteristics. Hydraulic knee
system designs traditionally use
turbulent fluid flow characteristics
through various orifices to manipulate
resistance to swing and stance control
and therefore inherently maintain some
amount of fluid drag or resistance. In a
rotary magnetorheologic actuator, a magnetic field manipulates iron particles
suspended in a very small amount of fluid. The varying magnetic field strength
changes the electromagnetic bonds between particles thus simulating a braking
action in the joint. In a study by Johansson et al in 2005, the zero-pressure, lowdrag Rheo Knee was found to decrease metabolic cost by 5% compared to the
Mauch SNS and by 3% compared to the C-Leg. More specifically, they found that
there was less work required by the hip musculature to initiate knee flexion during
terminal stance5. The effect of adding intelligent ‘user adaptive’ controls to the Rheo
Knee (Össur’s ‘Dynamic Learning Matrix Algorithm’ – based on a concept by Herr
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and Wildenfeld published in 2003) furthered functional advantages by allowing the
knee to self-adjust knee flexion resistance during swing phase based on a
programmed goal or ‘target’ thus limiting the need for continual adjustment of the
system by the prosthetist6. The combination of these features allows users of the
device to walk with great efficiency and ease while the knee self-optimizes its swing
phase programming with changes in user activity and footwear. Use of force and
angle sensors in the Rheo Knee combined with specific parameters for stance
engagement and release enhance safety when compared to mechanical position deactivated systems like the Mauch SNS. This is especially important for users when
engaging in ramp and stair descent activities. This was confirmed further by a recent
study in Germany in 2011 by Greitemann et al which highlighted significant benefits
for Rheo Knee users such as easy cadence adaptation, increased ease of walking
on uneven ground, stairs, slopes, and a reduction in both the fear of falling and
concentration when walking7.
Initially released for trans-tibial
amputees, Össur’s Proprio Foot has
been found to have a number of
mobility, safety and health related
benefits for amputees. Through
incorporation of a linear actuator,
accelerometers, angle sensor,
intelligent software and a carbon fibre
foot module, the Proprio Foot allows for
automatic dorsiflexion or plantarflexion
changes based terrain and activity.
Accelerometers and the angle sensor
determine limb and joint position and
identify motion. By recognizing motion
patterns and limb position, the software
is able to recognize level ground,
slopes, stairs, and sitting. As a result,
the system is able to adjust ankle
position through the linear actuator to
match needs. The unit is calibrated to
the individual to dorsiflex 5° during midswing thus allowing for 15-20mm additional clearance. The additional clearance
makes it easier to ascend ramps and to avoid tripping on uneven ground or when
avoiding obstacles. The ankle adapts gradually to the angle of a slope during ascent
and up to 25% of the degree of the slope during descent. During ascent, this allows
the user to better transition body weight forward over the prosthetic foot which
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reduces effort and allows for more normal distribution of socket pressures as found
in a study by Wolf et al in 20098. During descent, plantar-flexion of the ankle assists
with controlling descent speed as well as reducing knee flexion thrust and adverse
socket pressures while descending. A study by Fradet et al in 2010 found that the
increased dorsiflexion during ramp ascent enabled test subjects using the Proprio
Foot to walk in a more physiologically correct manner, and patients reported feeling
safer during ramp descent9. During stair ascent and descent, the ankle can be
programmed to dorsiflex to 4° or 6°. For ascending purposes, this makes it easier
once again to transfer weight forward and extend the knee to climb steps. For
descent, this feature allows the user to place more of the foot on the step lending to
a feeling of more stability vs. descending with just the heel on the edge of the step. A
study by Alimusaj et al in 2009 found that the Proprio foot reduced compensatory
movements during stair ascent and descent promoting increased knee flexion and
knee moment during stance10. It’s important to note that the stair/ramp adaptation
features are subjectively reported to be more valuable to those who are more
impaired as in bilateral trans-tibial cases. It makes the difference between complete
stair and ramp avoidance to being able to accomplish these mobility tasks with
relative ease. The impact of this device on the energy cost of walking is alluded to in
the study by Delussu et al in 2013. Despite the increased weight, they found that the
energy cost of walking with the Proprio Foot was significantly lower than when using
a conventional carbon fibre foot module11. The ankle unit also allows for
plantarflexion to accommodate sitting positions and dorsiflexion of the ankle to assist
in moving from sit to stand. Finally, the unit can be calibrated to self-align to the heel
height of any shoe. This allows amputees to always have walking alignment
optimized regardless of footwear or walking barefoot.
Össur’s Symbionic Leg is a fusion of the Rheo Knee and Proprio Foot technologies
into one system. The knee-ankle synergetic system is powered by one battery that
is housed in the knee unit and shared by the knee and ankle system. All of the
features mentioned with respect to the Rheo Knee and Proprio Foot remain available
in the Symbionic Leg.
One trial fitting of the Symbionic Leg has been completed in Australia in February of
2013 on a male transfemoral level osseointegration recipient and K3 level ambulator.
The fitting was completed by APC Prosthetics (Sydney) in cooperation with Össur
Asia-Pacific. Prior to receiving the osseointegration procedure, the user had been
wearing a narrow ML socket design with atmospheric suction suspension, Rheo
Knee and Re-Flex Rotate Foot since 2005. Follow up was conducted in May, 2013
in Sydney and by phone in September, 2013. Upon initial follow up in May of 2013,
activity data collected from the original Rheo Knee unit’s onboard computer
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compared to the data collected from the Symbionic Leg showed an overall increase
in walking speed range and frequency with the new device:
Walking Speed
<2.6ft/s
2.6-3.6ft/s
3.6-5.6ft/s
>5.6ft/s
Rheo Knee + RF Rotate
16%
72%
12%
0%
Symbionic Leg
13%
45%
33%
9%
The table above outlines walking speed vs. percentage of the overall step count recorded from
Rheo Knee and Symbionic Leg Bionic Workbench software programs at the time of first follow
up visit. Percentages are calculated as % of all steps recorded on the Rheo Knee (452,080
steps) and Symbionic Leg (235,113 steps) devices respectively. Note the relative increase in
% of steps taken for moderate to faster walking speeds, 3.6ft/s to >5.6 ft/s, using the
Symbionic Leg.
As of September 2013, he continues to use the Symbionic Leg with no functional
issues or need for adjustment to the system since initial fitting in February. He
chooses not to return to the pre-existing Rheo Knee and Re-Flex Rotate system.
He subjectively reports the following advantages moving from the previous system to
the Symbionic Leg:
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To the subject’s recollection,
not a single stumble has
occurred using the
Symbionic Leg. Further, the
user cannot recall a single
instance of catching the toe of
the prosthesis on the ground
while using the new device.
With the previous system, he
remembers several instances
of toe catching and then
stumbling and having to
recover balance to prevent a
fall. The Symbionic Leg has
eliminated these adverse
occurrences completely.
Much less effort is required to
ascend the ramps on the
affected limb, and it is easier to
step out with the sound limb.
More stable descent of slopes
with greater control and less
thought required.
Easy to change shoes to flats,
barefoot and working shoes while maintaining alignment of the prosthesis.
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Relaxed Mode (plantarflexion while sitting) is also appreciated for sitting in
terms of cosmesis and in more confined spaces.
A pilot study on 10 test subjects using the Symbionic Leg in Germany recently
released in March of 2013 by Merbold et al reports that ‘the most pronounced
improvements were seen in reducing the risk of falling, climbing slopes, reducing
compensatory movements, longer walking distance and clearly improved comfort
while walking.’12 Although it is early days in terms of available published peerreviewed clinical research to definitively support the synergistic combination of these
devices, preliminary investigations like these appear very positive and highlight the
great potential with respect to this design approach.
Suggested indications for use
The Symbionic Leg is presently recommended for K3 level transfemoral and knee
disarticulation amputees of low to moderate impact levels. Foot sizes are available
from 22-30cm and weight limit of 125kg. It is particularly recommended for those
individuals who are presently experiencing difficulty with ramp ascent and descent as
well as gait and stability compensation issues related to swing phase clearance.
More Information
The Symbionic Leg is expected to be released in Australia and New Zealand in
2014. Please contact Össur Asia-Pacific for more information:
Össur Asia-Pacific
26 Ross Street
North Parramatta, NSW 2151
Ph: (02) 8838 2800
www.ossur.com
References
1. Wilson AB: Hydraulics and Above-Knee Prosthetics. Clinical Prosthetics and
Orthotics 1983. Vol. 7, No. 4, 3-4.
2. Sapin E, Goujon H, de Almeida F, Fode P, Lavaste F: Functional gait analysis of
trans-femoral amputees using two different single-axis prosthetic knees with
hydraulic swing-phase control: Kinematic and kinetic comparison of two prosthetic
knees. Prosthetics and Orthotics International 2008, 32 (2): 201-218.
3. Sowell TT: A preliminary clinical evaluation of the Mauch hydraulic foot-ankle
system. Prosthetics and Orthotics International 1981, 5, 87-91.
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4. Koganezawa K, Fujimoto H, Kato I: Multifunctional above-knee prosthesis for
stairs walking. Prosthetics and Orthotics International 1987, 11, 139-145.
5. Johansson JL, Sherrill DM, Riley PO, Bonato P, Herr H: A clinical comparison of
variable-damping and mechanically passive prosthetic knee devices. Am J Phys
Med Rehabilitation 2005; 84: pp. 563-575.
6. Herr H, Wilkenfeld A: User-adaptive control of a magnetorheological prosthetic
knee. Industrial Robot: An International Journal 2003; Vol 30, No 1: pp. 42-55.
7. Greitemann B, Niemeyer C, Lechler K, Ludviksdottir A. Verbesserung der
Teilhabe durch mikroprozessorgesteuertes Kniegelenk – erste Erfahrungen einer
Kohortenstudie. Medizinisch Orthopädische Technik 1-2011, pp. 90-101.
8. Wolf SI, Alimusaj M, Fradet L, Siegel J, Braatz F: Pressure characteristics at the
stump/socket interface in transtibial amputees using an adaptive prosthetic foot.
Clinical Biomechanics (Bristol, Avon) 2009, 24 (10): 860-5.
9. Fradet L, Alimusaj M, Braatz F, Wolf SI: Biomechanical analysis of ramp
ambulation of transtibial amputees with an adaptive ankle foot system. Gait and
Posture 2010, 32 (2): 191-8.
10. Alimusaj M, Fradet L, Braatz F, Gerner HJ, Wolf SI: Kinematics and kinetics
with an adaptive ankle foot system during stair ambulation of transtibial amputees.
Gait and Posture 2009, 30 (3): 356-63.
11. Delussu AS, Brunelli S, Paradisi F, Iosa M, Pellegrini R, Zenardi D, Traballesi M:
Assessment of the effects of carbon fiber and bionic foot during overground and
treadmill walking in transtibial amputees. Gait and Posture, May 2013.
12. Merbold D, Hahnel T, Brandenburg J, Muller Ch., Tschernig M. Verbesserte
Sicherheit und Mobilitat durch ein mikroprozessorgesteuertes Beinprothesensystem
– Erste Ergebnisse einer Multicenter-Pilotstudie (Improved Safety and Mobility with a
Microprocessor-Controlled Leg Prosthesis System – Initial Results of a Multicentre
Pilot Study). Orthopadie-Technik Magazine 3-2013, pp. 1-5.
Acknowledgements
Thanks to the staff at APC Prosthetics (Sydney) especially Stefan Laux for
cooperating with Össur Asia-Pacific in conducting the Australian Symbionic Leg trial.