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Chapter 35: Lower Limb Prosthetic Components: Updated Classification and Passive, Body-Powered Components
Tab le 2
Overview of Prosthetic Feet and Ankles
Generic Class Basic Function Primary Indication Major Advantages Chief Limitations
SACH Simplicity Limited ambulation or
Single axis Rapid foot at To enhance knee
Multiaxial Hindfoot inversion/
Flexible keel Smooth, easy rollover To make ambulation
Dynamic elastic
response
eversion and internal/
external rotation
Dynamic push-o To increase activity
maximum durability
required
stability
To accommodate
uneven surfaces
easier
level
Inexpensive and
durable
Biomechanical stability
in early stance
Reduces stresses on
skin and prosthesis
Comfortable and
reliable
Subjective sense of
dynamic responsiveness
Rigid forefoot; not energy-
ecient
Abrupt dorsiexion stop in-
creases knee hyperextension moment; increased
weight, maintenance,
and initial cost
Increased weight, mainte-
nance, and initial cost
Limited push-o; increased
cost
Increased cost
SACH = Solid Ankle Cushion Heel
Copyright John W. Michael, MEd, CPO, Portage, IN
SACH foot as an element of the most
basic possible prosthesis and considers
it medically necessary for individuals
with very limited walking abilities.
The posterior third of a typical SACH
design consists of an open cell foam
rubber that readily compresses during
loading response (Figure 1). While the
heel is compressing, the sole of the foot
is gradually lowered to the ground,
effectively simulating plantar flexion
despite the lack of an ankle joint. As
momentum carries the body forward,
the heel cushion rebounds until the tip
of the rigid forefoot keel contacts the
ground. A flexible rubber toe segment
then permits passive rollover, and heeloff occurs.
In multiple studies across many
decades, the SACH design is the least
energy-efficient prosthetic option, particularly for individuals who can walk up
inclines or whose walking pace is closer
to normal.14 Therefore, its use is best restricted to infants and toddlers (when
no other alternative exists because of
size limitations) or for individ uals with
multiple physical comorbidities that prevent them from walking more than a
few steps or from walking at more than
a very slow pace.
In the past, a preparatory prosthesis
often routinely included a SACH foot,
but growing evidence indicates that
underprescribing prosthetic components can prevent an individual from
Figure 1
view of a Solid Ankle Cushion Heel foot with
its compressible foam heel and rigid wooden
keel. (Reproduced from Michael JW: Prosthetic suspensions and components, in Smith DG,
Michael JW, Bowker JH, eds: Atlas of Amputa-
tions and Limb Deciencies: Surgical, Pros thetic,
and Rehabilitation Principles, ed 3. Rosemont,
IL, American Academy of Orthopaedic Surgeons, 2004, pp. 409-428.)
Photograph of a cross-section
reaching his or her full rehabilitation
potential. For that reason, a modern
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
431
Section 3: Lower Limb
Figure 2
cross-section view of a single-axis ankle-foot
with a mechanical ankle axis allowing movement in the sagittal plane. Compressive bumpers positioned anterior and posterior to the
ankle axis regulate the rate and amount of
ankle movement. (Reproduced from Michael
JW: Prosthetic suspensions and components,
in Smith DG, Michael JW, Bowker JH, eds: Atlas
of Amputations and Limb Deciencies: Surgical,
Prosthetic, and Rehabilitation Principles, ed 3.
Rosemont, IL, American Academy of Orthopaedic Surgeons, 2004, pp. 409-428.)
Schematic illustration of a
initial prosthesis is often a definitive
quality design with biomechanically
appropriate components suitable for
long-term use. The adjustable design
of modern endoskeletal components
means that the socket can be replaced
when changes in limb volume and geometry require, eliminating the need
to replace the entire artificial limb at
frequent intervals.
The SACH foot is sometimes provided for clients living or working in
remote regions who do not have access
to prosthetic follow-up care and when
durability is more critical than function.
It also is prescribed at the request of satisfied previous SACH foot wearers.
Single-Axis Ankle-Foot
Having an articulated foot with a mechanical ankle joint that allows passive plantar and dorsiflexion motion
seems logical, and this design has been
used for centuries (Figure 2). However, when properly aligned and adjusted for the user, scientific studies have
shown that the primary biomechanical
distinction of this component is that it
reaches foot flat faster than alternatives
do,15 which has clinical advantages and
disadvantages.
Regardless of the component, the net
ground reaction force vector (GRFV)
originates at initial contact from the
most posterior aspect of the shoe in contact with the ground and then moves
forward in the line of progression during
ambulation. With a mechanical single-axis ankle, the GRFV moves forward
in a smooth, controlled manner until
foot flat occurs. At this point, the GRFV
instantly moves forward until it passes
through the ankle joint. This anterior
displacement of the GRFV generates a
knee extension moment that continues
until passive dorsiflexion motion is limited by an anterior stop, at which time
the GRFV gradually moves forward to
the toe region, further increasing the
knee extension moment.
This biomechanical characteristic
gives rise to the primary indication for
a mechanical single-axis ankle-foot,
which is to increase knee stability. If
the prosthetic knee joint lacks sufficient stance stability, or if the person
with a transtibial amputation has poor
quadriceps strength, this could be advantageous.16 A mechanical single-axis
ankle-foot also can be helpful when a
locked prosthetic knee is required because the ankle motion will allow the
sole of the foot to remain flat on the
ground throughout most of the stance
phase. Except for these narrow indications, most individuals with lower limb
amputations are better served by other
options.
For the individual who can walk reasonably well with a prosthesis, the use
of a mechanical single-axis ankle-foot
is not recommended because the center of pressure under the prosthetic foot
does not progress forward in a normal,
smooth manner. With a mechanical
single-axis ankle-foot, the center of
pressure “jumps” forward at foot flat
and then “stalls” under the ankle joint
until full dorsiflexion is reached, when
it again begins moving forward. The inconsistent progression of the center of
mass disrupts the individual’s forward
progression and may increase the effort required to walk. For the individual with a transtibial amputation, the
abrupt increase in knee hyperextension
forces raises concerns about long-term
damage to the surviving knee.
Multiaxial Ankle-Foot
In addition to sagittal plane plantar
and dorsiflexion, the mechanical multiaxial ankle-foot provides a limited
range of passive coronal and/or transverse plane motion, acting similar to a
universal joint that allows the foot to
remain in contact with the ground,17
even if the terrain is irregular, giving
rise to the classic indication for mechanical multiaxial ankle-foot components to accommodate uneven surfaces
encountered in the user’s vocational
or avocational activities. Multiaxial
components are routinely prescribed
for surveyors, golfers, hikers, and anyone else who regularly walks outdoors.
Many community ambulators prefer
the added ankle mobility offered by
the mechanical multiaxial ankle-foot
because it aids in crossing sidewalks,
parking lots, lawns, and other commonly encountered irregular surfaces.
A secondary indication for the mechanical multiaxial ankle-foot is to
contribute to socket comfort and skin
protection by absorbing some of the impacts of walking, which is particularly
useful for stepping off curbs; traversing
uneven surfaces; and descending stairs,
ramps, and similar declines. Although
the added weight and maintenance requirements for any articulated prosthetic foot has been a concern in the past,
contemporary design advances have
mitigated such concerns.
Mechanical multiaxial ankle-foot
components may be further subdivided according to the planes of motion
they permit. Some designs offer biplanar
movements: adding hindfoot inversion/
eversion in the coronal plane or passive transverse plane internal/external
rotation. Most mechanical multiaxial
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
432
Chapter 35: Lower Limb Prosthetic Components: Updated Classification and Passive, Body-Powered Components
ankle-foot components permit triplanar
movement, offering the greatest degree
of ground compliance. During walking
trials with a diagnostic prosthesis, the
specific model of the mechanical multiaxial ankle-foot is usually determined
collaboratively by the wearer and the
treating prosthetist.
A study of individuals with bilateral transtibial amputation observed
that this cohort typically preferred the
added passive motions of a mechanical multiaxial ankle and walked with a
more normal base of support than with
a solid ankle.18 The study participants
generally preferred additional transverse
plane movement as well, and most chose
to keep both the mechanical multiaxial ankle component and the transverse
plane torque-absorbing unit for use in
their own prostheses.
A recent study of individuals with
unilateral transtibial amputations who
had limited walking abilities (K1 and
K2 Medicare functional levels) found
improvements in gait speed, balance,
general comfort, and perceived satisfaction with their prostheses when the
legacy SACH foot was replaced with the
multiaxial ankle-foot.19 Similar findings
were reported in a rare double-blinded study on individuals with unilateral
transtibial amputations, where the patients tended to report a preference for
compliant feet over stiff or intermediate
prosthetic foot types.
20
Dynamic Elastic Response
In 1980, prosthetist-orthotists John
Campbell and Chuck Childs21 shattered the widely held assumption that
a prosthetic foot must have a rigid forefoot to provide adequate stability. Their
solid ankle–flexible endoskeleton foot
demonstrated that clever engineering
could result in a forefoot that had varying degrees of flexibility while providing
more than adequate support for ambulation and other normal activities of daily
living (Figure 3).
Figure 3
view of the solid ankle exible endoskeleton
foot. (Reproduced from Michael JW: Prosthetic suspensions and components, in Smith
DG, Michael JW, Bowker JH, eds: Atlas of Ampu-
tations and Limb Deciencies: Surgical, Prosthetic, and Rehabilitation Principles, ed 3. Rosemont,
IL, American Academy of Orthopaedic Surgeons, 2004, pp. 409-428.)
Since then, numerous keel designs
composed of differing materials and
offering varying degrees of forefoot
“springiness” have been developed into
commercially available options. Many
terms have been used to describe these
components, including flexible keel, energy storing and release, and dynamic
response, and they are now among the
most widely prescribed prosthetic components because of their good clinical
performance and positive acceptance by
users.
The term dynamic elastic response,
originally recommended by Lehmann
et al14 in 1993, is used in this chapter.
Research reports since the mid-1990s
demonstrate that the degree of keel stiffness in the forefoot of these prosthetic
feet varies widely, as does hysteresis,
which is partially dependent on the
materials used.23 Clinically, dynamic
elastic response components vary from
those with a relatively soft elastomeric
forefoot (which often is used for pediatric applications) to those with moderately stiff thermoplastic keels (generally
used for low to moderate activity applications; Figure 3), to those with stiffer
and lighter carbon fiber construction
(suitable for a broad range of activities
and ages; Figure 4). Feet with carbon
fiber spring keels, particularly those
with longer springs that extend above
the ankle into the shin region, have been
shown to offer the greatest efficiency in
Photograph of a cross-section
22
Figure 4
dynamic elastic response foot. (Courtesy of
Freedom Innovations, Irvine, CA.)
Photograph of a carbon ber,
mechanical energy return.22 However,
the available distance from the end of
the residuum to the floor may prohibit
the use of such high-profile designs for
some individuals, and mechanical ener
gy efficiency is rarely the sole criterion
for prescription.
Dynamic Shin Pylons
Since the mid-1990s, innovative designers have created several different
shin pylon components that permit
additional passive movement to augment the function of the four basic foot
types. Perhaps the most commonly
prescribed is the shock-absorbing pylon (Figure 5). Although these components are well accepted clinically,24
research suggests that the benefits of a
spring-loaded telescoping shin are not
yet fully understood. Torque-absorbing
pylons and tube clamps have long been
available, and both clinical acceptance
and research findings support the value of these components25 (Figure 6).
Some manufacturers offer pylons that
integrate both features into a single
component.
Hybrid Shin-AnkleFoot Components
Today’s lightweight and durable lower
limb prosthetic components can often
be creatively combined to provide an
individual with a design targeted to
his or her specific needs and activities.
Individualized treatment of this sort is
-
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
433
Section 3: Lower Limb
Figure 5
shock-absorbing pylon. (Courtesy of Ottobock
Healthcare, Austin, TX.)
Photograph of an endoskeletal
one of the hallmarks of contemporary
prosthetic rehabilitation. For example,
a resilient low-profile carbon fiber dynamic elastic response foot from one
manufacturer can be attached to a
multiaxial ankle with adjustable transverse plane resistances from another to
create a hybrid multiaxial-dynamic elastic response component. Alternatively, a
single manufacturer can engineer both
functions within a single integrated
component (Figure 7).
The further addition of a shockabsorbing pylon provides additional
passive movement to either of the hybrid
assemblies described earlier. Several effective prosthetic foot designs that integrate the function of a shock-absorbing
pylon and a multiaxial ankle with a dynamic elastic response foot are available
and have been well received, particularly by individuals who would like to participate in higher impact activities, such
as descending curbs or stairs or playing
recreational sports such as basketball
and volleyball (Figure 8).
Prosthetic component engineers continue to find innovative ways to simulate
Figure 6
torsion adaptor. (Courtesy of Ottobock Healthcare, Austin, TX.)
Photograph of an endoskeletal
the benefits of articulated motions using lighter, stronger, more resilient, and
more reliable carbon fiber monolithic
designs. The rapid improvement in prosthetic running components, used by a
small percentage of high-performance
athletes with amputations, offers a good
illustration of how effectively a passive
mechanical prosthesis—when optimally
fitted, aligned, and adjusted—can effectively simulate limited aspects of human
performance.
Hydraulic Ankles
The concept of fluid-controlled prosthetic ankles, whose motion is dampened
hydraulically, is not new. In the United
States, Hans Mauch began developing
the concept of a passive, fluid-controlled
ankle in the 1950s, but it did not enter
clinical trials until the late 1970s.26 The
superior damping and smoother motion
of the Mauch Ankle was well received
clinically, but chronic seal leakage problems ultimately resulted in its removal
from the market.
In this millennium, there is new
interest in exploring this concept further, and several commercially available hydraulic ankle-foot designs are
27
Figure 7
dynamic elastic response foot with a split keel
that enables multiaxial compliance. (Courtesy
of Freedom Innovations, Irvine, CA.)
Photograph of a carbon ber
currently available (Figure 9). Most
take advantage of the smoother damping that hydraulic resistance provides
compared with mechanical control, and
some automatically ramp up resistance
as cadence increases. In most designs,
the prosthetist can tune the hydraulic
resistance to better match the client’s
gait; some designs have independent
plantar flexion and dorsiflexion damping adjustments.
One subset of fluid-controlled ankle
designs permits up to 50° of passive ankle plantar-dorsiflexion, a larger range
than for nonhydraulic prosthetic ankle
designs. In addition, some hydraulic
ankles can be adjusted to permit an
extended range of passive dorsiflexion,
compared with the typical mechanical prosthetic ankle, although unconstrained dorsiflexion can induce a knee
flexion moment in the latter portions
of the stance phase. For the user with
a transtibial amputation, flexion can be
controlled by using active knee control.
For the user with a transfemoral amputation, flexion control can be provided
with an appropriately stabilized prosthetic knee.
Few objective studies of the most
recent generation of passive hydraulic ankles are currently available, but
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
434
Chapter 35: Lower Limb Prosthetic Components: Updated Classification and Passive, Body-Powered Components
some wearers report the perception of
a smoother, more comfortable gait pat-
28 -30
tern.
As more data and clinical reports emerge in the future, these variants
may require their own classification, but
at this time they may be considered another variable in hybrid configurations.
Some passive hydraulic ankles also are
available in MPC configurations.
Selecting Shin-AnkleFoot Components
Shin-ankle-foot components are prescribed based primarily on the client’s
desired activity level and functional aspirations (Table 3). From within
the functional classification, the prosthetist selects the specific commercial
product(s) believed to offer the greatest
value, function, and durability without
adding excessive weight. Suitability for a
client’s needs is typically verified during
walking trials with a diagnostic prosthesis that incorporates the components
before creating the definitive design.
Passive, BodyPowered Prosthetic
Knee Mechanisms
Historically, passive knee mechanisms
that are controlled only by their mechanical design have been classified into
a limited number of conceptual groups
based on their overall biomechanical
performance. This simple conceptual
approach remains useful because the
numerous commercially available components within each conceptual group
have very similar indications and limitations, making it relatively easy to
exclude poor choices and generate a
functionally appropriate prescription.
Single-Axis Constant-Friction Knee
Until World War II, the most widely
available prosthetic knee in the United
States was a very basic hinge design that
allowed the joint to bend freely during
the swing phase of gait. Adjusting a
screw that pressed on the knee bolt
provided rudimentary damping during
Figure 8
dynamic elastic response foot engineered with
multiaxial compliance and shock absorption.
(Courtesy of Freedom Innovations, Irvine, CA.)
Photograph of a carbon ber
swing. Because of the characteristics of
mechanical friction, constant-friction
swing control can be set to allow the
user to walk safely only at a single, reduced fixed cadence.
Because of its mechanical simplicity and lack of complex internal parts,
the single-axis constant-friction knee
remains one of the lightest, lowest cost,
and most durable knee options (Fig-
ur e 10). In the United States, Medicare
recognizes the single-axis constant-friction knee as an element of the most basic possible prosthesis and considers it
medically necessary for individuals with
very limited walking abilities.
Currently, the single-axis constant-friction knee is rarely used unless
rugged simplicity is the primary consideration (for example, by individuals
who live in remote areas and cannot arrange for regular prosthetic follow-up).
It also is sometimes used for very young
children when no alternative is available
because of size restrictions.
The single-axis constant-friction
knee should generally be avoided because it has two well-documented,
major biomechanical deficiencies. The
single-axis constant-friction knee has
no inherent stability; therefore, every
Figure 9
foot with an adjustable hydraulic ankle mechanism. (Courtesy of Freedom Innovations,
Irvine, CA.)
Photograph of a carbon ber
step must be carefully controlled by the
user’s hip extensors to prevent knee collapse and potential injury. To achieve
perfect voluntary control of a prosthetic
knee under all conditions is an unrealistic expectation for many individuals,
particularly those who are feeble or have
multiple comorbidities.
Equally important, because mechanical swing control is so ineffective, the
single-axis constant-friction knee is
essentially a passive pendulum during
the swing phase, with the rate of swing
limited by its length.31 Consequently,
anyone using this type of knee is forced
to walk at a constant, slow pace, which
is particularly aggravating for physically
fit individuals who might be able to use
the knee safely but would otherwise be
able to vary their walking cadence.
Stance-Control Knee
Stance-control knees have mechanical
design features that augment knee stability during the stance phase, usually by a friction-brake mechanism that
engages whenever weight is applied to
the prosthesis (Figure 11). Adding sufficient friction to the knee axis prevents
further motion, which eliminates the
risk of knee collapse. This component is
perhaps most frequently used as the initial prosthesis for an individual whose
physical condition limits ambulatory
potential.
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
435
Section 3: Lower Limb
Tab le 3
Overview of Prosthetic Knees
Generic Class Basic Function Primary Indication Major Advantages Chief Limitations
Single-axis/constant
friction
Stance-control Increased
Polycentric Positive stability and
Simplicity Single-speed walking
weight-bearing
stability
ease of exion for
saving phase; special design available
that provides sitting
cosmesis for long
residual limbs
only if hip control
is good or better
or when maximum durability is
required
General debility; poor
hip control
To enhance knee
stability; special
design available for
knee disarticulation
Inexpensive and
durable
Improved knee
stability
Stable without
disrupting swing
phase; special
design provides
cosmesis for long
residual limbs
Fixed cadence and low stability
Delayed swing phase; must
unload fully to ex or sit
Increased weight maintenance
and initial cost
Manual lock Knee of last resort Ultimate knee
Fluid-controlled Permits cadence
Copyright John W. Michael, MEd, CPO, Portage, IN.
However, the typical weightactivated, friction-brake design has
historically been limited by a notable
biomechanical limitation: It cannot be
flexed unless fully unloaded. For anyone capable of walking foot-over-foot
with a prosthesis, this characteristic
change; microprocessor control oers
most normal gait
pattern
substantially disrupts gait mechanics
because knee flexion during the preswing phase of gait is eliminated. For
this reason, the basic stance-control
knee is best limited for use by those
whose walking abilities are severely
limited, perhaps caused by multiple
stability
Able to vary walking
speed
Eliminates knee
exion
Variable cadence;
more natural gait;
hydraulic stance
control adds
stability
comorbidities in addition to amputation
issues. The individual who requires a
walker for balance and walks with a
slow, shuffling gait often does well with
this knee component.
knees may increase the risk of injury
Abnormal gait; awkward
sitting
Increased initial cost; may
involve increased weight or
maintenance
The bilateral use of stance-control
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
436
Chapter 35: Lower Limb Prosthetic Components: Updated Classification and Passive, Body-Powered Components
Figure 10
constant-friction knee. (Courtesy of Ottobock
Healthcare, Austin, TX.)
Photograph of a single-axis
during a fall because the brake mechanism makes it impossible to bend
the knees and control the direction of
collapse. Sitting down with bilateral
weight-bearing friction-brake prostheses is nearly impossible because the
wearer cannot simultaneously unweight
both sides to allow the knees to bend.
Consequently, the weight-activated
stance-control knee is best reserved for
limited unilateral applications.
Some more recently developed
stance-control knee designs now automatically release the stability feature
when the knee reaches full extension
under weight-bearing loads. This design
advancement improves biomechanical
function and makes them preferable
when mechanical stance control is desired for individuals capable of walking
foot-over-foot.
Polycentric Knee
Polycentric knees usually can be visually identified by the multiple articulations they have, with four axis points
being the most common configuration.
Because four linkage bars connect
the four axes, the polycentric knee
also is referred to as a four-bar design
(Figure 12).
Polycentric knee designs offer
multiple biomechanical advantages
Figure 11
a cross-section view of a weight-activated
stance-control knee. Weight bearing compresses the spr ing and causes the kne e to clamp
against the cylindric brake bushing. Unweighting the prosthesis allow the spring to open the
clamping mechanism so that the lower leg can
swing freely. (Reproduced from Michael JW:
Prosthetic suspensions and components, in
Smith DG, Michael JW, Bowker JH, eds: Atlas
of Amputations and Limb Deciencies: Surgical,
Prosthetic, and Rehabilitation Principles, ed 3.
Rosemont, IL, American Academy of Orthopaedic Surgeons, 2004, pp. 409-428.)
Schematic illustration of
over single-axis constant-friction and
stance-control configurations and are
increasingly popular as a result. One
important advantage is that the instantaneous center of rotation (ICOR), which
is the functional center of rotation of the
knee, does not have to be located within
the knee mechanism itself. In a typical prosthetic design, the ICOR of the
polycentric knee is located proximal and
posterior to the mechanical articulations
within the knee. The posterior location
of the ICOR, far behind the net GRFV
in standing, makes the knee inherently
stable because of the extension moment
that is created32 (Figure 13).
In most polycentric knee designs,
the position of the ICOR moves anteriorly and distally as the knee flexes,
following a curved pathway called the
centrode. After the knee has been flexed
Figure 12
centric kne e. (Courtesy of O ttobock Healthc are,
Austin, TX.)
Photograph of a fo ur-bar, poly-
a few degrees, the ICOR now falls in
front of the GRFV, and the knee flexes automatically. Clinically, this means
that a well-designed, properly aligned
polycentric knee can provide inherent
stability in early stance while still flexing
easily in late stance during the preswing.
Many users prefer this combination of
inherent stability plus ease of voluntary
swing phase initiation.
Some polycentric designs provide increased toe clearance at midswing because the linkage mechanically shortens
the shin during flexion. Actual ground
clearance can increase up to 3 cm for
specific designs, reducing the risk of
tripping on environmental obstacles.33
Because of these biomechanical advantages, stable polycentric knees are widely prescribed, and their use continues to
increase. They also work very well bilaterally and for individuals with higher
levels of amputation.
It is important to realize that not all
polycentric knees are inherently stable.
A second type of polycentric knee is
designed primarily to accommodate
long residual limb lengths by minimizing the protrusion beyond the socket
during sitting (Figure 14). Because they
are indicated for individuals with knee
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
437
Section 3: Lower Limb
Figure 13
stantaneous center of rotation (ICOR) for polycentric knees that typically falls proximal and
posterior to the mechanical axes. As the knee
is exed, the ICOR usually moves in an anterior and distal direction, as shown here, along a
characteristic arc. (Reproduced from Michael
JW: Prosthetic suspensions and components,
in Smith DG, Michael JW, Bowker JH, eds: Atlas
of Amputations and Limb Deciencies: Surgical,
Prosthetic, and Rehabilitation Principles, ed 3.
Rosemont, IL, American Academy of Orthopaedic Surgeons, 2004, pp. 409-428. Copyright
James Breakey, PhD, CP.)
Illustration of the initial in-
disarticulation or very long transfemoral amputations who have the bony
length and muscle strength to voluntarily control the prosthesis with the residual limb, many of these polycentric
knees do not offer substantial inherent
stability.
Linkages that are more complex,
with five or more bars, have become
popular in recent years. Most offer
additional biomechanical advantages
in addition to the features of a stable
four-bar polycentric knee. One design
includes a geometric lock feature that
automatically engages and disengages
Figure 14
knee designed for use in knee disarticulation
prostheses, where it folds under the socket to
minimize protrusion of the knee distal to the
socket in sitting. (Courtesy of Ottobock Healthcare, Austin, TX.)
Photograph of a polycentric
during ambulation (Figure 15). Others
provide a limited range of controlled
knee flexion during loading response,
simulating the shock-absorbing characteristic of the biologic knee (Figure 16).
Gait studies have confirmed that these
stance-flexion knees result in more biomechanically normal gait kinematics
during early stance phase.
34,35
Manual Locking Knee
Manual locking knees provide maximum stability by immobilizing the
knee in full extension throughout the
gait cycle. The user must unlock the
manual locking knee to flex it for sitting. Because swing phase knee flexion
is eliminated by the manual locking
knee, the prosthesis becomes functionally too long, and the user must hiphike, vault, circumduct, or abduct the
prosthesis for the toe to clear the floor.
These necessary compensations not only
result in abnormal gait patterns but are
also believed to increase the energy cost
of ambulation. It is customary to shorten
the prosthesis approximately 1 cm to
facilitate toe clearance with a manual
Figure 15
age polycentric knee with a geometric lock
feature that engages during loading response.
(Courtesy of Össur, Reykjavik, Iceland.)
Figure 16
linkage polycentric knee that provides a limited range of knee exion during loading response through the adjustable compression of
a bumper. (Courtesy of Ottobock Healthcare,
Austin, TX.)
Photograph of a multiple link-
Photograph of a multiple
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
438
Chapter 35: Lower Limb Prosthetic Components: Updated Classification and Passive, Body-Powered Components
locking knee; however, it then appears
as if the wearer is stepping into a hole
during the stance phase.
To avoid these undesirable characteristics, the manual locking knee should
be considered only as a last resort and
not be used when a polycentric or other
design would be effective. If a manual locking knee is needed temporarily
because of patient weakness or similar
considerations, a more functional knee
should be provided as soon as feasible,
before these gait faults become habitual.
Alternatively, several hybrid polycentric and stance-control knees include
a manual locking feature that can be
engaged when needed, such as in the
early phases of rehabilitation or during
negotiation of unfamiliar environmental
obstacles, and otherwise unlocked, taking advantage of their inherent mechanical stability mechanisms (Fi g ure 17).
The limited studies available suggest
Figure 17
tures that can be used as locking knees during early rehabilitation and/or mechanical articulating
knees with optional locking functions as user mobilization increases. A, Polycentric. B, Stance-
control. (Courtesy of Medi USA, Whitsett, NC.)
Photographs of hyb rid knee mechanisms wi th convertible mechanic al locking fea-
that the manual locking knee does not
provide a more energy-efficient gait than
a free-swinging knee, but it may be preferred by elderly patients who are feeble,
enabling them to walk more rapidly and
confidently than with a free-swinging
knee.36 The bilateral use of locked knees
is integral to the graduated length protocols of bilateral transfemoral stubby
prostheses to allow knee flexion in sitting as prosthetic height is progressively
increased.
37
Conventional wisdom has long dictated that manual locking knees be used
for toddlers and small children until
they have developed sufficient balance
to walk with a free knee. Available evidence suggests that this is unnecessary
because very young children who have
a polycentric or similar free-swinging
knee readily master its capabilities and
appear to develop a more mature gait
pattern at an earlier age than those who
transition from a manual locking knee.
Fluid-Controlled Knee
The term fluid-controlled knee refers
to a component that uses pneumatic or
hydraulic fluid to dampen knee motion,
typically in a piston/cylinder configuration. It is well established that fluid-controlled knees provide a smoother, more
normal swing phase movement than
knees with mechanical control, and they
automatically compensate for moderate
changes in a user’s cadence. They are
therefore indicated for anyone capable
of walking at variable cadences.
In theory, pneumatic knees may be
preferable for outdoor use in bitterly cold
conditions because extreme temperatures have little effect on their viscosity.
However, modern hydraulic knees use
silicone-based oils that do not thicken
very much in cold weather, so this is no
longer a major distinction.
A more important biomechanical
consideration is that gases (such as the
air in pneumatic knees) are readily com
pressible, whereas liquids (such as the
38
oil in hydraulic knees) are incompressible. The compressibility of the gas in a
pneumatic knee means that a specific
volume is required to provide adequate
resistance for swing phase damping of
the prosthetic knee. The incompressibility of hydraulic oil means that only a
small volume is needed to provide effective swing phase control, so a hydraulic
knee may be smaller and lighter than
a pneumatic knee equivalent. Incompressibility also means that a hydraulic
knee can offer a wider range of cadences
than a pneumatic design.
In addition, pneumatic control
cannot provide sufficient resistance
to create stance phase stability. Only
hydraulic knee designs offer effective
stance control, usually in the form of
a slowly yielding resistance to sudden
knee flexion that was pioneered in the
1950s by the Mauch Swing and Stance
knee (Össur).39 The Mauch knee and its
clones are still widely used in the United States, which is a testament to the
-
clinical effectiveness of this application
of fluid-control principles (Figure 18).
All fluid-controlled knees are more
complex and therefore more costly than
purely mechanical knees, and they require periodic servicing to replace worn
seals. However, many users consider the
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
439
Section 3: Lower Limb
Figure 18
knee regulated by a hydraulic cylinder. (Courtesy of Össur, Reykjavik, Iceland.)
increase in function well worth the additional cost and maintenance. Hydraulic
knees with a 36-month manufacturer’s
warranty are readily available, so the
incremental increase in cost is amor
tized over a longer functional life span
than would be the case with a typical
mechanical knee having a 12-month
manufacturer’s warranty.
Photograph of a single-axis
Hybrid Knee Components
Hybridization, which combines two
or more basic prosthetic knee designs
into an integrated assembly, is commonplace. One of the most popular
combinations adds the cadence response of fluid swing phase control to
the multiple biomechanical advantages of a polycentric mechanical design.
Some polycentric hybrids have two hydraulic cylinders: one for swing phase
damping and the other for stance phase
damping, to modulate the rate of stance
phase knee flexion during the loading
response phase of gait.
Another example is seen in the venerable Mauch Swing and Stance knee,
which has incorporated a manual locking mode since its inception for added
safety during activities such as climbing
ladders, making it the original hybrid
prosthetic knee.
Selection of Knee
Components
Table 3 summarizes the basic characteristics of knee and positional locking
components. They are prescribed based
primarily on an individual’s desired activity level and functional aspirations,
taking into consideration their stance
and swing phase abilities.
Michael40 previously described a simple algorithm for prescribing passive,
mechanical prosthetic knees, based on
the answers to four questions: (1) Is the
individual able to voluntarily control
the knee under all circumstances? (2)
Is the individual able to flex the knee in
a controlled manner during preswing?
(3) Is the individual able to vary his or
her cadence when walking? (4) Is the
individual able to walk at a moderate
or faster pace?
Fig ure 19 graphically illustrates the
-
logic tree that leads to a consideration
of articulation type (single axis versus
polycentric) and swing control (fluid
versus constant friction). The prosthetist selects the specific commercial
product(s) that meet these criteria and
are believed to offer the greatest value,
function, and durability without adding
excessive weight. The suitability of these
components for the client’s needs can
be verified during walking trials with a
diagnostic prosthesis before creating a
definitive design.
Positional Locking
Components
Although often overlooked because
their biomechanical functions are not
as complex as swing or stance phase
control, components that permit the
user to lock the prosthetic components
in multiple positions can substantially
enhance quality of life by making activities of daily living much less difficult.
Incorporating a locking positional rotator immediately above the prosthetic
knee joint allows the user to unlock and
rotate the knee internally or externally
(Figure 20). This facilitates dressing,
entering confined spaces such as automobiles, and sitting cross-legged. As a
general guideline, a locking positional
rotator should be considered whenever
there is sufficient space to install this
component between the socket and the
knee.
If the locking positional rotator is
placed between the transtibial socket
and the shin, the wearer can externally or internally rotate the foot 90° or
more. This facilitates kneeling or sitting
back on one’s legs, which can be important for specific occupations or cultural
reasons.
Locking ankle designs are also very
useful for people who wear prostheses.
Some are designed to permit the use of
swim fins while swimming, snorkeling,
or scuba diving. Others make snow skiing or water skiing easier for the person
with an amputation.
Many people benefit from being able
to adjust their prosthesis to accommodate differing heel heights (Figure 21).
Otherwise, changing to a shoe style
with a different heel height adversely
affects gait, comfort, and safety while
using the prosthesis. Several mechanical
and hydraulic adjustable ankle joints are
currently available that have a feature
to compensate for shoes with differing
heel heights. Some require manual adjustment, whereas others incorporate
MPC activation to automatically adjust
alignment to maintain maximum safety
and gait efficiency.
Summary
The prescription of prosthetic elements remains an art based on science. One of the most widely accepted
methods for developing prescription
recommendations is to match the biomechanical performance of each component to the individual’s functional
goals, physical abilities, and personal
aspirations.
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
440
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