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Section 2: Upper Limb
Figure 7
picting the exion motion of the wrist. (Courtesy of Fillauer, Chattanooga, TN.)
Photographs of the ETD, a nonanthropomorphic prehensor with a exion wrist, de-
that palmar prehension was the most
widely used static grasp, whereas lateral prehension was the most dominant
dynamic grasp.26 From the lateral and
palmar thumb positions, many more
power and precision grasps may be
accessed. Additional thumb and digit configurations are possible beyond
grasps and are classified as gestures.
Examples of unique gestures include
index finger pointing for typing, the
shaka (“hang loose”) sign, the “OK”
sign, or the “thumbs up” sign. Many
complex hands allow the creation of
custom gestures and grasps for each
user.
Although accessing many grasps and
Figure 9
a powered prehensor with thumb and ngers
with multiple degrees of freedom. (Courtesy of
Vincent Systems, Germany.)
Photograph of the Evolution 2,
gestures can increase the functional outcomes of the user, ease of access may
become a limiting factor.36 The gadget
tolerance of the user should be carefully
considered when prescribing complex
function. Complex hands have more
than one degree of freedom, with multiarticulating thumbs and digits. Complex
hands substantially increase the number
of functional grasping patterns available
to the user, with some having more than
30 unique grasps or gestures.
Unlike the singular palmar prehension of simple hands, complex hands
add lateral prehension, permitting the
thumb to be oriented in multiple planes.
Research on uninjured hands reported
hands because the user’s tolerance may
need to be fairly high, depending on the
number of grasps and gestures available
and how many will be used. Equally
important is how easily and rapidly the
grasps and gestures can be accessed.
There are multiple ways to access the
various grasps and gestures of complex
hands. Triggers, including single-peak,
double-peak, and co-contraction input
signals, will allow the user to switch
grasps. In the Evolution 2 hand (Vincent
Figure 8
tric Greifer DMC VariPlus, a nonanthropomorphic prehensor. (Courtesy of Ottobock, Austin,
TX.)
Photograph of the System Elec-
Systems), a single trigger allows access
to all of its grasps and gestures (Fig-
ure 9). The bebionic3 hand (Steeper)
allows access to grasps and gestures
with an external switch mounted on the
back of the hand and/or an input trigger (Figure 1). The grasps and gestures
of the i-limb quantum (Touch Bionics;
Figure 10) can be accessed with multiple input triggers, mobile devices, “grip
chip” proximity chips (Touch Bionics),
or with coordinated prosthesis movements (intelligent motion [i-mo; Touch
Bionics] gesture control).
In addition to grip chips, the Morph
system (Infinite Biomedical Technologies) can be placed inside the socket
proximal to the prosthetic wrist, providing proximity chip/tag compatibility
for many complex hands (Fig ure 11).
Desired grasps or gestures can be easily
and quickly accessed when the prosthesis is positioned within the sensitivity
of a strategically placed proximity chip/
tag (for example, accessing an index finger pointing gesture when near a keyboard for typing). The user is also able
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
182
Chapter 13: Upper Limb Externally Powered Components
Figure 10
has ngers and thumbs with multiple degrees of freedom. (Courtesy of Touch Bionics, Manseld,
MA.)
to access additional grasps by manually
stalling the desired finger(s) during actuation or manually moving the thumb,
as is done with the bebionic hand.
Although complex hands have much
more functional capacity than simple
hands, their designs may limit their application. With five to six motors compared with the single motor of simple
hands, opposition forces may be decreased and there may be an increase
in electrical current draw, increasing
intraday battery depletion. However,
larger capacity batteries, up to 2,000
mAh and 7.4 volts, are available to increase usage time if space and weight
tolerance permits. With many more
points of articulation, plastic instead
of metal frames, and/or a decrease in
thickness, complex hands are not as durable as simple hands. Manufacturers
generally recommend complex hands be
limited to light- or medium-duty activities. These factors should be considered
when prescribing these anthropomorphic prehensors.
An in-depth description of each
complex hand design, characteristics,
and features is beyond the scope of this
Photographs of the i-limb Quantum hand with a exion wrist. This complex hand
chapter, particularly because there are
many sizes and variations available.
However, four commercially available
complex hands will be briefly described:
the Michelangelo hand (Ottobock; Fig-
ure 12), the bebionic hand (Figure 1),
the i-limb hand (Figure 10), and the
Evolution 2 hand (Vincent Systems;
Figure 9).
The Michelangelo Hand is unique
from other complex hands because
of its simpler finger mechanisms. Although this hand has fingers with one
degree of freedom, the thumb increases
function beyond that of simple hands
because it has two degrees of freedom,
representing the anatomic thumb CMC
joint. Each finger consists of one single
rigid lever, with one revolute joint providing articulation similar to the MCP
joint of the human hand.
37, 38
All five
fingers are actuated simultaneously by
two electric motors driving a cam mechanism. Unlike the fingers, the thumb
can accomplish multiarticulations, with
two revolute joints arranged in series to
mimic the anatomic thumb CMC joint.
A small electric motor pre-positions
the thumb in either palmar or lateral
Figure 11
tem, a proximity tag system that allows easy
access to desired grasps and gestures. It can
be used with many multiarticulating hands; it
is shown here with a bebionic hand. (Courtesy
of Innite Biomedical Technologies, Baltimore,
MD.)
Figure 12
gelo Hand with wrist exion and extension.
This complex hand has ngers with one degree
of freedom and a thumb with two degrees of
freedom. (Courtesy of Ottobock, Austin, TX.)
Photograph of the Morph sys-
Photograph of the Michelan-
prehension before performing a task.36
Although the Michelangelo Hand is not
compatible with other quick-disconnect
prehensors, the AxonHook is available
as an interchangeable utility prehensor,
and the optional AxonRotation wrist rotator (Ottobock) provides powered pronation and supination.
Unlike the Michelangelo Hand, the
i-limb, the bebionic, and the Evolution
2 hand designs have fingers and thumbs
with multiple degrees of freedom. Unlike the three flexion degrees of freedom
of anatomic fingers, the fingers of these
complex hands are individually actuated
with two revolute single-degree-of-freedom joints (small hand sizes may have
only one degree of freedom).36 Generally, digits two through five articulate
similarly to the anatomic MCP joint
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
183
Section 2: Upper Limb
Figure 13
pomorphic one-degree-of-freedom Motion
Control ProHand with Multi-Flex Wrist. This
spring-loaded wrist provides exion and extension and radial ulnar deviation. (Courtesy of
Fillauer, Chattanooga, TN.)
Photograph of the anthro-
and distally at a single IP joint, representing both proximal interphalangeal
and distal interphalangeal anatomic
joints.37 The fingers of multiarticulating digit hands curl when flexed, allowing them to conform around the object
being grasped. These hands, excluding
some smaller versions, typically have
five to six motors, with one motor to
independently actuate each finger.
The thumbs of multiarticulating digit hands differ from the fingers in that
each have one revolute joint for circumduction to mimic the CMC joint and
one revolute joint for flexion to mimic
the MCP joint. The bebionic hand has
an additional single-degree-of-freedom
revolute joint to mimic the IP flexion
articulation of the human thumb. The
thumbs of multiarticulating digit hands
have the ability for circumduction into
opposed palmar or nonopposed lateral positions, increasing the number
of functional hand grasping options.38
Thumb circumduction pre-positioning
may be accomplished with an actuator
or manually by the user, depending on
the type of prosthetic hand.
The fingers of some multiarticulating
digit hands are not coplanar; instead,
they are arranged in an arc to adduct
during flexion, providing better grasp
of spherical objects or thin objects, such
as a credit card between the fingers.
Conversely, the overall width across the
fingers increases as the fingers abduct
during extension. To protect multiarticulating fingers and drive mechanisms,
each hand is available with a cosmetic
glove and has a built-in compliance to
permit joint articulation when an unexpected force is applied.
37
Many of the complex hands also have
the option to increase user function with
the addition of wrist degrees of freedom,
such as manual radial/ulnar deviation
and/or wrist flexion/extension (Figures
10 and 12).
Wrist Components
The recent technologic advances in upper limb powered prostheses have primarily focused on the anthropomorphic
prehensors, increasing the number of
degrees of freedom and grasp patterns.
However, it is also important to consider
the functional role of the anatomic wrist
and forearm. Although the human hand
is the most functional part of the upper
limb, its dexterity is highly dependent
on wrist function, particularly for tasks
requiring manipulation. For example,
many hand grasp patterns would either
be useless or require exaggerated compensatory movement from the proximal
joints and torso if it were not for the
exceptional positioning capabilities of
the wrist.
development of prosthetic wrist components have demonstrated increased
function, commercially available options offer limited functionality.28 In
a survey, transradial amputees ranked
improved wrist function as three of the
four highest desired functions, whereas
transhumeral amputees ranked it as two
of the highest four desired functions.
anatomic wrist motion, but also provide functional requirements unique
to individuals with upper limb amputations. Prosthetic wrists reduce compensatory movements that may occur in the
elbow, shoulder, and/or torso.
reduction of compensatory movements
29,39,4 0
Although research and
41
Prosthetic wrists not only mimic
29,39,4 0
The
is highly desirable because it may increase user comfort and reduce the risk
of secondary complications, including overuse injuries.
42,43
In addition,
prosthetic wrists may increase socket
comfort because of decreased reaction
forces acting on the residual limb.
5
Characteristics of Prosthetic Wrists
Just as anatomic wrist function occurs in both the hand and forearm,
prosthetic wrist function may occur
in several components, including the
forearm, wrist, and/or prehensor. For
example, an amputee may have wrist
function with two degrees of freedom,
with supination and pronation in the
wrist component and flexion and extension in the prehensor (Figure 13).
For this chapter, prosthetic wrist motion
includes all wrist motion regardless of
where it mechanically occurs, because
function supersedes location in fulfilling
user needs.
Prosthetic wrists may offer one, two,
or all three degrees of freedom in various
axial configurations. In their simplest
form, externally powered wrists provide at least one wrist degree of freedom
with a rotation-based quick-disconnect
mechanism. Additional degrees of freedom may increase function through
improvement of hand placement and
object manipulation, but this increased
function will likely come at the expense
of increased weight, length, complexity,
and cost. Longer transradial amputations may have space constraints that
further limit the possible degrees of
freedom.
For powered upper limb components, most prehensors connect using a
quick-disconnect wrist, an unofficial industry standard connection common to
all externally powered prehensors (excluding the Michelangelo Hand and the
AxonHook). The quick-disconnect wrist
enables easy exchange of prehensor devices; it is composed of a female housing
in the prosthetic forearm that secures to
a male ball retention mechanism in the
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
184
Chapter 13: Upper Limb Externally Powered Components
prehensor. The quick-disconnect wrist
provides power and communication for
the prehensor using a four-conductor
male coaxial plug in the wrist and a
mating female connector in the terminal device. The quick-disconnect wrist
releases by rotating the prehensor clockwise or counterclockwise approximately
330° until it releases. A prehensor may
be inserted into the quick-disconnect
wrist in any rotational orientation, requiring 330° rotation from the initial
position for release. Although prosthetic
hands are not frequently interchanged
with other hands, they are frequently interchanged with nonanthropomorphic
prehensors, such as converting from a
hand for social situations to a utilitarian
prehensor for occupational tasks and/
or hobbies. The externally powered
quick-disconnect wrist is not the same
as a body-powered quick-disconnect
wrist. The former has a larger diameter
and requires a unique coupler to connect body-powered terminal devices to
an externally powered wrist base.
The range of motion (ROM) of prosthetic wrists may vary by amputation
level and is accomplished by various mechanical forms. Prosthetic wrists typically have less ROM in both flexion and
extension and radial and ulnar deviation
than that observed in anatomic wrists;
however, all prosthetic quick-disconnect
wrists exceed anatomic function in supination and pronation because they
rotate a minimum of 300°. The three
degrees of freedom of the anatomic wrist
may be mimicked mechanically using
several joint mechanisms in various
configurations.
Prosthetic wrist motion may be passive or active, with the latter requiring
external power. The joint impedance
varies by joint motion. Passive wrist
motion may have minimal impedance
(free, unlocked), moderate impedance
(friction), high impedance (locked), or
minimal impedance with a centering
force (spring-loaded). The supination
and pronation of the quick-disconnect
wrist has moderate impedance, with
indexed positions every 15°. As the
quick-disconnect wrist does not have
a locking feature, powered prehensors
have limited supination and pronation
stability, even with active wrists.
Control strategies for passive wrists
impose a relatively low cognitive burden because the position of the wrist
can be rapidly achieved and is typically maintained throughout a task. In
addition, some passive wrists adapt to
applied forces, moving from a neutral
spring-loaded position when force is
applied and then returning to the neutral position when the force is removed;
this strategy further reduces cognitive
burden. Passive wrists typically provide
dual-state impedance with free and
locked indexed positions that allow the
prehensors to be prepositioned as desired, then secured during tasks.43 For
such wrists, the unilateral amputee may
position the prehensor using the intact
hand, by nudging the prehensor against
an object, or by securing the prehensor
to the object and using compensatory
motion to achieve the desired position.
For passive supination and pronation
of a quick-disconnect wrist, the unilateral amputee often positions the wrist
with the intact hand because this is the
quickest method.
To use an active wrist, there are frequently insufficient control sites to offer simultaneous control, although such
control is possible in a hybrid design or
with advanced surgical techniques such
as targeted muscle reinnervation. Subsequently, control strategies for active
wrists may impose a higher cognitive
burden because the amputee must mode
shift active control sequentially from the
prehensor to the wrist and vice versa.
44
Powered wrist function is currently
limited to single-degree-of-freedom
supination and pronation (Figure 14).
Although powered wrist rotators may
provide continuous motion exceeding
360°, limiting the supination and pronation end points using software may
Figure 14
Rotator, which allows supination and pronation. (Courtesy of Fillauer, Chattanooga, TN.)
Photograph of the ProWrist
increase user function by preventing
overshooting the ROM typically used
for accomplishing ADLs. The AxonRotation wrist rotator returns to a natural
hand position after not being used for
a set amount of time. Although wrist
rotators can provide substantial actuation torque, rotational stability is limited
to the friction of the quick-disconnect
wrist mechanism. In preparing for elective amputations, the length of a powered wrist rotator should be considered
if its use is anticipated because these
components are substantially longer
than other wrist components.
Discussion
It is easy to overlook the importance of
wrist function in positioning the prehensor for optimal function. Restoration
of wrist function is not only essential
for bilateral amputees,21 but also provides benefits for unilateral amputees.
Examples of common ADLs that may
benefit from the increased functionality of a wrist with multiple degrees
of freedom include threading a belt,
tucking in a shirt, eating, operating a
zipper, removing a wallet from a back
pocket, hammering a nail into a wall,
opening a door, holding a cup, driving a
car, tying shoelaces, holding a clipboard,
and carrying objects in the hand.
29,39,45
For amputations at or proximal to the
short transradial level, full wrist function is desirable and should be provided
if the user can tolerate the additional
weight. For longer transradial amputations, the user’s residual supination and
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
185
Section 2: Upper Limb
Figure 15
Digital Arm with a prosthetic revolute elbow
providing one degree of freedom. (Courtesy of
Liberating Technologies, Holliston, MA.)
Photograph of the Boston
pronation may be sufficient, eliminating
the burdens associated with the space
requirements, power, cost, and weight
that accompany restoration of full ROM.
When the provided wrist function is
limited to passive supination and pronation of the quick-disconnect wrist, it
should be placed in an optimal orientation to minimize unnecessary compensatory motions of the shoulder and
elbow. This optimal orientation may
not be perpendicular to the elbow axis.
Although research on anatomic wrist
function during ADLs suggests an orientation in ulnar deviation and extension as the most important alignment
considerations,
41,45
the optimal wrist
orientation for ADLs has yet to be identified because none of the tested wrist
orientations systematically decreased
compensatory motions.
17
Because prosthetic wrists currently
do not mimic the three degrees of freedom provided by an anatomic wrist, the
question of which degrees of freedom
to restore is important. Although priorities are not clearly defined, research
indicates that wrist flexion and extension substantially improves prosthesis
usefulness in more activities and with
a more natural motion.5 However, consideration may be given to sacrificing
hand function to increase wrist function
because research indicates that complex
prosthetic hands cannot be fully functional with a simple wrist rotator. Instead, a simple hand combined with
an advanced wrist may provide similar function at a substantially reduced
19
cost.
Certain movements, such as those
required for ADLs, would not be possible without prosthetic wrist function,
such as using wrist flexion to tuck a
shirt into pants.
5,40
Even the restoration
of two degrees of freedom may prove insufficient because flexion and extension
are not sufficient to enable all essential
activities.40 However, the combination
of flexion and extension and supination
and pronation enables amputees to perform many tasks commonly taken for
granted.
Elbow Components
The elbow is the simplest of the anatomic arm joints to mimic mechanically,
requiring only single-degree-of-freedom
forearm flexion and extension. However, the elbow has substantially higher
force requirements than the wrist and
hand because of the proportionally
longer forearm lever. Elbow function
presents design challenges for powered
prosthetic elbows because speed and
torque are often competing mechanical
design goals, especially when weight is
a priority.
Functionally, the elbow effectively
increases or decreases arm length, positioning the hand closer or farther from
the body.46 The elbow is essential for
many ADLs, permitting the hand to perform tasks close to the body, including
bathing, toileting, brushing teeth, and
holding a mobile phone. The elbow also
extends arm length to permit tasks away
from the body, including steering a car,
typing, and shaking hands. The elbow
also bridges the gap between distant objects and the body, which is essential for
tasks such as eating and opening doors.
The anatomic elbow has dynamic control, providing slow movement
throughout its ROM, maintaining a
flexed position, or supporting partial
body weight while increasing or decreasing arm length.46 The elbow is equally
adept at rapidly changing its position,
actively or passively (with gravity).
Although it may be easy to increase
prosthetic elbow actuator torque, it is
difficult to simultaneously keep the elbow lightweight and/or retain fast actuation. To statically maintain a flexed
position, prosthetic elbows must continuously draw electrical current or increase both complexity and weight with
clutch mechanisms. Thus, although the
anatomic elbow is functionally simple, mechanical replication requires
compromises.
In addition, although the anatomic
elbow has only one degree of freedom,
the prosthetic elbow adds an additional
degree of freedom, providing humeral
internal-external rotation proximal to
the elbow joint. The soft tissue of the
transhumeral residual limb, coupled
with the cylindric shape of the residual humerus, precludes an effective
coupling to permit transfer of residual humeral rotation to the prosthesis.
Many transhumeral socket designs
inhibit anatomic humeral rotation to
enhance socket comfort and increase
socket stability.
Characteristics of
Prosthetic Elbows
Because the elbow is essential for
ADLs, all prosthetic elbows seek to
mimic anatomic elbow function. Prosthetic elbows are one-degree-of-freedom revolute joints, with ROM from
0° (full extension) up to 150° (maximum flexion). Actuation is provided by
body power or electric motors (Figures
15 and 16); the latter being capable
of higher torque, but increasing the
overall weight of the prosthesis. Most
electric motors are housed in the proximal elbow segment to reduce functional weight and increase lift capacity,
although some may be located in the
forearm.
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
186
Prosthetic elbows suitable for use
with externally powered prostheses can
be divided into two general categories,
body-powered and externally powered
devices. Body-powered elbows that are
compatible with externally powered
components may be referred to as hybrid elbows. These hybrid elbows are
similar to traditional body-powered
elbows because they harness glenohumeral flexion and/or biscapular ab
duction for elbow flexion. They differ in
that they add cable routing for powered
batteries, inputs, and/or prehensors.
Hybrid elbows can be further distinguished by the type of elbow lock
they provide, body-powered (with a
harness) or electrical (with switches or
myoelectric inputs). Hybrid elbows become advantageous when weight, cost,
strength, ROM, and limb lengths are
concerned. However, as body-powered
elbow flexion requires 11.5 cm (4.53
inches) or more of cable excursion,
hybrid elbows have a limited working
envelope.
21
Externally powered elbows actuate
elbow motion using a combination of
electric motors, transmissions, clutches, and flexion-assist mechanisms. Although an in-depth discussion of elbow
drivetrain characteristics is beyond the
scope of this chapter, several features
will be discussed. Typically, brushless electric motors are used because
of their increased power density and
durability. To optimize motor power,
externally powered elbows may use a
higher voltage than found in prehensors, necessitating battery accommodation. A flexion-assist mechanism may
be present (also available in hybrid elbows) to provide a counterbalance to
the gravitational force on the forearm
and prehensor. Although a flexion-assist
mechanism adds to the overall weight,
the live lift (active lift force) capacity is
increased and shear forces on the residual limb are decreased.
21
Although elbow function during
powered motion is commonly
Chapter 13: Upper Limb Externally Powered Components
Figure 16
Utah Arm 3. C, The Utah Arm 3+. (Image courtesy of Fillauer, Chattanooga, TN.)
considered, passive function when the
limb is not actively engaged in prehensile tasks is also important. For example,
during ambulation, elbows may offer
a free-swinging mode to permit natural arm swing and reduce the extent to
which the weight of the elbow disrupts
the smoothness of the user’s gait. However, direct drive designs likely draw
electrical current when in free-swinging
mode.
Because some control strategies only
permit operation of one degree of freedom at a time, the user often positions
the elbow to a desired position before
switching to prehensor control. When
not under control, a mechanism must
stabilize the elbow position. Although
flexion-assist devices aid in stabilization, they lack sufficient force to “lock”
the elbow. Functional elbow stabilization may come from a locking mech
anism or from an anti-backdrive motor.
The latter may decrease battery capacity,
even though no elbow motion occurs.
Elbow stability when locked determines
the static lift capacity or the maximum
weight the user may lift while the elbow
is in a locked position (without overriding protective clutch mechanisms).
All commercially available prosthetic
elbows provide passive humeral rotation
Photographs of prosthetic elbow components. A, The Utah Hybrid Arm. B, The
providing internal and external rotation.
Joint impedance is typically moderate
(user-adjustable friction). Whether joint
friction will be adjusted internally or
externally should be considered in the
prosthesis design, because the former
may require an aperture for access. Although it would be mechanically possible to include forearm supination and
pronation along with humeral rotation
in a prosthetic elbow joint, this option
is not offered in any of the commercially available products. Experimental,
powered humeral rotational components have been developed; however,
because active control of this movement
is a lower priority than other functional
deficits, these components are not likely
to become commercially available until
control bandwidth is increased.
To provide increased humeral ro-
-
tational stability, an optional Locking
Humeral Rotator adapter (Liberating
Technologies) provides positioning with
intermittent high (locked) and low (unlocked) impedance in 15° increments.
The adapter can be used with existing
prosthetic elbows, adding 12 mm (0.47
inch) in overall height, and has lever or
cable release actuation such as a chin-actuated nudge switch or harness strap in
parallel with the elbow lock.
using a one-degree-of-freedom revolute
joint that is perpendicular to the elbow
axis. The humeral rotational joint is
positioned proximal to the elbow joint,
Discussion
Powered elbows may be appropriate for
many amputees. The most important
47
48
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
187
Section 2: Upper Limb
Figure 17
ponent with an electric lock actuator that permits changing the exion/extension axis from
locked to free swinging (Courtesy of Liberating
Technologies, Holliston, MA.)
Photograph of a shoulder com-
advantage is the elimination of control
harnessing for elbow joint actuation,
with a subsequent increase in wearing
comfort and expansion of the working
envelope. Users with short residual
limbs, limited strength, and/or limited
ROM may benefit from powered elbows,
particularly because of the increased
lifting capacity. However, some users
may find the increased weight, limited
battery capacity, and/or lack of simultaneous elbow and prehensor control too
burdensome.
Functional differences, including
maximum flexion angle, live lift capacity, weight, flexion speed, and compatibility with the desired prehensors, can
be considered when distinguishing between prosthetic elbows. The maximum
flexion angle determines how close the
user can bring the prehensor to his or
her face, such as when eating, without
the need for compensatory motion. Live
lift capacity determines the maximum
weight of objects that can be actively
picked up by the elbow; this characteristic is of particular importance for
bilateral prosthesis users. The weight of
the prosthesis can also be a determining
characteristic.
Although powered elbow control
strategies are similar to those of prehensors with proportional and digital sequential control variants, hybrid elbows
offer an advantage worthy of mention
for transhumeral amputees. By using a
body-powered harness to control elbow
flexion in parallel with the myoelectric
signals of the biceps and triceps for prehension, simultaneous two-degrees-offreedom elbow control and prehensor
control can be achieved. However, although simultaneous control is gained,
many advantages of myoelectric prostheses (such as an expanded working
envelope and increased live lift capacity)
are lost with increased harnessing requirements. Although it is also possible to have a hybrid combination of a
body-powered prehensor with an externally powered elbow, this configuration
is less common.
Shoulder Components
Shoulder disarticulation and other higher-level amputations are uncommon.49
There are few commercially available
prosthetic shoulder component options, none of which provide actively
powered humeral flexion and extension
or abduction and adduction. Although
actively controlled shoulder components
have been developed, the commercially available shoulder components only
have passive actuation and are similar to those used with body-powered
prostheses.
with an electric lock actuator is available, enhancing function for those with
high-level amputations and providing
essential function for bilateral amputees
if mechanical release levers are not a viable option (Fig u r e 17 ).
Prosthetic shoulder components offer
two degrees of freedom, with flexion/
extension and adduction/abduction. Anatomic humeral rotation is not provided
because prosthetic elbows provide this
degree of freedom. The shoulder component may be functionally described as a
revolute low impedance (free-swinging)
50,51
A shoulder component
joint with 36 high impedance (locking)
positions (at every 10°) for the flexion/
extension axis. In adduction/abduction,
the joint has a second revolute axis
with user adjustable impedance (friction) positioned perpendicular to and
functioning in parallel with the flexion/
extension axis. An optional abduction
ratchet is available to permit incremental abduction positioning and may be
disengaged when friction adjustment is
preferred. The shoulder component can
be adapted for use with endoskeletal or
exoskeletal systems and has a central
aperture for proximal electrode and/or
battery cabling.
Although joint activation for both
axes is passive, the user may activate
an electric lock actuator to change the
flexion/extension axis from locked to
free swinging. To passively position
the flexion/extension axis, the user
activates the electric lock actuator
using an appropriate input (such as
a switch or myosite) and uses trunk
flexion to facilitate gravity-assisted humeral flexion to achieve the desired
shoulder flexion/extension. After the
desired position is attained, the user
again activates the electric lock actuator to secure the joint in the next
extension increment. During ambulation, the flexion/extension axis may
remain unlocked for natural, passive
arm swinging motion.
Summary
Powered prosthetic components have
advanced substantially over the past
decade, particularly with the recent
commercial availability of multiarticulating digit hands. It is hoped that the
advanced powered wrist and shoulder
components that have been developed
for research will soon become commercially available. Despite the advancements, much work is still needed
to restore individuals with upper limb
amputations to biologic normal states,
as evidenced by current prosthesis abandonment rates.
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
188
Chapter 13: Upper Limb Externally Powered Components
As more complex components become available, it will become increasingly difficult for clinicians to remain
abreast of individual component features. Understanding the important
functional characteristics of upper
limb prosthetic components may help
clinicians to combine such components
synergistically to achieve an optimal
prosthetic solution for their patients.
Acknowledgment
The authors thank Craig W. Heckathorne, MSc, who authored the chapter
on this topic in the third edition of the
Atlas of Amputations and Limb Deficiencies: Surgical, Prosthetic, and Rehabilitation Principles, for laying a foundation
for this chapter.
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