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Section 2: Upper Limb
possible only if there is sufficient anchoring of the elbow lock strap. To accomplish this, a strap is fashioned that
travels across the user’s midsection,
between the inferior ribs and superior
to the iliac crest. The elbow lock cable
runs proximally through cable housing
mounted to the anterior socket wall at
axillary height and then reflects distally where it connects, through a control
strap, to the waist strap (Figure 38).
The Use of Pulleys
Pulleys have been used in a variety of
applications to create a mechanical advantage for a given action, and the same
concepts of leverage can be applied in
body-powered harnessing. For force
amplification, the control cable runs
through a pulley that is connected to
the terminal device. This configuration
doubles the required amount of excursion of the control cable to create the
same overall movement but reduces the
force required. This configuration is
used when ample excursion is available
and the chosen action requires substantial force.
When excursion amplification is
needed, the control cable runs through
a pulley that is connected to the harness.
This configuration doubles the required
amount of force at the control cable to
create the same overall movement, but
reduces the excursion required. This
pulley configuration is used when
there is sufficient force present but a
limited amount of excursion is available. The latter is more commonly used
(Figure 39).
For example, when using biscapular
abduction as a control motion with a
shoulder disarticulation prosthesis, the
motion generates good power but has
limited excursion. Using a pulley, the
configuration allows the user to generate approximately double the amount of
Figure 38
a distal waist strap as an anchor point for the
elbow lock cable, routed through a retainer
positioned at axillary height. This approach
allows scapular elevation as a control motion
Figure 37
body-powered transhumeral harnessing. (Courtesy of David B. Rotter, CPO, Scheck and Siress, Chicago, IL.)
Figure 39
when ample e xcursion is available. B, An excursi on amplier is used when amp le force is available. (Reprodu ced with permission fro m Below and Above
Elbow Harness and Control System. Evanston, IL, Northwestern University Prosthetic-Orthotic Center, 1966.)
Anterior (A) and posterior (B) photographs of a patient demonstrate bilateral
Illustrations of the use of pulleys to create force and excursion ampliers in body-powered harnessing. A, A force amplier is used
for elbow locking and unlocking for a shoulder
disarticulation prosthesis. (Courtesy of David B.
Rotter, CPO, Scheck and Siress, Chicago, IL.)
Photograph of a patient using
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
172
Figure 40
sion amplier in the harnessing of a body-powered shoulder disarticulation prosthesis.
(Courtesy of David B. Rotter, CPO, Scheck and
Siress, Chicago, IL.)
Photograph show ing an excur-
Chapter 12: Harnessing and Controls for Upper Limb Body-Powered Prostheses
components when fitting a patient with
bilateral shoulder disarticulation. There
are, however, some instances in which
some individuals with bilateral shoulder disarticulation do not have access
or resources to be fit with externally
powered components. In such cases,
the prosthetists should creatively use
the body-powered resources that are
available. An example of such a device
is shown in Figure 42, A. Two excur-
Figure 41
that is used when a control motion cannot be
provided through a harness. (Courtesy of Hosmer Fillauer, Chattanooga, TN.)
Photograph of a nudge switch
sion amplifiers are mounted opposing
each other on the posterior aspect of the
sockets. This split-socket design maximizes the individual’s ability to use
maximum biscapular abduction and
chest expansion. This control motion
enables movement of the elbows and
terminal devices. It should be noted
that the elbow must be locked on one
side or this configuration will cause
both elbows to flex simultaneously.
The practical application of such devices uses one side as the prime mover
of the system, with the other side primarily used in positional assistance.
Many switches and nudge controls are
necessary to activate the many elements
of these types of body-powered devices
(Figure 42, B).
Harnessing and Externally
Powered Switches
The same motions available to an individual for activating a body-powered prosthesis can be used to activate
Figure 42
lation prostheses. The bilateral excursion ampliers control elbow exion and terminal device pre hension. B, Photograph showing a nudge switch and series of mouth pull switches that are used to
control the shoulder lock, ve -function wrist lock, elbow lock, and elbow turntable lock . (Courtesy
of David B. Rotter, CPO, Scheck and Siress, Chicago, IL.)
A, Photograph of a patient wearing a bilateral body-powered shoulder disarticu-
switch controls responsible for activating electric components. For example,
biscapular abduction and glenohumeral
flexion are frequently used to pull a
linear potentiometer rather than using
a body-powered cable to operate an
electric elbow (Figure 43, A). The ad-
cable travel per the amount of biscapular
abduction and chest expansion exerted
(Figure 40).
Nudge Control
If sufficient excursion or force is unavailable using harnessing options, a nudge
control device allows the individual to
use his or her chin to push down on
a lever to create an action (Figure 41).
Harnessing a Bilateral
Shoulder Disarticulation
It should be noted that, when possible,
it is preferable to use a combination of
body-powered and externally powered
vantage of this control strategy is that
it gives the user positional feedback as
he or she initiates the movement to pull
the switch. It also requires substantial-
ly less force to initiate the movement.
The same principles can be applied in
more elaborate harnessing schemes
(Figure 43, B).
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
173
Section 2: Upper Limb
Figure 43
electric components. A, A linear potentiometer is used to control the position of an electric elbow
using glenohum eral extension and bis capular abduction. (Cour tesy of David B. Rotter, CPO, Scheck
and Siress, Chicago, IL.) B, In this hybrid setup with a triple control harness, the anterior suspensor
strap controls a ma nual elbow lock, the mid dle strap controls an elec tric switch regulating the p osition of an elec tric hook, and the distal strap provides body-powered control of the elbow position.
(Courtesy of Ryan Spill, CP, Philadelphia, PA.)
Summary
Body-powered devices continue to be
effective tools for patients with upper
limb amputations. As further advances
are made in externally powered control, it is critical that continued development of body-powered components and
strategies are not overlooked or abandoned. There will always be a need for
body-powered devices in the developing world, and these devices continue to
prove their merit in the industrialized
world.
References
1. Pursley RJ: Harness patterns for upper-extremity prostheses. Artif Limbs
1955;2(3):26-60. Medline
2. Williams III TW: Review: Control of
Upper-Limb Powered Prostheses. e
Academy Today 2014;10(1).
Photographs show ing the application of b ody-powered co ntrol motions to control
3. Simpson DC: e choice of control
system for the multi-movement
prosthesis: Extended physiological
proprioception (EPP), in Herberts P,
Kadefors R, Magnusson R, Peterson
I, eds: e Control of Upper Extremity
Prostheses and Orthoses. Springeld,
IL, omas, 1974, pp 146-150.
4. Garber M: e Perfect 3,000-year-old
Toe: A Brief History of Prosthetic
Limbs. e Atlantic 2013;Nov 21.
Available at http://www.theatlantic.
com/technology/archive/2013/11/theperfect-3-000-year-old-toe-a-briefhistory-of-prosthetic-limbs/281653/.
Accessed September 14, 2015.
5. Kuniholm J: Prosthetic history:
e body-powered arm and
William Selpho. Available at: ht tp://
openprosthetics.ning.com/proles/
blogs/prosthetic-history-the.
Accessed September 1, 2015.
6. William S: inventor. US patent
US18021 A. August 18, 1857.
Available at: http://www.google.
com/patents/US18021. Accessed
September 15, 2015.
7. Dorrance DW: inventor. Articial
hand. US patent US1042413 A. October 29, 1912. Available at: http://www.
google.com/patents/US1042413.
Accessed September 15, 2015.
8. Hosmer products: Hooks. Available
at: http://hosmer.com/products/
hooks/index.html. Accessed Septem-
ber 15, 2015.
9. Johnson K, Musicus M, Davis AJ:
Upper extremity prosthetic sockets,
suspension systems, and component
option to fulll prescription criteria,
in Spires MC, Kelly BM, Davis AJ,
eds: Prosthetic Restoration and
Rehabilitation of the Upper and Lower
Extremity. New York, NY, Demos
Medical Publishing, 2014,
pp 179-194.
10. Below and Above Elbow Harness
and Control System. Evanston, IL,
Northwestern University ProstheticOrthotic Center, 1966.
11. BAHA: Better Performance for BP
Prosthesis Wearers. Available at:
http://www.oandp.com/articles/
NEWS_2006-11-27_02.asp. Accessed
September 15, 2015.
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
174
Chapter 13
Upper Limb Externally Powered Components
M. David Beachler, CP LeRoy H. Oddie, CP
Abstract
Externally powered prosthetic components are a viable solution for individuals with upper
limb amputation. Because there are many possible component combinations, selection of
an optimal combination can be challenging. Factors to consider in the selection of powered
upper limb components include availability, weight, cost, cosmesis, power requirements,
noise, durability, repairability, and compatibility, along with the patient’s limb length,
gadget tolerance, hand dominance, and the anticipated environments where the prosthesis
will be used. Upper limb powered prehensors, wrists, elbows, and shoulders can be characterized by the mechanical joint type, degree(s) of freedom, joint impedance, and power
requirements. By comparing these characteristics with the lost anatomic function and
unique requirements of the user, an appropriate combination of externally powered and/
or body-powered prosthetic components may be prescribed.
Keywords: externally powered prostheses; hybrid prostheses;
myoelectric prostheses; prosthetic hand; prosthetic elbow; prosthetic
wrist; upper limb prosthetics
Introduction
There have been steady advancements
in the field of externally powered upper
limb prosthetic components. In large
part, the field of prosthetics has benefited from technological developments of
other industries in areas of electronics,
software, communication standards,
batteries, actuators, manufacturing
methods, material science, and mobile
communication devices. Consequently,
current externally powered upper limb
prostheses offer increased hand dexterity, longer battery life, more intuitive
Neither of the following authors nor any immediate family member has received anything of value
from or has stock or stock options held in a commercial company or institution related directly or
indirectly to the subject of this chapter: Mr. Beachler and Mr. Oddie. e views expressed in this
chapter are those of the authors and do not reect the ocial policy or position of the Departments
of the Army, Navy, or Defense, or the US Government. e identication of specic products or
scientic instrumentation does not constitute endorsement or implied endorsement on the part of
the authors, the Department of Defense, or any component agency. Although references to products,
companies, manufacturers, organizations, etc. are generally excised in government-produced works,
the abstracts produced and other similarly situated research presents a special circumstance when
such product inclusions become an integral part of the scientic endeavor.
control, increased function, and new
methods of user interaction.
1
The intact upper limb effortlessly performs both fine and gross motor tasks and even subtly contributes
to communication.
2,3
It should be remembered that the intact upper limb is
marvelously capable of a minimum of
28 simultaneous degrees of freedom,4
with sightless proprioception (including
position, heat, moisture, and pressure),
with substantial strength for gross motor
tasks and delicate dexterity for fine motor tasks, all with seemingly unlimited
energy and unconscious control in an
visually appealing, lightweight, waterproof package with self-healing
properties.
In contrast, the upper limb prosthesis is primarily restricted to a supportive role, is often underactuated, and is
generally limited to performing gross
motor functions. All upper limb prosthetic components are engineering exercises in compromise, with limitations
in power, size, aesthetics, strength, cost,
and durability. As prosthetic functionality is increased, the cost, weight, and
complexity of the components are also
increased.5 In addition, media reports
often highlight outcomes of new prosthetic technologies, with no inclusion of
important limitations; therefore, older
prosthetic technology, which may not
only be appropriate but may represent
the best solution, is often ignored.6
Thus, the expectations of individuals
with upper limb amputations and their
families may be greater than a prosthetist is capable of providing. The clinical
team and the prosthetist, in particular,
must carefully inculcate realistic functional and aesthetic expectations as early
in the rehabilitation process as possible.
Because upper limb prosthetic components are modular and highly compatible, a large number of component
combinations are possible. In addition,
these components are often complex in
design and function, requiring prosthetists to remain abreast of technological
developments and consult with prosthetic manufacturers and other prosthetists who specialize in upper limb
components. Prosthetists who are inexperienced in treating patients with
complex upper limb amputations should
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
175
Section 2: Upper Limb
consider referral to a prosthetist who
specializes in upper limb care. Many
manufacturers of upper limb prostheses
employ prosthetists and/or occupa tional
therapists with upper limb expertise
who can serve as valuable resources.
Factors to Consider in
Component Selection
Selecting the ideal combination of components is arguably the most difficult
aspect in the provision of prostheses for
upper limb amputees. Factors to consider in the selection of powered upper
limb components include availability,
weight, cost, cosmesis, noise, durability,
repairability, and component compatibility, as well as patient-related factors
such as limb length, gadget tolerance,
and the environments where the prosthesis will be used.
Availability
Because the market for powered upper
limb components is small, there are substantial product gaps such as the lack of
powered wrists with multiple degrees
of freedom. With knowledge of both
what is and what is not commercially
available, prosthetists and occupational therapists specializing in the care of
individuals with upper limb amputation can serve as excellent resources for
recommending appropriate prosthetic
components.
Weig h t
Weight is an important factor in selecting an upper limb prosthesis. Multiple
surveys have indicated that excessive
weight is a cause for prosthesis abandonment.
ponents inherently have more mass
than other upper limb components
because the source of actuation (motors) and power (batteries) are usually
contained within the prosthesis. However, this increased weight often results
in increased function, which may be a
priority for some users. Because weight
tolerance is dependent on the strength of
7,8
Powered upper limb com-
the individual amputee, weight is more
likely to affect smaller individuals. Decreases in component mass are seldom
proportional to decreases in component
size.
Importantly, not all weight is equal.
Static weight (component weight measured on a scale)9 does not equate to
functional weight (the weight of the device perceived by the amputee during
use). The more proximal the placement
of the weight, the less the user perceives
it. Depending on the length of the residual limb and component selection,
the prosthetist can strategically decrease
functional weight. For example, in a
patient with a short transhumeral amputation, the prosthetist may place the
battery proximal to the prosthetic elbow
or on the posterior side of the socket,
rather than within the forearm where
the additional weight would decrease
elbow capacity and increase perceived
weight with humeral flexion. The overall functional and static weight must be
considered when selecting a suspension
method because heavier components
may eliminate the preferred methods
from consideration. Generally, less
weight is preferable, because less weight
is often perceived as increased comfort.
Cost
The costs of powered prosthetic components are substantially greater than
body-powered alternatives. Often, the
end user does not pay for the prosthesis
because the prosthetist is reimbursed by
a third-party payer such as Medicare,
private insurance, or workers’ compensation. However, third-party payers
may provide only one prosthesis during
the lifetime of an amputee or may not
cover the cost of powered prosthetic
components because such technologies
are considered experimental or investigational.10 In addition, the amputee’s
medical coverage may change and the
future costs of replacements and/or repairs must be considered. Because cost
is not always reflective of function, more
expensive components should not be
selected unless they provide a better
solution for the user’s needs. Similarly, costs should not be a limiting factor unless available financial resources
are exceeded. A long-term perspective
should be used when considering costs
because higher initial costs may provide
justification for reduced future costs.11
Knowledge of local and state laws and
insurance regulations and the amputee’s
financial situation should be understood
before a prosthesis is prescribed because
some component costs may exceed
available funding.
11,12
Cosmesis
The importance of cosmesis (lifelike
appearance) is highly personal and often culturally influenced.13 Some amputees want prostheses with a lifelike
appearance, preferring detailed custom
silicone prostheses, even to the point of
sacrificing essential function.13 Other
individuals concede that those around
them will realize that the limb is a pros
thetic device and prefer anything but
prostheses with a lifelike appearance.
Other amputees adopt an intermediate
position in which lifelike appearance is
appreciated, but a device that does not
draw the attention of a casual observer
is also acceptable.
14
Cosmesis can be further differentiated into static and dynamic cosmesis.
Static cosmesis, which is the appearance of the prosthesis when not in motion, may be convincing to the casual
observer. However, dynamic cosmesis
considers the visual normalcy of both
movement and appearance.15 Dynamic
cosmesis is not limited to the appearance
of prosthetic components, but also considers how naturally the residual anatomic movement appears, such as the
lack or minimization of compensatory
movements.
16,17
Dynamic cosmesis often supersedes static cosmesis in importance because unnatural movement will
alert the casual observer that the prosthesis is not a natural limb. Dynamic
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
176
Chapter 13: Upper Limb Externally Powered Components
cosmesis may also refer to the motion
of a powered prosthetic component,
such as direct synchronous supination
movement of a wrist rotator in contrast
to the smooth motion of the anatomic
forearm.
18
Noise
For some individuals, noise can be just
as important as cosmesis. Noise can
draw undesirable attention to the prosthesis, diminishing realism.19 For such
individuals, demonstration of powered
components before prescription can
identify if actuator volumes are tolerable.
Durability
The importance of durability varies
by individual needs. Individuals with
upper limb amputations who work in
occupations involving manual labor require durable prosthetic components.
Unlike the anatomic arm, all prosthetic
components lack self-healing properties and have a limited useful life that
necessitates eventual replacement. In
general, powered upper limb components are less durable than body-powered components. In some instances,
it may be challenging to pay for or to
obtain reimbursement by third-party
payers for the repair and replacement
of powered components. The amputee
should be encouraged to take personal
responsibility in caring for his or her
prosthesis to reduce both the cost and
inconvenience of repairs.
Repairability, Warranty,
and Service
In some instances, powered upper
limb prosthetic components may require repair by the manufacturer. In
such cases, consideration must be given
to how the amputee will perform essential activities of daily living (ADLs)
while repairs are made. Solutions may
include loaner components from the
manufacturer for the duration of the
repair, or the provision of a secondary
spare prosthesis.
Compatibility
Because upper limb prostheses are assembled from various components, the
patient’s outcome will only be as successful as the compatibility of such components. Because there is only a limited
number of manufacturers making upper
limb prosthetic components,20 the available selection of components is limited,
and compatibility between components
from different manufacturers cannot be
assumed. Some manufacturers only ensure compatibility between components
in their own product lines, whereas
other manufacturers develop products
with the specific purpose of overcoming
the incompatibility problem.
Compatibility can manifest itself
in several forms. The most obvious is
mechanical compatibility. The quickdisconnect–type wrists made by several
manufacturers allow interchangeability
with most externally powered terminal devices, although a formal industry
standard has not been established. Although most upper limb components
have mechanical compatibility, they
may not be electrically compatible.
Components may operate at different
voltages or the connecting cables may
not be compatible. Some manufacturers
offer adapter cables to enable electrical
compatibility. Given the complexity of
upper limb components and potential
compatibility issues, a prosthetist with
upper limb expertise is valuable in understanding appropriate component
combinations.
Limb Length
The length of the residual limb can
have a substantial effect on the selection of appropriate components and,
subsequently, available functions. In
general, the restored prosthetic limb
segments should be matched as closely as possible in function and cosmesis
to the contralateral limb in those with
unilateral upper limb loss.21 In patients
with bilateral upper limb amputation,
anthropomorphic ratios serve as a guide,
and left/right limb segment symmetry
should be restored if possible.21 In elective amputations or revision surgeries,
the clinical team should consider potential components when planning the
amputation length.
For transradial amputations, there is
a trade-off between a longer limb that
preserves physiologic supination and
pronation and a shorter limb that allows space for wrist components with
flexion features or powered pronation
and supination. For transhumeral procedures, a length 12 cm (4.72 inches)
shorter than an elbow disarticulation is
ideal to permit selection of any elbow
components. Although powered elbows
can accommodate amputations as long
as 5 cm (1.97 inches) short of elbow
disarticulation, longer lengths are less
desirable because the humeral segment
may be longer than ideal.22 Elbow disarticulations and transhumeral amputa
tions that are less than 5 cm shorter than
elbow disarticulations will require a
body-powered elbow or outside hinges.
Gadget Tolerance
An amputee may have a physical or
cognitive limitation (referred to as gadget tolerance) that prevents the use of
a prosthesis, or possibly a particular
prosthetic component. Each individual’s
ability to adjust emotionally to wearing
and using a prosthesis is unique and
dependent on his or her personality
and psychological stage of loss.23 As
prosthetic components become more
complex, a higher gadget tolerance is
needed. Modern upper limb powered
prosthetic components have many features, which makes them highly adjustable; however, such components may
require interaction with mobile phones,
computers, or other devices to access
all the possible features. For example, a
prosthetic hand may offer several dozen
possible grip patterns, but may require
the user to interact with an external device such as a mobile phone to access
a certain grip pattern. In addition, all
-
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
177
Section 2: Upper Limb
Figure 1
tool and anthropomorphic multiple- degreesof-freedom bebionic hand. Prosthesis manufacturers can provide clinicians with prosthetic
components so that function can be demonstrated to a pati ent before compone nt prescription. (Courtesy of Steeper, San Antonio, TX.)
Photograph of a demonstration
powered prosthetic components currently require battery power, often re
quiring daily or intraday charging with
frequent use. Some amputees may not
have the cognitive or physical ability to
access the desired features and/or may
not be willing or responsible enough to
charge batteries daily. For such individuals, simpler externally powered and/
or body-powered components may be
more appropriate.
Environmental Factors
Powered components have environment
limitations preventing their use in certain situations. Batteries and electronic
components can be damaged or compromised by temperature extremes and
water exposure. Although some components may be water resistant, most
components are not waterproof, and
wet environments should be avoided.
Sand, dirt, or particle exposure also
may compromise function or increase
wear rates.24 If an amputee frequently
encounters such adverse environments,
the manufacturer’s specifications for the
selected component should be consulted
to ensure component warranties will not
be voided with normal use.
Ancillary Equipment
Unlike body-powered prostheses, which
require only the power of the amputee,
externally powered prostheses require
additional equipment for setup, maintenance, and use. Prosthetists and occupational therapists need testing equipment
to determine if viable myosites are available, to refine electrode placement within a prosthetic socket, and to provide
muscle training. A computer, tablet, or
mobile phone with appropriate software
may be necessary for programming the
features of microprocessor components.
Demonstration tools are available for
actuating prehensor devices to demonstrate component function to the amputee before component prescription
(Figure 1). Amputees may also need
a smartphone or other mobile device
to actuate additional programmable
-
modes. Although the patient’s access to
software typically does not incur additional costs, hardware costs should be
considered when providing powered
components.
Preparatory Fitting
Because of the many factors involved
in component selection and the possibility of prosthesis abandonment,
a preparatory fitting can be a highly
effective tool to ensure the amputee
is capable of using the selected components and that their functions are
suited to his or her needs. A diagnostic
socket (test or check socket) not only
ensures appropriate socket fit, it also
allows component alignment, lengths,
and electrode placement to be adjusted.25 After powered components have
been prescribed, component manufacturers should be consulted to determine if demonstration components or
no-obligation trial periods are available to permit flexibility in selecting
the final components.
Hybrid Configurations
Upper limb prostheses can be classi
fied into the three broad categories of
cosmetic, body-powered, or externally
powered; however, hybrid designs, with
body-powered, passive, and/or externally powered components, are commonly prescribed for patients. Hybrid
prostheses combine the advantages of
both body-powered and externally powered designs, particularly for individuals
with high-level and/or bilateral upper
limb amputations.26 Hybrid prostheses
may reduce weight, provide simultaneous control of multiple degrees of
freedom, reduce costs, increase grasping force and lifting capacities, and/or
permit finer control.26 However, hybrid
prostheses also may compromise working envelopes and increase harnessing
requirements.
In higher-level upper limb amputations, the critical deciding factor is often the choice of whether to power the
elbow or the prehensor. Although both
configurations are feasible, powering the
prehensor seems to be the most advantageous choice.27 Because the number of
possible hybrid configurations is vast,
an in-depth discussion is beyond the
scope of this chapter. However, some
hybrid upper limb components are discussed as appropriate.
Characteristics of Externally
Powered Components
Joint Types
Joints are essential for functional movement. To understand how prosthetic
components mimic the anatomic upper limb, it is helpful to describe the
function of the anatomic joints from
the perspective of simple mechanical
joints (Figure 2). Anatomic upper limb
joints may be revolute with one degree
of freedom, universal with two degrees
of freedom, or spherical with three
-
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
178
Chapter 13: Upper Limb Externally Powered Components
Figure 2
of freedo m and can be arranged as rot ator (A) or exor (B) joints. C, A prismatic j oint has one degree
of freedom and is capable of linear motion. D, A universal joint has two degrees of freedom. E, A
spherical joint has three degrees of freedom. (Reproduced with permission from Bajaj N, Spiers
A, Dollar A: State of the art in prosthetic wrists: Commercial and research devices. Presented at
IEEE International Conference on Rehabilitation Robotics (ICARR), 2015. Available at: . Accessed
November 23, 2015.)
degrees of freedom.
ious joints can be combined in series
or in parallel to increase the degrees
of freedom.
Photographs of various types of mechanical joints. Revolute joints have one degree
28
In addition, var-
force is removed. To prevent actuator
damage, some components have clutch
mechanisms to allow low impedance
29
(break-away) after a predetermined force
is exceeded.
Joint Impedance (Compliance)
In the anatomic limb, joint impedance
(resistance to external force) is variable
depending on the task. Although variable impedance is feasible for powered
prosthetic components, it is very inefficient. Consider the elbow holding a
book while reading. The impedance of
the arm equals the force of the gravity
acting on the book and forearm. In a mechanical system with continuously variable impedance, continual energy would
be required to maintain elbow joint position. To conserve energy resources,
prosthetic components typically adopt
a dual-state impedance control with low
impedance (free motion) to move the
joint to a desired position and high impedance to lock a joint into position.30
Movement Quality
Movement quality can refer to the extent
to which the movement of powered actuators replicates lifelike motion. Many
upper limb component actuators have
crude singular velocity movements,
which fall substantially short of the
natural rapid accelerations and decelerations of normal human movement.
In addition, it is common for amputees
to overshoot the desired joint position,
which increases task time and cognitive
load and ultimately leads to frustration.
As technology advances, the quality of
movement of prosthetic components
may receive more attention, improving
functional outcomes and behavioral
appearance.
Other prosthetic components, such as
the multiaxial wrist, have a springloaded impedance, returning the joint
to a neutral position after an external
External Power Source
Although unusual power sources, in-
cluding compressed gas, rocket fuel,
Figure 3
a exible lithium-ion polymer battery for use in
upper limb pro stheses. (Courtesy of I nnite Biomedical Technologies, Baltimore, MD.)
Photograph of the FlexCell Mini,
methanol, spinal fluid, and implantable
glucose fuel cells, have been considered,
all current available powered components rely on battery power.
31,32
A user
may tolerate daily battery charging but
may find intraday battery replacement
inconvenient, particularly if the second
battery must be carried in a pocket. Typical daily battery use expectancy should
be considered in component prescription because the charging frequency
may influence prosthesis acceptance.
Battery performance is dependent
on several variables. Battery technology
determines the relative power density,
with lithium-ion batteries being the
current standard. High power density
is preferred because longer performance
can be achieved with a similar weight.
From a user’s perspective, it is desirable
to have a battery last an entire day before
requiring recharging. The capacity of
various batteries can be evaluated by
comparing the milliamp-hour (mAh)
rating, with a higher number representing increased time performance.
26
Polymer batteries are packaged in
a soft pouch, which decreases weight
and rigidity. These batteries are advantageous for prosthetic applications because multiple thin flexible cells can be
strategically located, such as proximally
placed cells that are contoured to the
socket to decrease perceived functional weight (Figure 3). Polymer batteries
are not the same as lithium-polymer liquid electrolyte batteries, although both
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
179
Section 2: Upper Limb
lithium- ion and lithium-polymer chemistries are available in flexible polymer
packaging.
33
Powered prosthetic systems can operate at various voltages. For prostheses
with multiple powered components, the
voltage of one component may vary from
another to maximize actuator perfor
mance. For example, the elbow requires
much higher torque than the thumb
or fingers because of the substantially
longer forearm lever. To accommodate
such complexity, dual-voltage batteries
are available, or (less preferably) two
batteries may be used.
User convenience is affected by the
type and the placement of the battery.
Some batteries are internal and must be
recharged using a charging port before
the prosthesis is usable. Other batteries are external (removable) to permit a
charged replacement battery to be used
while the depleted battery is being recharged. Batteries should be placed as
proximally as possible to diminish perceived functional weight and maximize
actuator capacity. Proprietary batteries
may offer increased performance, but
come at the expense of limited local
availability. Battery charging times may
become a factor if the amputee is not
willing to carry a second battery when
the capacity is insufficient for a full day
of use.
Prehensors
The compactness of the hand and its
many functions make it difficult to replicate in a powered prosthetic prehensor.
To adequately replicate the core grasps
of the human hand, a prosthetic handlike prehensor requires three or four
degrees of freedom: two for the thumb,
one for the index finger, and one for the
remaining three digits.6 Limited space
availability restricts the size and number
of motors available for use. Thus, prosthetic hands are often underactuated,
attempting to provide similar function
with fewer degrees of freedom and/or
actuators.
34
Electrically powered prehensors were
developed more than a half a century
ago and remain commercially available
in several forms. Anthropomorphic
prehensors take the general form of an
anatomic human hand (Figures 4 and
5), whereas nonanthropomorphic pre-
-
hensors may be formed in the shape of
a hook or gripper (Figures 6, 7, and 8).
Initially, prehensors only provided one
degree of freedom, with two or three
digits in opposition.35 Recent developments in powered prehensor devices
have focused on more anatomically
influenced functional designs with the
thumb and the fingers having multiple
degrees of freedom (Figure 9).
Similar to other powered components, most prehensor control strategies
have proportional and digital control
variants and are programmable to the
user’s needs, whether using a single- or
dual-site control scheme. Regardless of
type, all prehensors are limited to basic
grasping functions, without independent control of digits. For example, although complex hands may offer several
dozen grasps, hand function does not
allow for complex behaviors such as independent digit control when typing or
playing an instrument.
Anthropomorphic prehensors are
available in various sizes to improve
cosmesis and minimize unnecessary
weight. In individuals with a unilateral
amputation, the width of the contralateral metacarpophalangeal (MCP)
joints is typically measured to determine
the appropriate hand size.
Single Degree of Freedom:
Anthropomorphic Prehensors
Anthropomorphic prehensors with
one degree of freedom (simple hands)
were the first commercially available
electrically powered hands. Current
designs more closely mimic the shape
of the human hand and have revolute
joints with one degree of freedom, with
simple opening and closing prehension. The mechanical digits and thumb
Figure 4
Electric Hand, an anthropomorphic singledegree-of-freedom prehensor. (Courtesy of
Steeper, San Antonio, TX.)
Photograph of the Select Myo
interphalangeal (IP) joints are rigid,
with articulation located at the MCP
joints and the carpometacarpal (CMC)
joint, respectively (Figures 4 and 5).
Simple hands are typically constructed of an inner actuator mechanism, an
outer hand-shaped form, and a cosmetic
glove as a cover. Simple hands generally have one or two motors to oppose
the thumb against the first and second
digits. The third and fourth digits are
passive and follow the motion of the first
and second digits.
As the digits and thumb of simple
hands are oriented in fixed palmar opposition, they are limited to a single palmar prehension grasp (also referred to as
three-jaw chuck or tripod prehension).
To form palmar prehension, the tips of
the thumb, second digit, and third digit oppose until they close or secure an
object. This fixed configuration allows
for powerful, consistent, and durable
thumb and digit operation when grasping. Objects may be grasped using the
palmar distal aspect of the digits for a
more precise-type grasp or by using a
combination of the palmar aspect of the
digits and palm to create a power or cylindrical grasp.
36
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
180
Chapter 13: Upper Limb Externally Powered Components
Figure 5
(right) without hand shells. These are anthropomorphic single-degree-of-freedom prehensors.
(Courtesy of Ottobock, Austin, TX.)
The functionality of simple hands
has been increased with the addition of
intelligent grasping. Sensors located in
the thumb tip or actuator transmission
can limit grasp force or detect an object slipping and rapidly respond with
increased grasping force. Examples of
prosthetic hands with intelligent grasp-
Photographs of the MyoHand VariPlus Speed (left) and the SensorHand Speed
basic grasping functions. Simple hands
provide unsurpassed durability because
of their simplicity and, when funding is
limited, simple hands offer an affordable
choice. The function of simple hands
may be increased with the addition of
passive radial ulnar deviation and/or
wrist flexion/extension units.
ing are the SensorHand Speed (Ottobock) and the Motion Control ProPlus
hand with Force Limiting Auto Grasp
(Fillauer). Intelligent grasping substantially increases user function, prevents
dropping objects, and permits grasping
of delicate objects. These types of hands
should be considered when prescribing
simple hands because amputees have
substantial proprioception deficits.
Although simple hands may not be
appropriate for all amputees because of
their singular grasping capability, they
remain an appropriate option for many
individuals. Simple hands are appropriate for both dominant and nondominant
hand loss because the prosthesis primarily provides a supportive role with
Single Degree of Freedom:
Nonanthropomorphic Prehensors
Nonanthropomorphic powered prehensors (utility prehensors) were influenced
by the desire to overcome functional
deficits associated with simple hands.
Utility prehensors, often referred to as
electric hooks or grippers, are similar to
simple hands because they simply open
and close. Utility prehensors have one or
two motors and a single-degree-of-freedom rigid orientation of revolute joints
for oppositional grasping. These prehensors sacrifice cosmesis for robust design,
strong prehensor pinch force, and increased visual feedback for finer motor
skills and heavy-duty work.
35
Figure 6
an example of a nonanthropomorphic prehensor with revolute, angular opening and closing
mechanisms. (Courtesy of Ottobock, Austin,
TX.)
Photograph of the AxonHook,
Utility prehensors provide basic tip,
lateral, and cylindrical grasping. The
opening and closing configuration of
these prehensor devices varies, with
the Greifer (Ottobock) having a parallel
opening and closing mechanism with
a relatively large opening (Figure 8),
whereas the AxonHook (Ottobock; Fig-
ure 6) and the ETD hook (Fillauer; Figure 7) have revolute, angular opening
and closing mechanisms. Both the Greifer and the ETD feature a safety release
lever to disengage the fingers from the
gear train and passively release the grip
of the device. The ETD is also available
with Force Limiting Auto Grasp. Utility
prehensors may have additional functions, including passive radial/ulnar deviation and/or wrist flexion/extension.
Multiple Degrees of
Freedom: (Multiarticulate)
Anthropomorphic Prehensors
Recent developments in myoelectric anthropomorphic multiarticulating prehensors (complex hands) have resulted
in a new generation of more anatomically influenced designs with increased
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
181
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