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Section 2: Upper Limb
Figure 5
Model 5XA Hook (Hosmer) with canted and nitrile-lined ngers. B, The Model 555 Hook with lyre-shaped, nitrile-lined ngers. C, The Model 5 Hook,
a stainless steel hook with canted ngers and no lining. D, The Cable-Activated Hook (left) and All-Purpose Hook (right) show two types of spring
tension closures. (Panel B and C courtesy of Hosmer, Austin, TX, and panel D courtesy of Ottobock, Austin, TX.)
Photographs show voluntary- opening hooks. A, An example of the most commonly prescribed design of a split hook is the aluminum
amount of hook tension preferred by
an individual user. The desire for increased grip strength must be balanced
by the realization that, in VO designs, a
patient must overcome his or her maximum grip strength every time the hook
is opened, regardless of the grip strength
required for the task.
Alternatively, several variations of
Figure 6
sor with composite construction and a variable tension knob. B, The Retro Classic Hook with
corr osion-resistant n ish and spring closure. (Cour tesy of ToughWare Prosthetics, Westmins ter, CO.)
Photographs of hooks made of composite materials. A, The Vari-Grip Prehen-
adjustable-tension VO hooks are commercially available. Two-load hooks
allow the user to choose between two
prehensile strengths by positioning a
a back-lock feature, which allows the
hook to be opened only by a cable pull;
the fingers cannot be pried apart. This
feature prevents the hook from opening
inadvertently when grasping or lifting
a heavy load and ensures that the hook
remains closed without additional effort
from the user (Figure 7, B).
Variation also exists in the gripping
surfaces of VO hooks. The most common is a replaceable nitrile coating that
lines the inner surface of the hook to
increase the tackiness and compressibility of the gripping area (Figure 5,
A). Pediatric hooks can be coated with
a polyvinyl chloride plastic or covered
with removable rubber sheaths to protect the user and the environment from
incidental abrasions that occur from
rubbing against the hook (Figure 8).
In some instances, users prefer uncoated hooks because the gripping surface
is less prone to degradation over time
(Figure 5, C).
The most common means of creat
ing hook tension is with rubber bands
(Figure 5, A, B, and C), although some
hooks use springs (Figures 5, D and
6, B). It is generally accepted that one
rubber band creates approximately 1.5
lb of grip force. The application of additional or replacement rubber bands
is somewhat difficult and is facilitated
by a specialty tool; many patients defer
this procedure to their treating prosthetist. Tremendous variability exists in the
switch at the base of the hook (Fig-
ure 9). In a related strategy, the Retro
Classic Hook (ToughWare Prosthetics)
and the Vari-Pinch Prehensor (V2P;
ToughWare Prosthetics; Figure 6)
allow the user to choose among sev-
-
eral prehensile strengths by sliding a
knob on the underside of the device.
In addition to this mechanical adjustment, the Vari-Pinch Prehensor allows
the user to exchange elastic bands to
further modulate the available grip
strength. As in two-load hook designs, these features allow users to
maintain a low cable tension when a
reduced pinch force is sufficient, with
the ability to raise this force when
necessary.
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
142
Most hooks have a cable attachment
for a ball terminal, but some have a ring
for tying a nylon cord or a Spectra Cable
(Allied-Signal) (Figures 5, A and 8, B).
VC Hooks
The Grip 3 and Adept series (TRS) of
terminal devices are unique prehensors available for adults and children in
aluminum, steel, and titanium versions
with or without a urethane coating.
They feature multiple cylindric gripping surfaces within the fingers for gross
grasping. Fine prehension is provided
by the finger tips (Fig u re 10). Patient
acceptance has been highest among children and sports-minded adults whose
primary concern is function.
The Army Prosthetic Research Laboratory (APRL) Hook (Hosmer) is a split
hook with lyre-shaped, replaceable aluminum fingers (Fig ure 11). An internal locking mechanism automatically
engages when closed around an object
and cable tension is removed. To unlock
the grasp and open the hook, the user
must apply a pull force greater than was
used to close it. The inherent problem
with this design is that a user holding a
delicate object or another person’s hand
must close the hook slightly further to
release the grasp. If the user already
had a firm grip, he or she may damage
a delicate object or hurt the other person. The user can choose between two
possible opening ranges of 0 to 1.375
inches (3.49 cm) or 0 to 3 inches (7.62
cm) by flipping a small switch on the
base of the unit. A smaller range permits
faster grasp and release and requires less
excursion to fully close. Unfortunately,
the mechanical complexity of this device makes it costly to manufacture and
prone to breakdown.
Hands
Although several body-powered hands
are available, few are used as active terminal devices. These hands have many
drawbacks, including frictional loss
of force, glove restriction of motion,
Chapter 11: Upper Limb Body-Powered Components
Figure 7
the Model 7LO Work Hook (right). B, The Model 6 Work Hook with backlock feature. (Courtesy of
Hosmer, Chattanooga, TN.)
Figure 8
a pediatric size hook coated with a polyvinyl chloride plastic coating. This hook is also available in
smaller (Mode l 10P) and larger (Mode l 99P) sizes. B, A pediatric ho ok with removable rubb er sleeves
and a tie connection. (Part B courtesy of RSL Steeper, Leeds, England.)
limited pinch force, and contours that
block visual inspection. These factors
substantially limit their usefulness for
tasks requiring grasp and release.
Many individuals with recent loss of
an upper limb desire an interchangeable
hand for social occasions in addition
to a utility hook for general use; this
Photographs of f armer’s hook terminal dev ices. A, The Model 7 Work Hook (le ft) and
Photographs show pediatric vinyl-coated hooks. A, The Model 12P Hook (Hosmer),
request is the most common indication
for prescribing a body-powered hand.
Hooks require a longer cable length
than hands because of their different
attachment points, so a user intending
to interchange the two must be provided
with a hook-to-hand adapter cable that
extends the control cable for hook use.
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
143
Section 2: Upper Limb
Figure 10
closing Grip 3 Pre hensor. (Courtesy of TRS, Bo ulder, CO.)
Figure 9
Voluntary Opening Hook. (Courtesy of Hosmer,
Chattanooga, TN.)
Figure 12
ternal hand mechanism. B, A three-jaw chuck body-powered hand with palmar cable exit. C, A
pediatric cable-operated hand with ve moving ngers and dorsal cable exit. (Courtesy of RSL
Steeper, Leeds, England.)
Body-powered hands are rarely appropriate for those with bilateral upper
limb loss because of their functional
limitations. Externally powered hands
offer far greater pinch force and improved grasp-release function and are
Photograph of the Sierra 2-Load
Photographs of cable-operated hands. A, An example of a three-jaw chuck in-
preferable when maximum prosthetic
hand function is required.
Most cable-operated hands have the
thumb and the first two fingers moving
in a three-jaw chuck prehension pattern (Figure 12, A). Often, the metal
Photograph of the voluntary-
Figure 11
thetic Research Laboratory (APRL) Voluntary
Closing Hook (Courtesy of Hosmer, Boulder,
CO.)
Photograph of the Army Pros-
structure of the three fingers is covered
by a plastic hand shell (Figure 12, B).
These first three fingers open and close
the hand shell, and the fourth and fifth
fingers move passively with the others. Some designs have a solid outer
layer with three to five moving digits
when the hand opens and closes (Fig-
ure 12, C).
Body-powered hands are heavier, less
versatile for handling objects, and block
the user from seeing the object during
manipulation. They cannot fit into
pockets or grasp buttons and zippers.
The three-jaw chuck grip pattern positions the thumb to line up between the
first two fingers, but is not directly opposed to either one. This makes it much
more difficult to pick up small objects
because the thumb and first finger do
not meet to form a stable pinch surface.
Body-powered hands are less efficient than hooks because of frictional
energy losses in the hand mechanism,
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
144
Chapter 11: Upper Limb Body-Powered Components
Figure 13
untary Opening Hand with a rigid exterior. The
rst and second ngers move in a three-jaw
chuck pattern with an automatic back lock and
a two-position stationary thumb. (Courtesy of
Hosmer, Chattanooga, TN.)
Photograph of the Sierra Vol-
so an equivalent amount of input force
results in a lower output pinch force. As
a result, both VO and VC hands require
more power to operate than VO or VC
hooks. A few studies have compared activation forces and efficiency and found
that hooks outperform hands in most
measures.
3,4
Most VO hooks obtain a
pinch force of more than 20 N, with the
ability to generate a greater force with
the addition of rubber bands. None of
the hands achieved a grip force greater
than 18 N, and most forces were less
than 15 N. The gloves used to cover
the hands further increase the effort
required for operation.3 VC hands were
also found to be inefficient, because the
substantial frictional losses during hand
operation require increased activation
force and work by the user.
4
Hands are sized by the circumference of the palm. Smaller hands have
a smaller maximum opening width,
and fewer objects can be grasped. The
control cables can exit on the palmar
(Figure 12, B) or dorsal (Figure 12, C)
Figure 14
Mechanical Hand, with a thumb that moves
away from the other t wo ngers when opening
to create a wider opening width. (Courtesy of
Hosmer, Chattanooga, TN.)
Photograph of the Dorrance
aspect of the hand. Some hands allow
the cable to be rerouted through the
threaded stud and interior of the socket,
resulting in a more cosmetic prosthesis;
however, this method of routing eliminates the ability to interchange the hand
with a hook.
VO Hands
Several manufacturers offer VO hands
with standard internal aluminum or
nylon frames covered by a soft plastic
shell. In some models, the pinch force is
adjustable. Some are also available with
automatic back-locking mechanisms
that lock the fingers in the closed position to prevent inadvertent opening.
Alternatively, hands may be constructed with a hard, solid exterior. Thumbs
that move away from the other fingers,
either during opening or by virtue of a
wider, stationary position, allow for a
greater opening width (Figures 13 and
14). Some designs feature fingers comprised of coiled springs in a bent resting
Figure 15
Voluntary Opening Hand (Hosmer) with coiledspring ngers and automatic back lock to prevent inadvertent opening.
Photograph of the Lock Grip
position. Pulling on the cable control
straightens the springs and causes all
five fingers to open (Figure 15). Relaxing cable tension allows the fingers to
close in cylindrical prehension.
VC Hands
VC hands theoretically offer the same
advantage of graded prehension as
hooks, but the frictional losses in the
mechanism are much greater. As a result, the user must exert more effort to
close the hand and much of the control
is lost. As with all cable-driven hands,
the thick fingers block visual feedback
at the fingertips, and the cosmetic glove
further impedes motion. To compensate for the increased force required for
closing, some VC hands have a locking
mechanism to maintain grasp after closure. After the user releases tension, a
second cable-pull releases the lock and
the hand opens. Several manufacturers
offer VC hands with the familiar threejaw chuck grip pattern.
The APRL hand, like the hook, fea-
tures the same back-locking mechanism
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
145
Section 2: Upper Limb
Figure 16
thetic Research Laboratory (APRL) Voluntary
Closing Hand (Hosmer) with the ngers closed
and the thumb in the open position.
Photograph of the Army Pros-
that automatically engages to prevent
opening when cable tension is released.
It also shares the same disadvantage of
requiring increased grip force before
the fingers will release. Like the Sierra Voluntary Opening Hand (Hosmer;
Figure 13), it has a two-position thumb
for opposition to the first two fingers or
to create a wider opening width (Fig-
ure 16). Pushing the thumb inward toward the palm from its open position
secures the thumb in the closed position. Slightly pushing it in again will release the lock, and the thumb will spring
into the open position. The thumb of the
APRL hand can be detached and used as
a two-position opposition post in partial
hand prostheses.
Cosmetic Gloves
A cosmetic glove protects the hand
mechanism from contamination and
provides the external appearance of the
prosthesis. It is applied over the shell of
a passive or mechanical hand and must
be replaced at regular intervals when it
deteriorates from wear. Prefabricated or
custom-made gloves are available and
vary in material, thickness, appearance,
and detail.
Figure 17
in dierent skin tones are shown. (Panel A courtesy of Ottobock, Austin, TX.) B, The Regal, a male,
high-denition prefabricated glove, is shown. (Panel B courtesy of RSL Steeper, Leeds, England.)
Prefabricated gloves are the least
costly and most commonly prescribed
covering. The glove is chosen based on
hand size, and the approximate skin
tone is matched to the patient from color swatches available from each manufacturer. Gloves are available in generic
male, female, adolescent, and child
contours across a range of skin tones
(Figure 17, A). Male and female gloves
of the same size differ in nail, hair, and
vein appearance.
Prefabricated, off-the-shelf gloves
have improved in quality in recent
years. Most are now made of silicone
instead of easily stained polyvinyl
chloride and are available with more
sophisticated color depth and realistic skin texture. Some product lines
have a wider selection of skin tones,
Photograph shows prefabricated gloves. A, Several female prefabricated gloves
Gloves from different manufacturers are often not compatible with
hands made by other manufacturers,
even when the hand size is technically
the same. Differences in finger shape,
orientation, and length result in extra
material in the palm, web space, or fingertips. This allows movement of the
glove over the hand shell and creates
wrinkling in the palm or sponginess at
the fingertips, which can further impede
hand function.
Custom-made gloves offer the most
natural appearance. They are hand
made from a sculptured reverse copy
of the remaining hand. Skin tones and
color may be matched in person or by
using a calibrated photo of the noninvolved side. Hairs and other details are
applied individually or painted on.
with optional artistic painting of hair
and fingernail details that add more
customization and realism (Fi g ur e 17,
B). Some manufacturers offer gloves
in longer lengths that extend to the
elbow. When used with an internal
cable hand, these gloves can offer improved cosmesis, but they restrict full
pronation and supination unless cut
and separated at the wrist.
Wrists
The purpose of the wrist unit is to attach
the terminal device to the prosthesis and
allow appropriate positioning for activities. Wrist units primarily facilitate pronation and supination, but some permit
flexion, extension, or other movements.
The patient must have a full range of
wrist rotation so the terminal device can
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
146
be positioned in the most functional orientation for specific tasks. The degree of
voluntary pronation and supination the
amputee can produce depends on the
preserved anatomy and length of the residual limb and how well its movements
are transmitted to the terminal device.
When an amputation occurs through
the proximal half of the forearm, little
or no voluntary pronation or supination
can be captured by the socket. In the
case of longer limbs, an ill-fitting socket
that allows the limb to rotate inside it
will not effectively translate anatomic
motion into usable motion of the prosthesis. Self-suspending, supracondylar
socket designs can also restrict rotation
because the epicondyles lock the socket
in a certain orientation. Amputees usually compensate for the loss of range of
motion by adjusting their body position,
often through shoulder movement.
Terminal devices are connected to the
wrist unit by one of two mechanisms.
The most common is a threaded stud
that extends from the base of the terminal device and screws into the prosthetic wrist. European manufacturers
sometimes use a baseplate mechanism
with a plunger that inserts into the wrist
unit and is held in place by a variety
of mechanisms. Some terminal devices
and wrist units are available with either
option, but the two mechanisms are not
compatible with each other.
Some wrist types are available in
round and oval shapes. Oval shapes provide a smoother transition to the socket
for long transradial amputation levels
(Figure 18). If used with a prosthetic
hand with an oval base, there will be a
smooth, even contour to the prosthesis
when it is in neutral rotation. When the
hand is rotated into pronation or supination, however, a noticeable prominence
will be seen at the wrist because the geometries are no longer congruent. Many
hands have a round base to align with
the more common round wrists; these
hands have irregular contours when
used with oval wrists.
Chapter 11: Upper Limb Body-Powered Components
Figure 18
quick-disconnect wrist unit. (Courtesy of Hosmer, Chattanooga, TN.)
Photograph of an oval
Friction Wrists
The simplest wrist units use friction to
hold the terminal device in place. The
user manually positions the terminal
device wherever he or she prefers along
the 360° rotational axis. A set screw
or spacer washers are adjusted so that
sufficient friction is applied to prevent
rotation of the terminal device under
encountered loads, but manual rotation
of the device is still possible with the
uninvolved hand. Bilateral amputees
may pre-position such friction wrists
by striking one terminal device against
the other or by gripping a stable object
such as a table edge and rotating the
device into the desired position. Friction
wrist units are available in aluminum or
stainless steel in a full range of infant,
child, and adult sizes. Variable friction
wrist units are durable and economical
but, by nature of their simple design, do
not provide a consistent resistance to
rotation. Friction is generated by a rubber washer that compresses and applies
increasing resistance to rotation as the
terminal device is screwed in. Progressively less resistance is applied as the
terminal device is unscrewed from the
wrist unit and as the mechanism wears
out (Fig ure 19).
Constant-Friction Wrists
Constant-friction wrist units generally are preferred because they provide
constant friction throughout their range
of rotation (Figure 20). Most units of
Figure 19
Wrist, which has a variable friction mechanism
that uses compression on inner rubber washers to provide friction and resistance to rotation. B, Photograph of the Economy Wrist. This
device also is available with metal extensions
to facilitate use in trial prostheses or for socket
lamination for extra strength. (Courtesy of Hosmer, Chattanooga, TN.)
A, Illustration of the Economy
this type use a recessed, nylon- threaded
opening that is machined to accept the
threaded posts of standard terminal
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
147
Section 2: Upper Limb
Figure 20
the threaded stud of the terminal device, providing constant friction that can be adjuste d with a set screw. B, Photograph of a round WE Fric tion Wrist
(Hosmer), which also is available in an oval shape. C, Photograph of a Delrin Wrist (Panel C courtesy of RSL Steeper, Leeds, England.)
A, Illustration of a Hosmer constant-fric tion wrist mechanism. A clamp assembly tightens evenly around the mechanism that receives
Because friction wrist units do not
lock, they may present difficulties for
users who engage in work or avocational
activities that exert high rotational loads
on the terminal device. Friction joints
of all kinds can be difficult to keep in
proper adjustment and tend to permit
unwanted rotation when subjected to
very high torsional loading.
Quick-Disconnect Wrists
Quick-disconnect wrists are named for
Figure 21
Wrist. This constant-friction wrist uses a metal
wedge that is resis tant to breakdown. (Courtes y
of Hosmer, Chattanooga, TN.)
Photograph of the WedgeGrip
Figure 22
friction wrist unit. (Courtesy of RSL Steeper,
Leeds, England.)
Photograph of a constant-
their ability to facilitate rapid exchange
of different terminal devices. However, even if this function is not necessary, they are often used because they
can freely rotate for positioning and
devices. Turning a small, set screw in
the body of the wrist causes the nylon
thread to be tightened evenly against
the stud of the terminal device, thus
creating constant friction. When the
threads wear out, the insert can be replaced to restore function. Designs that
use a mechanical wedge to apply pressure to the terminal device stud tend
to resist wear and thermal breakdown
better than nylon inserts (Figure 21).
Constant-friction wrist units are available in several styles, round and oval
configurations, and in a range of sizes.
Although low-profile friction wrists are
available for longer residual limbs, the
standard length of the threaded stud
on the terminal device may need to be
shortened to facilitate their use.
In a unique approach, one type of
constant-friction wrist allows the user
to manually adjust the degree of friction
by simply rotating the exterior housing
to regulate the pressure on the friction
mechanism around the end of the terminal device (Figure 22).
lockdown in the desired degree of supination or pronation. More locking positions allow a more precise positioning
of the terminal device.
The most common design consists
of an adapter that is screwed onto the
base of each terminal device and inserted into the prosthetic wrist until it engages against an initial stop. This first,
unlocked position permits free rotation
and positioning but prevents the terminal device from falling out of the wrist
unit. An additional axial force into the
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
148
Chapter 11: Upper Limb Body-Powered Components
Figure 23
(Hosmer).
Figure 24
tesy of Hosmer, Chattanooga, TN.)
Photographs show the FM Quick Change Wrist (right) with an adapter (left)
Photographs of a ring-typ e quick-change wrist (right) with an adapter (left). (Cour-
wrist activates a geared mechanism that
securely locks the terminal device in the
desired orientation. Pressing a lever releases the adapter to the unlocked position that allows free rotation. A heavier
pressure ejects the adapter and terminal
device from the wrist (Figure 23).
An alternative ring-type quickchange wrist uses a similar adapter, but
the locking mechanism is operated by
rotating the outer housing (Figure 24).
Turning the ring in one direction unlocks and releases the terminal device
from the wrist. Rotating it the other
direction locks the terminal device in
place. This mechanism is slightly less
durable than the button type, and it
is very difficult for bilateral users to
operate.
Other quick-disconnect designs use
a baseplate and plunger rather than a
screw-on adapter. These wrists use a
different locking mechanism in which
a combination of ball bearings and a
snap blade fit into holes or detents in
the baseplate of the terminal device
(Figure 25).
Friction disconnect wrists combine a
user-adjustable, constant-friction mechanism with the free-rotation quickdisconnect feature (Figure 26). These
units benefit users who like adjustable
friction but need easy interchange of
terminal devices.
Rotational Wrists
Rotational wrists facilitate hands-free
positioning of the terminal device
Figure 25
wrist unit (referred to as the Zed Rotary) that
uses a baseplate and plunger-design quickdisconnect mechanism. (Courtesy of RSL
Steeper, Leeds, England.)
Figure 26
Disconnect Wrist. (Courtesy of Hosmer, Chattanooga, TN.)
Figure 27
Wrist. (Courtesy of Hosmer, Chattanooga, TN.)
Photograph of a heavy duty
Photograph of the Friction
Photograph of the Rotational
(Figure 27). Pressing a lever releases
a lock and causes spring-loaded pronation. If the lever is held down, the
wrist remains in free rotation, and a
cable pull will supinate the terminal
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
149
Section 2: Upper Limb
Figure 28
Friction Wrist. (Courtesy of Hosmer, Chattanooga, TN.)
Photograph of the FW Flexion
device to the desired position. Releasing the lever locks the wrist in 1 of 18
locking positions.
Flexion Units
Wrist flexion is particularly useful for
activities at the midline, such as toileting, eating, shaving, and dressing. For
many users, such activities are usually
performed more easily with the unaffected hand than with a prosthesis. The
prosthetic forearm can also be biased
radially (preflexed) and toward the midline (canted) to reduce or eliminate the
need for a wrist with full flexion capability. For these reasons, flexion wrists
were not traditionally prescribed for the
unilateral amputee unless a restricted
range of motion is present in the more
proximal joints or on the contralateral
side. However, some studies suggest
that wrist flexion can improve performance, speed, and ease of use of the
prosthesis for those without additional
pathologies.
5-7
Restoring wrist flexion is essential
for individuals with a bilateral upper
limb amputation who perform all daily
functions with prostheses. Because the
mechanism adds weight at the distal
end of the prosthesis, it is sometimes
prescribed only for the dominant side.
Flexion units are fully functional only
with hooks, because the edges of the
hand base will hit the wrist unit and
impede movement.
Figure 29
with a threaded stud. B, A pediatric version of the Sierra Wrist Flex Unit. (Courtesy of Hosmer,
Chattanooga, TN.)
Figure 30
Flex. (Courtesy of Ottobock, Austin, TX.)
Photographs of the Sierra Wrist Flex Unit. A, The unit attaches to another wrist
Photograph of the Robo-Wrist.
Photograph of the MovoWrist
Figure 31
This ball-and-socket wrist has quick-disconnec t
and locking capabilities. (Courtesy of Medical
Bionics, Alberta, Canada.)
The Flexion Friction Wrist (Hosmer)
contains a constant-friction mechanism
for rotation inside a button- activated
flexion hinge that permits manual
pre-positioning of the hook in neutral,
30°, or 50° of volar flexion (Figure 28).
The Sierra Wrist Flexion Unit (Hosmer)
is a dome-shaped device with similar
locking flexion positions (Figure 29).
Rotation with this unit occurs only if
it is installed distally to another wrist
unit, such as a friction or quick-disconnect unit. The entire unit rotates
where it mounts to the second wrist,
allowing the terminal device to be flexed
in any direction around the 360° axis
and to cover a wider work envelope
than the first alternative. This can be
advantageous for the bilateral amputee
struggling to perform midline activities;
however, the added weight and length
from having two wrist units limit its
applications. The MovoWrist Flex (Ottobock) combines locking rotation
with locking wrist flexion and extension in a much lower-profile construct
(Figure 30).
Most ball-and-socket joints offer in-
finite positioning in any direction, but
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
150
are held in place only by friction. They
do not lock, which poses a problem for a
user engaging in more than just light duties. They are used most often in endoskeletal systems. An exception is found
in the Robo-Wrist (Medical Bionics),
an innovative locking ball-and-socket
joint with a quick disconnect capability
(Figure 31). The unique locking mechanism enables hundreds of positions in
varying degrees of rotation, flexion, and
extension, along with radial and ulnar
deviation up to 43° from the center in
any direction.
Multifunction Wrists
The N-Abler V wrist unit (Texas Assistive Devices) combines the largest
number of wrist functions. The unit
is a five- function wrist combining
spring-loaded pronation; cable-controlled supination; lever-activated
flexion in 0°, 30°, or 50°; rotational
locking; and a quick- disconnect mechanism (Figure 32, A). The N-Abler II
(Texas Assistive Devices) uses a ringtype quick-disconnect mechanism for
locking rotation and a knob for precise,
incremental adjustment of flexion and
extension (Figure 32, B).
Ultimately, the purpose of the wrist
unit is to allow the user to position the
terminal device in an orientation that
maximizes assistance to the other hand
when performing bimanual activities.
Differences in a patient’s anatomy, limb
length, range of motion, comorbidities,
and activities will influence wrist unit
recommendations; such individual factors should be of primary consideration
in prosthetic prescription.
Hinges for Transradial
Prostheses
Flexible Hinges
Amputation through the distal third of
the forearm usually preserves a limited
amount of physiologic supination and
pronation. Flexible hinges permit active use of this residual forearm rotation,
thereby reducing the need for manual
Chapter 11: Upper Limb Body-Powered Components
Figure 32
ve-function wrist. B, The N-Ab ler II and accessories availabl e for use alone or as distal at tachments
to the N-Abler V or another wrist unit. (Courtesy of Texas Assistive Devices, Brazoria, TX).
Figure 33
prosthesis. (Courtesy of Jim Skardoutos, C-Fab.)
pre-positioning of the terminal device
(Figure 33). Although flexible hinges of
metal cable or leather are commercially
available, custom-made flexible hinges
made of polyethylene terephthalate webbing are most commonly used. They are
attached proximally to the triceps pad
and distally to the prosthetic forearm.
Hinges of the proper length keep the
socket secured onto the limb when the
elbow is flexed and prevent distal mi-
Photographs of devices in the N-Abler wrist series. A, The N-Abler V unit is a
Photograph of hi nges made of exible p olyethylene terephthalate o n a transradial
device with a wrist mechanism. However, amputations at or above the level
of the midforearm effectively eliminate
the possibility of transmitting active supination or pronation to the terminal
device, and the addition of rigid hinges
would not substantially detract from the
available range of motion. Rigid hinges
add width at the elbow, and this added
bulk often is visible in the lamination or
through clothing.
gration. Flexible hinges should permit
at least 50% of the anatomic residual
pronation and supination.
Single-Axis Hinges
Single-axis hinges are designed to pro-
vide axial and rotational stability be-
Rigid Hinges
An advantage of rigid hinges is their
ability to stabilize the elbow and prevent
rotation of the prosthesis during heavy
loading. Because the forearm segment
cannot rotate, the patient cannot use
voluntary pronation and supination and
must manually preposition the terminal
tween the prosthetic socket and the
residual forearm during active pros-
thetic use (Figure 34). Correctly aligned
single-axis hinges should not restrict the
normal flexion-extension range of mo-
tion of the anatomic elbow joint. The
joints should be set in a modest amount
of preflexion to load the stops of the joint
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
151
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