Orthosis for Improvement of
Arm Function in C5/C6
Tetraplegia
M. Margaret Wierzbicka, PHD
Allen W. Wiegner, PHD
ABSTRACT
People with spinal cord injury at the
C5/C6 motor complete level typically
have relatively well-preserved biceps
function but minimal or no voluntary
control of triceps. The authors previously have shown the lack of voluntary control of triceps results in deficiencies in
speed and accuracy of elbow movements
as well as reduction in reachable workspace. The authors also have shown
these deficiencies can be corrected by the
addition of constant extensor torque at
the elbow. The purpose of this article is to
describe a prototype constant-torque elbow orthosis and illustrate the improvements in function it affords people with
C5/C6 tetraplegia.
Introduction
Spinal cord injury causes the loss of voluntary control of muscles and varying
levels of decreased mobility and dexterity depending on the level and completeness of injury. People with injuries
at the C5 or C6 motor complete level
whose biceps function is relatively well-preserved often have little or no voluntary control of their triceps. A flexorextensor muscle imbalance such as this,
combined with even a small amount of
biceps spasticity, can lead to the development of a flexed and supinated forearm.
A number of orthoses have been designed to prevent and help correct elbow-flexion postures or contractures.
For example, static orthoses use a turnbuckle or other means to permit the elbow to be locked at a preset angle of
extension that gradually is increased as
contractures are reversed (1,2). Dynamic plastic (3) and spring-loaded orthoses such as Wire-Foam? a (4) and
Dynasplint(r) (5) provide a continuous,
low-intensity extension force to reverse
flexion contractures. The Wire-Foam
orthosis obtains force from a pair of coil
springs located on the lateral and medial sides of the axis of elbow rotation.
The Dynasplint consists of two stainless
steel struts placed laterally and medially on the upper extremity, with adjustable compression-coil springs incorporated within the distal forearm struts.
Abrahams et al. (6) have combined a
pair of medial and lateral coil springs
with geared hinge joints to produce extension force and spring wire coupled to
the distal forearm cuff to provide pronation force. Itzkovich et al. (7) have described a dynamic orthosis comprised of
arm and forearm braces made of polyethylene and joined at the elbow using
flail arm hinges with one side locking.
Extension force is provided by a rubber
ring on the posterior of each brace. Static activities can be accommodated by
locking the lateral hinge.
Another problem related to weak or
absent triceps in C5/C6 subjects is the
deterioration of control of upper-limb
motion even when contractures at the
elbow are absent. Controlling the arm
without the triceps muscle is like driving a car without brakes: It is difficult
to stop moving. For example, one patient complained about hitting himself
with his fork while attempting to feed
himself. In general, without use of the
triceps (which is the primary elbow extensor), tetraplegic subjects face limits
not only to their range of accessible
space but also to their ability to perform tasks in which the biceps muscle
does the work.
The authors recently showed tetraplegic subjects with weak or absent triceps performed elbow-flexion movements substantially slower than did control subjects and made larger overshoot
errors when making flexion movements
to a target (8). When a torque motor was
used to aid the weak triceps by providing a constant extensor torque of 2.5
Nm, which is approximately 6 percent of
maximum "normal" triceps strength of
39 Nm on the authors' apparatus (8), patients performed elbow flexions more
quickly and with accuracy indistinguishable from that of the control subjects.
Improving upper-limb function of a
tetraplegic person is important because
it allows an injured individual to lead a
more independent life. However, to
date, the application of elbow orthoses
to tetraplegic subjects largely has been
for the prevention or correction of elbow contractures. The broader problem
of improving tetraplegics' control of
arm movements has not been addressed. All commercially available
spring-driven dynamic elbow orthoses
use a linear spring, allowing the extension torque to change with elbow angle,
meaning extension torque is maximal
when the elbow is most flexed and minimal when most extended.
The authors have developed an orthosis that provides constant extensor
torque at the elbow. The design is based
on the authors' studies showing an improvement of arm function in C5/C6
tetraplegia with the use of a torque motor (8,9). The new orthosis differs from
existing orthoses in two significant ways.
First, it allows constant extension
torque during activities performed
close to the mouth (when the arm is
flexed) and when reaching overhead
(when the arm is fully extended). Since
torque does not increase with flexion,
fatigue of the biceps when working
with flexed postures is limited; yet sufficient torque is available to fully extend the arm in any position. In contrast, a linear spring does not provide
enough force at "full extension" to allow such extension overhead.
Second, subjects are more comfortable and more adept when controlling
arm movements against a constant
torque than against the progressive
torque from a linear spring or rubber
band (9).
Two alternative methods of restoring
triceps function that do not involve orthoses are posterior-deltoid-to-triceps
tendon transfer surgery (10) and functional electrical stimulation (FES) (11)
Tendon transfers are an attractive option because they are completely internal and sometimes can provide sufficient strength for lifts for skin pressure
relief. However, tendon transfer is no
feasible if the posterior deltoid has in
sufficient strength, and the procedure
does require surgery plus a substantial
period of postsurgical immobilization
and rehabilitation.
FES is effective only in subjects with
out extensive triceps denervation. Because FES of a muscle relies on electrical activation of the peripheral motor
nerve, the muscle cannot be electrically
activated if death of its spinal motor
neurons (denervation) has led to loss of
peripheral motor axons. The FF5 approach to restoration of triceps function is cosmetically advantageous as it
avoids the use of a bulky orthosis; however, this approach requires the application of skin electrodes, tolerance of
sensations associated with stimulation
and conditioning of the triceps muscle
to reduce the problem of muscle fatigue. An orthosis remains an option
for patients who want improved function but for whom the alternatives are
unfeasible or undesirable.
The overall objective of this study is
to design and test a purely mechanical
orthosis that improves arm motion in
tetraplegic subjects who have injuries at
the C5/C6 level. The purposes of this article are to: 1) present a prototype of the
constant-torque elbow orthosis, and 2)
demonstrate improvement of arm function in C5/C6 subjects using the orthosis.
Description of the Orthosis
The prototype one-hinge orthosis
shown in Figure 1 consists of four cuffs
(see Figure 1, c1-c4
) and one hinge incorporating a spring (see Figure 1,s
) on
the lateral side of the elbow. The cuffs
are fabricated from two layers of carbon-fiber lamination braid C designed
for light weight and high stiffness and
lined with 3/8-inch blue T-Foam? for
customized fitness and comfort. Having
a pair of cuffs both proximal and distal
to the elbow allows for uniform distribution of torque generated at the elbow
joint and helps keep the orthosis in
place. The hinge arms (see Figure 1, h
)
are made of stainless steel tubes 7 mm
in diameter and 0.11 mm thick.
The hinge on the lateral side of the
elbow incorporates a spring mechanism
(see Figure 2
). A power spring C (see
Figure 2, a
) is used to provide torque in
the direction of elbow extension. The
inner end of the spring is attached to a
ball-bearing mechanism to allow for
low-friction rotation. A lightweight
spring cage (see Figure 2, b
) is provided
to hold the spring in place, and a mechanical stop (see Figure 2, c
) prevents
extension of the elbow beyond the
anatomic limit. The hinge and tubes are
welded together to avoid unnecessary
connecting parts that would add
weight, the result of which can be fatiguing to subjects with proximal weakness in the upper limb. The weight of
the authors' prototype is about 600 g.
Torque can be changed in this prototype by winding or unwinding the power spring using a ratchet, much as one
would wind a clock. A removable handle (see Figure 2, d
) is provided for convenience. When the orthosis is fitted to
the arm, the spring is prewound to an
appropriate torque, usually to 2 Nm
(four full turns). At this level, users can
extend their upper limbs overhead. If
the spring is fully wound, it will produce
constant torque (± 10 percent) over the
full range of elbow flexion, 0-150 degrees. Torque can be decreased (see
Figure 2, e)
when donning or doffing the
orthosis, making those actions much
easier, then restored once the orthosis is
in place. A second advantage of this
ratchet is it allows the user to change
the spring torque at any time, increasing it to provide greater function or decreasing it to provide greater comfort.
A second spring, used to enhance
pronation, also can be seen in Figure 1
.
Individuals who have injuries at the C5
level retain their biceps muscle, which
can act as a supinator of the wrist, but
lose the ability to pronate the wrist. A
few patients with injuries at the C6 level may retain some ability to pronate.
The prototype orthosis assists pronation by means of a second spring (see
Figure 1, w
) attached at one end to the
lower hinge arm (h) and at the other to
the distal forearm cuff (c4). Pronation
torque is enabled by winding up the distal cuff (rotating it clockwise in the case
of the right arm) around the hinge arm
several times prior to locating the cuff
on the wrist. As the cuff unwinds (counterclockwise) around the hinge arm, it
assists pronation of the hand. The pronation torque is weak because of the
small diameter of the hinge arm (7
mm) and some unavoidable misalignment of the spring axis and the pronation/supination axis of the forearm. The
forearm spring is set to be approximately one-tenth as strong as the elbow
spring. Although this spring did appear
to facilitate pronation, this function was
not evaluated quantitatively and will
not be discussed further in this article.
An earlier prototype, a two-hinge orthosis (illustrated in Reference 12), has
springs incorporated into hinges on
both sides of the elbow. The hinge on
the medial side makes the orthosis
stiffer and more stable as all torques
are in the plane of elbow flexion. In
that model, cuffs retain rotation along
the pitch axis (see Figure 1, p
), ensuring
automatic positioning of the cuffs to
distribute pressure evenly onto the
skin. However, two users noted the medial-hinge spring interfered with manual propulsion of the wheelchair.
Modifying the prototype by removing the medial hinge, which is absent in
the one-hinge prototype, makes the orthosis lighter and less intrusive. However, the modified orthosis becomes subject to twisting torques and requires
more attention to orthosis rigidity. The
two-hinge orthosis might be preferred
by patients who use a powered wheelchair and thus do not find the medial
hinge intrusive.
Part of the challenge of designing a
one-hinge orthosis is ensuring proper
balance of the various forces acting at
the elbow so the orthosis will remain in
place on the arm when the user flexes
or extends his/her arm. The center of
rotation of the orthotic hinge should remain aligned with the anatomic hinge
of the elbow. However, an orthosis designed to provide torque to extend the
arm at the elbow using only one hinge
will tend to twist around the arm to
straighten itself without straightening
the arm. Increasing the rigidity of the
orthosis minimizes the tendency to
twist; however, this increased rigidity
also decreases comfort.
The authors' solution to the problem
of discomfort was to provide sufficient
degrees of freedom in the attachment
of cuffs to the hinge so the cuffs could
be adjusted to provide for appropriate,
equal distribution of forces as well as
comfort. The first one-hinge design allowed for two rotations: one around the
axis of the hinge arm, a "roll" rotation
(see Figure 1, r
), and another perpendicular to it, the "pitch" rotation (see
Figure 1,
p).
In addition, translation (sliding) of
the cuff along the hinge arm was possible. Cuff pitch was permitted using only one screw to mount the cuff to the
hinge arm. Cuff roll and translation
could be limited after proper adjustment by tightening set screws (see Figure 2,
)(f, g) on the hinge arm to provide
the best fit and performance. Although
these set screws are adequate for a prototype, more robust connectors may be
required for continuous clinical use.
Disallowing rotation of the cuffs
along the pitch axis was necessary since
this degree of freedom contributed
strongly to the twisting of the orthosis.
Cuff pitch was eliminated by attaching
the cuffs to the hinge arms with two
screws, above and below the hinge arm
(see Figure 1
). In the absence of cuff
pitch, the cuffs could still be translated
more proximally and distally along the
hinge arm and rotated around the
hinge arm. The latter adjustment was
especially useful for donning or doffing
the orthosis: A cuff could be rotated
out of the way and then rotated back
into place. After tightening the set
screws to make the orthosis rigid, the
balance of forces acting on the arm was
surprisingly good but still not as good
as in the two-hinge orthosis (12), which
stayed on the arm even if no Velcro?
straps (see Figure 1,
(v) were used.
Methods and Results
Selection of Subjects
Subjects were recruited from the outpatient population of the Spinal Cord
Injury Service at the Brockton/West
Roxbury VA Medical Center. Three
tetraplegic men with motor-complete
spinal cord injury at the C5/C6 level
whose biceps were relatively functional
[4/5 to 5/5 by manual muscle testing
(MMT)] and whose triceps were weak
(215 or less) participated in the study.
Subjects' ages were 46,32 and 19 years,
and times since injury were 2,9 and 0.3
years, respectively.
Individuals were excluded from the
study if they had severe spasticity, very
limited range of motion, unhealed bone
fractures or other significant medical
problems in addition to SCI. The number of subjects participating in each test
is given in the description of that test.
Description of Test Procedures and
Results
The evaluation tests were designed to
investigate improvements in: 1) controlling arm motion (tests 1 and 2), 2)
expanding the reachable workspace
(tests 3 and 4), and 3) functional ability
to perform daily tasks (test 5).
All tests were performed in a quiet
setting in the Motor Control Laboratory of the Spinal Cord Injury Service.
Tests were scheduled at the convenience of the subjects and always were
conducted by the same research team.
Each subject was seated in his own
wheelchair and performed the required
task without outside help. Because the
one-hinge and two-hinge orthoses discussed above produced similar functional results when adjusted to provide
similar torque, the results of testing
have been pooled.
Test 1: Single-Joint Speed and Accuracy
This test was designed to isolate the
function of the elbow flexors and extensors. Subjects performed fast elbow-flexion movements in which the biceps
muscle accelerated the limb and the triceps acted as a brake to decelerate the
limb and stop the arm at the intended
place (13,14). To quantify improvement
in controlling arm motion with use of
the orthosis, rapid movements aimed at
a visual target were compared with and
without the orthosis on the arm (10 trials under each condition).
Subject I (age 46; two years since injury) sat in his own wheelchair with his
elbow supported on a table at shoulder
level and strapped to a manipulandum,
allowing horizontal rotation of the forearm at the elbow but limiting any contribution of shoulder muscles to the movement. An oscilloscope in front of the
subject displayed two horizontal lines:
one representing the initial and then final target, and the other indicating the
subject's actual arm position. The subject's task was to align his arm position
with the initial target line at the beginning of each trial and then move his arm
"as quickly and accurately as possible''
to the final target position when the target line shifted. Movements of 10 and 30
degrees were performed.
The angular position of the arm was
recorded with a potentiometer attached
to the axis of the manipulandum. The
position data were sampled at 1 kHz for
one second after a target presentation.
Velocity and acceleration were calculated by numerical differentiation of the
position data. Parameters including
peak displacement, peak velocity,
movement time, and duration of the accelerative and decelerative phases were
evaluated off-line according to predefined criteria detailed in Reference 8.
Errors in performance were measured
as the difference between actual peak
displacement and the intended target
and were expressed as a percentage of
the target amplitude. Constant error
(mean percent overshoot) and variable
error (standard deviation of within-subject constant errors) were evaluated.
The results of this test indicate the
orthosis facilitated faster movements;
average (± SD) movement times decreased (p<0.0) from 312 (± 34) ms to
24 (± 31) ms in 10-degree movements
of one subject and 468 (± 49) ms to 294
(± 35) ms in 30-degree movements (see
Figure 3a
). Acceleration and deceleration times also were improved (see Figure 3b
). The variability of movements
(standard deviation of errors) was statistically similar with and without the
orthosis (see Figure 3c
) although average overshoot (constant error) was de
creased during 30-degree movements
with the orthosis. However, the subject
moved faster while wearing the orthosis. These faster movements, if made
without the orthosis, would have resulted in considerably larger errors in overshooting the target. The improvements
in motor performance with use of the
orthosis are similar to those reported in
previous studies using a torque motor
rather than an orthosis (8,9).
Test 2: Multijoint Coordination
This test examined improvement in interjoint coordination enabled by the
orthosis, reflecting the improved ability
of the user to compensate for the force
of gravity acting on the arm. The subject was asked to draw horizontal and
vertical ellipses in the air in front of
him (frontal plane) with the palm of his
hand. A drawing of an ellipse, with long
axis 16 cm and short axis 7 cm, was
shown to the subject to indicate the approximate size and shape of the ellipses
he was to draw. The trajectory of the
arm movement was recorded by means
of two miniature accelerometers (each
weighing only a few grams) mounted
perpendicularly to each other and attached to the subject's hand.
Data from these accelerometers
were transferred to a computer program that calculated the displacements
of the movement trajectories in the
frontal plane. Only subject 1 participated in this study. As can be seen in Figure 4
, use of the orthosis allowed the
subject to draw less distorted vertical
(see Figure 4b
) and horizontal (see Figure 4d
) ellipses in comparison to the
drawings he made without the orthosis
(see Figure 4
a,c), when his hand
could not maintain its intended path
through space because of his lack of
triceps strength.
Test 3: Front Reaching
The purpose of this test was to show
whether wearing an orthosis would allow for an increase in the subjects' range
of arm movements. Three subjects performed reaches, with and without the orthosis, to a 60-cm high, 120-cm wide "target board" (see Figure 5
) placed on a
table 45 cm in front of their chests. Targets were 4-cm diameter holes arranged
in four rows of seven columns, spaced 18
cm apart. A lightweight pen was
strapped to each subject's index finger.
Each subject was instructed to bring
his hand from the initial resting position on the table to each target. Success
was based on ability of the subject to
place the pen into each hole and maintain that position. Requiring the subject
to hold the hand at the intended target
for a couple of seconds allowed the authors to differentiate targets that could
truly be reached from targets that
could only be touched by "flinging" the
arm momentarily to the target.
After the drill described above, reach
speed was tested. Each subject inserted
the pen in each target hole, one after
the other, returning the hand to the
resting position on the table after each
insertion. The entire series was timed,
and only those holes the subjects could
reach both with and without the orthosis were included in the speed test.
Results of the front-reaching test
varied among subjects because a test of
three-dimensional reaching requires
coordinated movement and strength of
several muscle groups, including shoulder muscles. For example, in lifting the
arm overhead, both triceps and shoulder muscle strength are important.
Since the pattern of weakness following spinal cord injury varies from person to person, subjects' performances
on three-dimensional tasks are expected to reflect not only the effect of the
orthosis but also differences in strength
of other muscles. In contrast, during
single-joint tasks (Test 1) in which motion is restricted to elbow rotation, subjects' performances more closely reflect their triceps weakness (8).
Subject 1, who had no triceps
strength (0/5 by MMT) and fairly weak
deltoid muscles, showed marked improvement in his ability to reach the
higher and contralateral targets while
wearing the orthosis (see Figure 6b)
;
the targets were not accessible without
it (see Figure 6a
). For example, when
attempting to reach the higher targets
without the aid of the orthosis, the subject's hand often collapsed onto his
head. These targets became easily accessible with the aid of the orthosis. In
the timed task, the subject's arm moved
quickly to 15 targets (the lower three
rows and inner five columns that also
could be reached without orthosis)
when wearing the orthosis. This task required 37 seconds without the orthosis
and 30 seconds with the orthosis.
Subject 2 (age 32; nine years since
injury) had no useful triceps strength
(0/5 by MMT) and was limited by a 20degree flexor contracture in the tested
(left) arm; nevertheless, he showed the
ability to reach two additional targets
(one in the highest row directly in front
of him and another slightly contralateral to him in the second row from the
top) while wearing the orthosis. However, contralateral reaching to the leftmost column was not restored by the
orthosis because of the contracture.
The timing test, which in this case involved 16 target locations, indicated the
subject could move faster with the orthosis (32 seconds) than without it (42
seconds).
Subject 3 (age 19; four months since
injury) had a weak triceps muscle (2+/5
by MMT) but strong deltoids and
could reach all targets even without
wearing the orthosis. This subject also
had some control of elevation of his
arm above his head but could not fully
extend his arm overhead. Subject 3
might not be a suitable candidate for
wearing the orthosis because he would
show little functional improvement
from its use.
In summary, this test showed the elbow orthosis extended subjects' reaching space, allowed faster movements
and abolished arm collapse associated
with placement of the hand above
shoulder level.
Test 4: Side Reaching
In this test, subjects were asked to extend the upper limb to the side and
abduct it as high as possible both with
and without the orthosis while seated in
a wheelchair. The distance from the
Floor to the subject's hand was measured at full abduction or when the elbow collapsed into flexion. Subject 1 increased his vertical side reach from 129
to 142 cm, subject 2 from 112 to 127 cm,
and subject 3 showed no improvement.
Test 5: Propelling the Wheelchair
This test, simulating wheelchair propulsion movements, was performed only
by subject 1 (subject 2's contracture
precluded a meaningful test, and subject 3 had full range of backward reaching). The task was to sit in the wheelchair and reach back as far as possible
to contact the push rim. Without the orthosis, subject 1 could extend his arm
only to a gravity-assisted 48 degrees of
flexion before touching the rim. With
the orthosis, he could further straighten
his arm to 23 degrees of flexion, allowing a longer power stroke.
Conclusion
In earlier studies of the role of flexor and
extensor muscles in controlling fast single-joint movements to a target (13), the
authors found the extensor muscle, in this
case the triceps, plays an important role in
facilitating faster movements. Such basic
research in neuromuscular motor control
recently has led to recognition of deficits
in control of arm movements of C5/C6
tetraplegic subjects who retain little or no
voluntary triceps activity even in the absence of biceps spasticity or flexion contractures (8).
These findings suggest the applicability of elbow extension orthoses to the
C5/C6 tetraplegic population may be
far wider than anticipated. In general,
orthoses designed to treat contractures
at the elbow are designed to increase
the elbow range of motion after therapy but are not designed to improve active upper-limb motion.
Even spring-loaded designs are not
suitable for this purpose since the force
of their linear springs declines in proportion to elbow extension. In overhead
reaching, the extension torque provided
by a linear spring would decline with
arm extension to the point of inadequacy. A second problem with linear springs
is they contribute to control instability
(tremors) that subjects themselves can
recognize (9). For these reasons, the key
innovation of the present design is the
use of a spring that provides constant
(or nearly constant) torque over the full
range of elbow movement.
The authors' goal is to develop a
lightweight orthosis composed of passive mechanical elements (primarily a
constant torque spring) to improve
control of the upper limb. The orthosis
is not intended to provide the substantially larger forces required for transfers (e.g., from bed to wheelchair) because the added bulk and weight this
would require would be unacceptable
for ordinary use. The orthosis also is not
intended for the reversal of flexion contractures; but used regularly, it may assist in the prevention of contractures by
promoting elbow movement through
the full range of motion.
The functional tests indicate the prototype orthosis allows for faster, better coordinated upper-limb movements and
provides increased reaching capacity for
people with C5/C6 tetraplegia. Although
the current prototype meets the authors'
goals for improved function, the design
could be improved by reducing its bulk,
increasing stability and cosmesis, and,
ideally, eliminating dependence on outside help for donning/doffing and adjusting spring-force levels.
The authors have chosen not to seek
a patent on the concept of constant extension torque so other practitioners
can creatively use it in their own orthotic designs. As opposed to splints developed for temporary clinical purposes, an orthosis intended for continuous
use at home must excel in terms of convenience, long-term comfort and attractiveness.
Acknowledgments
This work was supported by grants from the Paralyzed Veterans of America Spinal Cord Research Foundation. The authors are grateful to Professor Tomasz Wierzbicka for his valuable help in mechanical design of the orthosis. The authors also appreciate the ideas and manufacturing skill of machinist Steven Rudolph and Dennis Amtower, CPO.
M. MARGARET WIERZBICKA, PhD, is
assistant professor of neurology at Harvard
Medical School in Boston.
ALLEN W WIEGNER, PhD, is assistant
professor of neurology (biomedical engineering) at Harvard Medical School and a
biomedical engineer in the Spinal Cord Injury Service, West Roxbury Department of
Veterans Affairs Center, Boston, MA 02132;
(617) 323-7700, ext. 5387.
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