Hybrid Arm Orthosis
Nisim Benjuya, Ph.D.
Steven B. Kenney, B.S.M.E.
Abstract
The complexity of upper extremity control
of the paralyzed patient offers a great challenge to the rehabilitation engineer, orthotist/prosthetist, or others. However, researchers can approach the problem of disabled arm motion, for the sake of simplicity,
as a series of links with minimum pivots and
minimum gravitational forces. In addition,
when the degrees of freedom are reduced
from four to three as in the case of this study,
then some daily functions can be achieved by
using simple braces and splints that are
powered externally and internally.
In our design of a hybrid arm orthosis
(HAG), we emphasized simplicity and active contribution of the user. The orthosis
takes full advantage of the wheelchair frame
without changing its profile. The HAG offers body powered shoulder abduction and
elbow flexion of one degree of freedom. A
motor-powered wrist supination and three-joint jaw chuck pinch offer the remaining
two degrees of freedom.
Introduction
The target population for this device consists of patients who are wheelchair bound
due to post-poliomyelitis, high level spinal
cord injury (SCI), or stroke. This population
is one whose needs are the most difficult to
accommodate when it comes to fitting an
external device (orthosis) that will assist in
performing the activities of daily living. This
target population, once limited in number,
has recently increased due to improved intensive care and surgical methods. Once the
patient survives, nursing care becomes necessary due to the complete dependence and
need for continuous assistance of the majority of high level (C3-5) quadriplegics. While
low level (C6-8) quadriplegics are rarely in
need of orthoses to provide useful upper extremity function because of residual hand/
arm control, C3-4 patients with the aid of
orthotics may become more productive, motivated, and less dependent on nursing care.
A 1965 review by Chyatte et al of four patients with muscle disease compared performance in ADL with and without orthoses. The orthosis used was a mobile arm
support (MAS). 1 They reported that performance in self-hygiene, grooming, feeding, and diversional activities improved with
MAS, and concluded that the orthosis, when
properly selected and adjusted, could increase the functional capacity of muscle disease patients.
Current HAG designs most often take advantage of the wheelchair to minimize the
hardware attached to the patient, and to conveniently house power sources and actuators. These devices typically employ a control system that uses a joy stick,2,3 sip and
puff switch, tongue switches,4 biological
(myoclectric) signals,5 or residual motion of
the patient's limbs.6
While the literature shows that current
HAG designs are adequate, an acceptable
control system has not yet been found and is
central to the issue of maximum compliance
by the user. The literature also states that, as
with any adaptive device, simplicity and cosmetics dictate immediate and long-term acceptance of an HAG by the patient, and still
need to be improved upon. The researcher
maintains that recent advances in the industrial development of thermo-plastic materials, miniaturized power sources, and micro-processor controls encourage fresh, new attempts in the design and control of HAGs.
The purpose of this paper is to describe a
novel HAG design for C3-4 patients that restores hand/arm functions. The proposed solution aims to integrate a patient's upper extremity movement with rehabilitation intervention and to withstand the test of time and
use by the patient.
Description of the Hybrid Arm
Orthosis (HAO)
The proposed HAG achieves two major
functions by using two different power
sources. The shoulder and elbow joints are
interconnected and simultaneously abduct
and flex, respectively, by contralateral
shoulder elevation. The wrist supination and
three-point jaw chuck pinch is generated by
two separate switchable DC motors in sequence. This section details the materials
and calculations used to achieve the four de
sired motions of the HAG: shoulder abduction, elbow flexion, wrist pronation/supination, and finger prehension.
Shoulder Abduction
Figure 1A
and Figure 1B
F shows a single pivot mechanical
shoulder joint mounted on the upper frame
of the wheelchair. This pivot houses a torque
spring (7.5 in*lb. at 180 degrees). The center
of the pivot is adjustable so that it can be
aligned with the center of rotation of the
patient's shoulder joint. Attached to the pivot bearing is an upper arm support bar
(UASB), which is motivated with a cord. A
motivating cord is attached to the UASB at
an extension (lever arm) 3" above the pivot
center. This lever arm rotates 450 with respect to the vertical when the upper arm is at
rest along the body (Figure 2)
. The motivating cord travels through a system of pulleys
and a compression spring on the contralateral side of the target arm to minimize the
force needed to elevate the motivating
shoulder (Figure 3A
and Figure 3B
).
The following formula summarizes the
torque analysis of the pulley system.
T [2Fs + Kc(0.051) (45-RGM)]
3SIN(THETA) + [(110-RGM)/180] Kt
where Kc is the compression spring constant
(6.2 lb.*in) and (0.051) (45-ROM) is its deflection over a 450 range of motion (65-110).
The total displacement generated is 2.31" or
0.051"/° (2.31"/45°). RGM is the range of motion 1,2,3, . 45° and 3" is the extension
(lever arm) length of UASB. Kt is the torque
spring constant. Adding the shoulder force
2Fs, the left portion of the formula ([]) describes the tension in the cable. The remaining part of the formula describes the contribution that is due to the added torque from
the torque spring located at pivot, assuming
an initial position of 300 that corresponds to
an arm along the body, that is, resting position. Once the torque due to external forces
is calculated, the required Es can be solved,
as follows:
Fs= T/6 (SIN(THETA)) - (110-RGM/180)
Kt/6SIN (THETA) - Kc(0.051) (45-RGM)/2
Adapting from Dempster (1955), the location of average centers of gravity and the
weights and lengths of body segments, we
generated a graph that represents the optimal required shoulder torque vs. abduction
angle (theta) (Figure 4)
. Because the force
characteristics of both abduction and torque
spring have negative linear slopes with respect to theta, we matched the required
shoulder torque curve over theta for a given
shoulder displacement by varying the spring
constants, loading the torque spring and
varying the lever arm length.
We assumed no energy loss due to inertia
force or friction. The elevation force produced by the motivating shoulder transfers
to the shoulder pad that is attached to a
shoulder lever arm. The motivating shoulder
lever arm travels along a shaft via a linear
bearing and pulls the mobile pulley block
which also travels along a shaft on a linear
bearing (Figure 3)
. The compression spring
positioned between the base of the shaft and
the mobile pulley block assists this pulley
unit. The compression spring initially is chosen to balance some of the gravitational
forces due to the weight of the whole arm at
rest. However, we also wanted to take advantage of the stored energy in the spring, to
aid the patient in abduction. Once the mobile pulley system moves upward it pulls the
cable attached to it, which travels through
the lower and upper fixed pulleys on its way
to the lever arm above the UASB. For 2" of
shoulder elevation the patient generates 380
of horizontal abduction of the upper arm.
Elbow Flexion
A push-pull cable, on one end attached to
the upper seat bar at 4.5 cm away from the
center of shoulder (Figure 2a
and Figure 2b
), and on the
other end fixed to the lower arm support bar
(LASB) 5 cm away from the elbow joint
(Figure 5a
and Figure 5b
), offers a simultaneous flexion in
relation to shoulder abduction. The LASB is
attached to a custom made elbow support
positioned at the end of the UASB (Figure 2a
and Figure 2b
). As for shoulder abduction, we used a
torque spring (3.5 lb.*in at 1800) inside the
bearing block of this joint. We found the
contribution of flexion force to overall external forces that act on the shoulder to be negligible. The torque spring works to balance
the weight of the lower arm in the main activation range and to assist in flexion as its
stored energy is released.
Wrist Pronation/Supination
A miniature 6V DC motor (Portescap)
drives the spur gear fitted to a wrist brace
(Figure 5a
and Figure 5b
). The location of the motor and the
design permit normal, unrestricted pronation/supination at the rate of 30°/sec. The
motor is powered by the wheelchair battery,
together with the control circuitry. The control circuitry consists of CMOS logic gates
and a current feedback motor controller.
The logic gates are switched to activate the
motor by air pressure sensitive relays. The
relays are controlled by three air buttons
fixed on the head rest of the wheelchair (Figure 6)
. To activate the pronation/supination
motor the patient slightly presses the center
button (>12.1 mm Hg) which powers the
control circuitry, then depresses one of the
more sensitive buttons (>3.7 mm Hg) at the
side of the central button to activate the selected motor. Each consecutive depression
of the button changes the direction of the
motor.
Finger Prehension
A second miniature 6V DC motor located
on the Velcro™ strap that secures the lower
arm to the LASB, energizes the prehension
gearbox (Figure 5a
and Figure 5b
). The gearbox via the flexible shaft translates the motor shaft rotation
to the fingers. The transfer of rotation to
finger prehension is accomplished by a worm
and spur gear combination (Figure 7a
and Figure 7b
). The
index and middle finger are guided from
their interphalangeal joints to oppose the
fixed thumb. The thumb harness, made of
thermoplastic materials, is custom fitted to
the patient. The maximum opening is adjustable up to 10 cm and the pinch at finger tips
when it closes reaches up to 5 lbs. of force.
This force is adjustable at motor controller
level of circuitry. The second half of the head
control circuitry is devoted to the control of
finger prehension motor with a similar excitation method. The air button which signals
the "go" and "direction" is opposite the pronation/supination button.
Discussion
The benefits of an upper extremity orthotic system for severely paralyzed patients
have been proven. However, a system previously reported elsewhere (Lehneis and Wilson, 1972), such as the IRM electric arm
orthosis, suffers from cumbersome control
and bowden cable power transmission. The
Ranchos electric powered orthotic system
(Nickel et al., 1969) and the Rehabilitation
Institute of Montreal orthotic system both
suffer from conspicuous control sites. The
Burke modular orthotic system (Stern and
Lauko, 1975), on the other hand, suffers
from overly complex control modules and an
unfavorable cost-benefit ratio.
The researchers of the proposed HAG
feel that the use of commercially available
materials and a design that is easily assembled, fits most wheelchairs, and uses the patient's residual body power, would be desirable and affordable to patients. We have attempted to use a minimum and the simplest
of controls. Our clinical evaluation so far was
limited to two quadriplegics at C3-4 level.
The HAG was compatible with our expectations of providing simultaneous shoulder abduction and elbow flexion and sequential
wrist and finger manipulation to the patients. The patients tested accomplished activities of daily living, such as self-feeding,
with relative ease after minimal training (an
hour to two hours). In developing our HAG,
we also tested the feasibility and ease of
modifying parts of the system for individuals
with different needs with success. Modularity of the system permitted a wide range of
flexibility as to location of various parts and
their size.
To summarize; the HAG enables the user
to carry out some activities of daily living
previously not possible, attempt others, and
feel increased independence, self-confidence and self-esteem.
We are currently investigating a means to
provide simple pinching force feedback display mounted on the transmission box of the
prehension unit.
Acknowledgements
The authors wish to acknowledge the contributions of Jonathan Dietz and Vincent Durso, C.G.
to this report.
This work was supported by the Veterans Administration RR&D Service
Nisim Benjuya, Ph.D., is project director at
Rehabilitation Engineering R&D Lab, VA Medical Center, W. Roxbury, MA 02132.
Steven B. Kenney, BSME, is currently attending the Graduate School of the University of Arizona.
References:
- Chyatte, S.B., C. Long II, and R.J. Vignos,
Jr., "Balanced Forearm Orthosis in Muscular
Dystrophy," Arch. Phys. Med. Rehab., 46, 1965,
pp. 633-636.
- Stern, P.H. and T. Lauko, "Modular Designed, Wheelchair Based Orthotic System for
Upper Extremities," Paraplegia, 12, 1975, pp.
299-304.
- Lehnies, HR. and R.G. Wilson, Jr., "An
Electric Arm Orthosis," Bull. of Prosth. Res.,
Spring 1972, pp. 4-20.
- Nickel, V.L., A. Karchak, Jr., and JR. Allen, "Electrically Powered Orthotic System," The
Journal of Bone and Joint Surgery. 51 A(2), 1969,
pp. 343-351.
- Hamonet, C., D. Boulogne, S. Simon, and P.
Bedhket, "A Myoelectric-Controlled Orthosis!
Recent Development," The Hand, 7(1), 1975,
pp. 63-66.
- Engen, T.J., "Powered Upper Extremity Orthotic Development," Progress Report, VA
Grant RD-1564, September 1967.
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