Prosthetics Research and the Engineering Profession
Renato Contini, B.S.M.E. *
In the establishment of any program in
prosthetics,* whether it be a program devoted to research on and
development of new and improved devices, or whether it be a program for the
dissemination of knowledge in the application of these devices, guidance must
come primarily from the medical sciences. In any such program, one can
appreciate the role of the physician, either the surgeon involved in the
amputation or the physiatrist concerned with the physical rehabilitation of the
patient. To a lesser extent perhaps, the role of the physical and occupational
therapist, in implementation of the prescription established by the physician
for medical rehabilitation or re-education, also is generally
Since there can be no prostheses without
a limbmaker, the role of the prosthetist cannot be underestimated. Certain
attempts at the fabrication of artificial limbs may be traced back to the time
of the Roman Empire. Several ingenious devices made during the sixteenth century
(Fig. 1 and Fig. 2) still are in existence. The major impetus, however, was received
as a result of the Napoleonic Wars, of the War between the States, and of the
Franco-Prussian War. Improvements in medical practice had by then made it
possible to save a much larger number of men who had lost limbs than had been
possible earlier. There thus developed a well-defined craft which reached its
peak during World Wars I and II and which established with the medical
profession a working relationship directed toward the fabrication of acceptable
To the efforts of these three
professional groups - medicine, therapy, and limbmaking - there have been added in
more recent rehabilitation programs the efforts of the social worker, of the
psychologist, of the psychiatrist, and of the counselor in vocational guidance,
the over-all purpose being to return the amputee to a more successful and
better-adjusted position in society. The organization and functions of a modern
prosthetics clinic team, as most usually accepted, have been fully and ably
described by Bechtol.
Important as is the role of each of these
disciplines, the progress that has been made in prosthetics in recent years may be
attributed, in large measure, to the interest the problem has aroused in a
substantial number of engineers. The role of engineering in a prosthetics
program is not as yet well understood or fully appreciated by the general
public. We speak of the role of engineering, rather than of the role of the
engineer, because we are concerned more with the application of certain basic
physical principles than with the particular individual who applies them. When
these principles are well understood and applied by the physician, therapist, or
prosthetist, each will function better in his own role. Unfortunately, in our
present system of education no provision is made for imparting the basic
principles of engineering in courses of instruction for any of these other
disciplines. As a consequence, until recently such advances as were made in
prosthetic devices came about primarily as a result of much trial and error
rather than as the outcome of a planned approach.
Any program directed to the development
of new prosthetic devices may be divided into three major stages. The first is
concerned with basic research. Second is the translation of knowledge gained in
the basic research stage into a specific design for a particular device. And
third is the application of the device to the amputee and the evaluation of
functional gain. But of course a program does not necessarily proceed in such an
orderly fashion. Before a device is finally accepted for general application, it
may be necessary, and in fact it often is, to retrace the sequence not once but
many times in order to gain additional information and understanding. We shall
consider later the role of engineering in each of these stages.
Man performs activities in a variety of
ways controlled by physical law. The manner in which he does so has thus
interested scientists since the time of Leonardo da Vinci (1452-1519), who made
the first systematic study of human movements and described them in his Note
on the Human Body. In 1679-1680, Borelli, a pupil of
Galileo, published De Motu Animalium, the first treatise which applied
the sciences of physics and mathematics to human and animal activity. The
mathematicians and physicists of the eighteenth century - Bernoulli, Euler, and
Coulomb - tried to develop rational mathematical formulae for determination of the
capacity of human work.
The number of investigators increased
greatly in the nineteenth and early twentieth centuries, and the two World Wars
gave still greater impetus to research in the general field of human locomotion
and activity. In Germany, France, England, Russia, and the United States, with
different objectives perhaps but directed toward the same general problems,
Fischer, Fick, Gilbreth, Amar,
Martin, Schlesinger, Schede, Bernshtein
, Steindler, Elftman, Henschke and Mauch
, and the groups at the University of California and at
New York University have studied human performance. Each,
individually or as groups, contributed to the increasing knowledge both in the general areas of
human activity and in the specific application of this knowledge to
From the time of Leonardo, almost every
investigator in this field either was primarily a physical scientist, or, if
not, had a very intimate knowledge of physics and mathematics. In the later
period particularly, the major contributors to the increasing knowledge of human
performance have been engineers, physical scientists, or anatomists and
physiologists with training in the physical sciences. A more comprehensive
review of the investigators in this field is that of Contini and Drillis
Basic Research in Human
In the design of any structure or
mechanism, for whatever purpose, the engineer usually proceeds from a set of
established specifications. These specifications may describe the function of
the device, the space it may occupy, the activity it must perform, the forces
which may be applied to it and which it must withstand, the chemical and
physical damage to which it may be subjected, the working life expected of it,
how often it should be overhauled or maintained, and what it may cost. To design
a prosthetic device properly, similar specifications should be prepared. Some of
the requirements for a satisfactory prosthesis may be developed from known data,
that is, from information obtained empirically over extended periods of time and
from the experience of countless amputees. Other information, however, and
perhaps the more important in the design of prostheses, can come only after
systematic experimentation. To supply this information, then, is the purpose of
the program in basic research.
Every human movement takes place in time
and space and is controlled by external and internal forces and by the mass of
the parts involved. The internal forces are generated in the muscles and
transmitted through the limbs to tools, controls, instruments, or other objects.
The external forces are those of gravity, inertia, ground reaction, and air
resistance. When the body is at rest, the external and internal forces are in
equilibrium; when it is in motion, the resultant of these forces has some value
other than zero.
Of course human movements may be observed
and the pattern of movement described subjectively. But unless these movements
can be recorded and measured precisely, no true understanding of the movement
can be had, nor can repeated movements be compared objectively in the same
individual or between different individuals. As technology has moved ahead,
engineering knowledge has made it possible to develop instruments and techniques
for recording and measuring movements and the forces which affect these
movements. Although it would be interesting, as an historical aside, to review
the methods used by earlier investigators, it is more profitable to describe
some of the recent developments.
Methods of Measurement
The invention of photography in the
middle of the nineteenth century, and the subsequent improvements in
photographic techniques, have made it possible to record motions and
displacements exactly (Fig. 3). The development of motion-picture photography,
of interrupted-light photographic techniques, and of a combination of the two as
obtained in the gliding cyclogram has made it possible to measure not only
displacement but also the rate and change in rate at which movements occur. By
these techniques, then, we can obtain displacement, velocity, and acceleration.
Once these quantities are known, and when the mass of the total moving body or
of its segments can be obtained by other measures, the forces acting on the
body, the energy costs, and the power requirements can all be
Of the photographic techniques mentioned,
motion-picture photography is used perhaps most universally. By mechanical or
electromechanical means, a light-sensitive film is transported at a known, fixed
rate past a lens and shutter. The film-transport mechanism is synchronized with
the shutter so that a picture is taken each time the film is advanced one frame.
The speed at which pictures are taken may be varied between sequences to suit
the particular need, and the shutter speed may be varied to stop the action down
to the smallest fraction of time consistent with the
particular apparatus and with the object being photographed.
With conventional motion-picture
equipment, frequencies of up to 128 frames per second have been photographed,
action being stopped down to the order of one five-hundredth of a second. Within
these limits most human activities may be photographed adequately. A timing
device - in effect a large clock, driven by a synchronous motor, and with the dial
subdivided into hundredths of a second - permits measurement of the variability in
time between frames and in exposure time (Fig. 4). Sometimes x-ray and
motion-picture photography have been combined. By this means it is possible not
only to record the motion of a limb but also to observe any relative motion
between the activating skeletal structure and the external surfaces.
Although this method of motion
recording has been used extensively, and even
though it may be quite adequate for some measurements, it has certain
disadvantages which detract from its general usefulness. In the reduction of
data, for example, each frame must be registered in two of the three major
coordinate axes, some point being maintained as a control. The location of each
moving segment must be determined from a constant frame of reference, a matter
which introduces possible sources for error. And it has been found that the
transport mechanism does not always respond at the same rate, so that the
interval of time between frames, on which the computations depend, may not
always be constant.
When the activity to be recorded is not a
repetitive one, as in jumping, or is repetitive but progresses along a linear
axis, as is the case with the walking pattern of a leg amputee, interrupted-light photography can be
used. In this system the film is stationary in the camera. The lens shutter is
kept open, while a slotted disc, driven at the desired speed by a synchronous
motor through a gear or pulley system, rotates before the shutter in such a way
as to admit and exclude light alternately. The speed at which the disc rotates
and the number of slits in the disc together determine the time increment
between exposures. The width of the slit (that is, the size of the angle
included in the slit) and the rotation speed of the disc determine the time of
exposure. In the studies conducted at New York University in conjunction with
the Veterans Administration's Prosthetic Testing and Development Laboratory, the
disc rotates 20 revolutions per second and the slit is 14 degrees wide, so that
the exposure time is of the order of one five-hundredth of a second and each
revolution results in one exposure (Fig. 5). These conditions are optimum for
the particular application, but they can be modified for other applications. In
the system developed by the Prosthetic Devices Study, Research Division, New
York University, working with the VA's PTDL, the light is supplied by a single
photoflood bulb and is returned by reflective tape, such as Scotch-lite,
which marks the points to be photographed. Similar results might be achieved
with an open lens and a strobe-flash source of light.
The obvious advantage of this system is
that it provides a complete pattern of a total movement, such as the forward progression
of an amputee for two or three strides, all of which may be recorded on one
film. Reduction of data is greatly simplified, since the measures of vertical
and horizontal displacement are taken directly from a single set of axes. The
error then is only that which the operator may make in measuring. The time
increment is as constant as permitted by the variation in speed of a synchronous
The Gliding Cyclogram
When the motion to be recorded is
repetitive in limited space, the interrupted-light method cannot readily be
employed, for the pattern of points cannot then be distinguished as to
occurrence in time. To overcome this difficulty, Bernshtein in Russia and
Drillis in Latvia developed the gliding cyclogram. This method is
similar to that previously described except that here the film is transported
across the field at a constant rate but at one that may be varied to suit the
particular activity being recorded. Under these circumstances, the position of
any point can be identified both in space and time. Even if, in a repetitive
motion, a point on a moving segment is returned to an original position, the
image in the initial and succeeding instances will be displaced on the film by
the distance the film has been transported in the elapsed time increment. If,
for example, a point were moving in a circular path, its locus would appear on
the film as a cycloid. Although this method increases the amount of work to be
done in data reduction,* suitable graphic shortcuts reduce this work differential to a minimum. As will be apparent (Fig. 6), the gliding cyclogram
has special advantages in recording the motion of arm activities, many of which
are repetitive and overlapping.
Although each of these methods permits
the measurement of displacement, velocity, and acceleration, other methods of
instrumentation give direct measurement of velocity and
acceleration in certain situations. Velocities along one axis may be measured
with a tachograph, a device consisting of a fine cable connected to a moving
body, continuing in a closed loop, and driving the rotor of a generator (Fig. 7). Since the voltage is proportional to the angular velocity of the rotor,
which in turn is proportional to the velocity of the body, the voltage generated
is a direct measure of the linear velocity.
Another electrodynamic device, the
accelerometer, measures accelerations directly. Essentially, this instrument
consists of a small, compact mass supported by a spring device. When the mass is suddenly accelerated,
its inertia deflects the spring by an amount dependent upon the acceleration and
the spring constant. By suitable means, such as by differential transformers,
the deflection is converted into a change in voltage proportional to the
displacement and thus proportional to the acceleration imparted to the
accelerometer. More recently, accelerometers have been devised employing strain
gauges (see below).
Displacement and velocity permit us to
describe a motion; acceleration and mass permit us to compute the forces which
affect the motions. Sometimes it is possible and desirable to measure forces
directly. A number of such force-recording devices have been made possible by
technological advancement in the past 20 years.
The Strain Gauge. The strain
gauge, which has been used in innumerable applications, is such a device.
Essentially, it consists of a fine wire of known cross-sectional
diameter and electrical resistance, arranged in a packet (not unlike a
Band-Aid) so that it may be attached directly to some structural element.
When the structural element is stressed, it either elongates or shortens,
depending upon whether it is in tension or in compression. The filament of the
strain gauge follows the structural element to which it is attached, and its
cross-sectional area is reduced or increased, with consequent stretching or
compression along its length. The electrical resistance is thus increased or
decreased from the normal or zero-load position. By suitable electrical
magnification and instrumentation, and with proper initial calibration,
instantaneous changes in load can be measured and recorded.
The Capacitor. Another device for
measuring loads or forces directly is the capacitor, a small capsule consisting
of a dielectric material between two layers of electrical conducting material.
When a voltage is applied across a capacitor, an electric charge is stored. The
capacitance of the unit varies directly as the area of the surface plates and
inversely as the thickness of the dielectric. When pressure is applied across
the faces of the capacitor, the thickness of the dielectric is reduced and the
capacitance is changed.
Pressure gauges based on this principle
have been developed at the Franklin Institute. In these instruments,
the construction is loose so that appreciable changes in spacing between the
plates, and hence in capacitance, occur with changes in loading. Springiness is
achieved by impressing a waffle pattern of indentations into the steel discs which serve as the
plates of the capacitors. The gauge is used as one arm of a bridge circuit in
which a high-frequency signal is supplied and the unbalance is amplified and
recorded on an oscillograph. The degree of unbalance is calibrated in terms of
load on the gauge.
Other Force-Recording Devices. Still other techniques for the measurement of loads have been used widely.
For example, the principle of equal distribution of pressure in pneumatic and
hydraulic systems has resulted in the development of various types of pressure
gauges. The property of springs - leaf, helical, or torsion types - in maintaining,
within certain limits, a direct ratio of load to deflection has been used in
other force-measuring units. Still other devices have been developed making use
of other known physical phenomena to obtain data desired in specific
Many of these principles, techniques, or
devices have been applied in the basic research program to obtain the data
needed to develop new and better prostheses. The same applications also have
been used to evaluate the prostheses on the amputee, and in some instances
special adaptations of certain of these principles have been used as aids in
amputee training. Some of the more important experimental units merit further
The Lower Extremity
In 1945 the Prosthetic Devices Research
Project at the University of California, Berkeley, initiated a program of basic
research directed toward the gathering of information on locomotion, both in
normal subjects and in leg amputees. It was desired to obtain data on the
individual factors which contribute to the pattern of human gait - the
displacements of the head, arms, and torso; the displacements and rates of
displacement of the thigh, shank, and foot; the moments at the hip, knee, and
ankle joints; the pressure at the point of ground contact; and the shift in
apparent point of pressure application. Using the techniques already described,
the engineers participating in this program developed a variety of ingenious
To record the displacements of the
segments of the body, motion-picture techniques were adopted. The appropriate
control points on the body were identified by targets, in some instances the
motions of small magnitude were magnified by target extensions, and in other
instances the pattern of locomotion was photographed at intervals varying up to
3000 per second. To obtain the components of motion along the three axes of
space, a glass walkway and tilted mirror were used. By this expedient, side and
plan pictures were taken simultaneously on one film, thus minimizing the time
required for reduction of data and also reducing the possibility of error as
compared to the use of two synchronized cameras. From these photographs the
motions of the leg segments, heel and toe rise, degree of knee flexion, phasing
of the step, and all other desired details could be analyzed. Forces during the
swing phase could be determined, as could also the moments at the
To measure ground reaction, two force
plates were designed using strain gauges in various combinations to measure
vertical, fore-and-aft, and lateral components of foot pressure at ground
contact. Through appropriate electronic combinations, the strain pickup also
could give the apparent instantaneous center of pressure and the torsional
moments exerted by the rotation of the foot at ground contact. In a similar
study conducted by the Research Division, College of Engineering, New York
University, the same elements, strain gauges, and structural beams were combined
in another variation of the force plate. Both the UC and the NYU force plates represented a
refinement of those conceived and used by Elftman, who, in his
earlier studies in human locomotion, had used springs and dial gauges to record
components of forces.
The Upper Extremity
The University of California at Los
Angeles, through its Engineering School, was entrusted with basic research in
the upper extremity. To study the range of movement required by arm prostheses
in the performance of selected daily activities, a photographic procedure was
established. A subject was placed within an enclosure composed of vertical,
horizontal, and lateral grids. Two mirrors permitted views in the horizontal and
lateral planes (Fig. 8). When the subject was photographed, the motion of the
targets on the joints could be pictured simultaneously in all three planes,
together with the coordinate grids, thus permitting rapid data reduction. An
ingenious mannikin enabled the duplication of motions photographed for further
study of particular combinations of angular displacement of segments.
It is difficult to indicate clear
boundaries between the basic research and the evaluation stages in the
Artificial Limb Program, for many of the tools used to obtain basic data also
are useful to the group at New York University engaged in the evaluation of
prostheses. These techniques and others now being used in the evaluation program
are discussed later (page 65). As the measuring and recording instruments become
more generally applied, scientists other than engineers will become equally
proficient in their use. When the need arises, the engineering profession
undoubtedly will produce even more refined devices for measuring more complex
Important as is the role of engineering
in the development of instrumentation and equipment for basic research in human
motion, it is in the second stage of any prosthetics program - the design of the
prosthetic device - that the engineer is pre-eminent. Among the many factors he
must consider in the design of a prosthetic device we may include
safety, function, control, efficiency, appearance, comfort, simplicity, and
durability. These features can scarcely be assigned any order of importance;
since they are all interdependent, the design usually must end up as a
Safety, function, control, efficiency,
and appearance require a knowledge of the means - mechanical, pneumatic,
hydraulic, or electrical - by which the desired performance can be accomplished
and also a knowledge of the forces available, of the forces applied, and of the
proper distribution of masses in the device. Comfort requires a knowledge of the
limits and distribution of pressure that can be tolerated by body tissues and
vessels without damage and without distress to the amputee. Simplicity and
durability, both important in the cost and maintenance of the device,
require a knowledge of the breakdown that
may occur owing to perspiration and body acids, continuous use, temperature
changes, and abrasion and chemicals from external sources and, in addition,
knowledge as to what materials and combinations of materials may be used to
minimize such deterioration.*
This kind of problem is the true test of
engineering. All the physical sciences
which contribute to the substance of engineering may be called upon in evolving
the final product. The mechanical engineer contributes his knowledge of
mechanisms - cams and gears and linkages, which together may reproduce a motion.
With the hydraulic and electrical engineer, he devises means for the operation
or control of the prosthesis, for damping a swing, or for magnifying the power
available within the amputee. The metallurgical engineer develops the alloys
which go into the joints and prescribes methods of treatment to bring out the
maximum qualities desired - strength or ductility or resilience or wear. The
chemical engineer makes available the new synthetic substances which so
handsomely replace the natural substances heretofore the only materials
available. Plastics, whether they be the strong, structural resins used in the
lamination of shanks and arms, or whether they be the plastics
used for cosmetic purposes, have radically changed the appearance,
weight, and sanitary properties of prostheses.
The design engineer must combine all this
knowledge into the most effective whole. He must bring to the job all of the
experience and ingenuity he possesses so that the ultimate product will not only
produce the desired function, be strong enough, and last an adequate period but
will also be relatively inexpensive and simple enough to be maintained locally
with a minimum of special tools. The making of artificial limbs can now be based
on well-established scientific principles; it can cease to be empirical and can
become a branch of engineering and medical activity. But without the necessary
technical skills, progress in prostheses will return to the trial-and-error
system from which it has so recently emerged. Some of the specific problems to
be solved, and the methods for their solution, which have occurred in the design
of upper- and lower-extremity prostheses, deserve to be discussed in some
The Lower Extremity
The scientific basis for lower-extremity
prostheses is provided by biomechanical investigation of the functions of the
lower limb in human locomotion. Man is an erect biped, that is, he has two supporting limbs and
the mass of his body is carried in a vertical plane. The human body, then, may
be represented as an upper mass upheld by two supporting columns. The upper mass
consists of the head, arms, and trunk. The supporting columns are the two lower
limbs. Of complex character, they each consist of three segments, superposed and
movable on each other. To meet the needs of standing, the three movable segments
form a quasi-rigid column by virtue of their superposition.
The standing position includes standing
on both feet and standing on one foot, as in the stance phase during locomotion
when the weight is borne on one foot only. The vertical line passing through the
center of gravity of the body passes behind the line connecting the centers of
the two hip joints and in front of the axes of the knee joints. Extension of the
trunk relative to the thigh and of the thigh relative to the shank is thus
maintained by gravity and limited by powerful ligaments. The two lower limbs
therefore remain rigid with a minimum use of active muscle groups. But
locomotion demands that the lower limbs be composed of movable, superposed
segments. This requirement appears irreconcilable with the demands imposed by
the standing position, but the natural arrangement of the lower limbs meets both
requirements. Mobility of the hip and knee joints is essential in performing a
normal step, a motion which can be divided into four alternating phases, two
phases of support on both feet and two phases on each foot
During single support on one foot, the
supporting leg bears the weight of the body while the other swings in the
sagittal plane like a pendulum suspended from the trunk. Since the two lower
limbs are of precisely the same length, the swinging leg must become shorter
than the supporting one, or else the swinging foot would drag on the ground.
Shortening of the swing leg is effected by flexion of the thigh on the trunk, of
the shank on the thigh, and of the foot on the shank.
The geometry of the hip joint, and
particularly that of the knee and ankle joints, is very complex. Not all
authorities are in agreement as to the movements of the segments of
the lower limb in flexion and extension, but
enough is known to provide information as to how stability and mobility are
provided both in standing and in walking. In the manufacture of artificial legs,
it is desirable to reproduce insofar as possible the static and dynamic
characteristics of the sound limb.
The Above-Knee Case
With notably rare exceptions, the design
of artificial legs proceeded along a fairly well-defined pattern. Generally,
until the middle of the nineteenth century, and now still so in many
underprivileged countries, it was considered adequate to supply the leg amputee
with a peg-leg. For above-knee amputations, it consisted of a pylon supported
below a pad, corset, or socket, which in some fashion was attached to the stump
or suspended from the shoulders. For below-knee amputees, the stump was flexed
and the peg-leg attached below the flexed knee.
Such an artificial leg satisfied
completely one of the two functions of the normal leg. It provided a column
which, together with the sound leg, allowed the individual to stand erect. It
also enabled the wearer to walk, although, since there was no knee joint, it
affected the amputee's gait considerably. In the swing phase, the wearer was
required to raise the hip on the amputated side in order to swing through; in
the stance phase he necessarily had to vault over the pylon. Although such a
device is simple, strong, inexpensive, and quite serviceable, the amputee is
subjected to excessive stress during walking, his gait is asymmetric and
unnatural, his performance in walking is inefficient, and his physical
appearance is far from cosmetic.
Next in order of development was the
so-called "conventional" leg (Fig. 6, page 11). In general, this prosthesis was
made to look like the sound leg, that is, it possessed some cosmetic appearance.
The knee was hinged and could be flexed, although in the earlier devices a knee
lock was provided to assure stability in standing. The foot was attached to the
shank with either a rigid or a jointed ankle.
This order of devices had many advantages
over the peg-leg, but it introduced other problems. Because of the knee hinge,
it was possible to sit or kneel or to perform in a
more natural manner other activities requiring knee flexion. Moreover, because
of the knee joint, when not provided with a knee lock, the amputee was able to
walk with a better gait. Knee flexion permitted a certain amount of leg
shortening in the swing phase, thus reducing the amount of hip elevation
required to clear the ground. But the knee and ankle joints introduced
instability in the stance phase, particularly at heel contact. The free-swinging
leg resulted in an exaggerated back swing and forward swing with a pronounced
shock at each stop. Later compromises were effected by setting the knee bolt
forward of the weight line of the body, by addition of check straps to
decelerate the shank at toe-off and to provide some assistance at the beginning
of the forward swing, by introducing friction devices at the knee bolt, by a
combination of both, and by limiting ankle motion through the use of bumper
With minor and individual exceptions,
this was the general state of development at which the above-knee prosthesis had
remained until the end of World War II. As a result of the research initiated
thereafter, engineers began to devote time to the application of old and new
knowledge to the design of lower-extremity prostheses. Among the features which
had been demonstrated as desirable were flexion at the knee but with some
stabilizing control at the time of heel contact and immediately thereafter, some
measure of support in an emergency situation such as in stubbing the toe, a
controlled swing of the leg, an ankle joint which would permit rotation in a
horizontal plane as well as in the sagittal and transverse planes and yet not be
so flexible as to increase instability, and a toe-lift device for ground
clearance in the swing phase. All this was to be accomplished without
substantially increasing weight, sacrificing durability, or increasing initial
and maintenance costs of the device. By combining known engineering principles
with newly developed materials, a substantial gain was achieved in the
above-knee prosthesis, with consequent improvement in the performance of many
The U.S. Navy above-knee leg
developed at the U.S. Naval Hospital, Oakland, California, is an example of such
an improved prosthesis. Controlled swing with terminal deceleration was achieved
by the use of friction devices which come into operation in the last portion
only of the forward and backward swings. New plastics and molding techniques
provide a much more natural appearance. New methods of bonding rubber and a new
method of attaching the foot to the shank allow for greater flexibility at the
ankle without serious problems of instability.
Proper application of mechanical and
hydraulic engineering principles have resulted in two improved devices, the
Stewart-Vickers and the Henschke-Mauch hydraulic legs, both for above-knee
amputees. The Stewart-Vickers leg (Fig. 9) provides some resistance to knee
flexion and hydraulic damping or deceleration at the terminal portion of the
forward and backward swings. By a controlled cycle of operation of valves and
cylinders, it provides coordinated hip-knee-ankle flexion in the swing phase so
that adequate ground clearance is obtained, gives to the gait a more natural
appearance, and apparently results in less effort on the part of the amputee.
Whenever it has been tried by an amputee, it has generally resulted in favorable
The Henschke-Mauch leg, which
most nearly duplicates the swing pattern of the sound limb, has been designed to
provide stability at heel contact, both at the beginning of the stance phase or
in the event of a sudden forward acceleration as in stumbling. A carefully
designed, pendulum-type valve controls the passage of hydraulic fluid within a
cylinder, the added stability being maintained long enough for the amputee to
regain his balance but not long enough to impede knee flexion in the stance
phase or to increase the risk of a fall. By other valving arrangements the
hydraulic cylinder also controls the leg in the swing phase by providing
adjustable constant friction in the full cycle plus terminal
The human knee joint flexes by a
combination of rotation and sliding, so that a simple, single-axis joint cannot
duplicate the relative positioning of the tibia and femur. A number of attempts
have therefore been made to duplicate this articulation in so-called
"anatomical" knees by means of various complex mechanical devices, of which one is the
four-bar linkage. In Fig. 10, links AD and BC attach thigh to
shank. Links AB and CD are formed by the shank piece and the thigh
piece, respectively. A is the center of rotation of the ankle; K
is the center of rotation of the knee; H is the center of rotation of
the hip joint. The locus of the instantaneous center of rotation of the knee is
0-5-10-20-30-45-90, the centers being at the point of
intersection of projections of the links AD and EC. Each number
indicates the angle of knee flexion which places the instantaneous center at the
point shown. As extension takes place, the effect is as if the shank were
lengthened and the thigh shortened, a feature which aids stability in the stance
phase and reduces the force required to start flexion at the beginning of the
In the design shown, maximum elevation of
the center of knee rotation occurs prior to full extension, so that initial knee
flexion at toe-off is difficult. An improved design, with maximum knee elevation
at full extension, is to be found in the University of California
four-bar-linkage knee . It attempts to simulate the path of the
instantaneous centers of rotation of the knee joint so as to provide maximum
stability and maximum flexibility at the proper times in the walking
The Below-Knee Case
It is this complex articulation of the
knee joint that poses a major problem in the design of an adequate below-knee
prosthesis. Since the below-knee amputee retains his natural knee, and since
each individual knee follows an individual pattern in flexion, it has thus far
been impossible to provide between the thigh corset and the below-knee socket an
articulation that will not introduce some displacement between the stump and the
Methods of Suspension
The suspension of the above- or
below-knee prosthesis has been another area for research and design. Above-knee
prostheses had been suspended either by shoulder harness or
by some sort of pelvic band. The former did not maintain an adequate positioning
between the stump and the socket, since by its very nature it could not adjust
to the varying relationship between the shoulder and the leg in different
activities. Although the pelvic band retained the leg more securely, it in turn
imposed an artificial restriction on possible thigh movements, especially
rotation and abduction.
A novel method of suspension by suction
was patented by Parmelee in 1863, but the idea apparently was
abandoned in this country although it continued to be used occasionally in
Europe. Increasing experience with the suction socket in Germany after 1933
brought it to the attention of medical and engineering scientists in other
countries, including the United States. After World War II, in a coordinated
program sponsored by the Veterans Administration and directed by the Advisory
Committee on Artificial Limbs of the National Research Council, all aspects of
suction-socket suspension were studied carefully. The results of this study
proved the merits of the suction-socket method of suspension, and it is
gradually being adopted for all above-knee prostheses where the
limbmaker is certified to make such a socket and where there are no medical
contraindications. A similar method of suspension is being worked out for
below-knee prostheses with increasing evidence of success.
The Upper Extremity
The upper limb is the limb of contact. It
consists of three segments - the hand, the forearm, and the arm. Of these, the
hand is the most highly differentiated and the most important, since the
essential upper-extremity function is grasp, which is mobile and variable in
quality, power, and duration. Although its primary function is that of
prehension, the hand is also one of our major sense organs. Through it we sense
temperature, pressure, surface quality, and the shape of objects. For the blind
it serves as substitute for the eyes by providing a sense for discriminating
form and texture and, together with the forearm and arm, for determining spatial
relationships. The forearm and arm serve merely as mobile attachment for positioning the hand in
space. Since most of the hand movements and its different articulations are
dependent on arm and forearm muscles, they provide a reserve of active power for
hand activation. A detailed analysis of the functional mechanism of grasp
furnishes the basis for construction of the more scientifically
conceived artificial hands.
The Mechanism of Prehension
The natural grasp and manipulation are
wholly dependent upon the muscular action controlling movement of the fingers.
The nature of muscular action therefore determines the nature of the grasp, and
the two properties governing the mechanical phenomena of muscular function are
contractility and elasticity. Contractility of the muscle is controlled at will.
It can be graduated voluntarily in power, extent, and duration, so that the
fingers can be closed firmly or gently, as in holding a tool or an egg, or partially or wholly, as in
holding a book or a sheet of paper (Fig. 11.). Similarly, the fingers can be
moved or closed for very short or very long increments of time, as in fingering
the violin or in holding a telephone receiver. Muscle normally is in a state of
tone, which may be defined as the property possessed by muscle of preserving,
either by voluntary or by reflex action, a state of contractility. This
contractility may be long or short in duration, greater or less in extent,
strong or weak in power. By means of muscle tone, the hand can be kept in a
convenient position for long periods of time.
Since the hand is so important in
everyday activities, and since its functioning is so complex and so dependent
for mobility on the two other segments of the upper limb, surgical and
orthopedic treatment of the upper-extremity amputee is extremely important in
restoration of functional loss. It should be directed toward preservation of the
maximum amount of natural mobility. Since it is not yet
possible to create artificial muscle, it is necessary to reproduce as well as
possible by indirect processes the effects of normal muscle action on the
fingers. Prostheses for this purpose are successful in such proportion as the
mechanical effects produced approximate those of the natural
Until the present, and even now with all
the currently available technology, the most adequate substitutes for the lost
muscle activators are muscular substitutes, self-powered agents which induce the
movement of the artificial fingers by means of artificial tendons, that is, by
control cords. The latter are, as a rule, attached by some appropriate means to
the shoulder on the amputated side or on the normal side or both. The movement
produced by them is thus entirely dependent upon the shoulder group of muscles.
Improvements in surgical techniques and extensive research in muscle
physiology recently have reawakened interest in the use of
cineplastic procedures to provide other muscle motors (Fig. 12). Both the biceps
and pectoral muscle groups have been used for this purpose.
Since the action of the controlling
muscles must continue for such periods as required for the particular grasp
function concerned, the muscular substitute can become heavily
burdened. It is therefore absolutely necessary to arrange for release of the
muscular substitute once the fingers have been placed in the appropriate
position. This is achieved by mechanisms which produce in the artificial fingers
the same effect as that produced by muscle tone in the natural
Prior Art in Upper-Extremity
Although the basic concept of an
artificial arm and its terminal device has not changed materially from that of
the first arms made many years ago, recent technological developments in
materials of construction and a better application of known mechanical
principles have together resulted in arms of improved appearance and greatly
improved function. As in the artificial leg, the materials most commonly used
for the artificial arm and forearm have been wood and leather. Control was
achieved by shoulder harness operating through control cords, usually leather,
connected to the terminal device, which was usually a split hook, that is, a
pair of iron or steel fingers bent in the shape of a hook and so hinged as to
close on each other. For different applications the shape of the hook was
modified as appropriate. Since in general the closed position required for
grasping an object is of longer duration than is the open position
for approaching the object, opening was
effected by the shoulder muscles and closing was brought about by some spring or
elastic medium. Cosmetic appearance was neglected or, in those few cases where
it was attempted, a passive hand was the usual result.
To return to the arm amputee some measure
of productive capacity, there were devised a great many one-function terminal
devices, each intended for some particular occupational need (Fig. 13). Such
"tools" could be inserted and attached to the distal end of the artificial arm.
The practice was predominantly European, and we see in their
"armamentaria" hooks, rings, hammers, knives, brushes,
and a multiplicity of other designs intended to enable the amputee to function
in his customary occupation as smith or carpenter or metal worker
Present-day technology and a formal
approach to the design of both arms and terminal devices has since effected vast
improvements in upper-extremity prostheses. Although most of the newer designs
have been described in detail in available literature, it is
appropriate here to review these developments in a very general way as they
relate to engineering practice.
New Arm Substitutes
The developments in plastics and in
methods of fabrication have resulted in greatly improved arms. By proper
lamination, molding, and coloring, arms and forearms can be made lighter,
stronger, and with much better cosmetic value. Shoulder caps for
high above-elbow amputations and for shoulder disarticulations (Fig. 14) can be
molded successfully to provide a good base for attachment of the prosthesis.
Similarly, plastics of a different character and with other molding methods
produce the flexible artificial gloves which cover the active hand to provide
With regard to elbow and wrist
articulation, basic research had indicated the desirability of certain ranges of
arm motions. To provide the necessary mobility, multipositioning
elbows and wrists have been devised. The use of ratchet mechanisms, friction
clutches, and alternator devices enable the above-elbow amputee to position the
forearm by voluntary control through the shoulder harness. Wrist units have been
designed both for positioning the terminal device in flexion and rotation and
for quick disengagement of the terminal device.
New Hand Substitutes
The improvements effected by sound
engineering approach are particularly evident in the terminal device (Fig. 15).
Since control resides in the shoulder muscles, it appears logical that voluntary
control should be available for closing the fingers rather than for opening the
device. Such an arrangement, characteristic both of the APRL hook and of the
APRL hand, permits some measure of control of the force
applied. An alternator mechanism provides for alternate opening and closing of
the fingers, locks the fingers in the closed position with the desired pressure,
and thus relieves stress on the shoulder muscles while an object is held. The
extent of opening of the fingers can be set in either of two positions,
depending upon the particular operation being performed, and in repetitive
operations the lock can be eliminated, thus reducing the amount of work to be
done by the shoulder muscles. The development of these voluntary-closing devices
has, moreover, permitted the more successful fitting of cineplasty cases.
For other situations, where an amputee
may prefer a voluntary-opening hook, the Northrop two-load hook is
available. Using springs rather than elastic bands, it permits the
fingers to close with either one of two available spring loads. The hook fingers
of this terminal device as well as of the APRL hook were shaped in accordance
with the findings of basic research into the frequency of hand prehension
The whole technique of harnessing has
undergone extensive revision as a result of applied engineering principles. One feature concerns the fact that the power available at the
shoulder should be transmitted to the terminal device with a minimum of loss,
that is, with maximum efficiency. Replacing the older leather thongs is the
Bowden cable adapted from the aircraft industry. The cable is attached to the
harness, directed along the arm by an appropriate number of suitably located
cable-housing retainers, and ends at the terminal device. In this circuitous
path are friction losses owing to passage of the cable through its housing,
especially at points of flexion around joints. Proper
selection of points of load application, however, and judicious design of
various components make it possible to reduce frictional losses to a minimum
(Fig. 14 and Fig. 16).
The successful harnessing of cineplasty
cases requires the intelligent use of applied mechanics and biomechanics. The terminal device and the control system by which it is operated
must be adapted both to the end-uses desired by the amputee and to the
physiological characteristics of his muscle motor.
External Power Sources
A more or less radical departure in the
design of upper-extremity prostheses has been the application of engineering
science to the utilization of external power sources for activation of arms and
terminal devices. Although pneumatic and hydraulic applications have been
attended with little success, the development of miniature, compact, and
powerful electrical components has made it possible to develop an electrically
actuated arm. Elbow flexion, wrist
rotation, and prehension can all be operated electrically, but thus far it has
not been possible to develop completely suitable methods of control. The
individual components, such as the electric elbow lock, may, nevertheless, have
useful application in more conventional arms. Study of such
possible applications is now under way. There can be little doubt that, in some
future study, with even newer materials and more advanced methods, externally
powered arms, discretely controlled and respondent to the will of the amputee,
may be developed.
Techniques of Evaluation
The real merit of a prosthesis cannot be
judged solely on the basis of mechanical and cosmetic elegance of the design or
by the number of functions it incorporates. It can be evaluated in true
perspective only when it is fitted to the amputee and when his over-all
performance with and acceptance of the device is appraised. In the Artificial
Limb Program, the Prosthetic Devices Study, Research Division, College of
Engineering, New York University, has been charged with the evaluation of
prosthetic devices. To conduct this work, the roster of personnel includes
physicians, psychologists, physiologists, therapists, and engineers, and the evaluations
consider both the subjective and objective aspects of the biomechanical
Although in much of ordinary engineering
practice the objective evaluation of a mechanism is the only valid criterion, in
prosthetics practice, because of the close relationship between the human and
mechanical elements, the importance of subjective evaluations cannot be
discounted. As has been demonstrated repeatedly, what appears to be
a very distinct and sound advance in a prosthesis may not in fact be acceptable
to the amputee. A proper understanding of the attitudes of amputees, how they
are affected by their own experience and by the characteristics of a device, and
how these factors can be translated into the design is altogether necessary. The
psychologist therefore has an important role in the evaluation process. So, too,
the therapist, trained to observe human performance, and with a knowledge of the
physiology and function of the human organism, can render a sound opinion with
respect to the relative merits of various amputee-prosthesis
But these methods of evaluation are
subject to all the limitations of personal judgment. The experience and acuity
of the particular observer, the relationship between the observer and the amputee, and other
individual factors will in some way affect the evaluation. To a certain extent
these variables are controlled by a comparison and correlation of judgments of
different observers, but even under the most favorable conditions there may
always be areas of disagreement as to what has been observed.
When positive criteria of performance
with a prosthetic device can be established, it becomes very important to be
able to measure and record accurately those factors which constitute the
criteria. Instrumentation and methods developed on the basis of engineering
knowledge provide the tools for obtaining objective data. They enable the
investigator to compare the performance of a particular amputee with different
prostheses, of a given amputee with the same prosthesis at different times, or
of different amputees wearing identical prostheses. The recording instruments
and techniques available can record more rapidly, more accurately, and more
permanently than can any human observer. All the devices useful in the basic
research program are equally useful in the evaluation program.
The Lower Extremity
Symmetry in the Walking
In establishing criteria for the
evaluation of lower-extremity prostheses, it has been postulated that the
pattern of normal locomotion is symmetrical and, therefore, that the behavior of
the normal side may be the legitimate measure of performance of the affected
side. That is to say, the more nearly the amputee achieves a symmetrical pattern
of locomotion the better the prosthetic device and the better the adjustment to
it. Further, it is assumed that, in the performance of activity, the human
organism adjusts itself to perform at a minimal level of stress. The measure of
performance of normals, then, can be a guide to the relative merits of
amputee-prosthesis combinations. Such criteria as stability in the erect position, variability of stride
time, and other biomechanical factors may be used as indices of performance.
Lacking proper instrumentation, no objective evaluations of this character could
The investigations of Hettinger and
Muller indicate that the walking cadence favored by a normal human
being is usually that which requires the minimum expenditure of energy.
Deviations from this optimum cadence require increasing amounts of energy.
Psychologists indicate that, in a repetitive operation which may be performed at
varying tempos, the average person will perform the operation with least
deviation at some one tempo best suited to him. On the strength of these two
premises, the variations in stride time at different cadences were recorded and
curves plotted (Fig. 17). The assumption is made that the nearer the curve of
the affected leg approaches that of the normal leg, and the nearer the two
curves approach those of a normal subject, the better the prosthetic device.
Such data can be taken with the tachograph (Fig. 18), force plates, and
Fig. 19 represents a typical plot of
vertical load versus time during ground
contact from heel contact to toe push-off. By means of stick diagrams and
force-plate records, this over-all curve may be resolved into one for
heel-contact impact and another for toe push-off momentum. When the separation
is correct, the area C should be equal to the area D. Used in conjunction
with other criteria, these curves give useful information regarding the effect
of a prosthesis on the amputee's gait.
Marey and Demeny determined that the time of double support in the walking cycle is inversely
proportional to cadence. The NYU studies indicate that it is also related to the
ratio of swing-phase time to stance-phase time r and that, moreover, at
optimum cadence the stance-phase time in normals is approximately twice the
swing-phase time. A criterion was established that, given the relationship between double-support
time and cadence, plotted against a family of curves for varying ratios of
swing-phase time to stance-phase time, that amputee-prosthesis combination was
best which enabled the amputee group more nearly to approach the normal
Fig. 20 shows the average trend line
for a group of normals and for a group of above-and below-knee amputees. From
the equation indicated, a series of hyperbolas may be plotted for varying values
of r. The observed double-support times for normals, for below-knee
amputees, and for above-knee amputees at three different speeds were plotted,
and straight lines were fitted to these observed points. A line for double-support time
crosses each of the hyperbolas at two points. The mean abscissa of these points
indicates optimum cadence. Since a deviation from this optimum causes an
increase in energy consumption, the increase in the value of r can be
used as an indicator of higher energy requirement. The validity of this
criterion appears to be borne out, since the below-knee group, having more of
their natural limbs, more nearly approach the normals. Again, such data can be
obtained only because adequate instrumentation, force plates, tachograph, and
camera are available.
Stability in the erect position is used
as another criterion. The normal individual keeps himself erect by
the interaction of muscle and skeletal groups responding to sensory cues. In the
amputee some of the normal cues have been destroyed and new ones, such as
pressure on the stump, or pain, have been introduced. Besides this, the amputee
has fewer muscle groups available with which
to compensate for the effect of external forces tending to throw him off
balance. Because the human anatomical structure is not truly rigid, the
equilibrium of a normal erect subject will be disturbed by a force of lower
magnitude than that which will unbalance a rigid body of the same general mass
distribution and with the same general support base (Fig. 21). Since the amputee
cannot compensate for the effect of unbalancing forces as readily as can a
normal, and since in fact poor alignment or fit of the prosthesis may exaggerate
the unbalancing effect, the measure of stability is highly important.
Three methods are used for obtaining
information on stability. In one, the subject is placed in a known position on
one force plate and the center of the base of support on the force plate is
determined geometrically. The extent and frequency of deviation in the sagittal
and transverse planes are recorded simultaneously (Fig. 22). Mean values of
recorded oscillations determine the location of
the center of pressure, which at the same time is also the projection of the
center of gravity on the force plate. Distances measured from the center of
pressure of the axis of each foot give an indication as to how the body weight
is distributed between the two legs.
Since the reduction of force-plate data
alone is not sufficient for the purpose of determining stability constants, a
simple device, the stability platform shown in Fig. 23, has been fabricated
for imposing upon a subject known accelerations and recording that
one at which he is unbalanced. The support
base is known, the center of mass of the subject vertically above the platform
can be established, the acceleration when the platform is suddenly released can
be controlled by the known weights in the suspended basket, and thus it can be
determined at what acceleration the subject is unbalanced. Stability trapezoids
for normals and for above- and below-knee amputees (Fig. 24) have been prepared
on the basis of available data. It will be noted that thus far only four
positions have been recorded - accelerations tending to unbalance the subject in
the forward, rearward, right, and left directions. No positions along
intermediate axes have been studied, but it seems likely that, if more positions
were measured, the envelope would assume some oval shape. This criterion too
seems validated by results, since, although there are differences between
individual amputees as well as between normals, as a group the below-knee
amputees more nearly approach the normal group.
Another simple device which has been used
to corroborate acceleration data is the inclined platform. A kymograph records
the increasing angle of tilt, and the recording is interrupted when the subject
Standardization of Fit and
It is not amiss at this point to mention
two devices, developed at the University of California, which are indispensable
in the evaluation procedures. The alignment devices for above- and below-knee
prostheses and the transfer jig are tools useful in assuring that
different prostheses on the same amputee are alike in physical dimensions and
positioning, and they make it possible to measure the effects of known changes
in position or alignment in the same prosthesis. A third device, developed at
the Prosthetic Testing and Development Laboratory of the Veterans
Administration, makes it possible to duplicate sockets, a matter of importance
when shanks requiring different sockets are needed. The internal contours of the
socket can be maintained and their effect on changes in performance thus
Measurement of Force
Engineering knowledge makes it
possible also to study special characteristics of
a device or of a method of fitting. In evaluating the relative merits of the
"soft" and hard sockets for below-knee amputees, three new techniques have been
evolved. It is desirable to observe changes which occur in the stump as a result
of wearing the socket. Accordingly, there has been devised a jig which will hold
the amputee in a given position while an impression or cast is made of his
stump. Since a rigid pattern of posture is thus imposed, the impression or cast
reflects only physiological changes over a period of time. The contours of the
stump are then obtained by using a contour tracer or perigraph, also developed
for this special purpose. Small variations in contours at known levels can be
recorded and compared.
The second technique involves the use of
the capacitance gauges previously described. In a study at New York University,
in cooperation with the Prosthetic and Sensory Aids Service of the Veterans
Administration, they have been applied in an attempt to answer once and for all
the question among limb-makers as to the proper distribution of forces within a
below-knee socket. Several gauges are attached at points of particular interest
on the stump of a below-knee amputee (Fig. 25). The subject then walks at
different speeds for a distance of 30 to 40 feet while the unbalance of the
gauge bridges is recorded. In this way, simultaneous indications of pressure are
obtained at six points on the stump. Although it is still too early to make a
general statement, it is evident that great differences exist in the forces
exerted by the stump on the socket wall at different points. A composite record
of the forces involved during a single stride (Fig. 26) shows the relative
magnitudes of forces at a number of points. The maximum observed pressure was 65
lb. per sq. in. at the relatively insensitive patellar tendon. Eventually it is
intended to map the total stump contact area for pressure distribution during
different phases of the walking cycle.
In addition to the research applications
of the pressure gauge, it is likely to find use in the routine fitting of
sockets. For this purpose, gauges would be attached to the stump at critical
points, such as weight-bearing areas, sore spots, or relieved areas, when a new
socket were tried on. A meter reading would give
the magnitude of the pressure at the points in question and would tell
objectively whether the pressure were excessively concentrated or well
distributed when the subject stood or walked.
The third technique specially
developed makes use of the strain gauge also described previously. By means of
this instrument it has been possible to attack the problem of determining the
relative distribution of body weight between the sidebars and the socket of the
below-knee amputee. In the experimental procedure developed, modified sidebars
(Fig. 27) are substituted for the original ones of the test subject. So
constructed that the subject's gait is unaffected by the substitution, these
modified sidebars permit the mounting of the strain gauges so as to simplify
determination of axial and bending strains. In the test procedure, wires are run
from the gauges on the bars to a recording oscillograph by means of an
eight-conductor cable. Stick diagrams and force-plate records are taken
simultaneously with the recording of the dynamic sidebar strains (Fig. 28).
Thus, at any particular instant, the position of the leg in space,
the forces it exerts on the ground, and the strains in the sidebars all are
known. From the knowledge of the axial sidebar loads, plus some logical
assumptions and some simple kinematic relationships, the components of socket
load along the axis of the shank and normal to the shank axis can be found. At
the present time, runs have been made on two test subjects, one unilateral and
one bilateral, both wearing conventional wooden sockets.
The Upper Extremity
Engineering techniques have been employed
in the evolution of upper-extremity prostheses also, though not to the same
extent. The refinements in lower-extremity prostheses are such as to require
discrete, fine, and rapid measurements, while those in the upper extremity are
comparatively gross and subject, in many cases, to visual observation and
judgment. Moreover, the increased performance with the newer arms and terminal
devices can be appreciated quite readily by both the amputee and the observer. In the upper
extremity, therefore, the employment of measuring devices is required only in
those special situations where human observations fail.
The efficiency of an upper-extremity
control system, from the point of load application at the harness to the point
of pressure applied by the terminal device, cannot be obtained other than with
measuring instruments. For such measurement, the strain gauge, applied to
appropriately designed devices, can be used to measure the pressure at the tips
of the fingers or the force applied at any point along the cable of an actuating
system. In the course of some of the NYU studies, a channel-shaped structural
element was designed in such a way that it could be inserted as a link in the
cable system at different points along the cable. Tension in the cable causes deflection in
the elements, and the extent of deflection is recorded as a change in voltage
through strain gauges cemented to the crossbar of the channel.
A similar principle has been used for
measuring hook-finger pressures. Elements resembling tuning forks were designed,
the beams being so shaped as to accommodate different grasps. Strain gauges
cemented to the crossbar measure the bending stress in the fork, the stress
being proportional to the pressure applied by the amputee at the tips of the
hook fingers. With knowledge of the linkages involved in the system, it is
possible to determine what harness combination is most efficient.
At the Army Prosthetics Research
Laboratory, a "grip" meter has been developed for the purpose of measuring normal grips and
the grips that can be achieved by amputees with artificial hands. The grip is
resisted by a spring calibrated to be read directly on a dial
Range of Stump Motion
During the course of development of the
electric arm, an unusual instrument was developed by Alderson to
measure the range of motion of the various muscle groups which later were to
actuate the controls of the electric arm. The simul"arm"ator permits the
designer and fitter to estimate the range of control available to the amputee in
the various muscle groups - biceps, triceps, pectoral, etc., and to allow for this range in designing
the control switches of the prostheses.
The Future in Prosthetics
As more and more improvements are
incorporated into upper- and lower-extremity prostheses, the relative merit of
one prosthesis as compared to another will become more and more difficult to
evaluate without appropriate instrumentation and recording. The development of
recording and measuring devices must therefore keep pace with the combinations
to be evaluated. Hence the engineer must continue to function in his role in the
evaluation phase of the program.
The contributions of engineers and the
role of engineering in all stages of prosthetics design and application now have
been well established. But this turn of events could scarcely have materialized
without the cooperation of the Government. The program established by the U.S.
Congress, supervised by the Veterans Administration, and coordinated
by the Advisory Committee on Artificial Limbs of the National Research Council
assured a continuity of operations - of research, design, and evaluation - in which
engineers and engineering groups could become interested.
Theretofore engineers had been interested
in prosthetics in a desultory fashion only, and engineering principles had been
applied only to the extent that that knowledge was available to the individual
limbmaker concerned. Engineers have brought to the Artificial Limb Program a
curiosity as to the physical principles involved in human performance and an
appreciation of the scientific method in approaching the problems. They have
contributed their knowledge of measurement and of instrumentation to obtain
necessary data, they have translated the results into terms of new needs, and
they have applied their knowledge of materials and of mechanisms toward the
fulfillment of those needs.
It cannot be expected that the present
program, born of World War II and under the pressure of veterans' demands, will
continue indefinitely. And yet it may be anticipated that more and more amputees
will continue to need truly functional artificial limbs. Records indicate that
annually there arise from disease and other natural causes - industrial and
traffic accidents and accidents in the home - many times more amputees than were
produced in all Service-connected activities throughout World War II. And these
include the weak and the old and the very young, not alone the average, healthy
male represented by the veteran amputee. As in all science, the problems which
yet require solution are much more numerous than are those already solved.
Programs must therefore be established which will be broad enough in scope and
long enough in duration to attract engineers. The limb industry must continue to
upgrade itself, to create the positions which require engineering skills, and to
offer commensurate rewards. Rehabilitation agencies and all those groups
interested in the welfare of the disabled should consider how the role of the
engineer and of the physical scientist can be integrated into their
As an alternative it has been suggested
that a cross-discipline should be evolved, with courses of instruction available
to the engineer, the physician, and the rehabilitation specialist to enable each
to understand each other's problems. Such a curriculum in biotechnology could
offer the engineer instruction in physiology and psychophysiology useful
as well in applications other than prosthetics. It could offer the physician and
rehabilitation specialist instruction in the physical sciences, instrumentation,
and measurement. For such an integrated course of instruction there are already
precedents. Physicians have studied engineering for a better understanding of
orthopedics. Engineers have studied the physiology of human activity to develop
better operational methods in industry. In Europe, particularly in Germany,
Russia, and the Scandinavian countries, a whole new science of "work physiology"
or "work science" is being developed. In England the Ergonomics Society brings
together physiologists, psychologists, and physical scientists interested in the
problems of human performance, and their contributions are having effect on the
design of equipment and operational processes. A scientist from whatever field,
trained in biomechanics, can bring to a prosthetics program a much greater
appreciation of the problems to be solved. He will be better equipped to
evaluate the solutions that will be offered. But it seems inevitable that the
solutions in their final development will be offered only by the
In the preparation of this article a
number of people were exceptionally helpful. Special mention needs to be made of
Rudolf Drillis, of the Prosthetic Devices Study, New York University, who
provided much of the raw data and who was of particular assistance in review and
discussion of the technical aspects of the material. Martin Koenig and Seymour
Kaplan, both also of the staff of PDS-NYU, supplied the sections on capacitors
and on be-low-knee sidebars, respectively. Various other members of the PDS-NYU
staff read critically several sections of the manuscript. The Prosthetic Testing
and Development Laboratory of the U.S. Veterans Administration supplied a number
of the photographs, and George Rybczynski worked up all of the line drawings
from rough sketches. To all these, and to others not mentioned specifically,
sincere thanks are extended.
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- Parmelee, Dubois D., U.S. Patent 37,637, February10, 1863, and reissue patents 1,907 and 1,908, March 4, 1865.
- Public Law 729, Eightieth Congress, Second Session,Approved June 19, 1948.
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