The Biomechanics of Control in Upper-Extremity Prostheses
Craig L. Taylor, Ph.D. *
In the rehabilitation of the
upper-extremity amputee, structural replacement by prosthetic arm and hand is an
obvious requirement, and it poses a comparatively easy task; functional
replacement by remote control and by substitute mechanical apparatus is more
elusive and hence infinitely harder. For the purposes of functional utility,
remaining movements of upper arm, shoulder, and torso must be harnessed, and use
must be made of a variety of mechanical devices which amplify remaining
resources by alternators, springs, locks, and switching arrangements. The
facility of control attained through this apparatus is the key to its ultimate
value.
The future of upper-extremity prosthetics
depends upon an ever-increasing understanding of the mechanics of the human body
by all who minister to the amputee-prosthetist, surgeon, and therapist alike. It
must always be stressed that the final goal is an amputee who can function. Too
often there is a tendency to put undue faith in the marvels of mechanism alone,
when in fact it is the man-machine combination that determines performance. It
is in this broad frame of reference that the biomechanical basis of
upper-extremity control must be approached.
Prosthetics Anthropometry
Surface Landmarks
If successful control is to be obtained,
the various components of the prosthesis must be positioned with a good degree
of accuracy.
To do so requires reference points on the
body, of which the most satisfactory are certain bony landmarks. Most of these
skeletal prominences protrude to such an extent that location is easily possible
by eye. Others require palpation, and this method should be used to verify
observation in every case. The bones most concerned in upper-extremity
anthropometry are the clavicle, the scapula, the humerus, the ulna, and the
seventh cervical vertebra. Surface indications of protuberances, angles, or
other features of these bones constitute the landmarks, the locations and
definitions being given in Fig. 1.
Arm and Trunk Measurements
The typical male torso and upper
extremity are shown in Fig. 2, which, together with Table 1., was derived from average measurements on Army personnel. Such an average form serves
to establish harness patterns and control paths. The arm, forearm, and
epicondyle-thumb lengths constitute the basis of sizing prostheses. (In everyday language the
word "arm" is of course taken to mean the entire upper extremity, or at least
that portion between shoulder and wrist. In anatomical terms, "arm" is reserved
specifically for the segment between shoulder and elbow, that between elbow and
wrist being the "forearm." Although in the lower extremity the word "leg"
commonly means the entire lower limb, whereas anatomically the "leg" is that
segment between knee and ankle, confusion is easily avoided because we have the
special word "shank." No such spare word is available to describe the humeral
segment of the upper limb.-Ed). Arm length places the artificial elbow; forearm length locates the
terminal device. The epicondyle-thumb length is an important over-all sizing
reference because in the unilateral arm amputee it is customary to match hook
length (and, in the case of the artificial hand, thumb length) to the length of
the natural thumb (Fig. 3).The bilateral arm amputee can be sized from body
height by means of the Carlyle formulas, which employ factors derived
from average body proportions.
Functional Anatomy
The human torso, shoulder, and upper
extremity are exceedingly complex structures. In any dealing with these elements
of anatomy, therefore, it is desirable to sort out from the mass of detail those
features important to the particular area of study and application. Where
prosthetic controls are concerned, the mechanism of movement is the central
subject of consideration. This functional anatomy treats of the aspects of bone,
joint, and muscle structure that together determine the modes and ranges of
motion of the parts. It is a descriptive science, and while to escape dependence
upon nomenclature is therefore impossible, the purpose here is to convey a basic
understanding of the operation of the upper-extremity mechanisms without undue
use of specialized terminology. In any case, the reader should have available
basic anatomical references such as Gray's Anatomy or kinesiology
texts such as those of Steindler and of Hollinshead.
Elementary Motions of the Upper
Extremity
The geometry of each joint is complex,
and most movements involve an interaction of two or more joints. Consequently, a motion
nomenclature based on joint movements would be unnecessarily complicated. More
simply, the motion of each part upon its proximal joint may be described with
respect to the principal planes which intersect at that joint. In this system,
moreover, one may define a standard position in which the trunk is erect, the
arms hang with their axes vertical, the elbows are flexed to 90 deg., and the
wrist planes are vertical to assume the "shake-hands" position. Fig. 4
presents the angular movements possible in the three planes of space. The
shoulder-on-chest, arm-on-shoulder, and hand-on-wrist actions take place through
two angles, as if moving about a universal joint. Geometrically, the arm motions
are more precisely defined by a spherical coordinate system where the segment
position is given by longitude and colatitude angles. For descriptive
purposes, however, the anatomical nomenclature is commonly used. It should be
recognized that, for multiaxial joints, flexion-extension and
elevation-depression angles describe motions in the major orthogonal planes
only, and intermediate angular excursions must be thought of as combinations of
these motions.
The simplified movement system depicted
in Fig. 4 is incomplete in many ways. Not included are such movements as
twisting of the shoulder due to various scapular movements, anterior-posterior
swings of the arm in positions of partial elevation, and the slightly conical
surface of revolution of forearm flexion.(It deserves to be noted here
that, taken literally, expressions such as "forearm flexion-extension," "arm
flexion-extension," and "humeral flexion-extension" represent questionable
nomenclature. To "flex" means to "bend." Limb segments do not bend very
readily without breaking. Joints are designed
for flexion. In the lower extremity, for example, one speaks not of "shank
flexion" but of "knee flexion," not of "thigh flexion" but of "hip flexion."
That is, one uses "flexion" or "extension" not with reference to motion of the
distal segment but with reference to the more proximal joint. Although Webster
accepts the expression "to flex the arm," he obviously uses the word "arm" in
the everyday sense of meaning the entire upper extremity, or at least that
portion between shoulder and wrist. Because this loose terminology in the upper
extremity is so widely established, not only among workers in prosthetics, it is
used throughout this issue of Artificial Limbs, with the understanding that
"forearm flexion" means "elbow flexion," "arm flexion" and "humeral flexion"
mean "flexion of the glenohumeral joint (and associated structures) " See page 9
et seq.-Ed.). These details may,
however, be ignored in the interest of the simplicity
of description that is adequate for the purposes of upper-extremity
prosthetics.
The Shoulder Girdle
Skeletal Members and
Joints
The scapula and clavicle are the chief
bones making up the shoulder girdle. Secondarily, the proximal portion of the
humerus may be included, since the close interarticulation of all three bones at
the shoulder joint gives a considerable degree of coordinated activity among
them and also extends to the complex as a whole the actions of many of the
muscles inserting on the individual members.
Details of the skeletal anatomy involved
are shown in Fig. 5. There are in the system two joints and one pseudo joint.
In the sternoclavicular joint, the clavicle articulates with the sternum in a
somewhat saddle-shaped juncture recessed in a concavity within the sternum. The
biaxial surfaces permit movements in two planes. Ligaments crossing the joint
prevent displacement of the clavicle anteriorly and laterally. The
elevation-depression range is 50 to 60 deg., the flexion-extension range from 25
to 35 deg.
In the acromioclavicular joint, the
distal end of the clavicle articulates with the scapula in an elliptical
juncture which permits a ball-and-socket type of action. The acromioclavicular
ligaments bind the joint directly. Strong ligaments from the clavicle to the
coracoid process give important additional stabilization. The range of movement
is small, being only about 10 deg. in the frontal and sagittal
planes.
The pseudo joint, the scapulothoracic, is
a muscular suspension which holds the scapula against the thoracic wall but
which at the same time permits translatory and rotatory movements. A large
factor in maintaining this joint in position is barometric pressure, which is
estimated to act upon it with a force of 170 lb.
Muscles and Movements
The complex arrangement of bony elements
is rivaled by the involved nature of the muscles of the shoulder girdle and by
the intricate ways in which they act upon it. The schematic view of Fig. 6
presents the fundamentals. Elevation of the shoulder is seen to be brought about
principally by elevators and downward rotators of the scapula, such as the upper
trapezius, the levator scapulae, and the rhomboids. Although the rhomboids
assist in elevation, they do not contribute to upward rotation. Depression of
the shoulder is mediated by muscles inserted on the scapula, the
clavicle, and the proximal end of the
humerus. Anteriorly the lower fibers of the pectoralis major, the pectoralis
minor, and the sub-clavius, and posteriorly the lower trapezius and latissimus,
act as depressors.
Rotation of the scapula upward (i.e.,
right scapula, viewed from the rear, rotates counterclockwise) or downward
(i.e., right scapula, viewed from the rear, rotates clockwise) is brought
about by a special combination of the elevators and depressors. As shown in
Fig. 6, two portions of the trapezius, together with the serratus, cause
upward rotation. Conversely, the pectorals, the latissimus, and the rhomboids
cooperate to cause downward rotation. As will be seen later (page 13), the
mechanical principle of the couple applies in these rotatory actions upon the
scapula.
Flexion and extension of the shoulder
involve as principal elements the abduction and adduction, respectively, of the
scapula. The flexor muscles acting on the shoulder complex are the pectoralis
major and minor, which swing the clavicle and acromion forward. The serratus
anterior aids strongly by abducting the scapula. The extensors, placed
posteriorly, include the latissimus, which pulls posteriorly and medially on the
humerus, and the trapezius and rhomboids, which pull medially on the
scapula.
The forward and backward shrugging of the
shoulders with abduction and adduction, together with some upward and downward
rotation of the scapulae, constitutes a major control source. Even in
above-elbow amputees who use humeral flexion for forearm lift and for
terminal-device operation at low elbow angles (page 22), scapular abduction is
utilized for terminal-device operation at large angles of elbow flexion
(e.g., when the terminal device is near the mouth). In shoulder amputees,
both these operations depend wholly upon scapular abduction augmented by upward
rotation.
The Arm
The Humerus and the Glenohumeral
Joint
The humerus, together with its joint at
the shoulder, comprises the skeletal machinery of the arm. As noted in Fig. 4,
it is capable of flexion-extension, elevation-depression, and rotation upon its
proximal joint. The glenoid cavity, a lateral process on the scapula, receives
the spherical surface of the humeral head. The glenohumeral articulation is
therefore of true ball-and-socket character. The fibrous joint capsule is
remarkable in that it envelops the humeral head and the glenoid margins in
complete but rather loose fashion, so that a wide range of movement is possible.
To some extent barometric pressure, but to larger extent the musculature
spanning the joint, is responsible for keeping the articular surfaces together
in all angular positions. A group of muscles including the subscapularis, the
supraspinatus, and the infraspinatus function principally in this holding
action.
Muscles and Movements
The kinesiology of the arm is closely
associated with that of the shoulder girdle, nearly all natural movements
involving a coordinated movement between arm and shoulder. It is helpful,
however, first to describe the pure movements of the arm. Schematics of the
muscles acting upon the arm are presented in Fig. 7. Elevation is effected by
the lateral deltoid and the supraspinatus, depression by the latissimus, the
pectoralis major, the long head of the triceps, and the teres major. In both
actions, the contributions of individual muscles differ according to the angle of
the arm. And it should be noted that, with insertions near the pivot point of
the humeral head, the rotatory moments are proportionately small, thus
accounting for the large number of muscles necessary to give adequate joint
torques. Arm flexion and extension are brought about by two groups of muscles.
The biceps, the coraco-brachialis, the anterior deltoid, and the clavicular
fibers of the pectoralis major mediate flexion, while the posterior deltoid, the
long head of the triceps, the latissimus, and the teres major effect extension.
Rotation of the arm depends upon muscles that insert on the surface of the
humerus and then pass anteriorly or posteriorly around it to impart medial or
lateral torsion. As would be expected, rotational forces are greatest when the
arm hangs at the side; torque is reduced drastically when the arm is elevated
over the head and the twisting angles of the muscles tend to
disappear.
Combined Arm and Shoulder
Movements
In most natural arm movements, such as
arm elevation, arm flexion, forward reaching, and to-and-fro swings of the
partially elevated arm, both arm and shoulder girdle participate. In full arm
elevation of 180 deg., for example, 120 deg. are contributed by rotation of the
arm on the glenohumeral joint, 60 deg. are contributed by upward rotation of the
scapula.In forward reaching, involving partial arm flexion, the
shoulder flexes and the scapula abducts and rotates slightly. Properly managed,
this motion, the common flexion control motion of both the above- and the
below-elbow amputee (pages 19-22) can give marked gracefulness to prosthetic
operation.
The Forearm
Skeletal Members
The radius and ulna together constitute a
forearm lever which can rotate about the elbow axis. By virtue of the
arrangement at the proximal head of the radius and at the distal end of the
ulna, the forearm can also carry out torsion about its longitudinal axis to
produce wrist rotation. With the aid of the mobility at the shoulder and at the
wrist, it is possible to place the hand in space in an almost unlimited number
of positions. The skeletal anatomy of the elbow is shown in Fig. 8, the
articulations being the ulno-humeral and the radiohumeral. Participating in
forearm rotation is the radioulnar joint at the wrist.
The ulnohumeral joint has an unusual
structure. The complex surfaces of articulation between ulna and humerus are
such that the axis of rotation of the forearm is not normal to the long axis of
the humerus. As the elbow is flexed or extended, therefore, the forearm does not
describe a plane. Instead, the ulna swings laterally as the elbow is extended,
until at full extension the cubital angle is about 170 deg. Xevertheless, only
small error is involved in considering the motion to be essentially that of a
simple hinge with an axis of rotation perpendicular to ulna and humerus and
allowing the ulna to swing through about 140 deg. of flexion.
In the radiohumeral joint, the slightly
concave proximal end of the radius
articulates with the hemispherical capitulum placed somewhat laterally on the
anterior surface of the distal end of the humerus. The radius is free to move
with the ulna through the complete range of flexion and, in addition, to rotate
with forearm pronation and supination. In the radioulnar joint, the distal end
of the ulna forms a curved surface against which the radius opposes an
articulating concavity. As the forearm goes through a
pronation-supination range of about 170 deg., the radius "swings like a gate"
about the distal end of the ulna.
Muscles and Movements
As shown in Fig. 9, the musculature for
providing forearm flexion and extension is comparatively simple, while that for
pronation-supination is somewhat more involved. Flexion of the forearm is
effected principally by the biceps, originating on the scapula and inserting on
the radius, and by the brachialis, spanning the elbow from humerus to ulna.
Secondarily, the brachioradialis and other muscles, originating distally on the
humerus and coursing down the forearm, contribute to flexion. Extension is
largely the function of the triceps, originating on both the scapula and humerus
and inserting on the leverlike olecranon process of the ulna. A small extensor
action is added by the anconeus.
Rotation of the forearm is a function of
many muscles. Some, such as the supinator, evidently are designed for the
purpose, while others, as for example the finger flexors, have different
principal functions, the contribution to forearm rotation being only incidental.
Fig. 9 presents the major rotatory muscles only. Supination is mediated by the
brachioradialis, the supinator brevis, and the biceps, pronation by the
pronators quadratus and teres. Of great importance to
upper-extremity prosthetics is the fact that rotation of the forearm is a
function of total forearm length. With successively shorter stumps, not only are
the rotation limits of the radius and ulna reduced, but also the contributions
of muscles are eliminated as their insertions are sectioned.
Musculoskeletal Mechanisms
The upper extremity having been
considered from the standpoint of functional and descriptive anatomy, attention
may now be turned to a more mechanical view of its operations. Typical elements
of mechanism in the upper extremity include joints (bearing surfaces),
joint-lining secretions (lubricants), bones (levers and couple members), tendons
(transmission cables), and muscles (motors). The arrangement of these elements
makes up a complex machinery capable of such diverse activities as precise
orientation in space, performance of external work, fine digital manipulations,
and so on.
Typical Joint Mechanics
The elbow joint embodies the essential
structures of diarthrodial joints. The bearing surfaces are covered with a thin
layer of articular cartilage that is continuous with the synovial membrane
lining the whole joint capsule. Subsynovial pads of fat serve to fill up the
changing spaces that occur during movement of the joint (Fig. 10). It is
believed that these fatty deposits serve as "pad oilers" to maintain the
continuous film of synovial fluid over the articular surfaces. This
fluid contains mucin (a glycoprotein which serves as a lubricant for the joint)
and other material constituting a nutritional medium for the articular
cartilage. Considerable uncertainty exists concerning the method of formation
and distribution of the fluid to the joint, but its mechanical function is clear
and the normal joint performs as a well-oiled bearing.
Bones and Their Mechanical
Function
The bones of the upper extremity, besides
forming a support for soft tissue, provide a system of levers which makes the
arm an important mechanism for the performance of
gross work, such as lifting, slinging, and thrusting. The arm bones serve
further as positioners of the hand, in which other, finer bones constitute the
intricate articulated framework of the manipulative mechanism. Two main features
of bones merit discussion here-their internal composition and construction and
their external shape and adaptations that permit them to serve as members of
mechanical systems.
Internal Structure
There is much evidence that the gross
internal structure of bone is eminently suited to withstand the mechanical
stresses placed upon it by the compressive loads of weight-bearing, by the
tensions of tendons and ligaments, and by the lateral pressures of adjacent
tissues.The nature and orientation of the trabeculae in cancellous
bone have, for example, long been held, in theory, to provide the maximum
strength along the lines of major stresses. This idea, originally suggested by
von Meyer, has been championed by many, including Koch, who carried out a stress
analysis on the femur. Objections to the von Meyer theory have dealt largely with the
frequent and incautious extension of the concept. It is now believed that
genetic and growth factors determine the essential form and dimensions of bone.
Mechanical stresses serve secondarily to mold and modify it to give added
strength where stresses are greatest. One must grant from even a superficial
examination of the internal structure of bone that Nature has done an admirable
job of designing for maximum strength with minimum weight.
Members of Mechanical
Systems
The second principal feature of bones,
that of serving as rigid members in a complex of mechanical systems, is the one
that has engaged the most attention. It is surprising that the simple lever
concepts of Archimedes have persisted in anatomy and kinesiology texts to the
present day. Thus, the forearm-flexor system is said to act as a third-class
lever, the extensor system as a first-class lever. Although these assertions are
of course true, both of these systems are, in the more complete language of
Newtonian mechanics, parts of force-couple systems in which equal and opposite
components of force are transmitted through the bones and joints (Fig. 11).
Elft-man has emphasized this view. The magnitude of the couple is given by
the product of the force (either of the equal but opposite forces) and the
distance between them, which also is numerically equal to the torque of the
muscle force. The concept of the couple calls attention to the existence of the
equal and opposite forces in joints and emphasizes the loads placed upon them by
muscular work. Another and more complicated application of the couple is seen in
scapular rotation. Here, as described by Inman el al. and as shown
in Fig. 12, the pull of the lower fibers of the serratus anterior upon the
scapula is such as to give it upward rotation, while the thrust of the
clavicle, acting through the acromioclavicular joint, holds a pivot for the
rotation. Simultaneously, the pull of the upper trapezius fibers causes the
clavicle to undergo angular rotation about the sternoclavicular joint. The
result is that, at least through the first 90 deg. of arm elevation, the motion
is shared by coordinated angular rotations of scapula, clavicle, and humerus. As
a basic part of this rotatory action, the scapula acts as the moment arm of a
force couple, the trapezius and serratus providing components of force which are
equal and opposite.
Tendons and Muscles
The specific functions of tendons are to
concentrate the pull of a muscle within a small transverse area, to allow
muscles to act from a distance, and in some instances to transmit the pull of a
muscle through a changed pathway. The mechanical importance of this tissue is
nowhere more evident than in the arm, where a large degree of versatility of
motion in the segment distal to each joint is preserved by "remoting" the action
of muscles through slender, cablelike tendons over joints. By this means lines
of pull are brought near the joint axes, thus providing a lever arm consistent
with the tensile force of the muscle at all joint angles and also giving at low
joint angles an increased angular motion for a given linear contraction. Other
advantages of remoting the muscles are seen in the forearm and hand. In order to
afford the variety and complexity of interdigital movements, many
independent muscle units are necessary, and critical
space problems are avoided because muscles such as the common flexors and
extensors of the fingers are placed at some distance up the forearm.
The predominant function of tendon as a
tension member in series with muscle, which is a tension motor, is seen in early
growth stages. An undifferentiated cellular reticulum of connective tissue is
everywhere found in embryonic tissue. The parent cells are fibroblasts; they
elaborate and extrude the collagenous material of which white fibers are made.
At this point the presence of mechanical tensions in the tissue
influences the rate, amount, and direction of the resultant fiber formation. At
maturity the tendon is composed almost entirely of white collagen fibers,
closely packed in parallel bundles, to form a cablelike strand. It is contained
within a sheath which forms a loose covering lubricated continuously by a
mucinous fluid to reduce friction with surrounding tissues.
Mutual adjustment of the characteristics
of muscle and tendon is shown in many respects. The musculotendinous juncture
varies with the arrangement of the muscle fiber. It shows a simple series
arrangement for fusiform muscles like the biceps, or it comprises a distributed
attachment zone by continuation of the tendon into intramuscular septa where
pinni-form fibers may insert (Fig. 13). In some unexplained way the relative
lengths of muscle and associated tendon are so composed that the shortening
range of the muscle is that necessary to move the segment distal to the joint
through its maximum range. The capacity to adapt the ratio of muscle
length to tendon length has been demonstrated in an experiment in which the
pathway of the tibialis anterior tendon in the rabbit was shortened. The result
was that the tendon shortened while the muscle lengthened to regain the normal
joint range.
The relative strengths of muscle and of
tendon also show an approximate compatibility, the tensile strength of tendon,
measured at from 8700 to 18,000 lb. per sq. in., being greater than
that for muscle. Strength tests of excised muscle-tendon systems show that
failure commonly occurs in the belly of the muscle, or at the musculotendinous
juncture, or at the bone-tendon juncture, but never
exclusively in the tendon itself. Analysis of clinical cases indicates that
muscle is still the site of failure even when it is maximally tensed.
It is clear, then, that of the muscle-tendon combination the tendon is
normally always the stronger.
Forearm-Fexor Mechanics
The forearm-flexor system is well suited
to serve as an example of biomechanics because the bone-joint system comprises a
simple uniaxial hinge while the flexor muscles, though five in number, can be
reduced to a single equivalent muscle whose geometry and dynamics can be
specified from measurement data. Fig. 14 illustrates the lever system
on which the equivalent muscle acts. The
angle between the axis of the muscle and that of the forearm bones, i.e.,
the "angle of pull," theoretically ranges from 0 deg. at full extension to
90 deg. at 100 deg. of elbow angle, and since the moment arm is continuously
proportional to the sine of the angle of pull the mechanical advantage of the
lever also is proportional to it.
There are of course departures from this
idealized geometry. For one thing, the angle of pull and the elbow angle are not
exactly equal. Moreover, at small elbow angles the torque component does not
actually drop to zero because the muscles must always pass over the elbow joint
at some finite distance from its center. Finally, the force-length curve of the equivalent muscle must also be taken intoaccount in expressing the effective torque. For these and other reasons, actual torque measurements take
precedence over theoretical calculations, and the composite curve of Fig. 14
has been plotted from the results of a number of investigators. Whereas the
moment arm peaks at an elbow angle of 100 deg., the muscle force is declining
throughout the elbow-flexion range, and the net effect, as reported by Miller
, is a maximum torque of about 625 lb.-in. at from 80 to 90 deg.
Clarke and Bailey found a peak of about 400 lb.-in. at between 70 and 80
deg., and the author has obtained 550 lb.-in. just under 90 deg. in a group of
subjects. Wilkie's data give a value of about 525 lb.-in. at 80 deg., measured on himself. These variations can be explained as resulting from the effect of a limited
sampling of an inherently variable characteristic. Greater consistency probably
could be obtained in a larger series of measurements.
Maximum Torques in Major
Aactions
Because they express the fundamental
output characteristics, and because they are most easily measured, the muscle
torques about the major joints represent the most significant and practical
aspects of the statics and dynamics of the musculoskeletal system. Not only is
muscular power a concept of uncertain validity but also it is very difficult to
measure. The combined effect of muscle and lever, however, can easily be
measured in many subjects, so that statistical stability can be achieved in the
results. Because muscle agonists change length with joint angle, and because
they are thus caused to work on different parts of their length-tension
diagrams, joint torques vary as a function of joint angle. As demonstrated by
Clarke, this phenomenon, shown in Fig. 14 for the forearm-flexor system,
holds more or less for all major actions about the joints. But these details may
be neglected in summarizing the maximum torques throughout the upper-extremity
system (Table. 2).
The Functional Role of Sockets
The socket is the foundation of the
upper-extremity prosthesis. It obtains purchase upon the most distal segment of
the remaining member and should be stable, though comfortable, in its fit with
this member. The socket must bear weight both axially and in all lateral
directions. It is the attachment member for mechanical components and for
control guides and retainer points. Hence the socket must be a sound structural
member as well as a custom-fit, body-mating part. Finally, the socket extends
the control function of the member to which it is fitted, giving movement and
direction to the prosthesis. In any discussion of prosthetic controls,
therefore, the starting point is the socket.
The requirement of formability and
strength in sockets has been met satisfactorily by the introduction of polyester
laminates. These materials permit close matching of the stump
impression, and variations in strength can be introduced by increasing the
number of laminate layers. The double-wall construction provides a
stump-fitted inner wall, with an outer wall that can be designed to structural
uniformity and cosmetic requirement. Sizing to achieve this aim has now been
reduced to standard practice. Finally, the texture and coloring of
the plastic laminate can be controlled to achieve satisfactory cosmetic
results.
The Below-Elbow Socket
The peculiar feature of the forearm, that
pronation-supination is a function of the whole forearm length, places a special
limitation on the below-elbow socket. Although for stability in flexion the
whole remaining forearm stump is best sheathed in the socket, to do so prohibits
forearm rotation. In the case of the longer below-elbow stumps, therefore, some
sacrifice in stability can be afforded in the interest of retaining forearm
rotation. The proximal portion of the socket is fitted loosely to give freedom
for forearm rotation while the distal portion is fitted snugly to provide a
stable grip. Fig. 15 shows the amount of forearm rotation available at various
levels of the natural forearm and that remaining in below-elbow amputees of
various types. Because of torsion of the flesh, however, and because of slippage
between the skin and the socket, effective socket rotation is lost in stumps
which are only 50 percent of forearm length. The effective socket rotation
remaining in the wrist-disarticulation case is only about 90 deg.
Further adaptations of below-elbow
sockets to suit the functional requirements at the various levels are shown in
Fig. 16. In the long below-elbow stump, the elliptical cross-section of the
forearm near the wrist permits a "screw-driver" fit of the socket to yield
the maximum in rotational stability. With the
shorter stumps, the possibility of effective rotation is reduced and is lost
completely at about 50 percent of forearm length. At this level, the problem of
forearm rotation is outweighed by that of providing flexion stability.
Dependence upon a rigid or semirigid hinge system is necessary in the short
below-elbow stump, and finally, in the very short stump, effective forearm
flexion is so reduced that a split socket with step-up hinge becomes a
necessity.
The goal of below-elbow socket design is
to regain as completely as possible the control function of the forearm, which
includes (a) positioning of the hand by forearm flexion and (b)
hand rotation by means of pronation-supination. In the below-elbow
prosthesis, adequate forearm flexion is obtained rather easily; rotation is
limited to the potential available in the longer stumps. Manual wrist rotation,
of course, supplements the remaining natural rotation. In the below-elbow
prosthesis, then, control of the terminal device in space depends in fair
measure upon the role of the socket in preserving the residual flexion and
rotation of the below-elbow stump.
The Above-Elbow Socket
Unlike the below-elbow case, the
above-elbow stump presents no problem of diminishing rotation with diminishing
stump length because arm rotation is confined wholly to the gleno-humeral joint.
Socket design for the above-elbow case is therefore related principally to the
requirement of fitting the stump closely so that the humeral lever can be fully
effective in controlling the prosthesis. Fig. 17 shows the minor variations
corresponding to above-elbow type, including the elbow disarticulation. Sockets
for the latter must take account of the bulbous end of the stump. They must
provide snug fit around the epicondyle projections but maintain sufficient room
in the region just above, where the stump cross-section is reduced, to
permit insertion of the stump in the socket. In
both the elbow-disarticulation and the standard above-elbow cases, the upper
margin of the socket is terminated below the acromion for freedom of movement at
the shoulder. In the short above-elbow case, the socket is carried up over the
acromion to obtain additional stabilization and suspension from the shoulder, as required by the very
limited stump area. The control function of the above-elbow socket is twofold.
As in the below-elbow case, the socket extends the slump to the next more distal
joint and thus gives range and direction to this component upon which the
positioning of the still more distal segments depends. But in addition to this
feature, the above-elbow socket also has a power function. Through its
attachments to shoulders and torso, it provides the forces and displacements
needed to produce forearm flexion, terminal-device operation, and elbow lock. To
fulfill these functions, the socket must have stable purchase on the stump in
both flexion and extension. Hence, for elbow-disarticulation and above-elbow
types, the socket should continue to the axillary level; for short-above-elbow
amputees, it should come up over the acromion (Fig. 17). Finally, medial and
lateral rotation of the socket are necessary for further functional positioning.
Close fit and good suspension are required to give stability in these
actions.
The Shoulder Socket
In the range of amputation sites from
transection of the humeral neck to complete removal of the shoulder girdle, the
socket form changes from shoulder cap to thoracic saddle. As displayed in Fig. 18, the bearing area increases as the remaining shoulder elements are reduced;
similarly, the amount of "build-out" needed to preserve shoulder outline
increases with increasing amputation loss. With disarticulations and all more
extreme losses, sectional plates may be introduced at the axillary parasagittal
plane. This arrangement makes it possible to fabricate the prosthesis in two
sections, a matter of considerable advantage to the limbmaker, and it also
affords the functional advantage of a preposition swivel of the humeral section
upon the saddle section to simulate flexion-extension of the arm.
The functional aspects of the shoulder
socket are to some extent secondary to the structural; yet there are certain
definite functional ends to be served. Shoulder and scapular mobility in
elevation, flexion, and extension should be preserved to the highest possible
degree. In humeral-neck and shoulder-disarticulation cases, aid can be given to
the shrug control (biscapular abduction), and at least a small range of motion
can be given to the elbow, but of course no such function can be expected in
forequarter or partial-forequarter amputees.
Major Arm and Shoulder Controls
The common method of operation of
upper-extremity prostheses is by means of shoulder harness which provides
suspension and which also transmits force and excursion for control motions. In
this manner such operations as forearm flexion-extension, terminal-device
operation, and elbow lock are managed. Fig. 19 presents the essential features
of the major harness controls. In principle, each effective control must begin
with a point stabilized on shoulder or torso, pass
over a voluntarily movable shoulder or arm part, and thus provide relative
motions with respect to the origin. At the movable point, the control cable
enters the Bowden-type housing, which transmits the relative motion independent
of movements of the distal segments. Controls may be used singly or in
combination, depending upon the level of amputation, amputee preference, and
other practical considerations.
Besides the relative motions between
various segments of the human body, still another source of energy for operation
of upper-extremity prostheses can be made available by the surgical procedure
known as cineplasty, in which a skin-lined tunnel is fashioned in
the belly of a muscle group. In various experimental programs conducted both
here and abroad, muscle tunnels have been made in the forearm flexors, the
forearm extensors, the biceps, the triceps, and the pectoralis major.
Of all the various combinations tried,
the biceps tunnel in below-elbow amputees has proved to be the most successful.
Failure of other cineplasty systems has been due in some cases to inability of
designers to overcome the mechanical problems involved in harnessing the energy
thus provided and in other cases to the inherent properties of the particular
muscle group concerned. In the below-elbow case, use of the biceps tunnel
eliminates the need for shoulder harness and permits operation of the prosthesis with the stump in any
position. It has given excellent results in many instances and has been made
available to those beneficiaries of the Veterans Administration who can make
effective use of the procedure.Fig. 21
The cineplasty tunnel in the biceps of
the average male will provide sufficient force and excursion to operate modern
terminal devices-an average maximum force of 50 lb. and 1 1/2 in. of useful
excursion. It is not unusual for some individuals to be able to
build up the force available to a value in excess of 100 lb., but such a high
force normally is not required.
The Nature and Operation of Ccontrol
Systems
The Below-Elbow Single-Control
System
The single control for the below-elbow
amputee is powered by arm flexion to provide terminal-device operation. This
control motion, used by the above-elbow amputee also, depends upon a coordinated
flexion of the humerus and abduction of the scapula on the amputated side;
little shoulder activity is required on the sound side. It is substantially the
same motion as that used in normal unilateral reaching. The displacements of
humerus and scapula are additive, so that the resulting motion is quite natural.
With full Bowden-cable transmissions of power from arm cuff to forearm socket,
there is no influence of elbow angle, and the operation is mastered easily by
all amputees with stumps of 35 percent or more of normal forearm
length.
The Below-Elbow Dual-Control
System
(Although the terminology
commonly used to describe the several control systems could well afford to be
better systematized, it is adopted here because it is now so well established
throughout the field of prosthetics. One may think of "dual control" as
meaning that two control sources are involved in the provision of all necessary
functions, but according to convention it means that two functions, specifically
elbow flexion and terminal-device operation, are provided by a single control
source, the third function, elbow lock, if needed, being managed by an
additional control source. Yet "triple control" (page 22) in the accepted sense
means not that three functions are furnished by a single control source but that
three control sources are used to provide three functions, one for
each.-Ed.)
In harnessing below-elbow stumps shorter
than 35 percent of normal forearm length, it generally is necessary to use an
auxiliary type of lift to help the amputee flex the forearm. This procedure is
applicable to a split-socket type of prosthesis. It merely is an adaptation of
the above-elbow dual-control system (page 22) using a lever loop positioned on the
forearm section so that arm flexion may be utilized to assist in forearm lift.
The cable housing is split and assembled so that when the arm is flexed the
elbow will flex. The elbow hinge has no locking mechanism, the short below-elbow
stump being used to stabilize the forearm. Normally, sufficient torque is
available about the elbow axis to give adequate stability in all usable
ranges.
In prescribing for a new amputee with
this level of amputation, it might be advisable first to have the amputee try a
split-type prosthesis without the below-elbow dual-control system. If, at time
of initial checkout, the amputee cannot lift his forearm, or if he complains of
painful contact with his stump, then of course the dual system is indicated.
After the assist lift has been worn for some time, the remaining muscles of the
stump may have hypertrophied, in which case the amputee might be able to discard
the dual system and convert to the below-elbow single control.
The Below-Elbow Biceps-Cineplasty
System
Force and excursion provided by the
biceps muscle tunnel are harnessed by inserting into the tunnel a cylindrical
pin of a nontoxic material and attaching a cable to each end of the pin. As in
the other types of control systems, the Bowden-cable principle is employed to
maintain a constant effective distance between the source of energy and the
mechanism to be operated, regardless of relative motions occurring between body
segments. In order that conventional terminal devices may be employed, it is
necessary to join the two cables before attachment to the mechanism. Several
devices for making this coupling are available commercially.
Suspension of the socket is provided by
an arm cuff, which is attached to the socket by any of the various hinges
normally used in fabrication of below-elbow prostheses. The arm cuff is
fashioned in such a manner that forces tending to pull the prosthesis from the
stump are absorbed by the condyles of the elbow rather than by the muscle
tunnel.
The Above-Elbow Dual-Control
System
In above-elbow amputees, the humeral
stump furnishes the motive power for the three operations of the
prosthesis-flexion of the forearm, operation of the terminal device, and
management of the elbow lock. The first two operations are so linked
mechanically that a single control motion, arm flexion, produces either
terminal-device operation or forearm flexion, depending on whether the elbow is
locked or unlocked (Fig. 20). Although the control motion by arm flexion in the
above-elbow case is similar to that described for the below-elbow amputee, there
are several differences. Because the cable passes through a lever loop on the
forearm to give torque about the elbow, it is affected by elbow position. As the
forearm is flexed, arm-flexion excursion is used up, and the excursion needed to
operate the terminal device must come from scapular abduction (shrug), as in
shoulder cases. Typically, the above-elbow amputee manages a full range of free
forearm flexion by a normal arm-flexion movement. But in the elbow-angle range
of from 90 to 135 deg., with elbow locked for terminal-device operation, he must
call upon supplementary excursions from biscapular abduction. With the terminal
device at the mouth, practically all operation depends upon shoulder
shrug.
In the above-elbow dual-control system,
operation of the elbow lock depends upon humeral extension and associated
coordinations. When the forearm has been flexed to the position desired, the
elbow lock is engaged by the arm-extension movement. Skill is needed to maintain tension on the arm-flexion
cable so that the arm does not drop during the locking control motion.
Well-trained amputees elevate the arm moderately to compensate for the humeral
extension and thus maintain the elbow angle. The extension control motion is
complex. The humerus is simultaneously extended and elevated so that it moves
obliquely to the side. During this phase, the point of the shoulder must be
stabilized, or even moved forward, and the trapezius is bulged by downward
rotation of the scapula (Fig. 21).Fig. 22
The Above-Elbow Triple-Control
System
The triple-control system has been
devised to separate terminal-device operation from forearm lift. When the
dual-control system is used, the amputee must select, by the use of the elbow
lock, either terminal-device operation or forearm lifting. By separating forearm
flexion and terminal-device operation, the triple control makes it possible for
the terminal device to be controlled by an independent body motion. Although in
general an above-elbow amputee fitted with triple control has an elbow lock, a
few such cases are able to separate prehension from forearm flexion without use
of the lock.
A control cable from the terminal device
is so attached and positioned that biscapular abduction or merely shoulder shrug
will operate the terminal device through its full range of prehension. To lift
the forearm the amputee uses arm flexion. Elbow-lock operation is accomplished
in the same manner as in the dual-control system, that is, by arm
extension.
It is apparent that this arrangement will
work best with a comparatively stable socket and a relatively long above-elbow
stump. The chief advantage of the triple-control system is that at full forearm
flexion the terminal device may still be operated through its complete
range.
The Shoulder Dual-Control
System
In the absence of the humeral lever, the
shoulder becomes the major power source, biscapular abduction controlling both
forearm and terminal device in the dual-control system. The control path courses
horizontally across the scapulae, and either opposite-axilla loop or basic
chest-strap harness (page 46) captures the action satisfactorily. The
combination afforded by the dual principle also is illustrated in Fig. 20.
The shoulder amputee has a special
difficulty in obtaining the combination of full forearm flexion and
terminal-device operation because, unlike the above-elbow amputee, who can add
the excursions of humeral flexion and scapular abduction, he must obtain all
movement from biscapular abduction. Shoulder amputees with broad shoulders and
wide chests usually achieve this action satisfactorily; others must accept the
limitation of partial terminal-device operation at full forearm flexion.
Partial-shoulder and fore-quarter amputees must depend upon the sound shoulder
entirely, and in this case the action range of the terminal device typically is
limited to not more than 90 deg. of forearm flexion.
In shoulder amputees, operation of the
elbow lock must be managed by various special arrangements. The waist control,
utilizing shoulder elevation; the perineal strap, based on relative motion between shoulders and
pelvis; the nudge control, requiring either manual or chin operation; extreme
shoulder flexion on the sound side; and extension of the shoulder on the
amputated side complete the array of known feasible possibilities. It is evident
that with this class of amputees control motions will be slower and deliberately
sequential. They are therefore necessarily more noticeable and
awkward.
The Shoulder Triple-Control
System
The harness required for the
triple-control shoulder-disarticulation system consists of a chest strap for
forearm flexion, a waist strap to operate the elbow lock, and an
opposite-shoulder loop for prehension. The amputee must have excellent scapular
abduction and must be able to separate it from extreme opposite-shoulder shrug,
and he must have available good shoulder elevation on the amputated side. The
chief advantage of the triple control in the shoulder-disarticulation case is
identical to that of the triple control in the above-elbow case, namely, that
the terminal device may be operated fully in the vicinity of the mouth. To
operate the prosthesis from an extended position, the amputee first produces
biscapular abduction, thus raising the forearm. Then, with the forearm held in
place, he elevates the shoulder on the amputated side to lock the elbow. To
operate the terminal device, he then flexes the sound shoulder. Excursion for
terminal-device operation is thus unaffected by forearm flexion.
Unfortunately this system must be
restricted to humeral-neck and shoulder-disarticulation cases. For lack of
sufficient excursion on the amputated side, it is unlikely that a forequarter
amputee would be able to use triple control.
Mechanical Application of the Major
Controls
To elucidate practical amputee
biomechanics, it is necessary to refer to several aspects of the connecting
mechanism between amputee and prosthesis in the power-transmission system. Of
first importance are the proximal retainers, which are located at the point where the cable from the shoulder
harness enters the cable housing. These retainers are the beginning points of
the transmission systems indicated in Fig. 19. In both below- and above-elbow
cases, the proximal retainer is positioned in accordance with the ratios shown
in Fig. 22. For all above-elbow stumps of greater than 50 percent of
acromion-to-epicondyle length, the proximal retainer point is placed slightly
lower than half way down the arm, the reason being that the control passes
naturally through this point in its course from opposite shoulder, across the
scapula, and thence to the lever loop on the forearm shell. The humeral lever
power is quite adequate at this point (Table 3), and no practical
advantage is gained by a lower placement. With above-elbow stumps less than 50
percent as long as the normal arm length, acromion to epicondyle, the proximal
retainers must be placed at the level of the stump end in order to prevent undue
tipping of the socket, as would occur if forces developed beyond the end of the
stump.
In shoulder cases, the control path is
directed horizontally at approximately the midscapular level and brought to the
arm section at the axilla. The control motion is purely biscapular abduction,
and consequently the proximal retainer is placed on the prosthesis at the
midscapular level. The resulting force and excursion are given in Table 3.
Arm-extension forces are potentially
quite high, as also shown in Table 3. Because only 2 to 6 lb. of force and
1/2 in. of excursion are required to operate an elbow lock, normally
there is a generous power excess. The principal concern in harnessing
arm-extension control is to obtain operation with minimal movement and thus to
avoid awkwardness.
Conclusion
The central purpose of this article has
been to outline the biomechanical basis of control in upper-extremity
prostheses. Consequently, emphasis has been placed upon the normal and residual
functional anatomy and kinesiology underlying this service. The particularized
biomechanics of prosthesis control has been defined, and the limitations
incurred in amputations at high levels have been stressed. The major message is
that a thorough understanding of the motions of control available to each type
of patient is necessary to the proper prescription, fitting, and training of the
upper-extremity amputee. Thus only can full advantage be taken of the improved
functional features to be found in modern arm components.
References:
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- Carlyle, L. C, Using body measurements to determine proper lengths of artificial arms, Memorandum Report No. 15, Department of Engineering, University of California (Los Angeles), 1951.
- Carlyle, Lester, Fitting the artificial arm, Chapter 19 in Klopsteg and Wilson's Human limbs and their substitutes, McGraw-Hill, New York, 1954.
- Clark, W. E. Le Gros, The tissues of the body; an introduction to the study of anatomy, 3rd ed., Clarendon Press, Oxford, 1952.
- Clarke, H. Harrison, and Theodore L. Bailey,Strength curves for fourteen joint movements, J. Assoc. Phys. & Ment. Rehab., 4(2):12 (1950).
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- Inman, Verne T., and H. J. Ralston, The mechanics of voluntary muscle, Chapter 11 in Klopsteg and Wilson's Human limbs and their substitutes, McGraw-Hill, New York, 1954.
- Inman, V. T , J. B. deC M. Saunders, and L. C. Abbott, Observations on the function of the shoulder joint, J. Bone & Joint Surg., 26:1 (1944).
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- Miller, D. P., A mechanical analysis of certain lever muscles in man, Ph.D. dissertation, Graduate School, Yale University, New Haven, Conn., 1942.
- Newman, R. W., and R. M White, Reference anthropometry of Army men, Report No. 180, Quartermaster Climatic Research Laboratory, Lawrence, Mass., 1951.
- Steindler, Arthur, Kinesiology of the human body tinder normal and pathological conditions, Charles C Thomas, Springfield, Ill., 1955.
- Taylor, Craig L., The biomechanics of the normal and of the amputated upper extremity, Chapter 7 in Klopsteg and Wilson's Human limbs and their substitutes, McGraw-Hill, New York, 1954.
- Taylor, Craig L., Control design and prosthetic adaptations to biceps and pectoral cineplasly, Chapter 12 in Klopsteg and Wilson's Human limbs and their substitutes, McGraw-Hill, New York, 1954.
- University of California (Los Angeles), Department of Engineering, Manual of upper extremity prosthetics, R. Deane Aylesworth, ed., 1952.
- Unpublished data, UCLA.
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