Construction of the Patellar-Tendon-Bearing Below-Knee Prosthesis
Bryson Fleer *
A. Bennett Wilson, Jr. *
The first and most obvious requirement of
any below-knee prosthesis is to furnish a suitable extension of the stump to the
ground in such a way as to provide adequate support for the body weight with as
little involvement as possible of other parts of the residual anatomy. In the
interest of appearance as well as of function, there is a need secondarily for
some reasonably faithful simulation of the normal leg, otherwise known as the
"shank." Each of these requirements may be met in either of two ways. In one the
structural member may be endoskeletal (the pylon), in which case the skeletal
form may be covered with some suitable camouflage designed to give natural
appearance. In the other, the structural element may be exoskeletal
(crustacean), in which case the shell-like supporting member may itself be so
shaped as to provide the desired appearance of naturalness. In either case,
there is needed some acceptable means of attaching prosthesis to stump in
a way that will satisfy the additional
requirements of weight-bearing, comfort, and stability both in standing and in
the stance phase of walking. As has been found through several centuries of
observation and experiment, this is best accomplished by attaching the
prosthesis via the medium of a sleeve, or socket, so shaped and so fitted as to
accommodate prevailing features of local anatomy and physiology and into which
the stump may be inserted.
Of all the methods, and variations of
methods, that are available for the construction of sockets advantageously
fitted to the irregular surfaces of the below-knee stump, most fall into one or
another of three classes. One of these involves the forming, or
shaping, of materials (such as aluminum or other metals). A second involves the
negative carving, or excavation, of some suitable material (such as wood). And
the third involves the molding of some material (such as leather). Because the
hand-shaping of metals, like the hand-carving of wood, is at best difficult and
time-consuming, and also because the skill needed for doing either may be
developed only through long periods of apprenticeship, metals and wood have in
recent years both been on the decline as materials of choice in the fabrication
of sockets. Although the molded leather socket has persisted owing to its
comparative ease of fabrication, it too is being displaced because of
undesirable properties (such as its tendency to deform under load and its
inclination toward perspiration absorption and consequent odor). Profoundly
encouraging this transition has been the advent of plastics technology and the
introduction of plastic-laminating techniques into the field of limb
prosthetics. The lighter, cleaner, stronger sockets of plastic
laminate, much more easily made and with considerably more precision, have now
all but replaced other types of sockets in new fittings of below-knee
prostheses.
Fabrication of the plastic-laminate
below-knee socket involves the taking of a suitable impression (the negative cast) of the
particular stump concerned; the preparation of a positive model (male replica)
from the negative mold; modification of the model in such a fashion that in the
final socket (to be made from the rectified model) the weight of the body will
be distributed over the respective areas of the stump according to their
relative tolerance, or lack of tolerance, for weight-bearing; and, finally, the
layup, lamination, curing, and finishing of the plastic socket itself. Should
liners or other special features be wanted for particular cases, they are
incorporated in the layup, as will be seen later.
While the method of construction
described here is applicable in the fabrication of a variety of below-knee
sockets, it is intended more specifically for the construction of the plastic
below-knee socket in which the purpose is to utilize to fullest extent the
patellar ligament as one of the principal weight-bearing areas.
Construction of Socket and
Liner
Taking the Negative Cast
Unlike numerous other below-knee sockets
heretofore recommended, the socket for the patellar-tendon-bearing (PTB)
prosthesis is intended to remain at all times in intimate contact with the
entire surface of the below-knee stump. The stump is therefore contained firmly
in the socket throughout its length, and accordingly the cast is taken not while
the patient is bearing weight on the stump (as has sometimes been done in the
construction of certain "open-end" sockets) but while he is seated, relaxed, the
leg hanging naturally over the edge of the support (say a table), and the knee
flexed naturally about 30 deg. Whatever special effects are induced by the hands
of the operator as he takes the cast are intended not to produce a
"weight-bearing shape" but to emphasize the special points of weight-bearing to
be anticipated in a PTB socket.
Although of possible impression materials
there is available a substantial number, the most suitable, the least expensive,
and the most workable for the present purpose is the old orthopedic standby,
plaster of Paris. Judging from past practice, and from long usage in limb
prosthetics generally, one may suppose that there are a number of
satisfactory ways of taking a plaster impression, each perhaps with certain
advantages and disadvantages peculiar to itself. Experience seems to suggest
that for PTB sockets the most useful and practical means of cast-taking is to
wrap the stump with plaster-impregnated bandage. Use of the bandage offers,
among other things, the opportunity of regulating the tightness of the cast by
controlling the tension applied to the bandage while it is being
wrapped.
With the amputee seated appropriately,
somewhat as in Fig. 1A there is applied to the stump a thin cast sock
of such size and length as to fit snugly and to come up well over the knee. To
the top of the sock on either side of the thigh are attached, by harness clamps,
the ends of a piece of 1-in. webbing passing around the patient's waist and just
long enough to support the cast sock under comfortable tension. As in the
cast-taking technique commonly used to produce other forms of below-knee
sockets, the prosthetist must now identify and outline the bony prominences and
other landmarks, both those known to be unusually sensitive to pressure (and
hence requiring buildup in the model in order to give relief in the socket) and
those especially well adapted to weight-bearing (those requiring reduction of
the model and hence buildup in the socket), in this case particularly the
patella and the patellar ligament (Fig. 1B). To do so, the fitter
moistens the cast sock and outlines the areas concerned with indelible pencil so
that, subsequently, the tracings will be transferred first to the negative mold
and then to the positive model.
Fig. 1. Preparations for taking the
negative cast. A, Patient seated with stump relaxed and knee flexed
easily (about 30 deg.), cast sock applied and retained well above knee,
prosthetist identifying (by palpation) bony landmarks and other pertinent
features to be outlined by indelible pencil; B, the areas generally
marked out for later use in modification of the model-some expected to be
weight-bearing, some more or less pressure-sensitive and hence in need of relief. See text.
|
|
In all cases, at least nine areas are identified. These include the patella itself (Fig. 1B, a), the mid-point
(Fig. 1B, b) of the patellar ligament (approximately at the level of the
medial tibial plateau), the tubercle of the tibia (Fig. 1B, c),
the head of the fibula (Fig. 1B, d), the anterior crest of the tibia
(Fig. 1B, e), the distal end of the fibula (Fig. 1B, f), the
antero-distal end of the tibia (Fig. 1B, g), the medial flare of the
tibia (Fig. 1B, h), and the medial border of the tibia (Fig. 1B, i).
Marked only if they are prominent or sensitive to pressure are the anterior
prominences of the lateral and medial tibial condyles, the lateral
border of the tibia, and any other sensitive areas that might suggest the
presence of bone spurs, adherent scar tissue, neuromas, or similar
conditions.
When the necessary marking has been
completed, the patient having maintained his stump as much as possible in the
original position of knee flexion without external rotation of the femur, a few
rolls of 4-in. plaster bandage are laid out conveniently beside a basin of
clean, cool water. As needed, each strip of plaster bandage is immersed in the
water for about four seconds, squeezed to remove excess water, and applied to
the stump over the marked cast sock. The wrap is begun with one or two layers of
bandage running lengthwise (Fig. 2A), beginning in front and just above
the top of the patella, passing down and around the end of the stump, and
continuing up the back of the stump to the posterior crease of the knee.
Thereafter a series of circumferential wraps (Fig. 1B) is begun at the
upper border of the patella and made to spiral down, then up, the stump so that
half the width of the bandage (2 in.) overlaps each successive layer. Each layer
is smoothed carefully as it is applied, and the wrapping is continued until the shell thus formed has a
thickness of about 1/8 in. in the proximal third. Additional layers are applied
over the distal portions until about six rounds have been completed.
Fig. 2. Taking the negative cast. A,
Beginning of the wrap with plaster bandage, strips extending well above
knee, front and rear; B completion of the spiral wrap (see Fig. 3).
|
|
While the amputee continues to maintain
the original angle of knee flexion with relaxed musculature, the plaster is
smoothed over the surface and worked in around the prominences and depressions
by means of the hands until the plaster begins to harden. At this point, the
fingers and thumbs of the operator are called upon to outline the patellar
tendon and to compress the popliteal tissues, as shown in Fig. 3, and
considerable experience and judgment are required to establish just how much
pressure should be applied and in what direction. The thumbs are placed in such
a position as to make a 45-deg. angle with the long axis of the tibia, and their
ends are directed upward and inward midway between the lower edge of the patella
and the tubercle of the tibia. Meanwhile, the fingers, wrapped around the knee, force the cast into the
popliteal area, the forefingers being at the level of the posterior crease of
the knee. Contact with the sides of the knee is maintained to prevent bulging,
but distortion of the sides and pressure on the hamstring tendons are to be
avoided. Pressure should be firm but not so great as to cause finger fatigue (a
sign that too much pressure is being exerted). Both prosthetist and patient
attempt to remain as motionless as possible while the plaster hardens beyond the
possibility of permanent deformation.
Fig. 3. Use of the hands to shape cast
while plaster is hardening. Thumbs compress bandage in and around patella,
fingers force cast into popliteal area.
|
|
Casting the Positive Model
When the plaster has hardened completely,
finger pressure is released, but the cast is allowed to remain in place for an
extra minute or two, whereupon the harness clamps are released and the cast sock
is reflected down over the cast, the amputee flexes his knee to 90 deg., and the
prosthetist, with his hands in the same position as when forming the cast,
removes the whole cast from the stump by an anteroposterior rocking motion
induced while simultaneously pulling downward (Fig. 4). The cast sock, bearing
the indelible markings, is allowed to remain in the cast, and the latter is then
filled to the top with fluid plaster of Paris of the usual consistency. Into the
center of the still-liquid plaster is inserted lengthwise (to a depth of not
more than 6 in.) an 18-in. length of 1/2-in. iron pipe (approx. 1 in. O.D.) to
serve as a mandrel in future bench operations. When the plaster has set for 20
to 30 minutes, the wrap cast is stripped off after it has been cut lengthwise
down the posterior surface, and the model is ready for modification in accordance with the outlines
originally marked on the cast sock.
Fig. 4. Removal of the cast. Because of
the depressions made in the cast purposely, a rocking motion is required to get cast off stump. Knee flexion helps.
|
|
Modification (Rectification) of the
Positive Model
With the exception of those areas where
the wrap cast was purposely distorted by the prosthetist's fingers and thumbs
(around the patellar ligament, just under the lower edge of the patella, in the
popliteal space, and so on), the positive plaster model now constitutes a
faithful reproduction of the stump. It remains to revise the model in such a way
that, when a socket is laminated over it, the shape of the socket will be that
required to distribute the weight of the body over those areas best suited to
weight-bearing while at the same time relieving sensitive areas from
responsibility for bearing more weight than will be comfortable. This is
accomplished by carefully carving away plaster where additional force transfer
will be acceptable and by building up the model (with shaped patches of leather
or other suitable material) in areas expected to be incapable of accommodating
any appreciable part of the load. Guidance in this operation is to be had from
the indelible outlines previously transferred first from cast sock to cast and
then from cast to model.
Although the original compression of the
cast in the vicinity of the patellar ligament and around the tibial tubercle
represents a preliminary step in shifting the anticipated load in the direction
of the ligament midway between the lower border of the patella and the upper
margin of the tibia, further modification of the model in this area is now
required to intensify the effect. Accordingly, the model is cut away, as shown
in Fig. 5, to form a channel at least 1/2 in. deep, on a radius of
about 1 in., and extending horizontally across the front about 1 1/2 in.,
just short of the thumb prints on either side of the tibial crest. Smooth
contours are obtained by sanding rough spots with a piece of wire
screen.
Fig. 5. Initial step in modification of
the positive model - undercutting to enhance support on patellar ligament.
|
|
Another stump area normally capable of
bearing a portion of the body weight is the anteromedial flare at the proximal
end of the tibia. As shown in Fig. 6A, then, the model is shaved down
in this area. At the deepest point of the resulting concavity, at least 1/8 in. should be removed (depending at least in
part upon the amount of soft tissue overlying the stump in this area), and the
edges should be smoothed out into continuous surfaces of gentle curvature. Since
adequate vector forces cannot be exerted upon the anteromedial surface of the
tibial condyles without corresponding vector forces on the lateral side, and
since in any event the PTB socket is designed to provide, if possible,
mediolateral stability without the necessity for sidebars, knee joints, corsets,
and so forth, the lateral surface of the model is now also shaved down, as shown
in Fig. 6B. Depending upon the individual characteristics of the
particular stump concerned, 1/8 in. to 3/8 in. of plaster is removed,
beginning about 3/4 in. below the border of the head of the fibula and
continuing to within 1/2 in. of the end of the fibula.
Fig. 6. Successive steps in modification
of the positive model. A, Reduction for enhanced support on medial tibial condyle; B, the same to provide lateral support against fibula; C, the
same to avoid pressure on anterior crest of tibia.
|
|
Just as the PTB socket is expected to
furnish adequate mediolateral stability, so it also must provide enough
anteroposterior stability to come under full control of the knee of the wearer
on the side of the amputation. Relatively comfortable and yet adequate fixation
of the stump within the socket in the anteroposterior direction is effected by
trimming down the anteromedial and anterolateral surfaces of the model almost
throughout the length of the remaining tibia (Fig. 6C). The result is a
wedgelike support along both sides of the front of the tibia, which, then,
must be backed up by corresponding but opposite forces to the rear of the socket
in the popliteal area. As seen in Fig. 7, the popliteal area of the model is
thus shaved down to the depth of the fingerprints, the upper portion of the
model in this vicinity being rounded out to give a flare to the posterior brim
of the socket. Finally, should it be the intention that the ultimate socket
provide some amount of end-bearing, thin layers, up to about 1/4 in., of plaster
may be shaved from the end surface of the model. If only the closed socket with
no appreciable end-bearing is sought, the end of the model is simply smoothed
with sandpaper, as is the whole model in any case to provide a finished
job.
Fig. 7. Further modification of the
model. Popliteal area is shaved away to provide countersupport against forces
from the front, thus improving anteroposterior stability of socket.
|
|
The model having been thus reduced to
obtain the proper distribution of the loads to be anticipated in the socket, it
is now equally necessary to build up those areas needing more or less relief
from the pressure of weight-bearing. These ordinarily include the head and the
end of the fibula, the prominent crests of the medial and lateral tibial
condyles, the tibial crest throughout its length, and the
antero-distal end of the tibia. In general they will already be outlined on the
model from the indelible markings on the cast sock. Skived patches of leather
carefully trimmed to fit (Fig. 8) are used to provide the modification needed.
They are bonded to the plaster in the places needed, and the rectified model is
then ready for use in fabrication of the plastic-laminate socket. The drawings
of Fig. 9 present for comparison the shapes of stump, original stump model,
and stump model after rectification.
Fig. 8. Build-up of positive model to
furnish relief in pressure-sensitive locations. Skiving of the leather patches
provides a smooth transition from plaster to build-up.
|
|
Fig. 9. Contours at successive levels
overlaid to show comparative shapes of stump, of stump model as made from the
cast, and of stump model after suitable rectification (modification). The
specific shapes vary from patient to patient, of course, depending upon
individual differences.
|
|
The Soft Insert
To accommodate any inadvertent
irregularities in the socket, or any minor incongruities between stump and
socket, and because in general it has been found desirable to provide a
comparatively soft and pliable liner in below-knee fittings, lamination of the
socket itself is preceded by fabrication of an insert made of medium-weight
horsehide (4 to 6 oz.) and 1/8-in. sponge rubber. Although the making of the
liner and the lamination of the socket may be reviewed as two separate
operations, they are, as will be seen, actually carried out as two successive
steps in the layup, reinforcement, and lamination of the socket. Since the
socket and its liner are both prepared over the rectified model, the
innermost layers are the ones designed first, and hence the first step is to lay
up the leather insert.
The modified plaster model having been
placed in the bench vise upside down and held there, in the vertical position,
by means of the mandrel of iron pipe, there is cut from medium-weight horsehide
a piece in the shape of an isosceles trapezoid such that the two parallel sides
are 2 in. longer respectively than the proximal and distal circumferences of the
model, the other dimension being about 2 in. longer than the model, and the
direction of stretch of the leather being in the same direction as are the
parallel sides (Fig. 10A). With the smooth side in, the leather is fitted
to the model, the intended seam line being so placed as to follow the posterior
centerline. While the leather sheet is held in place by a suitable number of
harness clamps (Fig. 10B), the seam is marked with pencil. The sheet
having then been removed from the model, it is sewed
along the mark, the clamps being removed one at a time as the sewing proceeds.
After the seam has been trimmed neatly throughout its length to within 1/8 in. of the stitching, the leather sleeve is replaced on the model, the work
is removed from the vise, and the proximal extension of the leather is tucked
and stapled to the top surface of the model (Fig. 10C). An approximation
of the final trim line of the socket is now drawn around the top of the
leather-covered model (Fig. 11), and the whole is replaced in the vise, the
mandrel again serving as the means of support.
Fig. 10. Preparation and layup of the
leather insert, or socket liner.
|
|
Fig. 11. Proximal trim line of the
leather liner.
|
|
To form an end pad for the socket, there
is now cut from a 1/8-in. sheet of sponge rubber (Kemblo) a disc large enough to
fit neatly over the end of the model, the diameter of the disc being usually
equal to the average diameter of the stump (Fig. 12A). The distal end of
the liner and one side of the rubber pad are now coated with cement (Stabond
T-161), allowed to dry until the cement is tacky, and then placed together so
that the pad will conform to the shape of the end of the model. Unless the
curvature of the model is extreme, the pad will conform when pressed into place.
Should it not conform well, a dart or two will suffice to correct any difficulty
in arriving at a smooth transition between rubber and leather. In either event,
the periphery of the Kemblo end pad is now skived with a sanding drum (Fig. 12B) so that the outer edge will be flush with the horsehide.
Fig. 12. Construction of the socket end
pad.
|
|
Padding of the sidewalls of the model is
now undertaken by the successive application, beginning on the anterior surface, of a
circumferential series of fitted strips of Kemblo running the length of the
model. To begin, there is first cut a strip of Kemblo 2 in. wide and long enough
to overlap the end pad 1/2 in. and to extend beyond the model about an inch
proximally. The anterior surface of the leather liner and of the end pad are
coated with cement,* as is also one surface of the first strip of
Kemblo. When the surfaces are tacky, the Kemblo strip is placed in the
position representing the anterior crest of the tibia and allowed to extend over
the end cap about half an inch (Fig. 13A). Carefully pressed into place so as to
conform to all of the irregular areas, the edge of the first strip constitutes
the pattern for one edge of the second. So that when finally cemented in place
the second strip will fit as snugly as possible against the edge of the first,
one edge of the applied first strip is marked with chalk (Fig. 13B), and
the second strip is laid along the model parallel to the longitudinal axis and
so that one edge just overlaps the chalked edge (Fig. 13C). The chalkline thus
transferred to the new strip marks the trim line for tailoring to the contours
of the model (Fig. 13D). When the new strip has been trimmed as marked,
it is cemented in place, and the process is repeated until the entire surface of
the liner has been overlaid with a smooth covering of Kemblo. Where the
strip ends overlap the end of the model, they are skived on the sanding drum,
and a second end pad, like the first, is cemented over the end of the padded
model. Skiving of the second end pad to be flush with the longitudinal strips of
Kemblo completes the layup and fabrication of the soft insert (Fig. 13E).
Fig. 13. Layup of the soft liner of
sponge rubber (Kemblo). One edge of the first strip (A) becomes the
pattern (B, C, D) for the second, and so on, until the entire model is
overlaid with a smooth and neatly fitted covering (E).
|
|
The Plastic Shell
The next step is the lamination of the
plastic shell over the soft liner but readily separable from it after
construction of the shell is complete. As in the case of plastic-laminate
sockets for other levels of amputation, use is here made of sleeves fabricated
from sheeting of polyvinyl alcohol (PVA). Since in the construction of the
below-knee socket it is desired to keep the liner separate from the plastic
shell, two sleeves are used-the first to form a separator between liner and
shell and the second, as usual, to enclose the whole
layup-and-resin combination as a means of impregnating the reinforcing
materials. Since neither sleeve need be more than an approximate fit for the
model, two identical ones are fabricated to the dimensions shown in Figure 14.
After the outer surface of the socket liner has been coated liberally with talc
(to prevent sticking), the first PVA sleeve is stretched over the model and
liner and trimmed around the distal end where it parts company with the surface
of the liner (Fig. 15A). A half-inch annular area of PVA adhesive is now painted
around the cut edge (Fig. 15B), and the open section is covered with
another piece of PVA neatly bonded to form an end for the sleeve (Fig. 15C). At
the proximal end of the model the other end of the PVA sleeve is tied tightly
about the mandrel, and any loose material is trimmed away to give a neat layup
(Fig. 15D).
Fig. 15. Application of PVA separator
over socket liner and model
|
|
The model and overlying liner, thus
covered with the PVA separator, are now ready for layup of the laminations and
reinforcing materials to be incorporated into the
plastic shell, or socket. Three pieces of 1/2-oz. Dacron felt, cut to the
same pattern as used for the leather liner (Fig. 10A), are sewed as shown
in Fig. 16A and pulled over the model one after the other, the seams
lying on the posterior aspect of the model. Then, under the last layer of felt,
in the vicinity of the postero-proximal margin, there are placed five
rectangular pieces of Dacron felt (Fig. 16B) measuring 2 in. by 4 in., the
purpose being to thicken and reinforce the posterior edge of the
socket.
Fig. 16. Layup of reinforcing materials
for plastic socket. A, Layers of Dacron felt in place; B, extra material
added in posteroproximal area; C, application of Fiberglas cloth and cast
sock over Dacron.
|
|
A strip of Fiberglas cloth wide enough to
cover the proximal half of the model is now wrapped around the Dacron so as to
overlap itself by at least an inch, and a light cotton cast sock is slipped over
the distal end of the model to hold the Fiberglas reinforcement in place (Fig. 16C). When the second PVA sleeve has been stretched over the whole and tied
tightly about the mandrel, the layup is complete and ready for application of
the resin-catalyst mixture.
A quantity of the resin (200-400 grams,
depending on socket size), prepared according to the recipe given in Appendix A, is poured into the open, distal end of the second PVA sleeve and
thoroughly worked down into the fibers of the laminating materials. The open end
of the sleeve is tied off, and working is continued to remove air and to
complete impregnation by the familiar process of "stringing." To ensure that
undercut areas and all other irregular contours of the model are reproduced in
the final socket, the layup is now wrapped, as appropriate, with strips and pads
of sponge rubber or with pressure-sensitive tape, whichever is more convenient
(Fig. 17A). Left thus undisturbed, the resin will cure at ambient room
temperature in about 30 minutes, whereupon it is allowed to lose any heat of
reaction and to return to room temperature.
Fig. 17. Plastic lamination and initial
finishing of the PTB socket. A, Layup encased in second PVA bag,
impregnated well with resin, and undercut areas bound down by wraps of sponge
rubber; B, removal of socket and liner from model after curing of resin
is complete; C, specifications for trimming the top brim of the
socket.
|
|
It remains now but to free the socket and
liner from the plaster model. This is accomplished by trimming along the
proximal edge of the layup (Fig. 17B) at a 45-deg. angle until the underlying
sponge rubber is just exposed. The shell is then readily slipped off the model,
as the liner in turn may be slipped out of the socket. With liner removed
temporarily, the proximal brim of the socket is now trimmed as shown in Fig. 17C.
Preparation of Socket for Alignment
The socket thus produced must next be
properly aligned with respect both to the residual anatomy of its intended
wearer and to the rest of the prosthesis, including the prosthetic foot and the
shoe to be worn over it. Although the below-knee prosthesis may be so aligned,
as it has been for a great many years, by the simple expedient of "aligning by
eye" (that is, simply by trial and error and by observation of the static and dynamic
behavior of the amputee-prosthesis combination), the whole procedure is made
much easier (and the resulting relationships much more readily amenable to
duplication if need be) by application of one of the more modern tools of
prosthetics practice. Recommended for use in the present instance is the
below-knee adjustable shank developed at the University of California. As may be
seen in Fig. 18, the UC below-knee adjustable shank consists
essentially of a steel plate perforated with a rather large number of
countersunk screw holes and supported on a crossed-bar mechanism in which two
identical and graduated bars cross each other back to back at a fixed angle of
90 deg. and in which each bar is capable of sliding across the other at the
point of intersection, or of rotating about the longitudinal axis of the other,
or of doing both simultaneously in an infinite variety of combinations
of sliding and tilting. Each bar is held in
position by a pair of opposing setscrews, such that loosening of any one screw
permits both sliding of the bar to which that screw is attached and rotatory
motion about the companion bar. The net result is a kind of universal joint in
which, within the limits required, any combination of anteroposterior and
mediolateral shifting horizontally may be had together with any combination of
anteroposterior and mediolateral tilting. Included with the device is a pylon
shank for temporary service during alignment, and a clamp on the shank portion
provides for attachment of the foot and for adjustable foot rotation with
respect to socket orientation.
Fig. 18. The University of California
below-knee adjustable shank.
|
|
Attachment of Socket to Adjustable
Shank
Since the below-knee adjustable shank is
intended for use in combination with the socket shell, and since the latter is
asymmetrical in all directions on the outside as well as on the inside, there is
now required some practical means of attaching the socket rigidly to the shank.
Experience shows that such an attachment is best arrived at by first sinking the
socket into a hollow block of wood of suitable size and shape. For purposes of
reference, here and throughout the remaining stages of construction, the socket
is first marked with vertical centerlines representing,
respectively, the anteroposterior and mediolateral planes. As shown in Figure
19, the lines are established by connecting, in side and rear views, the
estimated center points of the top and of the bottom of the socket, the proximal
center point for the anteroposterior plane (Fig. 19A) being taken at the level
of the posterior brim of the socket while the corresponding center in the
lateral view (Fig. 19B) is taken slightly above the indentation provided for the
patellar ligament.
Fig. 19. Anteroposterior and mediolateral
center-lines of the socket, intended for reference in alignment. In each of the
two views, the approximate "center" of the brim and the estimated "center" of
the bottom of the socket are connected by straight lines, except that in the
lateral view the proximal center point is taken just above the level of the
indentation provided for the patellar ligament.
|
|
A cylindrical socket block of willow,
about 6 in. long and about 6 in. in diameter, is now drilled through along the
longitudinal axis of the cylinder (parallel to the grain) with a 2-in. bit, and
one end of the tubular aperture is carved out so as to receive the lower end of
the socket to a depth of 3 or 4 in. and in such a way that the socket will rest
easily in the block with 5 deg. of adduction (Fig. 20A) and 5 deg. of
initial flexion (Fig. 20B).
Fig. 20. Positioning of the socket in the
socket block to give 5 deg. of adduction and 5 deg. of initial
flexion.
|
|
The distal surface of the socket shell,
roughened to improve adhesion, is now bonded into the block in the predetermined
position by use of a mixture of resin and sawdust (or other filler). When the
bond has hardened thoroughly, the lower end of the socket block is sawed across
squarely at such a level as to leave only about an inch of
wood below the end of the socket shell.
With the socket attachment plate and the
slide-tilt unit of the below-knee adjustable shank (Fig. 18) centered and level,
the socket block is now set upon the attachment plate in an orientation such
that the mediolateral center plane of the socket (posterior reference line) lies
in the same direction as the lower pair of setscrews of the slide-tilt unit
(Fig. 21). Thereafter the socket block is moved upon the attachment plate in the
anteroposterior direction until a plumb line dropped from the anteroposterior
centerline of the socket at the level of the midpatellar tendon lies 1 1/2 in.
in front of the centerline of the upper tube clamp (Fig. 21A). Similarly,
the block is then moved in the mediolateral direction until a plumb line dropped
from the center of the posterior brim of the socket lies 1/2 in. lateral
to the centerline of the upper tube clamp (Fig. 21B). While the block is held in
this position temporarily, a pencil line is drawn about the attachment plate
onto the base of the block, the socket and block are removed from the adjustable
shank, and excess wood is cut away from the block to produce the result shown in
Fig. 22.
Fig. 21. Orientation of socket and socket
block upon adjustable shank using socket centerlines for
reference.
|
|
Fig. 22. Socket and socket block after
removal of excess wood from the latter. Circle on base marks position of
socket-attachment plate for reattachment of adjustable shank.
|
|
With the block thus partially trimmed,
the adjustable shank is replaced against the bottom of the block in the same
relative position as before, and the block is attached to the plate of the shank
by means of not fewer than six 3/4-in. flat-head wood screws
(No. 10), which, incidentally, will seat nicely into the countersunk holes in
the attachment plate. The particular position chosen in the individual case is,
of course, as already described and as shown in Fig. 20 and Fig. 21, and the net
spatial relationships of socket to adjustable shank shall be such that, to begin
with, all of the adjustment setscrews are near the middle of their ranges of
possible adjustment.
Choice and Preparation of the Prosthetic
Foot (With Shoe)
Although in the construction of the
patellar-tendon-bearing below-knee prosthesis use might be made of any one of a
variety of foot-ankle units commercially available,
the most satisfactory results are usually obtained with the nonarticulated SACH
foot (Solid Ankle, Cushion Heel), in which a heel wedge of compressible but
resilient material provides shock absorption and the equivalent of plantar
flexion at heel contact while a solid wooden core (or keel) properly shaped at
the ball of the foot furnishes needed support during roll-over and push-off in
the stance phase of walking. Fig. 23 presents schematically the familiar SACH
foot as seen through a transparent shoe properly fitted.
Fig. 23. The SACH foot, in transparent
shoe, schematic. A, Heel contact; B, plantar flexion immediately
after heel contact, heel wedge compressed. Rocker shape of keel at the ball of
the foot gives support during roll-over and furnishes needed assistance at
toe-off. Flexible toe piece permits normal toe-break in the shoe.
|
|
Generally, choice of SACH foot in the
individual case depends on three factors: shoe size, height of the patient, and
relative stiffness of the heel wedge. At present, oversize SACH foot blanks,
left and right, are available in three ranges of shoe size (6-8, 8-10, 10-12)
and two degrees of stiffness of the heel insert ("firm" and "medium"). As for
heel stiffness, "medium" is generally recommended for below-knee amputees
weighing up to 140 lb., "firm" for those exceeding 140 lb. As for Table 1, which
presents the recommended size of foot blank as related to shoe size and height
of patient, it should be noted that, as in most aspects of lower-extremity
prosthetics, no hard and fast rules exist and that in any case borderline sizes
have to be worked out as compromise. Ultimate choice of foot-blank size and
heel-cushion stiffness should always be based on evaluation of the needs of the
individual patient.
Once the foot blank has been selected,
it remains to shape the foot (Fig. 24) until
it fits properly into the intended shoe. Although in the oversize blank the
general contours of the foot are provided for by the manufacturer, so that in
general only slight modifications are required, certain precautions need to be
exercised. For example, the portion of the foot above the top of the shoe should
not be reduced until the final wooden shank has been installed. Similarly, no
material should be removed from the lower third of the heel contour lest the
distance from heel to toe-break be made too small for a tight fit. Conversely,
certain size reductions are usually essential, especially on the lower surface
of the arch of the foot, in the toe area, and in the heel cushion above the
lower third of the heel, all as shown in Fig. 24. In particular, the lower
surface of the arch of the foot must be so reduced that it can never come into
compression contact with the arch of the shoe (Fig. 23). Required here is a minimum
clearance of 1/8 in., for otherwise motion may be restricted or the shoe
damaged. In like manner, the dorsal surface of the arch of the foot should be
reduced until the lacing gap of the shoe matches that of the shoe on the
remaining normal foot, but not to the extent that fitting in this area might be
loose.
Fig. 24. Shaping of the SACH foot blank
to the requirements of the shoe. Failure to maintain tightness in the areas
indicated, or to provide relief in the others, leads to abnormal gait regardless
of the care taken in construction of the rest of the prosthesis
|
|
Just as the arch of the foot must be
prevented from binding against the insole of the shoe, so the toe portion of the
foot blank must be reduced so that expansion under compression will not restrict
motion in the toe of the shoe. Finally, the upper two thirds of the heel insert
must be shaped to give about 1/8 in. of clearance from the lateral,
medial, and posterior brims of the counter of the shoe, a feature which permits
the heel wedge to expand under compression without binding against the shoe
(Fig. 23).
A subtle feature in the shaping of the
heel wedge is that the rearmost point of the heel should be fashioned to lie 1/4
in. lateral to the anteroposterior midline of the foot (Fig. 25) so that later,
when the necessary toe-out is introduced, the point of the heel will
automatically return to a position directly in the line of
progression.
Fig. 25. Shaping of the heel of the SACH
foot to accommodate proper toe-out in the finished prosthesis.
|
|
All of these shaping operations are of
course best carried out by means of a cone or drum sander, the sanding being
done as much as possible in a direction parallel to the direction of the laminations at all points. A
spindle speed of at least 1750 r.p.m. is desirable; and in the course of
fitting, a thin sock should be placed over the boot whenever the foot is
inserted into the shoe for trial.
There remain now but two final
adjustments-the first having to do with heel elevation (distance between bottom
of heel and the surface upon which the ball of the foot rests when the top
surface of the foot is parallel to the supporting surface) and the second with
heel-cushion stiffness. Currently, SACH foot blanks are manufactured with a heel
elevation of 11/16 in. If, when the shaped foot and companion shoe are held on a
surface with top of foot parallel to that surface, there should be undue
compression of the heel wedge, the heel elevation may be increased (by not more
than 3/16 in.) by sanding the lower surface of the foam crepe shoe-sole material
in the heel area. Should compression of the heel wedge be inadequate under the
same circumstances, shims of crepe shoe-sole material, leather, or any other
firm but flexible material may be shaped and bonded to the bottom of the
heel.
If needed at all, the second adjustment
(heel-cushion stiffness) awaits attachment of the foot (with shoe) to the rest
of the assembly (i.e., to the bottom of the adjustable shank).
Accordingly, the foot-attachment plug of the adjustable unit is now bolted to
the flat, top surface of the foot, and the distance between foot and adjustable
unit is established with an appropriate length of aluminum-alloy tubing 1.625
in. O.D., 1.510 in. I.D. Attachment of the proximal end of the tube is
by insertion into the clamp at the bottom of the adjustable unit. To clamp the
distal end of the tubing about the foot-attachment plug, the lower end of the
tubing is split, the tubing is slipped over the plug, and the assembly is fixed
together with the tube clamp furnished with the adjustable shank. Preliminary
toe-out of the foot is obtained simply by loosening the tube clamp, rotating the
foot so that the line of progression is parallel to the anteroposterior (bottom)
slide bar of the adjustable unit, and resetting the tube clamp. Should the unit
be too short when tried on the patient, the foot is removed, annular spacers are
added, the foot is replaced, and the clamp tightened again. If the unit is found
to be too long, the foot is removed and a shorter length of
aluminum-alloy tubing is substituted.
With the socket-and-block combination,
the adjustable unit, the tubular pylon, and the foot-and-shoe combination thus
assembled, the amputee dons the socket and stands upon it, weight distributed
equally between heel and ball of foot. If all has been done well, the
orientation in the parasagittal plane will be such that, when the prosthesis
stands unloaded, the longitudinal axis of the shank will be inclined some 2 to 3
deg. anteriorly (Fig. 26, solid outline) whereas when the amputee stands upon
the prosthesis the longitudinal axis of the shank will rotate posteriorly until
it lies in a vertical plane (Fig. 26, dotted outline). The change in relative
position brought about by addition of the wearer's weight represents of course
an initial compression of the heel wedge. Over and above initial compression is
that needed and acceptable at heel contact during the stance phase of walking.
In general, the heel should compress about 3/8 in. at heel contact (Fig. 23B). Should, in any particular case, any of these values prove to be
appreciably larger or smaller than the recommended compression values, the heel
cushion must be replaced by a stiffer or a softer cushion, whichever applies.
The procedure for so doing is set forth in Appendix B.
Fig. 26. Trial below-knee leg showing
proper anterior tilt of shank (2 to 3 deg.) in the unloaded condition (without
weight of wearer). Dotted outline shows return of the long axis of the shank to
the vertical when amputee stands upon the prosthesis (initial compression of
heel wedge). Should these relationships not prevail upon examination, a change
in heel stiffness is indicated (Appendix B).
|
|
Making the Supracondylar Cuff
All prior conditions having been met
satisfactorily, the assembly shown in Figure 26 is now ready for preliminary
alignment on the amputee. But before any alignment can be undertaken it is first
necessary to fabricate the means of socket suspension-the supracondylar cuff
fitting about the distal flares of the femur and resting in front upon the upper
margin of the patella (Fig. 27). Though in some cases it may be necessary later
to resort to jointed sidebars and thigh corset, with or without still additional
paraphernalia, the simple cuff, with its side tabs attached to the socket
posteriorly, commonly suffices in actual prosthetic use and, in any case, serves
adequately the purposes of final fitting and alignment.
Fig. 27. Finished PTB prosthesis using
supracondylar cuff as only means of suspension.
|
|
To make the cuff, including the tabs,
a suitable piece of pearled elk leather is
first cut out along the pattern labeled a in Fig. 28. Since ultimately
closure of the cuff is to be by buckle on the lateral side, and since it is
desired to have the smooth side of the leather outside, the orientation of
pattern and material must be chosen properly. One side of the pattern is of
course for right amputees, the other side for left amputees.
Fig. 28. Patterns (one half actual size)
for preparing supra condylar cuff. a, Pattern for the cuff itself; b,
pattern for the buckle billet.
|
|
Rubber cement is now applied to the rough
side of the leather part just cut, and two pieces of Dacron webbing 1/2 in. wide and 4-1/2 in. long are bonded to the leather tabs (Fig. 29) as insurance against excessive stretching. A piece of horsehide large enough
to cover cuff and tabs is then selected, the rough side is covered with rubber
cement, and the horsehide is bonded in place as a liner. When this laminate has
set, the elk leather, Dacron webbing, and horsehide are sewed together along the
edges, and the horsehide and webbing are trimmed flush with the elk
leather.
Fig. 29. Application of Dacron webbing to
cuff side tabs to prevent undue stretching of the leather.
|
|
When the cuff itself has been completed,
a buckle billet is cut from a scrap of horsehide according to the pattern
labeled b in Fig. 28, the ends of the piece are skived on the rough
side, a slot for the buckle is cut out, a 5/8-in. buckle is inserted in the
slot, and the billet is lapped back on itself, rough side in, and bonded
together with rubber cement. The billet containing the buckle is then glued and
sewed to the pearled elk surface of the cuff, as shown in Fig. 30. Finally,
six or seven 3/16-in. holes are punched in the tabs at 3/8-in. intervals, and
buckle holes of suitable size are punched into the strap of the cuff on 1/2-in.
centers (Fig. 27).
Fig. 30. Installation of buckle and
buckle billet on condylar cuff.
|
|
Attaching Cuff to Socket
As will be noted in Fig. 27, one
intention of the condylar cuff is that it shall bring about tension in the side
tabs as the knee is extended throughout the range and that it shall permit the
side tabs to relax as the knee flexes in sitting or in the swing phase of
walking. Thus the points of attachment of the side tabs are pivots, the axes of
rotation being behind the anatomical knee axis. Since the cuff must pull in
against the patella over a full 60 deg. of knee flexion in the swing phase,
while for comfort in sitting the tabs must relax throughout an additional 30
deg. to give 90 deg. of knee flexion (Fig. 31), the optimum points of attachment
of tabs to socket must be arrived at by trial of the socket and cuff on the
patient for whom they are intended.
Fig. 31. Positioning of cuff side-tab
attachments such as to provide tab tension throughout 60 deg. of knee flexion in
the swing phase of walking and tab relaxation throughout an additional 30 deg.
to accommodate comfortable sitting with knee flexed a full 90
deg.
|
|
The amputee first dons the cuff so that
the tabs are on either side of the knee and fastens it comfortably. He then dons
the socket over a stump sock, being careful to obtain
proper seating of the stump, and stands on the prosthesis with weight evenly
distributed on two legs. While this condition is maintained, the tabs are pulled
down on either side of the knee and approximated to their natural position on
the sides of the socket. The hole nearest the level of the tibial plateau but
behind the average anatomical knee axis is selected on each side and the points
marked through the holes with a pencil (Fig. 32). By means of self-tapping
screws, the necessary buttons are attached temporarily at the points indicated,
pending final alignment and walking trials. When all adjustments are complete,
the buttons are attached permanently by means of rivets.
Fig. 32. Attachment of side-tab buttons
at position determined in Figure 31. The usual position, arrived at by trial and
error, is behind the average anatomical knee axis at the level of the tibial
plateau.
|
|
Preliminary Alignment
From the alignment established at the
time of assembly of socket, adjustable shank, and foot (pp. 36-42) it is now
necessary to arrive at the optimum alignment for the given case, a requirement
demanding ultimately the participation of the amputee himself. Since the
positioning of the socket in the block, the orientation of the adjustable unit,
and the characteristics of the foot are all mutually interdependent in defining
the "net" optimum alignment, it is imperative that no attempt be made to correct a fault at a given point
without considering the possibility of thus upsetting position relationships at
another. The whole process of alignment is in fact a series of checks and
rechecks, and it is the responsibility of the prosthetist to determine the site
of faults, if any, and to make appropriate corrections as the process advances
in stepwise fashion. As has been seen, use of the below-knee adjustable shank
makes it possible to orient a below-knee socket to any necessary combination of
fore-and-aft positioning, side-wise positioning, fore-and-aft tilting, or
side-wise tilting. But because each setscrew fixes not only the lengthwise
positioning of its own bar but also the rotatory positioning of the companion
bar, it is essential, in the course of successive adjustments, to reset the
same screw as was first loosened (not its opposing counterpart) and to
recheck any preceding adjustment to make certain that it has not been
disturbed.
The amputee having first donned the
socket-shank combination (together with the condylar cuff for suspension and
with the intended shoe on the prosthetic foot), a preliminary approach to
alignment on the individual is made in four steps, as shown in Figure 33. While
anteroposterior tilting is avoided, mediolateral sliding is accomplished. While
anteroposterior sliding is avoided, mediolateral tilting at the desired angle is
established. While mediolateral tilting is avoided, anteroposterior sliding is
carried out to the extent desired. While mediolateral sliding is avoided,
anteroposterior tilting is accomplished. To avoid any unintentional
disorientation, each operation is followed by a check of the previous setting.
Additional minor adjustments are made as needed until the alignment of the
prosthesis upon the wearer is such that the toe-out of the prosthesis matches
that of the normal foot, that the amputee can stand erect, hips level, with
weight equally distributed between the two feet and with heels not more than 4
in. apart, and that in standing in one position between parallel bars (or with
the aid of crutches) he can shift his weight comfortably with adequate control
of both mediolateral balance and of knee flexion-extension.
Of the principal faults sometimes
encountered at the time of preliminary
alignment of the trial prosthesis on the patient, some have to do with spatial
relationships in the frontal plane (Fig. 34), others with relative positioning
of parts in the parasagittal plane (Fig. 35). If, for example, there should be a
gap at the brim of the socket on the lateral
side, accompanied by undue pressure at the medial brim, the pylon of the
adjustable shank may be found to be either vertical (Fig. 34A, foot
necessarily flat on the floor) or tilted laterally (Fig. 34B, foot resting
incorrectly on lateral edge of sole). In the first case, the remedy consists in
shifting the socket medially by means of the adjustable unit (Fig. 34A). In the
second, elimination of the trouble is to be found in tilting the socket
laterally, again by means of the adjustable unit (Fig. 34B). When, in Fig. 34B, the pylon shall have assumed a vertical position in the medio-lateral
plane, the socket will have settled into a satisfactory fit near its proximal
end. Similar, but opposite, corrections are made should undue pressure be found
to prevail on the lateral brim of the socket, it being kept in mind that the
long axis of the shank pylon must always lie in a vertical plane (foot flat on
floor).
Fig. 34. Two faults in mediolateral
alignment sometimes found during initial trials of prosthesis on amputee. A,
Pylon vertical (foot flat on floor) but socket too far lateral; B,
same situation but with pylon tilted laterally so that foot rests on outside
edge of sole only.
|
|
Fig. 35. Faults in anteroposterior
alignment sometimes found during initial trials of prosthesis on amputee. A,
Knee forced backward, shank pylon tilted posteriorly so that too much weight
is borne on heel; B, knee forced backward but with shank pylon vertical
(foot flat); C, heel off floor, all weight borne on ball of foot; D,
knee forced forward by virtue of too much anterior tilt in
socket.
|
|
In the parasagittal plane, a number of
faults may be observed from time to time with individual patients (Fig. 35). For
example, it may be found that application of the wearer's weight forces the knee
backward, the shank pylon tilting posteriorly in one case (Fig. 35A), standing vertical in another (Fig. 35B).
Should a shift of the socket block
forward on the adjustable shank prove not to correct the difficulty shown in
Fig. 35A, it may be that the heel cushion in the foot is too soft, in which
case the heel wedge must be replaced by stiffer material according to the
procedure outlined in Appendix B. When, on the contrary, the knee is forced
backward while the pylon remains in a vertical plane (Fig. 35B), then adequate
correction should be obtained simply by tilting the socket-block combination
anteriorly upon the adjustable unit. Occasionally, the weight of the amputee
forces the socket forward while the pylon remains vertical (Fig. 35D).
When such a relationship prevails, it is usually corrected by tilting the
socket posteriorly. And finally it may happen that, when the amputee stands
erect in the prosthesis, the heel is not in contact with the base of support
(Fig. 35C), which of course means that all of the weight is borne on the ball of
the foot instead of being distributed equally between heel and ball. Tilting the
socket anteriorly usually corrects this undesirable arrangement.
Fig. 35
It should now perhaps be noted that, in
the process of preliminary trials on the patient, none of the indicated
adjustments should be more than a minor adjustment. The necessity for any gross
adjustment at this point in the procedure reflects some inadvertence in
the conduct of the preceding steps of construction, and in such a rare case it
may be better for the prosthetist to start over, or at least to retrace his own
performance from socket casting to assembly of the adjustable leg. In any event,
it will be obvious that the orientation of the socket in the wooden block, the
position of the block with respect to the adjustable shank, the orientation of
the adjustable unit itself, and the design of the SACH foot are all
interdependent and that each of these factors contributes to the final result,
so that a change in any one feature affects the behavior of all the others.
Accordingly, successful alignment of the PTB prosthesis is still partly a matter
of art and thus calls for extraordinary skill and judgment on the part of the
prosthetist. Throughout the preliminary tests it should be remembered that the
wearer of the PTB prosthesis is expected to walk with the knee on the side of
the amputation flexed some 5 to 8 deg. and with weight borne over the middle
third of the prosthetic foot in midstance. If any major changes are made in the
initial alignment, then over-all height should be checked, since an increase in
anterior tilt reduces the effective length of the prosthesis while an increase
in posterior tilt tends to increase it.
Dynamic Alignment
Despite the apparent implications of the
nomenclature, dynamic alignment of the PTB prosthesis is less an actual alignment as
such than it is a check to make certain that the alignment established in the
static condition of standing is satisfactory when the amputee undertakes normal,
level walking along a substantially straight line of progression. The features
sought in dynamic alignment are essentially the same as those sought under
static conditions, though the criteria are different. If, indeed, the
requirements of static alignment have been met fully, and if the particular case
involved presents no gross deviations from the characteristics of the average
below-knee amputee, then the chances are that dynamic alignment will amount to
no more than a confirmation, at most a minor revision, of the spatial
relationships already existing.
Since, however, no amputee-prosthesis
combination, however carefully worked out, can be expected to perform in an
optimum way without the active and cultivated participation of the wearer, no
attempt at checking out the dynamic alignment of a PTB prosthesis is apt to be
valid until the amputee has become familiar not only with what is to be expected
from the prosthesis but also with what responsibility he, the wearer, has in the
management of the limb. Accordingly, the patient is first encouraged to
experiment (at first between parallel bars) with simple weight-bearing on the
limb, with active knee flexion-extension, with standing and sitting, with short
and simple steps including roll-over on the prosthesis, and finally, when he
has gained some confidence, with straight and level walking without benefit of
parallel bars or crutches. Meanwhile, the prosthetist and trainer continue to
make such minor adjustments as seem indicated by observation of dynamic
conditions. Thus, the indoctrination of the patient and the final details of
alignment are carried out together, sometimes alternately, sometimes
successively, until both patient and clinic team are satisfied that the best
possible job has been done. Some of the problems that project themselves
occasionally during dynamic alignment are depicted in Fig. 36, Fig. 37, and Fig. 38,
and the final antero-posterior position of the socket with
respect to the shoe is shown in Fig. 39.
Fig. 36. Check of foot and socket in
mediolateral plane during walking. A, Proper alignment in front view;
B, correction for undue pressure at medial brim of socket, rear view.
Compare with Figure 34A.
|
|
Fig. 37. Check of anterior tilt of socket
(and hence of initial knee flexion). Too much initial flexion, as here, may
cause loss of knee stability at heel contact (A) or lack of support
(drop-off) at the end of the stance phase (B), or both.
|
|
Fig. 38. Check of posterior tilt of
socket. Too little initial knee flexion (excessive posterior tilt of the socket)
may cause early arrest of knee flexion after heel contact, or a prolonged period
of unstable weight-bearing on the heel, or an excessive shift of the body weight
to the ball of the foot accompanied by premature heel rise at midstance (A).
Inadequate knee flexion may also give rise to scuffing of the toe during
swing-through (B).
|
|
Fig. 39. Ultimate anteroposterior
position of socket with respect to shoe. A plumb line dropped from the
anteroposterior centerline of the socket at the level of the midpatellar tendon
should pass just ahead of the breast of the heel of the shoe.
|
|
Because in the practical matter of
walking comfortably, effortlessly, and with acceptable appearance the details of
alignment in the anteroposterior direction are more critical than those having
to do with the mediolateral, it is recommended that the latter always be
attended first, the anteroposterior adjustments being left until the very last.
As in all other steps of alignment, each successive change should be followed at
once by a check on the preceding one so that no correction coming later can
upset another made earlier, except with the full knowledge of the prosthetist
(as is sometimes necessitated in compromise situations where one advantage is to
be gained only at the expense of another). In all cases, the patient should be
allowed to walk upon the adjustable shank long enough (days, if need be) to
demonstrate that all adjustments are at an optimum for the particular
physico-anatomical circumstances then prevailing. When the prosthetist is
convinced that he has attained the best possible set of conditions, the
alignment is duplicated in the finished prosthesis by means of the UC adjustable
alignment-duplication jig.
Alignment Duplication
The so-called "alignment-duplication jig"
of the University of California, intended originally for duplication of the
alignment of above-knee prostheses, consists of two adjustable, viselike clamps so mounted side
by side upon a firmly fixed, tubular base as to be capable of being moved along
the length of the base as required or of being fixed in any selected positions
along the base in any chosen linear relationship to each other. One clamp is
intended to position and hold the thigh portion and artificial knee of an
above-knee prosthesis, while the other holds and positions the shank-foot
combination. To be interposed between the two clamps, mounted on the same base,
and movable along the base between the clamps, is a bracket intended as a guide
for a miter saw whenever the saw is needed. When the bracket is in place, it is
so oriented that the saw will make a cut normal to the long axis of the tubular
base.
Once the clamps have been set so as to
accommodate as precisely as possible a thigh socket, adjustable knee unit,
shank, and foot in the relative positions established in alignment trials, the
component parts of the final prosthesis may be substituted for the adjustable
devices without upsetting the prevailing alignment. Similarly, the alignment of
an existing prosthesis may be duplicated in a new prosthesis simply by setting
up the alignment jig to match the first limb and then making the second limb to
match the setting of the jig. When the desired orientation
of socket and knee block with respect to shank and foot has been attained, the
saw is used to cut the planes representing the intended juncture of the two
segments.
Application of this device to the
below-knee case, including the case of the patellar-tendon-bearing prosthesis,
is readily accomplished by introduction of a special fixture called the "ankle
bracket." Mounted on the base in the same way as the clamps, it is used in place
of one of them, that one being simply shoved out of the way temporarily (Fig. 40). Drilled
through the top of the ankle bracket is a 3/8-in. hole whose axis is such that,
when the bracket is in place, the axis is parallel to the base tubes of the jig.
When, in the below-knee case, static and dynamic alignment with the adjustable
leg satisfy both prosthetist and amputee, the SACH foot is removed from the
adjustable shank, and the distal end of the shank is attached to the ankle
bracket by means of an Allen-head screw (Fig. 41). Since toe-out of the foot must be
re-established after the final shank piece has been properly substituted for the
adjustable shank, the prevailing relationship of the foot to the socket is keyed
before the foot is removed from the adjustable leg. Using a straightedge and one
of the bonding lines of the foot for reference, the prosthetist first marks
points on the front and back brims of the socket (Fig. 42). Thus later, when the
final shank has been aligned and cemented into place, the foot may be replaced
in the same relative position of toe-out as established in the alignment trials
on the adjustable shank.
Fig. 40. University of California
alignment-duplication jig for above-knee prostheses, as adapted for below-knee
alignment duplication through substitution of the ankle bracket (cross-hatched)
for one of the adjustable clamps.
|
|
Fig. 41. Attachment of adjustable shank
(with socket and socket block) to ankle bracket of alignment-duplication
jig.
|
|
Fig. 42. Recording toe-out of foot before
removing foot from adjustable shank. Same toe-out must be reestablished later.
See Figure 50.
|
|
Since, when the ankle bracket is fixed to
the base, the axis of the hole through the bracket is parallel to the long axis
of the base, so also then is the long axis of the shank parallel to the base
tubes when subsequently the shank has been bolted to the ankle bracket. The
orientation of the socket being thus established, the socket clamp is brought up
into position alongside the socket (Fig. 43), care being taken to see that the
clamp is then not less than 10 in. from the end of the base tubes (so that later
it can be backed out of the way). The socket clamp is there locked to the base
tubes, and the clamping thumbscrews are run down carefully but firmly so as to
clamp the socket without at the same time placing any distorting strains upon
the shank. The relative positions of shank and socket are thereby established in
the jig for later reproduction in the finished prosthesis. To
establish the over-all length of the final prosthesis, the positions of the
ankle bracket and of the socket clamp are then recorded from the scale running
the length of the base tubes of the jig.
Fig. 43. Setting of socket clamp to the
orientation previously established by the ankle bracket. At least 10 in. should
be allowed at socket end so that socket clamp and socket may later be moved out
of the wav. See Figure 45.
|
|
With the socket thus fixed in the clamp
and with the clamp and ankle bracket secured to the base of the jig, the
adjustable shank is now removed, first from the ankle bracket and then from the
wooden base of the socket (Fig. 44). The saw guide is mounted near the base of
the socket (Fig. 45), and a cut (not more than 1/4 in. from the end of
the base) is made (Fig. 46) so as to produce a surface normal to the axis of the
jig. The clamp holding the socket is moved out of the way, a partly hollowed,
wooden shank block is now attached to the ankle bracket by means of the same
Allen-head screw as before (Fig. 47), and a cut is made to produce a surface
which, like the bottom surface of the socket base, will be normal to the long
axis of the jig (Fig. 48). When the sawing is completed, the saw guide is
removed from the jig, and shank and socket block are brought together by sliding
the socket clamp back to its original position on the tubular base.
Fig. 44. Removal of the adjustable
shank.
|
|
Fig. 45. Installation of saw guide on
same base as other fixtures.
|
|
Fig. 46. Making saw cut on bottom of
socket block. Remove not more than 1/4 in. at thinnest point about
periphery.
|
|
Fig. 47. Attachment of shank block to
ankle bracket.
|
|
Fig. 48. Making saw cut on top of shank
block. Length of block after cutting shall be such that it may be substituted
for the adjustable shank and pylon without significant change in over-all length
of the prosthesis.
|
|
If all has been done properly, the top
surface of the wooden shank and the bottom surface of the socket block will now
meet comfortably all around the periphery. When that is the case, the mating
surfaces are spotted with glue, brought together firmly, and held in place by
locking the fixtures to the base tubes (Fig. 49). To avoid inadvertent dripping
of glue onto the equipment, the base of the jig may be draped loosely with
scraps of paper, rag, or waste. When the glue has set firmly, the whole unit is
removed from the jig, and the foot is attached to the shank (Fig. 50) in the
same position (with respect to the socket) as before (reference lines match).
Thereafter the leg is ready for final shaping and finishing (Fig. 51).
Fig. 49. Shank and socket block glued
together in relationship established by jig fixtures. Dripping glue is caught by
waste thrown over jig base.
|
|
Fig. 50. Attachment of foot to shank with
same relative toe-out as existed in trial leg using adjustable shank. Compare
with Figure 42.
|
|
Fig. 51. Assembled prosthesis ready for
external finishing. Orientation of parts is that established in trials of static
and dynamic alignment.
|
|
Finishing the Prosthesis
Since it is inconvenient, if not actually
impossible, to determine in advance exactly how the shank block and the socket
block are going to line up in the finished prosthesis, and since ultimately, in the interest of weight-saving, it is
desirable to carve out the shank block to the thinnest possible shell compatible
with strength requirements, it is necessary to break apart the temporary
attachment of shank and socket, but not until essential landmarks have been
recorded for the purpose of later reassembly in the same relative positions as
established in the alignment jig. Similarly, finishing the foot and ankle
(distal part of shank) requires another removal of the foot, but not until the necessary
reference position has been recorded on the work itself. To begin, the toe-out
of the foot is marked with pencil, as shown in Fig. 52A, and the foot is
removed by unscrewing the attachment bolt. Because in the shaping of the distal
end of the shank, and in its preparation for the lamination to follow (page 56),
some material usually has to be shaved off the outside of the shank in the ankle
area, the pencil mark on the anterior aspect is carried onto the base with a
sharp tool, such as an awl or a penknife (Fig. 52B). In order that the
later plastic-laminate covering may form a smooth transition from shank to foot,
a line is now scribed around the periphery of the bottom of the shank about 1/16
in. from the edge (Fig. 52C), and the shank is ground down smoothly to the
line.
Fig. 52. Preliminary steps in the
finishing of the PTB prosthesis. A, Marking the established toe-out of
foot with respect to shank; B, foot removed, reference mark transcribed
to bottom surface of shank to avoid obliteration in next step; C, 1/16-in.
annular ring marked about bottom surface of shank block as guide line for
shaving down ankle area; D, ankle area shaved down, reference lines
marked to record orientation of shank and socket block (after whole limb has
been shaped on outside to match contours of the remaining leg of patient); E,
shank block removed from socket block and routed out to form shell uniformly
1/4 in. thick all around.
|
|
The rest of the external surface of shank
and socket block are now ground down to approximate the contours of the natural
counterpart (preferably to match the shape of the remaining leg of the
particular individual for whom the prosthesis is intended), and reference marks
are made front and rear to indicate the established relationship of socket and
shank (Fig. 52D). The temporary, glued attachment of socket block and
shank is now carefully broken apart by a sharp
knife, and the inside of the shank is routed out (by routing machine or by hand)
until the walls are uniformly only 1/4 in. thick (Fig. 52E). Thereafter
socket block and shank are glued back together, this time with intent of
permanency, the front and back reference lines being made to match up as in the
original attachment.
To provide additional strength and at the same time to give the prosthesis a
pleasant, perhaps even realistic, finish, the whole socket-shank combination is
now covered with a suitable plastic laminate of Fiberglas cloth, nylon
stockinet, and polyester resin, the latter appropriately tinted to simulate the
color of the human skin. The technique is essentially the same as in other
plastic-laminating procedures now in widespread use in prosthetics, for example
in the making of the PTB socket itself (page 73).
The socket-shank unit, less the foot, being supported on a mandrel held in a vise (FFig. 53), a disc of Kemblo is first bonded to
the bottom of the shank to protect it from resin and to close the foot-bolt
hole. Then a sheet of Fiberglas cloth wide enough to extend from the foot base
to within 2 in. of the socket brim is wrapped around the unit and is in turn
covered with two layers of nylon stockinet, the first being made to spiral in
the interest of increased strength (Fig. 54). A PVA sleeve made in the usual
manner is now pulled over the layup, and the fibrous layers are impregnated with
polyester resin in the fashion described earlier (page 36). When the resin has
cured, the excess (including the ends of the PVA sleeve) is trimmed off at top
and bottom (at ankle and at socket brim), and the
foot is replaced with the same degree of toe-out as before.
Fig. 53. Application of disc of Kemblo to
end of shank prior to layup and lamination.
|
|
Fig. 54. Layup for lamination of
socket-shank combination. A, First of two layers of nylon stockinet
twisted over layer of Fiberglas cloth; B, second layer of nylon stockinet
applied and tied off at both ends.
|
|
As a final finishing touch, the superior
plane of the foot (which will now be somewhat larger than the end of the shank)
is scored around with a pencil (Fig. 55), and the foot is sanded down in the
vicinity of the ankle to give a smooth transition to the shank.
The result is a finished prosthesis ready for trial on the amputee to determine,
among other things, the necessity, if any, for further support, or added
stability, or improved suspension in the form of conventional sidebars and thigh
corset. Should the supracondylar cuff already prepared prove adequate,
the amputee should be able to perform with an optimum of comfort, function, and
appearance both in standing and in normal walking on a level surface. In the
event it should not for any reason, the prosthetist proceeds with the
construction of additional equipment.
Fig. 55. Scribing foot at ankle line for
sanding to provide smooth transition between foot and shank.
|
|
The PTB Prosthesis in Special
Cases
The design of the so-called
"patellar-tendon-bearing" below-knee socket is such that, ordinarily, the socket
itself provides adequate stability in both the anteroposterior and the
mediolateral directions and is itself adequately suspended from the limb of the
wearer by no more than the supracondylar cuff already described. With proper
relief in the rear for the hamstring tendons, and with high enough side and
front walls, there develops no insurmountable problem in knee flexion-extension,
either in walking or in sitting, and the amputee is thus free of all impedimenta
otherwise characteristic of the articulated below-knee prosthesis. In a
comparatively small percentage of cases, however, special anatomical and/or
physiological circumstances invalidate the simple cuff suspension and the
equally simple means of support and stabilization typical of the true PTB
prosthesis. In such cases there is no alternative but to resort to the thigh
corset and metal sidebars, and sometimes even to the ischial seat and the waist
belt, despite the known advantages of the PTB socket. Since improvement of
weight-bearing characteristics and inherent stability as offered by the
patellar-tendon-bearing socket in no way alters the problem of the moving center
of rotation of the normal knee, and since single-axis mechanical knee joints are
for various reasons still found to be the most satisfactory under all conditions
of use, introduction of the thigh corset and sidebars to improve stability, or to assume some
of the weight, or both, presents the same problems as have prevailed heretofore.
To date the most useful approach to this problem, when corset and sidebars are
unavoidable, has been the development of an improved and simplified method of
arriving at the best compromise location of single-axis joints with respect to
the moving axis of the normal knee.
Use of Side Joints and Thigh
Corset
Theoretical
Considerations
Single-axis side joints must be aligned
on the shank and corset of the below-knee prosthesis so that they effectively
stabilize the prosthesis on the stump and allow the amputee to sit comfortably.
This is a complicated problem, first because the anatomic joint is not a
single-axis joint and, second, because the exact path of a series of "instant
centers," degree by degree, during knee motion is impractical to determine in
each specific case. Even an average anatomic center may be estimated only
roughly in the posterior portion of the femoral condyles. Thus at any one
position of the single-axis mechanical joints, the center of rotation of the
joints and the center of rotation of the knee will inevitably be incongruent
during part or all of knee flexion and will give rise to some
relative movement between the stump and the
components of the prosthesis as the knee and side joints move from full
extension to flexion at 90 deg. The task is to place the joints in a compromise
position that will offer the best function and eliminate discomfort resulting
from this relative motion. This may be done either by reducing the motion or by
having the motion relieve pressures which would otherwise cause
discomfort.
The effect of a particular position of
the side joints with respect to the socket and corset can best be understood by
investigating the effect of making a change from a position assumed to be the optimum one. Since
movements result from a combination of several factors, total motion is a
complex problem. In a hypothetical situation, it would be possible to have knee
flexion occur either with the stump held tightly in the socket* and
all motion occurring between thigh and corset or with the thigh fixed in the
corset and motion occurring between stump and socket. Of these two extreme
hypothetical situations, and the many possible variations in between, the one
which will be considered is that in which the stump is fixed in the socket and in which
relative motion occurs between the thigh and upper side arms of the joints. This
condition most nearly approximates the real situation and forms the basis for
the joint-location procedure described below.
Fig. 56A shows a hypothetical situation
in which the socket is held fixed and the stump is not allowed to move relative
to the socket. In the fully extended position, the upper sidebar is parallel to
the shaft of the femur, and the mechanical joint center is placed directly above
the average position of the anatomic center. The anatomic center, although it
actually varies in position from high in the thigh during hyperextension to near
the center of the femoral condyles at 90 deg. of flexion, is assumed to maintain
a single axis of rotation for comparison with the mechanical center during this
analysis. Alternatively, one may consider the effect of a tiny range of motion
and study the slight motion of the thigh corset on the thigh caused by a
mechanical joint center higher than the instant center of rotation during this
tiny knee motion. As the thigh flexes, the mechanical sidebar tends to move
relatively anteriorly on the thigh (for 90 deg. of flexion, distance A)
and to be drawn distally along the thigh (distance B). As a result,
pressure is created between the thigh corset and the posterior aspect of the
thigh because the stump is fixed in the socket. The stump might be forced
against the anterior part of the brim (the patellar-tendon area of the stump),
though by assumption the stump cannot move in the socket. Thus the conical thigh
corset moves distally away from the conical thigh, thereby releasing pressure by
allowing a greater perimeter of corset for a given level and perimeter of
thigh.
Fig. 56. Relative motion to be expected
between thigh and thigh corset (stump fixed in socket) during 90 deg. of knee
flexion when single-axis mechanical joints are placed in any of six positions
relative to a hypothetical average anatomic joint center.
|
|
Fig. 56B shows the effects of
placing the mechanical joint below the average anatomic center (or instant
center for a tiny motion). With flexion, the sidebar tends to move posteriorly
on the thigh (for 90 deg., distance C) and to move proximally on the
thigh (distance D). As a result, pressure is created anteriorly between
corset and thigh, or else by reaction forces the socket is pressed upward
against the stump. In this case, the conical corset is forced proximally,
engaging the thigh more tightly and thus further increasing
pressure on the thigh. Because such motion is sharply limited, the reaction on
the sidebars in effect attempts to push the socket forward and thus increases
pressure on the posterior popliteal area of the stump. Clearly this situation is
unsatisfactory.
Fig. 56C shows the effect of placing
the mechanical joint in front of the average anatomic center. With flexion, the
sidebar tends to be forced posteriorly (distance E) and distally
(distance F) with respect to the thigh. As a result, pressure tends to be
created anteriorly between corset and thigh, but the corset is withdrawn
distally down the thigh so that its fit is loosened and hence the anterior
pressure on the anterior portion is partially or wholly relieved.
Fig. 56D shows the effect of
placing the mechanical joint behind the average anatomic center. With flexion,
the sidebar tends to be forced anteriorly (distance G) and proximally (distance
H) with respect to the thigh. As a result, pressure is created
posteriorly between corset and thigh, and the conical corset is forced
proximally until it can go no farther, whereupon reaction forces the socket
forward to cause pressure in the popliteal area.
Fig. 56E shows an interesting special
case in which the mechanical joint is located on a 45-deg. anterior diagonal
through the anatomic center. In this case, the sidebar is drawn distally
downward on the thigh (distance I), but there is no tendency for the
sidebar to move either anteriorly or posteriorly with respect to the thigh. Thus
there is no anterior or posterior pressure between corset and thigh. The distal
motion would indicate that the corset might pull the stump anteriorly and cause
pressure on the patellar tendon. In practice, the conical corset merely moves
distally so as to relieve pressure on the thigh.
A similar analysis of the situation shown
in Fig. 56F would indicate that in this situation (posterior diagonal)
posterior pressure between corset and thigh would be created by the substantial
movement / (anterior movement of the sidebar). There would be no tendency for
the stump to be pushed anteriorly or posteriorly against the socket brim or for
the corset to move on the thigh.
Optimum Mechanical Relationship
Between Joint Axis and Average Knee Axis
Relative movement in the mechanical joint
position as compared with that in the anatomic joint position must first be
understood. The prosthetist can then establish the best position for the joint
axis by deciding what motions to suppress and what motions to allow. However,
when the conical corset is attached to the upper side arms of the joints,
proximal motion of the side arms will be suppressed so that reaction forces on
the arms will cause commensurate forward movement of the socket against the
stump and lead to pressure in the popliteal area. This factor must be borne in
mind when the motions of the upper side arms of the mechanical joints are
considered in establishing the best position. The hypothesis above of fixation
of the stump in the socket may now be modified.
There are two situations in which the
mo-Lions between the prosthesis and the stump are of particular significance:
when the amputee sits (a major fraction of the waking hours of most amputees)
and when the prosthesis is swinging through during walking.
Sitting.
When the amputee sits,
some motion between prosthesis and amputee will occur because of the inevitable
incongruity. This being so, it is better to permit joint movement to draw the
stump slightly out of the socket, and perhaps to move it forward so that roll
formation and pinching between the corset and the back of the socket are
reduced; yet forward motion should not press the rigid bony areas against the
socket wall. In order to lift the stump, the mechanical joints must pull the
corset up against the back of the thigh as the amputee sits. This will occur
when the upper joint arms move anteriorly with respect to the thigh (as in
Fig. 56A, D, and F). To move the slump forward or avoid forcing the
socket forward as the amputee sits, the upper joint arms should move distally
with respect to the thigh (as in Fig. 56A, C, and E). Thus, theoretically, a
satisfactory position for the mechanical joints will be directly above the
average anatomic joint axis, as in Fig. 56A, if it is assumed that the amount
of forward motion and upward motion should be approximately the same.
Swing Phase. For swing-phase
control, and freedom from chafing, there should be little or no motion between
the stump and the socket. Thus, the mechanical joint axis should be as close as
practical to the instantaneous anatomic joint axis during the 60 or 65 deg. of
knee motion in the swing phase. Because the instant center seems to move
substantially during full extension, and especially during hyperextension, the
alignment in slight initial flexion and the training of the amputee to maintain
slight flexion at heel contact are considered to be important steps in reducing
incongruities between axes and thus in reducing chafing.
If the prosthesis is to function
satisfactorily both during sitting and during the swing phase, the mechanical
axis should be above the average anatomic axis but not so far above as to
introduce too much relative motion between stump and socket during
walking.
All the foregoing analyses are based on
consideration of the knee as if it could be averaged over 65 deg. of swing or 90
deg. between sitting and standing to behave as a single-axis joint. But, as is
shown in the preceding article by Murphy and Wilson (page 4), the knee joint is
actually made up of two complex bony surfaces-the femoral condyles and the
tibial condyles. The femoral condyles are two convex surfaces separated by an
anteroposterior groove, while the tibial condyles are two concave surfaces which
fit their femoral counterparts. Further, these bony surfaces are separated by
cartilages and fluids and are connected in complex ways by ligaments, so that
analysis by x-rays alone may be inadequate.
The femoral condyles roll and slide on
the tibial condyles as the knee joint moves. The amount of sliding and rolling
determines the axis of rotation of the knee joint at any instant. A shift in the
axis of rotation may sometimes help and sometimes oppose required function. If
the path of the knee axis were exactly known, the best position for the
single-axis knee joint could be positively stated, and joints fully satisfying
the functional requirements could be designed. As noted above, such refinements
for each individual case seem impractical. However, experience has shown that the mechanical
joints can be located accurately enough when use is made of the procedures
proposed below, based on consideration of the knee as a single-axis joint at an
average location.
A typical relationship between socket,
joints, and thigh corset in the finished prosthesis is shown in Fig. 57. The
back brim of the socket will be trimmed to the patellar-tendon level. With the
joints flexed 90 deg., the posterodistal edge of the thigh corset will be 1 in.
behind the posterior brim of the socket and at the same level as or slightly
above the posterior brim of the socket. The joints are approximately on a
mediolateral axis parallel to the back wall of the socket, midway between the
patellar-tendon protuberance and the posterior wall, and the axis is
approximately 2-1/4 in. above the level of the mid-patellar
tendon.
Fig. 57. Typical relationship between
socket, joints, and thigh corset in a below-knee prosthesis.
|
|
Side-Joint Locating
Chart
Fig. 58 is a chart based on the
theoretical analysis given above. The chart can be used for correct positioning
of the side joints on a below-knee prosthesis. It indicates the motion to be
expected between the upper sidebar (the corset will be attached later) and the
femur (Fig. 59).
Fig. 58. Chart for determining location
of joint axis of sidebars in below-knee prostheses. The motion referred to is
that of the upper straps of the sidebars with respect to the thigh as the
amputee sits from the standing position (Figs. 59 and 60). Outline of distal end
of femur is considered to be mean actual size. The open circle, represents the
average anatomic knee center; the closed circle is the optimum position for the
mechanical joints.
|
|
Fig. 59. Compromise location of upper
sidebar straps in optimum position for comfortable walking as well as for
comfortable sitting.
|
|
Procedure:
- After the socket is aligned on
the adjustable leg and foot, the lateral lower sidebar is attached to the socket
temporarily in the position indicated in Figure 57 so that the center of the
joint is 2-1/4 in. above the midpatellar-tendon level and midway between the
patellar-tendon protuberance and the posterior wall of the socket. Only one
attachment point is used, namely, at the bottom of the sidebar, the bar being
secured above by wrapping masking tape around the socket. The single attachment
point at the lower end of the sidebar allows the joints to be moved back and
forth during trials and simplifies a change in position up or down. The upper
bar is not shaped or attached to the corset at this time.
- The amputee stands and extends
the mechanical joint. The position of the front and top edges of the sidebar on
the thigh is marked with a skin pencil.
- The amputee sits on a hard chair
with his knee flexed 90 deg., and a check is made to see that the posterior brim
of the socket and its lining are properly trimmed and that the stump is well
seated in the socket.
- While the amputee is sitting in
this position, the upper sidebar is moved until the front edge is parallel to
the line on the thigh marked in Step 2. A second mark is made on the thigh along
the front and top edges of the sidebar.
- The relative motion as evidenced
by the difference in position of the marks in Step 4 as compared with Step 2 is
measured.
- On the chart (Fig. 58) is
entered, in accordance with the scales shown, the data obtained in Step 5. This
information will indicate in true scale the approximate location of the
mechanical joint center with respect to the femur, as shown in typical
true size by the dotted outline.
- The direction in which to move the
joint to improve its position is now estimated. The optimum compromise position
is located a short distance above and slightly behind the average anatomic
center. On the basis of experience with adult amputees, the upper sidebars of
the mechanical joint should move distally on the thigh approximately 1/4 in.
with 90 deg. of knee flexion. A motion between 1/4 and 1/2 in. is allowable.
Motion greater than 1/2 in. results in the stump being forced forward
excessively or the corset moving distally excessively after the sidebars are
attached to the corset. The upper sidebars should move toward the front of the
corset approximately 1/2 in. with 90 deg. of knee flexion. This motion is
equivalent to a stump withdrawal with knee flexion after the sidebars are
attached to the corset.
If the movements are not within the
suggested limits, the joint is moved as indicated by the chart to bring them
within these limits, and a recheck is made by the same procedures.
When the joint has been properly located,
both sidebars are riveted to the socket so that a line connecting the centers of
the medial and lateral joints would coincide with the axes of the joints
themselves and would be parallel to the floor and to the posterior wall of the
socket. The upper sidebars are shaped to fit the thigh with the joints coaxial.
Particular attention should be paid to the shaping of the upper bars over the
femoral condyles because a close fit here helps to suspend the
prosthesis. At this point the corset is cut to shape and is temporarily attached
to the upper sidebars of the joints with binding screws.
Example (Fig. 60):
Fig. 60. Example of use of chart shown in
Figure 58, chart reduced from actual size.
|
|
- Step 5 indicates a relative
motion of 1 in. posteriorly and 1 in. distally along the thigh.
- Enter data on chart as shown to
locate point A. Point A represents the probable position of the
mechanical joint relative to the femur.
- The femur outline is
actual size in Fig. 58. Therefore the movement required to relocate the joint
in the assumed optimum position B may be scaled directly from the drawing
in Figure 58 (not in the reduced example, Fig. 60). In this example, the joint
axis shown is moved posteriorly a distance of 1-1/8 in. and proximally a
distance of 3/8 in.
Fabrication of Thigh Corset and Joint
Cover
Just as an encasement for any other part
of the body must be made to conform to the shape of the part and must have
enough elasticity and pliability to meet the requirements of necessary body
activity, so the thigh corset of the below-knee prosthesis must be custom-cut to
the particular size and shape of the thigh for which it is intended and it must
be strong enough and yet flexible enough to meet the changing demands placed
upon it. Because of its special combination of properties, leather has for many
years been the material of choice in the construction of thigh corsets, almost
to the exclusion of all other possible materials. Though from time to time in
the history of prosthetics there have been introduced a good many variations
intended to provide this or that beneficial feature, the basic construction of
the modern-day thigh corset remains unchanged. It amounts to the custom
fabrication of a comparatively long leather cuff, laced in the front, and
furnished with the usual tongue to protect the thigh from local compression and
constriction by the lacing. A common error is to make the corset too short, the
amount of purchase on the thigh then being inadequate to provide the degree of
stability required.
In the method of corset fabrication
currently recommended for use when corset and sidebars are needed with the PTB
prosthesis, the first step is to prepare, from appropriate measurements of the
patient, a suitable paper pattern of the surface of the thigh in the area
between the lesser trochanter and the condyles of the femur. While the optimum
length of the corset varies somewhat with the height of the individual, in
general it may be said that the pattern should extend upward some 8 in.
from about 2 in. above the midpatellar level on the lower end. Accordingly, the
circumference of the thigh is taken at these levels, and the corresponding
measurements are carried forward to the pattern step by step.
A square of paper of suitable weight and
texture (ordinary kraft wrapping paper, for example) and measuring 2 ft. on a
side is first folded in half (Fig. 61A). Along the fold are marked with pencil
the two points corresponding respectively to the top and bottom margins of the
corset (distance between points corresponds to intended length of corset). From
one mark there is extended, parallel to the edge of the paper, a line of length
equal to half the selected circumference of the proximal portion of the thigh.
From the other there is extended a similar line of length equal to half the
selected circumference of the thigh in the distal area. With the ends of these
two lines as reference, a third line is now drawn to join them, all as shown in
Fig. 61A, and a line (broken line in Fig. 61A) is then drawn to
connect the points of bisection of the proximal and distal circumference
measurements, the latter line representing the ultimate location of the upper
straps of the jointed sidebars.
Fig. 61. Preparation of corset pattern.
A, Paper folded in half, top and bottom margins of corset marked, lines
parallel to edge projected to the extent of half the circumference of thigh at
upper margin and half the circumference of thigh at lower margin respectively,
ends of lines connected by straightedge, top and bottom circumferences joined by
straightedge at points of bisection (broken line); B, paper opened at
centerfold, reference lines transcribed to opposite side, proximal margin
modified to match sine curve with maximum deviation of 1/2 in.
|
|
The paper pattern is now opened at the
fold to reveal the isosceles trapezoid shown in Fig. 61B, and the proximal
margin is cut roughly in the shape of a sine curve of 1/2 in. maximum deviation.
Similarly, the distal margin is cut to the dimensions shown in Fig. 62.
Fig. 62. Distal margin of corset pattern
outlined to match requirements of popliteal space, location of upper straps of
single-axis sidebars marked for future reference.
|
|
When the pattern has been completed, it
is laid upon a selected piece of 7-oz. cowhide (or English bridle) in such a
fashion that, when the leather has been cut out, it will fit upon the thigh
(left or right as required) with the rough side in, with opening toward the
front, and with the high side of the proximal margin lateral. By means of a
straightedge, the locations of the upper straps of the sidebars are transferred
to the leather for future reference in the construction of the corset, and the
leather is cut out along the lines of the pattern.
The piece of cowhide, shaped as already
described, is now applied to the thigh of the amputee smooth side out and held
in place by pressure-sensitive tape or some other
suitable means. The upper straps of the two sidebars are bent and shaped in such
a way as to follow as closely as possible the external contours of the thigh (to
assist in stabilization during the stance phase and in limb suspension during
the swing phase), and the proximal ends are trimmed off as necessary so that the
straps will extend to about 3/4 in. below the top of the corset (thus providing
maximum leverage while leaving room for finishing the top of the corset). Then,
for purposes of later attachment of the upper straps of the sidebars to the
corset, each upper strap is drilled with three holes 1/8 in. in diameter and so
spaced along the length of each strap that the first is 1/2 in. from the
proximal end, the second is about 2 in. above the center of the ballbearing race
on the distal end, and the third is half way between the other two (Fig. 63).
The two upper sidebar straps, thus drilled to accommodate screw-type fasteners,
are now placed against the corset, one on each side and each along one of the
two guide lines outside the centerline, and the positions of the two top holes
are marked through to the leather. The straps are removed, 1/8-in. holes
are punched through the leather at the points indicated, and the two upper
straps are attached, each by means of its top hole only.
Fig. 63. Preparation of upper sidebar
straps for later attachment to leather thigh corset.
|
|
To set temporarily, subject to later
revision if necessary, the bottom (distal) attachment holes of the straps, the
amputee stands, the prosthetist positions each strap directly over the
corresponding guide lines, and the bottom hole of each strap is marked through
to the leather with pencil (Fig. 64A). The amputee then sits with knee flexed 90
deg., the straps are once again positioned over the guide lines (Fig. 64B), and
the bottom holes are again marked through to the leather (at the new position).
The holes for the bottom attachments are now punched through the leather at the
proper height but midway between the two points marked on each side (Fig. 64C). The process amounts to bisecting the angle between the positions
of the bars in standing and their positions during sitting with knee flexed 90
deg. When the lower attachments have been completed, subject to final
adjustment, the prosthetist proceeds with the remaining details of corset
construction.
Fig. 64. Tentative attachment of upper
sidebar straps to corset.
|
|
While the amputee stands upon the
socket-shank-foot unit, the leather corset is wrapped about the thigh in the
intended position, edges in front, and the edges are marked for trimming so
that, thereafter, they will be 1-1/4 in. apart (Fig. 65). The corset is removed
from the patient, the edges trimmed as marked, and 1/4-in. holes for the lacing
are punched along each edge on 1-in. centers along lines 3/8 in. from the edges
(Fig. 65). Now the amputee dons the corset and laces it up with a suitable
length of nylon parachute cord singed at each end to prevent fraying. While
he stands thus, any necessary adjustments are made in the trim lines at top and bottom, the intent being to have the front lower edge fit closely about the patella and just above it while in the back there is enough relief to avoid bunching of the flesh when the patient sits. Should the alignment of the sidebar straps prove to be faulty for any reason, re-alignment should be carried out before
proceeding further.
Fig. 65. Trimming of front edges of
corset, placement of lacing holes in proper position.
|
|
When the fitting is thus far
satisfactory, a tongue is provided out of the same kind of leather (cowhide) as
was used for the corset itself, and the entire component is lined with cream
horsehide of medium weight (4 to 6 oz.). To form the tongue, a piece of cowhide
is cut long enough to extend from top to bottom of corset and wide enough to
extend 1 in. beyond the rows of eyelets on either side (Fig. 66). One of the
long edges is then skived so that, when that edge is later sewed to the body of
the corset, there will be a smooth transition from corset to tongue such as not
to cause any unnecessary irritation when the unit is worn. To line that portion
of the corset between the fixed side of the tongue and the edge on that side
(Fig. 67), a piece of medium-weight horsehide is cut 2 1/2 in. wide and
long enough to extend from top to bottom of lacer. One of the long edges is
skived, and the strip is then bonded (with rubber cement) to the inside surface
of the corset, smooth side facing in and skived edge lying 2 1/4 in. in
from the edge (which leaves about 1/4 in. of surplus horsehide for later
trimming).
Fig. 66. Relative size and shape of
corset tongue.
|
|
Fig. 67. Lining of corset tongue area on
fixed side of tongue.
|
|
The tongue of cowhide is now placed
smooth side out (toward the front of the corset) over the horsehide lining of
the edge of the lacer and with skived edge about 2 1/4 in. in from the edge of
the corset. When a smooth transition has thus been attained by whatever local
adjustment is necessary, both tongue and liner are sewed along the long side.
The smooth side of the lacer and the corresponding smooth side of the tongue
thus face each other to avoid any otherwise unnecessary bunching or wrinkling of
tongue or corset.
The next step is to line with
medium-weight horsehide the entire remaining internal surface of corset and
tongue. To do so, the corset (together with the tongue) is laid out flat on the
bench, rough side down. Thereupon is placed, rough side up, a piece of
medium-weight horsehide large enough to cover the entire piece of work. Thus
horsehide liner and corset-tongue combination are placed smooth side to smooth
side. When the liner has been cut out to correspond roughly to the shape of the
corset, the two pieces are sewed together across the top, the seam line starting
where the tongue joins the corset and ending about 1 in. short of the opposite
side. Thereafter the whole piece is inverted (Fig. 68) so that the horsehide
falls over the cowhide corset and tongue to form a smooth liner, smooth side of
horsehide in, smooth side of cowhide out. The entire facing surfaces are then
bonded together with rubber cement, the edges are sewed around carefully, and
any excess is trimmed close to the seams. On the side opposite the base of the
tongue, a final seam is sewed down the edge of the corset just inside the row of
eyelet holes, and the latter are then cut through the horsehide liner. Into
the punched holes are then installed the metal grommets for the lacing.
Fig. 68. Lining of entire internal
surface of corset and tongue.
|
|
To protect the clothing from excessive
wear, specially designed leather covers are commonly placed over the upper
flanges of the sidebars and over the housings of the ball-bearing races. For
this purpose use is made of cowhide one third the thickness of the leather used
to make the basic part of the corset. By appropriate use of the pattern shown in
Fig. 69, one cover is made for each side of the corset, one medial and one
lateral. When the sidebars have been riveted in place permanently through all
three holes on each side (with 1/8-in. copper rivets), the covers are set in
place, the distal portions being doubled back upon themselves and glued together
with rubber cement. After the upper portions of the covers have been sewed to the corset on
both sides, any excess is trimmed off, and a rivet is installed at about the
point shown in Fig. 70.
Fig. 69. Pattern for side-joint covers,
half actual size.
|
|
Fig. 70. Installation of side-joint
covers for protection of clothing.
|
|
Finally, as protection against the
effects of moisture and bacteria, all of the leather parts are coated with nylon
solution according to the usual techniques .
Auxiliary Belt Suspension
In below-knee prosthetics, the
conventional thigh corset (and sidebars) may serve any of three purposes to
varying extents and in varying combinations. It may be needed to provide
necessary additional stability not to be had from the below-knee socket alone.
It may provide needed suspension over and above that furnished by the
supracondylar cuff. It may be needed to furnish
additional weight-bearing over and above that provided by the PTB socket. Or it
may be required for any of these purposes in one combination or another.
Occasionally, additional suspension is needed for the PTB prosthesis with or
without the thigh corset, and in such cases use is made of the pelvic belt in
any of several forms. In all cases the belt fits about the iliac fossa on the
normal side and extends downward on the side of the amputation to connect to the
prosthesis itself. When, in addition to thigh corset and side joints, the pelvic
belt is needed, it is attached to the prosthesis above the mechanical axes of
the artificial knee joints. When the belt suspension is required on a limb
without thigh corset or sidebars, it is attached to the limb either just below
the brim of the socket or else to the supracondylar cuff, whichever is applicable. In general, the
pelvic belt serves to reinforce the suspension provided by the supracondylar
cuff, not the other way round. The supracondylar cuff is always tried first.
Whenever it suffices, no pelvic belt is required.
To prepare the pelvic belt and associated
suspensory attachments for the below-knee prosthesis, use is made of the
patterns shown in Fig. 70 and usually of one or the other of those shown in
Fig. 72. First there is cut from 2-in. cotton webbing a length 3 in. shorter
than the waist measurement. It forms the belt component labeled "waistband" in
Fig. 73A. Next a 7-in. length of 2-in. elastic webbing is cut to form the
tensile element of the vertical support (Fig. 74). Then there are cut from 6-oz.
cowhide or pearled elk one piece according to pattern A (Fig. 71), two pieces according to
pattern B (Fig. 71), and two pieces according to pattern C (Fig. 71). These form respectively the boomerang-shaped portion of the waistband
(section A in Fig. 73A), the buckle billets (5/8-in. buckles) to be
installed on the belt (B in Fig. 73B) and at the proximal end of the
elastic suspensor (B in Fig. 74), and the two elements labeled "sections
C" in Fig. 73C. When, in addition to the thigh corset and
sidebars, the pelvic belt is required, suspension is by virtue of the inverted
Y-strap shown in Fig. 74, the forked section being fashioned according to
pattern D of Fig. 72 and the ends of the fork being attached to the
prosthesis above the mechanical axes of the artificial knee joints, as already
pointed out (page 61). When pelvic suspension is required in the absence of
thigh corset and sidebars, section D (Fig. 72) is replaced by section
E (Fig. 72), or the elastic vertical suspensor (Fig. 74 and Fig. 75) may be
attached directly to the anterior aspect of the supracondylar cuff (Fig. 75)
without the necessity for sections D or E (Fig. 72). Details of
fabrication technique for these several variations in auxiliary suspension are
readily to be had from Figures 71 through 75.
Fig. 72. Patterns (one half actual size)
for suspension straps when (D) thigh corset and sidebars are used and
(E) when thigh corset and sidebars are not used.
|
|
Fig. 73. Details of assembly of the
pelvic belt.
|
|
Fig. 74. Details of suspensor strap when
pelvic belt is used in addition to thigh corset and sidebars. When thigh corset
and sidebars are not required, section D (Fig. 72) is replaced by section
E (Fig. 72).
|
|
Fig. 71. Patterns for construction of the
pelvic belt shown in Figure 73, half actual size.
|
|
Fig. 75. Arrangement of suspensor strap
when auxiliary support from pelvic belt is used in conjunction with the
supracondylar cuff but without thigh corset and sidebars.
|
|
As for details of actual construction,
section A (Fig. 73) is first bonded to the waistband with rubber cement
with an overlap of 1 1/2 in. the bonded side being on the side of the
amputation (Fig. 73C). The skived ends of the
leather sections B (Fig. 71) are lapped back on each other, each piece is
threaded with a 5/8-in. buckle, and the billets so formed are applied, one to
section A (Fig. 73) and one to the proximal end of the elastic vertical
sus-pensor (Fig. 74). The billets (B) having been fixed in place with
rubber cement, the forked section D (or the U-shaped section E) is
cemented to the distal end of the elastic webbing, as shown in Fig. 74, and
the ends of the fork (or of the inverted U) are attached to the socket just
below its brim on the medial and lateral sides. When belt suspension is intended
simply to supplement the cuff-suspension system, less corset and sidebars, the
vertical section shown in Fig. 74 is attached directly to the anterior portion
of the supracondylar cuff (Fig. 75). In every case all leather parts are backed
with a lining of horsehide, and all segments are sewed around, excess horsehide
being trimmed off close to the stitching.
Conclusion
In the construction or manufacture of any
piece of apparatus or equipment, for whatever purpose, there may occur to the
experienced craftsman any number of variations in technique to effect the same
result-some in the interest of economy perhaps, some possibly with the intent of
making the task easier, conceivably some with the idea of improving reliability
in a stepwise procedure and hence of reducing the possibility for error, some
perhaps for other reasons. Just so with the patellar-tendon-bearing,
total-contact, below-knee socket. The particular method herein described for
construction of the PTB socket, and of associated equipment for use in special
cases, is not, therefore, the only possible method. It is simply the one which,
in U. S. experience covering more than four years, has proved to be successful
and the one most widely used. It is entirely possible that desirable changes in
the recommended technique of construction, or with respect to the materials
used, will be apparent at once to prosthetists and others. There is, indeed,
nothing particularly sacred about the actual stepwise procedure described for
fabrication, or about the actual materials suggested, so that it is
reasonable to expect changes here and there as the application of the PTB
prosthesis comes more and more into widespread use.
Whatever changes in materials or
fabrication technique may in the future be found to be useful, however, it is
essential that the principles utilized in the PTB socket-in its design and in
its application with respect to the wearer and to the rest of the prosthesis- be
held inviolate if success is to be attained in the majority of cases. Features
such as the ledge for weight-bearing on the patellar tendon, the high sidewalls
for increased medio-lateral stability in standing and walking, the relief for
the hamstring tendons during knee flexion in sitting and in the swing phase of
walking, the firm but gentle contact of stump with socket throughout its length
as well as at the terminal end, the soft liner and end pad for shock absorption,
and the subtle aspects of alignment in slight adduction and slight initial knee
flexion are all based on systematic analysis of physical and anatomical fact and
are therefore indispensable to the usefulness of the true
patellar-tendon-bearing below-knee prosthesis. If, in the otherwise average
below-knee case, any one of these details is lacking, difficulty in one form or
another will ensue, in which case other and undesirable expedients have to be
devised and the inherent advantages of the PTB prosthesis-freedom from the
restrictions imposed by additional equipment-are at best seriously discounted
and may in fact be lost entirely.
Although precision and meticulous
workmanship are generally acknowledged to be essential requirements in the
successful construction and fitting of any limb prosthesis, they are in the PTB
limb especially in need of emphasis. Since the self-stabilizing, total-contact,
patellar-tendon-bearing, below-knee socket is intended to be manageable by the
wearer with little or no external assistance, all features of measurement, of
fit, and of orientation are particularly critical, so that even a minor fault
may result in gross deviation from proper performance. The eventual outcome of
any PTB fitting is thus not only a matter of formal instructions but also of
the exercise of sound judgment on the part of
the clinic team in each and every individual case. General experience to date
has indicated that the added investment in time and precaution almost always
results in a satisfied and successful wearer. Failure to attend details almost
always gives rise to failure and disappointment.
Appendix A
Formulation of Polyester Laminating Resin
(for Each 100 Grams)
Into 100 gms. of polyester resin mix thoroughly 2 gms. of ATC catalyst. Then mix in color paste according to manufacturer's recommendation. Add 10 drops of Naugatuck Promoter No. 3. Mix thoroughly.
Appendix B
Procedure for Changing Heel-Cushion Stiffness in SACH Foot
In the event the amputee, standing on the socket-shank-foot-shoe combination, demonstrates proper heel elevation (11/16 in.) but too hard or too soft a heel cushion during walking, the heel wedge must be replaced with another, either softer or harder as the case may be. The amputee first steps out of the socket, the shoe is removed from the foot, and the remaining unit is placed on a level bench with a block of wood 11/16 in. deep under the heel (Fig. 1A). By means of an ordinary carpenter's square, a vertical reference line is marked on one side of the socket block in the vicinity of the anteroposterior midline so that, after the wedge has been replaced, the prosthetist can be certain that the same orientation of the socket has been re-established.
The edge of the sole around the heel is not marked in such a way as to locate the anterior point of the existing heel cushion (Fig. 1B), the shank is clamped in a wood vise heel up, and the entire heel cushion is cut out with a sharp knife, the sole being peeled back first, the wedge itself later. Ant irregularities in the cut surfaces are smoothed with a fine file, and the new wedge is inserted, longest lamination next to the sole, and to such an extent that the point falls as nearly as possible into the position previously occupied by the point of the old wedge.
Thereafter the whole unit is remobed from the vise and placed upon the bench with the 11/16-in. heel block under the heel as before. Movement of the new wedge forward or backward, as required, re-establishes the original alignment, as indicated again by the square (Fig. 1C). When all is in order, the new wedge is cemented into place with Stabond T-161, and the heel is again shaped in the way previously recommended.
Fig. 14. Pattern for preparation of the
PVA sleeves.
|
|
Fig. 33. Preliminary alignment of trial
leg in four successive steps using the adjustment facilities of the UC
below-knee adjustable shank.
|
|
References:
- Anderson, Miles H., John J. Bray, and Charles A. Hennessy, The construction and fitting of lower-extremity prostheses, Chap. 6 in Orthopaedic appliances atlas, Vol. 2, Edwards, Ann Arbor, Mich., 1960.
- DeFries, Myron G., and Fred Leonard, Bacterio-static nylon films, Appl. Microbiology, 3 (No. 4): 238 (1955).
- Leonard, Fred, T. B. Blevins, W. S. Wright, and M. G. DeFries, Nylon-coated leather, Ind. Eng. Chem., 45:773 (1953).
- Murphy, Eugene F., The fitting of below-knee prostheses, Chap. 22 in Klopsteg and Wilson's Human limbs and their substitutes, McGraw-Hill, New York, 1954.
- University of California, Biomechanics Laboratory (Berkeley and San Francisco), Manual of below-knee prosthetics, November 1959.
- University of California, Biomechanics Laboratory (Berkeley and San Francisco), The patellar-tendon-bearing below-knee prosthesis, 1961.
References | 2. | DeFries, Myron G., and Fred Leonard, Bacterio-static nylon films, Appl. Microbiology, 3 (No. 4): 238 (1955). | 3. | Leonard, Fred, T. B. Blevins, W. S. Wright, and M. G. DeFries, Nylon-coated leather, Ind. Eng. Chem., 45:773 (1953). |
|
|
Footnote | Particularly if the socket wall were rigid and lacking a soft lining. |
|
|
Footnote | Application of cement within the 1/2-in. border around the estimated trim line is avoided at all times. |
|
|
References | 1. | Anderson, Miles H., John J. Bray, and Charles A. Hennessy, The construction and fitting of lower-extremity prostheses, Chap. 6 in Orthopaedic appliances atlas, Vol. 2, Edwards, Ann Arbor, Mich., 1960. | 4. | Murphy, Eugene F., The fitting of below-knee prostheses, Chap. 22 in Klopsteg and Wilson's Human limbs and their substitutes, McGraw-Hill, New York, 1954. |
|
|
A. Bennett Wilson, Jr. | Staff Engineer, CPRD, NAS-NRC, 2101 Constitution Ave, Washington 25, D.C. |
|
|
Bryson Fleer | Staff Editor, CPRD, NAS-NRC, 2101 Constitution Ave, Washington 25, D.C. |
|
|