Technical Note: Interchangeable Feet
Art E. Price, C.P.O.
Introduction
The ability to wear high heel pumps and
then switch to fashionable workout tennis
shoes is something that most women seldom
think about. The ability to wear normal cowboy boots is equally unremarkable for most
men. The amputee, however, is often
doomed to choosing a given heel height in
order that the original alignment characteristics of a given prosthesis can be preserved
and gait is not adversely affected.
There are feet available to compensate
somewhat for this inability to change heel
height. However, such compensation is not
exact and is often prone to failure due to the
forces encountered during normal gait.
Apart from this, simply changing the angulation of a given foot does not change the internal characteristics of the foot (i.e., keel
shape, toe break location, and plantar shape
of the foot as related to the footwear).
This being the case, this practitioner has
always resisted amputee requests for changing shoe styles and heel heights, considering
them not realistic. Such changes in these
alignment criteria could adversely affect the
gait and safety of the prosthesis, and it was
not considered sound practice to risk .such
fundamental points in order to allow changing shoes for cosmetic reasons. This reasoning has since been reconsidered in favor of
the view that a viable alternative for some
amputees involves the use of interchangeable feet. Such feet must be prepared to create the same (or reasonably similar) gait
characteristics.
Interchangeable Foot Preparation
A workable alternative for two aboveknee (AK) patients involved the use of interchangeable feet. These were obtained from
Kingsley Manufacturing. This manufacturer
has a well-established line of Solid Ankle
Cushion Heel (SACH) feet for various heel
heights and styles. A typical endoskeletal
system (Otto Bock with a safety knee and
SACH foot adapter) was used for both of
these young ladies. Both had a desire to use
various kinds of shoes (including pumps) and
both were of adequate residual limb length
to handle SACH feet. It was reasoned that if
the mating surface of each foot met the ankle
plate level (at the same height and angle),
then the artificial limbs should operate properly.
First, two prosthetic feet were selected on
the basis of heel heights. Once the feet were
obtained, there was the matter of making
them the same in relation to both height and
surface angulation. Kingsley is adamant
about not grinding or changing the mating
surface of the feet, so the tallest one governs
what to do. The higher heel Kingsley feet
have an X-height (i.e., height at the foot
bolt) that is roughly half an inch higher than
the lower ones. Placing this higher foot in its
shoe allows the measurement of surface angle in the Anterior-Posterior (AP) and Medial-Lateral (ML) planes. For the foot selected, it tilted posteriorly about three degrees and was level in the ML plane. Basswood was added to the lower foot shim to
compensate for the height and surfaced to
establish this same three-degree posterior
tilt and level ML reading. The shim was affixed with Barge cement. (Acrylic resin
would also have been sufficient.) For one
patient, a third foot was modified as she
wore one-and-a-half-inch high heels at
times. All the feet were shaped to meet the
plastic cover plate cosmetically on the
SACH adapter. They were then covered
with pigmented resin to match the foot color
and powdered to dull the residual shine (Figure 1)
.
Aligning Interchangeable Feet
Obviously, there are changes in stability
related to heel breadth. A pump has a spike
heel and affords little side-to-side stability on
heel strike. A tennis shoe is quite stable in
comparison. For this reason, the high heel
foot was aligned first, giving particular notice to heel strike and adjusting the pyramidal plate so that the heel was centered during
dynamic alignment. Thus, the shoe did not
try to shift either medially or laterally. Once
this less stable foot was aligned, the lower
(and wider) heels could only feel more stable. The use of an alignable SACH foot
adapter (such as the one available from Otto
Bock) was found to be absolutely essential in
this regard.
In order to keep toe out as a constant, it
was necessary to devise a foolproof method
for locking each foot in the same toe out
position (though some variance is certainly
possible). The optimal position is arrived at
during dynamic alignment. Once this position is arrived at, it must be locked in. To this
end, a one-eighth-inch hole is drilled
through the SACH adapter plate and into
each foot. The hole in the foot is made about
one inch deep and then enlarged with a number 29 drill. Ridges in the foot, formed by the
Bock SACH adapter, aid in keying the foot
into its final position (Figure 2)
. A one-eighth- by one-inch monel rivet is then prepared by removing its head and squeezing it
in a flat jaw vise to flatten it for about one-fourth inch of its top end. It can then be
pressed into the one-eighth-inch hole in the
adapter plate, and it will remain there projecting perpendicular and distal to the adapter plate (Figure 3)
. This pin will locate each
foot as it is being interchanged and assure
proper toe out (Figure 4)
. If more feet are
desired, the locating pin is simply punched
out (to drill the new foot's aligning hole) and
then driven back into place.
Patient Instructions
The patient should be given a 5/16-inch T-handle hex wrench to facilitate easy removal
and reapplication of the feet. These wrenches are available commercially at hardware
stores(Figure 5)
. The patient should also be
instructed to change the feet only with the
leg off so that the exposed pin will not become damaged when the foot is not in place.
Putting on and taking off the feet are demonstrated, and the patient is allowed to perform
the task until comfortable with the procedure (Figure 6)
. Cosmetic hose are used to
cover the entire prosthesis after the various
feet are interchanged (Figure 7
and Figure 8
).
Results and Comments
Both patients fitted were extremely
pleased with the freedom this kind of interchangeability afforded them. One patient
wrote an unsolicited thank you note saying,
"I can't tell you what it means to me to look
down and see two very nice legs in high heels
It has been a truly liberating experience, one which has greatly increased my
self-confidence." The second patient wanted
to be able to walk with her husband to stay in
shape. She currently reports walking four-and-a-half miles a day. She now wants a new
cover because her non-amputated side has
much better shape and muscular definition
than when the original cover was shaped.
As previously mentioned, some difference
will be noticed by the patient when going to
and from the different feet. However, this
difference is minimized, and it is acceptable
if the patient is motivated and of adequate
residual limb length to control a SACH foot.
The use of interchangeable feet is not advocated for all amputees, but it has a place
when questions of cosmetic freedom are at
hand. The desire for the ability to follow
current footwear trends and stay "fashion-
able" should not be considered capricious or
vain. It is a legitimate concern. Represented
here is a simple, reliable, and duplicable
method of addressing it.
Art E. Price, C.P.O., is with Orthomedics of
Santa Monica, 2222 Santa Monica Boulevard,
Suite 203, Santa Monica, Calif. 90404; (213) 4530011.
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