The Lange Silicone
Partial Foot Prosthesis
Lawrence R. Lange, CPO
Introduction
Though partial foot amputations are seldom
seen in most practices, and often do not require the use of a prosthesis, our office has
found a need for an improved prosthesis for
this amputation level. Currently, several
prostheses are available for this level, but
they have design problems. Foot plates with
toe fillers provide no means of securing the
residual limb to the prosthesis and require
shoes with lace closures (frequently high
top) to supplement this feature. Supramalleolar varieties are usually fabricated from
semirigid materials and are difficult to don
and doff, uncosmetic in appearance and do
not provide for residual ankle motion. This
article will offer an alternative to these supramalleolar varieties and correct some of
their detriments.
Development
Before 1984, partial foot prostheses were
primarily rigid devices of conventional design fabricated from leather and metal or
plastic laminate with a foam toe filler. These
devices did not replace anatomical motion
lost by amputation, but they did retain rollover in the toe section. In more distal amputation levels, the prosthesis was incorporated as part of the shoe, and no true socket
existed. This allowed wasted motion between the residual foot and the device.
There was no total contact in these devices,
and retaining the heel in the shoe often required the use of a lace closure and high top
shoes. The emphasis was always toward rigidity. Prosthetic failure-device or skin
breakdown-resulted from the inability of
these devices to follow residual anatomical
joint motion. For more proximal levels of
amputation, trimlines extended to the knee
to distribute the anterior pressure over a
greater surface area. These devices became
even more rigid due to increased breakdown
problems between the prosthetic socket and
toe lever.
Literature in this field before 1984 is scant
in reference to treating these levels of amputation. With increased surgical successes in
these distal amputation levels, and improved
limb salvage techniques, more patients now
have healed partial foot amputations. This
has resulted in the development of more orthotic and prosthetic devices to treat these
patients, and consequently, the O&P field
has become more concerned with preserving
motion and flexibility. The impact of energy-storing materials has produced significant
change in the treatment of amputees and the
expectations for their performance. Better
interface materials that behave like soft tissue supplements have emerged. All of these
improvements have resulted or played a part
in the development of the silicone partial
foot prosthesis.
The work on prototype flexible partial
foot prostheses began while the author was
working at a pediatric orthopedic institution.
We treated some patients with congenital
partial foot anomalies. Designs known
through education, experience and literature
were rigid, prone to failure or uncosmetic.
Cosmesis was a primary concern to the parents of the patients. Concerns also existed
about growth, fit and function. Attempts
were made to fabricate semirigid devices incorporating clear and fleshtone flexible plastics with foam toe fillers. The devices' fit and
function were fine but their appearance was
lacking. Plastic sockets were then implanted
into juvenile Symes SACH feet. This resulted in better cosmesis but failure of the device
became the primary consideration. A more
flexible, comfortable material as the socket
interface material was decided upon. More
flexibility did not appear to hamper gait, and
patient complaints and proximal trimlines
became fewer and lower, respectively. The
interface material, leather with a cowhide
liner, was comfortable but did not survive
well with children. It soiled quickly, and
odors could not be eliminated.
The final evolution of the basic device was
complete when silicone sockets were laminated to attach to the hollowed foot shells.
The socket was adhered to the foot shell with
silicone sealant, and Velcro closures provided the means for donning and doffing the
device. Improvements over time included
performing the second lamination over the
foot shell and initial socket to permanently
combine the two, and the use of a zipper in
the posterior of the socket to provide a more
cosmetic closure. With the introduction of
energy-storing feet, cosmetic, hollow foot
shells became available.
The present silicone partial foot prosthesis
requires two laminations and incorporates
the foot shell between the two. This permanently bonds the foot shell to the socket and
produces an elastic, resistive toe lever. This
toe lever can be further enhanced by adding
a variable durometer RTV foam in the hollow toe section. Thus, while more natural
ankle motion is achieved, there is resistance
to drop off past the midstance part of gait,
allowing for more normal ambulation. The
zipper closure allows very easy donning and
doffing, and the trimlines can be varied to
enhance suspension and control of the device for different levels of amputation.
Description
A supramalleolar negative impression is taken of the patient in a weightbearing or nonweightbearing position, depending on the integrity of the ankle joint and practitioner
preference. A positive model is developed in
the usual manner, and a parting lacquer is
applied. The desired thickness of sock to be
worn with the prosthesis is placed over the
model, and a PVA inner bag is applied and
capped. Three to four layers of perlon-tricot
stockinette are sewn and attached to the
model, and another PVA bag is applied. Silicone resin is poured into the sleeve and laminated into the stockinette. We laminate with
Otto Bock silicone for soft inserts. This resin
requires curing over a two-hour period at
greater than 100° F. This can be accomplished most easily by placing a heating pad
over the laminated positive model. Once this
inner lamination has cured, the outer PVA
bag is retained and sealed smoothly over the
distal end. This will act as a separator during
the second part of the fabrication.
For the second step of this procedure, a
Quantum (see Figure 1
), Carbon Copy System III or similar foot shell of the desired
overall length and width is selected. The attachment plate is cut out of the foot, and the
interior dimensions are enlarged to accommodate the newly laminated socket. The
socket, still on the cast with the previous
outer PVA bag intact, is fitted to the foot
shell until the toe out is correct, and the foot
shell is not constrictive around the socket.
The foot shell may be made more or less
rigid along the sides and posterior by varying
the thickness of the remaining shell. The toe
of the foot shell is filled with RTV foam
(available from Southern Prosthetic Supply), varying the durometer of the foam
based on the length of the residual limb. The
greater the length of the residual limb, the
lower the durometer necessary for a toe lever. The outer layer of PVA is now removed
from the inner lamination. Otto Bock silicone adhesive is applied to the interior of the
foot shell and over the cured RTV foam. The
model is then introduced into the shell, and
the adhesive is smoothed along the edges to
provide a cosmetic transition from shell to
socket. Once this has cured, the foot shell is
prepared for the second lamination.
A tie-off ring is carved into the shell at the
toe break using a Dremel-type tool. This ring
allows the stockinette used in the final lamination to be tied off at this point, providing a
smooth transition from the lamination to the
foot shell (see Figure 2
). The outside of the
foot shell is abraded proximal to the lamination ring. After this is done, silicone adhesive is applied over the entire outside of the
first lamination and the foot shell as far distal
as the toe break (see Figure 3
). A zipper is
positioned on the socket's posterior. The
zipper should be metal with fabric sides.
Polyester fabric is preferred so small holes
can be burned through the fabric to anchor
the adhesives through the material (see Figure 4
). After positioning the zipper on the
socket, it should be pressed into place and
adhesive allowed to extrude through the
holes burned into the fabric. Care should be
taken to keep adhesive from getting onto the
tracks of the zipper (see Figure 5
). Two to
three final layers of stockinette are applied
and pressed into the adhesive around the
zipper. The last layer or two should be of a
cosmetic shaded nylon stockinette to match
the patient's skin tone (see Figure 6
).
By using this technique, instead of pigmenting the silicone, curing problems have
been reduced. Once this has been done, the
outside PVA sleeve may be applied, and a
final lamination performed. The lamination
should be kept as thin and wrinkle-free as
possible to provide a light cosmetic finish.
After curing the completed prosthesis, the
outer PVA bag may be removed. The laminated silicone over the center of the zipper
may be cut. Do not trim silicone from the
attachment straps. The zipper may be
opened at this point. Using a very sharp
knife or scalpel, cut a straight line extending
from the base of the zipper to the proximal
trimline. Once this is done, the prosthesis
may be removed from the positive model
and trimmed. This fabrication procedure
will produce a light, soft, flexible, cosmetic
alternative to previously used varieties of
partial foot prostheses (see Figure 7
, Figure 8
and
Figure 9
).
For more distal partial foot amputation
levels, the height of the prosthesis may be
lowered to the height of the shoe counter.
This allows the finished prosthesis to be
donned and doffed without the use of a zipper. With this exception, the fabrication is
the same as is described above. The greater
the height of the prosthesis, the greater its
suspension and stabilization qualities.
Discussion
To provide better medio-lateral stability, we
have experimented with the addition of a
form of high-density polyethylene, Subortholene, as a sandwiched splint material. This
addition may be important for treating ankle
joints that suffer ligamental instability or loss
of segmental integrity. This can naturally be
the consequence of previously untreated
neuropathic dysvascular feet. This addition
need not interfere with normal tibial-talar
motion but would interfere with what has
become abnormal subtalar motion.
To date, more than 20 of these devices
have been fitted to patients and follow-up
maintained. Only two devices were rejected.
One rejection was due to uncontrollable
edema, the other to the patient's preference
for something more solid. (see Table 1
).
All levels of partial foot amputation have
been treated with this device. Modifications
may include higher trimlines or the incorporation of semirigid materials for increased
support at the more proximal amputation
levels.
Contraindications include the more proximal levels of amputation - i.e., Lisfranc and
Chopart-for patients who may wish to participate in very strenuous sports. Patients
with severe plantar surface ulcers secondary
to Charcot joints in the foot or those with
radiological evidence of bony disruption of a
severe nature would probably not benefit
from this treatment, but rather should have
weight relief of the lower extremity as their
primary care.
Summary
This article describes the rationale and fabrication procedures for a lightweight, flexible,
partial foot prosthesis fabricated using silicone resin. It was developed in response to
the need for a prosthesis that would provide
qualities not present in previously described,
less functional devices. The above-mentioned prosthesis allows for ankle motion,
interchangeability of shoes, good cosmesis,
comfort and excellent suspension.
Acknowledgements
I would like to acknowledge and thank the following individuals for their assistance in the development of the silicone partial foot prosthesis and the
collection of the data for this article: Monte
Chambers, RT (P); Michael O'Donnell, CPO;
Neil Slavkin, CP; and Ron Kidd, CPO.
Lawrence R. Lange, CPO, is vice president of
Hanger Orthopedics, 95 Bridge St., Wheeling.
W.Va. 26003.
References:
- Fillauer K. A prosthesis for foot amputation
near the tarsal-metatarsal junction. Orthotics and
Prosthetics September 1975 ;29:2:9.
- Young RD. A case study: special Choparts
prosthesis with custom-molded foot. Orthotics
and Prosthetics December 1984;38:4:79.
- Imler C. Imler partial foot prosthesis:
IPFP-the Chicago boot. Orthotics and Prosthetics September 1985;39:3:53.
- Bowker JH. Partial foot and Symes amputations: an overview. Clinical Prosthetics and Orthotics Winter 1987-88; 12:1:10.
- Stills ML. Partial foot prostheses/orthoses.
Clinical Prosthetics and Orthotics Winter 1987-88;
12:1:14.
- Collins JN. A partial foot prosthesis for the
transmetatarsal level. Clinical Prosthetics and Orthotics Winter 1987-88; 12:1:19.
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