Experience with Endoflex: A Monolithic
Thermoplastic Prosthesis for Below-Knee Amputees
Thomas J. Valenti, C.O.
Introduction
Over the past 10 years, exciting advances
have been made in the field of prosthetics
and orthotics. Such developments include
computerized office management, video gait
analysis, computer-aided socket design and
sophisticated fabricating techniques. All of
these have contributed significantly to our
profession and have enabled us to treat patients on a more professional level, as well as
improve the comfort and performance of the
amputee population.
This article will focus on only one aspect of
these advances, thermoplastic technology. It
is this author's opinion that thermoplastics
have provided more advantages in terms of
fabrication, than any other material available. The clear check socket is a primary
example and has come to serve as an important diagnostic tool during the evaluation
process. Understanding the properties of
these materials and the ability to choose and
handle them appropriately will direct the future of this industry.
In this article, a new prosthetic design for
the below-knee amputee will be presented.
This state-of-the-art prosthesis incorporates
a monolithic socket and pylon combination
and may be attached to commercially available prosthetic feet. It is suitable for a majority of amputees and its advantages include
increased flexibility, absorption of stress and
shear and reduced cost. Compared to composite materials, thermoplastics are relatively inexpensive and may offer the same advantages to the majority of our patients.
Description
The Endoflex is a monolithic socket-pylon
endoskeletal below-knee prosthesis with energy-storing capability (Figure 1
and Figure 2
).
What makes the Endoflex unique is its flexible pylon. The Endoflex wearer may expect
to gain a more normal gait than was previously possible with other prostheses. Until
now, the direction of new design concepts
had moved toward the production of prosthetic feet featuring energy-storing capability. These new designs may mean higher costs
and additional weight. The Endoflex is an alternative method of fabricating a below-knee
prosthesis offering similar functional advantages while minimizing the total weight.
There were several points considered
when designing this prosthesis. A majority
of the patients are older. For the geriatric
individual, even a modest decrease in the
effort required for walking is a blessing. The
need for reduced weight was essential. We
also saw a need to reduce patients' frustration and depression caused by having to
make repeated office visits before prosthesis
delivery. With this system, a structurally
complete prosthesis can, in most cases, be
delivered upon the first fitting, so the patient
can practice with the equipment for extended periods. This helps determine the accuracy of socket fit and alignment before finishing the prosthesis.
The Endoflex offers simulated normal
motion due to its thermoplastic fabrication.
This feature allows for use of the SACH
foot, which offers the patient the advantages
of lighter weight and lower cost. More active
patients, however, will perform better with
one of the dynamic feet in combination with
the Endoflex. The patient will experience
reduced impact and stress on the residual
limb, plus effective reduction and/or the
elimination of contractures.
Due to the flexibility of the Endoflex pylon, at heel strike, the pylon will bow slightly
anteriorly, thus simulating plantar flexion of
the foot. Conversely, during midstance and
push off, the thermoplastic pylon will bow
posteriorly, simulating the dorsiflexion angle
required for effective push off.
Other advantages include its reduced
weight. The complete Endoflex system with
cover, on average, weighs 2.3 lbs. The appearance is more realistic and cosmetically
pleasing due to the vinyl-coated soft foam
cover. It is a versatile system which can be
altered for use by patients in different weight
classes and activity levels.
The monolithic socket and flexible pylon
is fixed to a prosthetic foot by use of a thermoplastic foot clamp (Figure 3). Once the
correct length and toe out angle are determined, a hole is drilled through the pylon
and foot clamp and they are pinned together
to control rotation. One-fourth-inch copolymer is used for most patients, but 3/16-inch
copolymer can be used for light weight and/
or limited ambulators. There is a conical
transition from the socket segment to the
pylon segment. This configuration is critical
in controlling the degree and location of flexibility. The Endoflex can be fabricated into
any below-knee prosthesis incorporating a
variety of suspension techniques; cuff, supracondylar, supracondylar/suprapatella, thigh
corset or ischial weight-bearing, soft socket
designs and suction.
Background
Having previously worked with thermoplastics, the author was disappointed upon
entering the field of prosthetics and orthotics
in 1974, to learn that very little thermoplastic
technology was being applied in this profession. Despite the availability, prosthetists
were reluctant to experiment with this material. Although this author received valuable
experience in a large and reputable New
York facility, the frustration level grew, due
to the conventional methods employed. In
1977, the author opened his own orthotic
laboratory, which specialized in thermoplastic appliances. During the following years, as
more amputees were referred to our facility,
the need to develop skills in thermoplastics
as applied to prosthetics grew apparent. A
technique involving the fabrication of prostheses from thermoplastic materials was developed and employed in our practice, allowing greater ease in adjusting the alignment and a simplified method of fabrication,
which were important considerations. Initially, after dynamically aligning the prosthesis, we would apply a rigid foam and outer
lamination over the basic structure, which
consisted of a copolymer material. Copolymer, unlike PVC,1 offered more durability,
thus allowing the patient to use the equipment for an extended amount of time before
finishing.
One day, while attending clinic, a patient
who had previously worn a rigid shank, and
had been recently fitted with a thermoplastic
prosthesis, commented on the degree of flexibility he experienced in the prosthesis. He
demonstrated a new ability to run, which
brought home the fact that he had been provided with a dynamic prosthesis; a system
which would flex upon loading and spring
back an energy was required, thus eliminating the typical vaulting gait. Covering the
prosthesis with a rigid lamination, we came
to realize, was a mistake that destroyed the
functional advantage of increased flexibility.
The new concern was now that of durability.
Would this structure be able to withstand the
normal everyday abuse that a prosthesis
should endure?
We decided to test the durability of the
prosthesis with 32 of our patients over a two-year period. Each patient was fitted with an
unfinished thermoplastic prosthesis and
monitored at one-month intervals to check
for failure. After two years of testing, our
results were very favorable. We had experienced only two failures, both of which had
occurred in the foot clamps of very young
and active traumatic amputees. Our new
prosthesis was appropriately named ENDOFLEX, (ENDO - Endoskeletal, FLEX Flexible), and by 1985, 200 patients were
already using this prosthesis.
Development
At this point, we focused on designing a
foot clamp, (Figure 4)
which had to perform
two primary functions. First, it had to be
resilient and, second, it had to provide a
means of secure attachment between the
monolithic socket/pylon and the prosthetic
foot.
We performed experiments with various
thermoplastic materials; ultra high molecular weight polyethylene, polypropylene,
high density polyethylene, reinforced nylon,
copolymers, Delrin and other strong but
flexible materials. The shape of the foot
clamp is critical and through trial and error
we have concluded that a conical shape is
most effective in providing strength, yet allowing flexibility at the proximal surface of
the clamp.
In our previous experiences with an aluminum clamp, the fracture site occurred at the
proximal surface of the clamp, due to the
brittle nature of the metal itself.
Despite much experimentation with many
materials, we were still not satisfied with the
performance of the foot clamp. Thanks to
special friends in this profession, we were
introduced to a product unfamiliar to us, Nylon 66. This material, which has probably
been available for decades, has proved to be
most effective (Figure 5)
.
Another important aspect of the basic
prosthesis which requires discussion, is the
thickness of the pylon itself. Wall thickness
assures durability of the product and if the
wall thickness is diminished during the
thermo-forming procedure, the prosthesis
will not perform to specifications. Once we
developed a thermo-forming technique that
would allow us to consistently maintain wall
thickness, we began testing with patients
considered to be in high weight categories
and activity levels. The durability of the
prosthesis has now been proven. The prosthesis is suitable for almost any below-knee
amputee, from a cosmetic wearer to an active amputee. This excludes, however, the
super active amputee, i.e. the runner. This is
not a Flex Foot; it is an ideal walking prosthesis, not a running prosthesis. However,
this equipment is appropriate for probably
90 percent of the amputee population.
Fabrication
The cast or model is prepared in the usual
manner. A clear check socket is generated
and mounted on an adjustable prosthesis,
preferably using a structural system where
there is no distal (ankle) adjustment. The
flex pylon can only be mounted to the proximal surface of a foot in a perpendicular attitude. Any system which offers proximal slide
and tilt adjustments is acceptable. The prosthesis is assembled, fit to the patient and
statically aligned.
After dynamic alignment is complete, the
prosthesis is placed in a duplicating jig (Figure 6
and Figure 7
), which will align the mandrel, in
the socket, over the center of the pylon. The
socket is now filled with plaster, while the
mandrel is held in exact alignment. After the
plaster sets, the prosthesis is removed from
the jig and the check socket is removed from
about the model. At this point, any revisions
to the model determined by check socket
diagnosis during fitting are made. The mandrel is carefully loosened from the model,
and the model slid proximally up the mandrel. The mandrel now extends distal of the
model and becomes the mold for the pylon
segment of the prosthesis, with alignment
carried forward from the dynamic fitting
procedure (Figure 8
and Figure 9
).
The soft insert, or liner, is fabricated with
a 3/4-inch hole distally to accommodate the
mandrel. This hole is later plugged to ensure
total contact. An epoxy cone is selected and
placed on the mandrel distal to the insert.
The desired cone length is determined by the
total finished length of the pylon itself and
the patient's weight and activity level. This
conical segment will reduce the flexibility of
the pylon, due to its increased proximal circumference.
Material is selected for vacuum forming.
We have found a copolymer consisting of
approximately 90 percent polypropylene and
10 percent polyethylene to be the most durable and appropriate for the application.
Most patients require 1/4-inch copolymer;
however, for patients 120 pounds and less
and moderately active, 3/16-inch copolymer
is used. Also note, that only natural colored
materials are suitable. Any pigment added
during the extrusion of the sheet will weaken
it.
The thermoplastic is heated appropriately
and vacuum formed over the entire socket
and pylon mold. After initial suction is
achieved, the vacuum is reduced from 25
inches Hg to 12 inches Hg, to prevent extreme compression of the insert. When the
plastic is cooled, it is removed from the model using a modified socket puller. In some
cases, where deep concavities or bulbous
conditions exist, the plaster must be chipped
out from inside the shell.
The pylon and socket are trimmed posteriorly, cut to length and installed in the nylon
foot clamp, which has been previously bolted to the foot and secured with cyanoacrylate adhesive.
The prosthesis is refit on the patient. Any
slight angular alignment modifications, as
well as socket modifications, can still be
made with heat. At this point, the prosthesis
is structurally complete and can be delivered
to the patient in an unfinished state, if desired. This will allow the prosthetist and the
patient to more effectively determine whether further adjustments may be necessary before finishing the prosthesis.
Finishing may be accomplished using any
one of a number of endoskeletal finishing
techniques.
Case Studies
In 1988, more than 300 patients, mostly
new amputees, had been fitted with Endoflex. Case studies were compiled on the 46
patients among these who had previously
used rigid shank below-knee.
The patients were asked to compare the
two systems in five different categories.
These case studies revealed the following patient reactions:
- In the area of flexibility, 82 percent experienced greater flexibility with the Endoflex;
- 70 percent experienced greater comfort;
- 80 percent noted improved gait efficiency and extended ambulatory endurance;
- 63 percent commented on a reduction in
weight;
- 85 percent commented on the improved
cosmetic appearance of the Endoflex.
In all categories, the patients were more
satisfied with the performance of the Endoflex system as compared to their rigid shank
prostheses.
Conclusion
This ultralight, flexible, endoskeletal below-knee prosthesis, with its unique flexible
pylon, provides energy-storing capability.
The endoflex offers functional advantages
while minimizing weight. The patients can
receive the prosthesis upon the first fitting
and as case studies show, are satisfied with
its comfort, durability and cosmetic appearance (Figure 10)
.
Acknowledgements
I would like to thank my staff at The Ortho
Remedy, Inc. for persevering through this project. I know it has been difficult at times and I feel
fortunate that certain special people were always
willing to help. One staff member in particular
that I must express sincere thanks to is my office
manager, Tara Rockey, who has been by my side
for the past three years.
My sincere thanks are also directed to Donald
Liss, MD, and Howard Liss, MD, of the Physical
Medicine and Rehabilitation Center, Englewood,
New Jersey, and to Young S. Kwon, MD, of
St. Luke's/Roosevelt Hospital in New York
City, whose open minds and flexibility provided the opportunity to develop this product.
Their efforts were instrumental in its ultimate success.
And most important, an extra special thanks to
my wife, Maureen, whose energy and devotion
have turned my dreams into reality.
Thomas G. Valenti, C.O., is a graduate of New
York University, Post Graduate Medical School.
He is a member of the American Academy of
Orthotists and Prosthetists (The Academy), and
president of The Ortho Remedy, Inc., 522 Anderson Avenue, Cliffside Park, New Jersey
07010. He may be reached at (201) 943-3900 or
Fax (201) 943-9055.
References:
- Hittenberger, Drew A., C.P., "A Thermoplastic Endoskeletal Prosthesis," Orthotics and
Prosthetics: Journal of the American Orthotic and
Prosthetic Association, Vol. 37, No. 2, pp. 45-52.
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