Fabricating the Spiral and Hemi-Spiral
Orthosis Using a Composite Lay-Up
Technique
Michael Madden
Introduction
Conventional thermoplastic materials previously utilized in the fabrication of spiral
and hemi-spiral orthoses, have in many cases
proved themselves to be ineffective. Some
materials were found to be too brittle, leading to material fatigue and eventual failure.
Other materials were found to be less brittle,
yet too flexible to function as required by the
design of the orthosis.
The application of composite materials in
a semi-flexible acrylic lamination has made it
possible to achieve the results required while
eliminating the material fatigue and the
breakage associated with previously applied
materials. By incorporating the present materials technology in the spiral and hemi-spiral design, positive results have been
achieved. There is an added advantage in
that the orthosis can be custom tailored to
achieve the desired balance between rigidity
and flexibility, as required by a patient's specific needs. This can be easily accomplished
by varying the lay-up of materials, an option
not available in previously applied thermoplastics.
The only areas of concern addressed in
this article are materials application and fabrication. For casting, modification, design
and orthotic applications refer to the previously published materials available on the
spiral and hemi-spiral orthosis (see references).
Design
Because of the unique characteristics each
yields, a combination of three composite materials was chosen. The best results were
achieved when the three materials were
wrapped together into a "strut" design. This
allowed the unique qualities of the materials
to be balanced and to function together more
favorably than they would alone.
The strut begins with an "inner core" of
carbon. The carbon was utilized to control
the compressive loading, due, in part, to the
coiling action of the orthosis. It is necessary
to maintain some compressive resistance
during coiling to keep the spiral from collapsing under loading.
A layer of five ounce Kevlar-49(1) is used
as an outer sheath. The Kevlar-49(1) has the
unique property of being extremely flexible
under a compressive load, yet highly resistant to tensile strain. This helps to keep the
orthosis from stretching out during recoil.
In between the inner core and outer
sheath is a middle layer of 6-ounce S-2 glass
(2). This layer acts as the transition layer for
the stress changes which are inherent to this
design. The S-2 glass functions in both the
compressive loading and tensile strain
phases. Since carbon graphite is relatively
weak under tensile strain and Kevlar-49 is
relatively weak under compressive loading,
the S-2 glass becomes the balance between
the two layers aiding in the transfer of stresses between them.
The stresses on this particular design of
orthosis are quite unique, contributing to
failure of many previously applied materials.
There are not only the typical surface strains
associated with the changing stresses applied
through the gait cycle (i.e., heel strike to
foot flat, to toe off) which must be considered, but the coil and recoil action of the
orthosis as well as the constant rotational
torque applied must be considered as well.
Fabrication
After modification of the cast is complete,
it is extremely important to define the trimlines. Because of the lay-up technique, and
because of the difficulty in finishing any composite materials which might become exposed, it is important to predetermine all
trimlines. When applying the materials to
the model, keep them one-eighth inch to
one-quarter inch within the final trimlines.
The orthosis is laid up in three separate
sections; the cuff, the soleplate and the strut.
1. Cut four pieces of Kevlar-49, four
pieces of S-2 glass and two pieces of carbon
to the shape of the cuff.
2. Cut six pieces of S-2 glass and two
pieces of carbon to the shape of the soleplate.
3. Cut the material for the strut (Figure 1)
.
To do so, measure the length required from
about mid-cuff around the model to just superior to the plantar surface of the soleplate.
The strut should extend as distally as possible without creating a wedging of the heel.
Cut a piece of carbon four and one-half inches wide by the length required. Since the
Kevlar is more difficult to cut than most
composite materials, very sharp scissors will
be required. (Also, the use of 3M double
sided tape to mark out all cut lines will be
helpful. Leave half the width of the double
sided tape on the cut pieces. This will help in
the application of materials to the model as
well as the folding of the strut.)
Take the length of carbon for the strut and
fold it so that a flat length of carbon, one and
one-half inches wide by the desired length,
three layers thick, is achieved.
Lay out the fiberglass. Place the folded
length of carbon on the edge of the glass,
matching length for length, and fold the glass
around the carbon four times. This will result
in a length of material with an inner core of
carbon, an outer "wrapping" of S-2 glass
totaling seven layers. (Two layers of glass on
either side of the carbon.)
Lay out the Kevlar, placing the previously
wrapped material on the edge of the Kevlar,
matching length for length, and fold the Kevlar around the material approximately six
times. The results should be a strut with an
inner core of carbon (three layers thick), a
transitional layer of glass (three layers thick)
and an outer sheath of Kevlar six layers thick
(three on either side of the inner layers).
Though this may seem like a lot of material, using 25 inches to 30 inches of mercury
vacuum and stringing any excessive resin
from the lay-up will produce a strut about
three-sixteenths inch to one-fourth of an
inch thick.
By utilizing this technique of folding the
material around itself, we have found four
major benefits.
First, moving from the core outward in
either direction, we have created a smooth
transition of material from rigid, to semirigid, to flexible (under compressive loading).
Second, the uni-directional knit or a bi-directional (0 degrees - 90 degrees) weave of
the material allows the axis of the fiber to be
applied in the direction of the spiral's configuration without interrupting the length of
the fiber by cutting. This allows the stress to
be transferred along the long axis of the fiber, increasing the fiber's function and
strength.
Third, if the strut becomes exposed along
an edge during trimming, only one layer of
material is exposed, the outer layer. With
the typical sandwich lay-up, if the strut were
to be exposed in trimming, all 12 layers
would become exposed.
This leads to the fourth advantage. With a
bi-directional weave (0 degrees - 90 degrees)
the folding of the material will eliminate the
delamination which would occur between
layers. The wrapping eliminates the edges
where delamination would begin typically in
the sandwich lay-ups.
This "strut" design has also been utilized
with great success in the fabrication of the
rigid frames for the flexible socket systems in
prosthetics.)
Once the materials are cut and ready, prepare the model for lamination.
It is important that the materials be applied in the sequence noted to ensure best
results.
Lay-Up of Model- Inner layer of elastic stockinette over
entire model (Figure 2)
.
- Two of the Kevlar cuff pieces, followed
by two of the S-2 glass cuff pieces, and
three of the S-2 glass soleplate pieces.
The various pieces of material are held
in place by the double-faced adhesive
film (Figure 3)
.
- Wick layer of elastic stockinette over
entire model (to facilitate resin flow).
- One layer of carbon in both the calf and
soleplate.
- Apply strut to model-run from carbon
layer at mid-cuff to carbon layer just
superior to plantar surface of soleplate
(Figure 4)
.
- One layer of carbon in each the cuff and
soleplate positioned so that they cover
the ends of the strut.
- Wick layer of elastic stockinette over
entire model (to facilitate resin flow).
- Two layers of S-2 glass followed by 2
layers of Kevlar in the cuff and 3 layers
of S-2 glass in the soleplate.
- Outer cosmetic layer of elastic stockinette (Figure 5)
.
Laminate the model using an acrylic resin
mix of 70 percent flexible, 30 percent rigid
resin. It is important to remember that most
resins, if premixed, are already 20 percent
flexible.Example:
A 30 percent rigid, 70 percent flexible resin mixture will result in an orthosis which is
relatively compressible and best utilized in
light-to-medium duty applications. If a more
rigid orthosis is desired, the best results will
be achieved by increasing both the layers of
carbon and fiberglass and the resin rigidity
slightly.
It should be noted that as the resin mixture
is increased, the possibility of resin crazing
also increases. This is why the materials and
resin should both be increased together.
Summary
After extensive research into materials
and their applications, more than half a dozen prototype designs were fabricated. These
were used to test the materials in spiral and
hemi-spiral configurations and to see how
they reacted under stress. It was not until
satisfactory results were achieved here that
the first orthosis was fabricated for a patient.
Over a period of three years, more than
six orthoses have been prescribed. This includes bilateral spiral orthoses. Some of the
spiral and hemi-spiral orthoses fabricated
from composites have been applied to patients who had broken conventional thermoplastic orthoses. To date, none of the composite orthoses have undergone material fatigue or functional failure and all of the patients are in the original orthoses fabricated
for them.
Though the time involved in fabrication is
increased, so are the results. The application
of composite materials to the fabrication of
the spiral (Figure 6)
and hemi-spiral orthosis
may allow a more thorough look at its application and effectiveness in patient management.
Manufacturers- Kevlar-49 is an E.L. duPont De Nemours & Co. fabric used for its extremely high tensile
strength. It was developed for aerospace and military applications and is widely used in outdoor
sports equipment.
- S-2 glass is an Owens Corning fabric developed for military and aerospace applications. It
has superior laminate properties to standard fiberglass and is categorized by structural engineers
as an advanced composite.
Acknowledgements
I would like to acknowledge Y'shua Mashiyach
and his continual wisdom and guidance. Thanks is
not enough for what you have done.
I would like to thank Richard La Torre. It was
his laboratory, time and money which made it
possible for this project to be undertaken and
completed. It was his challenge that began it, and
his patience and direction which brought it to
where it is now. Thanks Dick! I truly appreciate
all that you have done for me in this and many
other projects, and many other ways.
On behalf of Richard La Torre, I would like
to thank Herbert Marx for the work he previously contributed to the spiral and its development.
Michael Madden is a Prosthetic/Orthotic Technician formerly with La Torre Orthopedic Laboratory, Schenectady, New York. He is a graduate of the 916 Area VO Tech technician program
in White Bear Lake, Minn. and holds a journeyman's certificate in prosthetics from the New
York State Department of Labor.
References:
- Lehneis H-R. Plastic spiral ankle-foot orthosis.
Orthotics and Prosthetics June 1974;28:3-13.
- Lehneis H-R, Marx HW, Frisina W, Goldfinger
GH. Plastic spiral below-knee orthosis. Orthotics
and Prosthetics Institute of Rehabilitation Medicine, New York University Medical Center
- Hexcel Corporation. Advanced composite reinforcements. Technical data sheets for structural
fabrics.
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