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A New Ankle Foot Orthosis With a Moldable Carbon Composite
Insert
Orthotics and Prosthetics, Vol. 35, No. 3, pp. 13-16, September 1981
Carlton Fillauer, C.P.O.
INTRODUCTION
The advent of thermoformed Ankle Foot Orthoses (AFOs) brought the demand
to fit all patients with the latest mode; weight reduction, cosmesis,
and greater control are the usual benefits derived; however, when ankle
motion must be eliminated the outcome is not always completely satisfactory.
Dorsiflexion cannot always be totally restricted by either altering the
material, or by changing the trim lines. Any increase in bulk to add rigidity
only adds to the problem.
Metallic reinforcing struts have been applied to the
lateral and medial sides of the AFO (1) resulting in a dramatic improvement
in resistance to dorsiflexion by a factor of four plus. Perhaps the inconvenience
of forming a close-fitting metal insert has deterred the general use of
this approach. Hopefully, the introduction of a new moldable high strenght
hybrid composite described here will open the way for wide spread application
of the improved design concept.
COMPOSITE
MATERIALS
Composite materials are a family of high performance materials consisting
of a matrix reinforced with a fiber. The matrix can be a thermosetting
resin such as an epoxy, polyester or polyimide, or a thermoplastic resin
such as nylon or polysulfone. The reinforcement can be carbon fiberglass,
Aramid, or boron fibers. The combination of a resin and a fiber results
in properties of a quite different character than either constituent.
These unusual properties are a result of the fiber being characterized
by single crystal properties which are five to fifty times greater than
those of the same material in polycrystalline form.
Composite materials are ideal for structural applications
where high strength-to-weight and stiffness-to-weight ratios are required.
The advantage of composites is that they usually exhibit the best qualities
of their constituents and often some qualities that neither constituent
possesses.
The advantageous properties include:
- Strength
- Stiffness
- Corrosion-resistance
- Weight
- Fatigue life
- Temperature-dependent behavior
- Thermal insulation
- Thermal conductivity
- Acoustical insulation
Naturally, not all of these can be optimized
at the same time.
APPLICATION
TO ANKLE FOOT ORTHOSES
Generally the trim line of an AFO is near the lateral mid-line of the
ankle but often it is moved further anterior in an attempt to achieve
ankle stability. The frequent result is continued bulging from buckling
of the sides at terminal stance and some loss of cosmesis from the increased
bulk, especially when the copolymer plastic is used. When this is unacceptable
we can resort to using a thicker polypropylene, with little benefit. It
is obvious that if we are to block ankle motion we must introduce a stiffener
in the area from proximal to the ankle to the arch area of the foot on
both the medial and lateral sides.
The carbon composite insert has made a dramatic impact
in solving all of the above problems. Without changing trim lines or increasing
thickness, almost complete ankle rigidity can be achieved by including
a pair of crescent shape composite inserts in the thermoform. A hybrid
composite of glass and carbon fibers in a thermoplastic resin matrix was
chosen as the ideal combination for the stiffness qualities and dimensional
stability desired. A two ply 40% carbon fiber panel 3/32” thick is used
for light duty requirements and a three ply 43% carbon fiber panel, 1/8”
thick is used for the medium and large size patients.
When the ankle angle is in the normal range of about
5¡ to 10¡ of plantar flexion one of four sizes of precut inserts are selected
(Fig. 1). The patterns are designed to fit just posterior to the usual
trim line, beginning about three to four inches proximal to the malleoli,
passing posterior to the ankle prominence and extending into the foot
area, terminating near the junction of the plantar surface with the medial
and lateral walls. This minimizes flexing in the foot insert and bulk
in the shoe.
Custom shapes (Fig. 2) are required occasionally for
other angles and these can be cut on a metal band saw and the edges smoothed
on a sand cone. It is important that the inner edge be undercut on a 45¡
angle to insure interlocking with the polypropylene wall.
There is no problem if a liner is used; the insert is
simply pasted to the liner or the plaster model prior to the thermoforming
procedure.
FABRICATION
To form the carbon composite insert to the model it should be heated with
a heat gun or in an oven until it is pliable (approximately 300¡ F.) and
then pressed in place with insulated gloves (Fig. 3). Attach the molded
insert to the liner or to the plaster cast with Scotch Mounts 1/32” thick
(Fig. 4). Scotch Mounts are urethane foam pads with an adhesive coating
on both sides. Place three pieces on the insert, one on each end and one
in the center, to provide extra spacing away from the liner which, along
with the under-cut edge, assist in the encapsulation of the inserts. Use
a ventilated foam liner or a nylon hose over the cast to assure complete
vacuum forming.
Presented here is a simple process added to an accepted
conventional laboratory fabrication procedure. It demands no new equipment,
skill or time consuming labor yet it adds a new dimension to the function
of the AFOs.
It consists of three steps: 1. Select precut or custom made inserts. 2.
Thermoform inserts to model. 3. Adhere inserts to model.
Several dozen floor reaction type orthoses have been
in use for over a year with excellent results and no reported failures.
When the inserts of adequate thickness are properly placed and secured
in the polypropylene walls, minimum deflection and gapping will occur.
We expect this conformable insert concept will find widespread use in
many areas of plastic orthoses where increased rigidity is required.
SUGGESTIONS
FOR SUCCESSFUL USE OF COMPOSITE INSERTS
- The ideal structure is achieved with polypropylene. High
density polyethylenes like subortholen do not flow sufficiently to encapsulate
the inserts.
- It is imperative that the composite piece be formed
uniformly to the contour of the cast, anteriorly and posteriorly, to
avoid a gap between the insert and the cast. Such a gap will cause the
interface to be irregular and could cause a fitting problem.
- Heat the polypropylene to the high level of the
plastic range before vacuum forming.
- Be sure the negative pressure equipment is adequate
and of sufficient capacity.
- If your cast has a polyethylene foam liner be sure
it is a ventilated material like our ventilated PE-LITE. This will assure
rapid evacuation of air.
- When vacuum forming over a bare or lacquer castÑcover
it with nylon hosiery before attaching the inserts.
- Place the composite insert on the A.P. midline
for maximum resistance to deflection.
- When custom making an insert be sure to locate
the area of maximum forming on a 45¡ angle to the weave of the glass
cloth seen on the surface of the composite sheet. This will avoid wrinkles
in the composite surface when it is being formed.
- Use the double stick mounts to space the inserts
away from the cast to facilitate encapsulation.
- When using a foam liner always peel back the liner,
after the vacuum forming is completed, to visually observe that the
insert is locked in.
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