A New Ankle Foot Orthosis
With A Moldable Carbon Composite Insert
Carlton Fillauer, C.P.O.
1
SUMMARY
INTRODUCTION
T
he a d v e n t of t h e r m o f o r m e d A n k l e Foot
Orthoses (AFOs) brought t h e d e m a n d to
fit all patients w i t h t h e latest m o d e ; w e i g h t
reduction, cosmesis, a n d greater control are
t h e usual benefits derived, h o w e v e r , w h e n
a n k l e m o t i o n m u s t b e e l i m i n a t e d t h e out­
c o m e is not a l w a y s c o m p l e t e l y satisfactory.
Dorsiflexion c a n n o t a l w a y s b e totally re­
stricted b y either altering t h e material, or b y
c h a n g i n g t h e trim lines. A n y increase in b u l k
to add rigidity o n l y adds to t h e p r o b l e m .
Metallic reinforcing struts h a v e b e e n ap­
plied to t h e lateral a n d medial sides o f t h e
A F O (1) resulting in a dramatic i m p r o v e ­
m e n t in resistance to dorsiflexion b y a factor
of four plus. P e r h a p s t h e i n c o n v e n i e n c e o f
forming a close-fitting metal insert has de­
terred the general use o f this approach.
Hopefully, t h e introduction o f a n e w m o l d able h i g h strength h y b r i d composite de­
scribed h e r e will o p e n t h e w a y for w i d e
spread application of t h e i m p r o v e d design
concept.
COMPOSITE MATERIALS
Composite materials are a family of h i g h
performance materials consisting of a matrix
reinforced w i t h a fiber. T h e m a t r i x can b e a
thermosetting resin such as a n e p o x y , poly­
ester or polyimide, or a thermoplastic resin
such as n y l o n or polysulfone. T h e reinforce­
m e n t can b e c a r b o n fiberglass, A r a m i d , or
b o r o n fibers. T h e c o m b i n a t i o n o f a resin a n d
a fiber results in properties of a quite differ­
ent character t h a n either constituent. T h e s e
unusual properties are a result o f t h e fiber
b e i n g characterized b y single crystal proper­
ties w h i c h are five to fifty t i m e s greater t h a n
those of t h e s a m e material in polycrystalline
form.
C o m p o s i t e materials are ideal for struc­
tural applications w h e r e h i g h strength-tow e i g h t a n d stiffness-to-weight ratios are
required. T h e advantage of composites is that
t h e y usually exhibit t h e best qualities of their
constituents a n d often s o m e qualities that
n e i t h e r constituent possesses. T h e advanta­
geous properties include:
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strength
stiffness
corrosion resistance
weight
fatigue life
temperature-dependent
t h e r m a l insulation
thermal conductivity
acoustical insulation
behavior
Naturally, not all of these can b e optimized at
t h e s a m e time.
APPLICATION TO ANKLE FOOT
ORTHOSES
Generally t h e trim line of a n A F O is near
t h e lateral mid-line of t h e a n k l e but often it
is m o v e d further anterior i n an attempt to
achieve a n k l e stability. T h e frequent result is
c o n t i n u e d bulging from buckling o f t h e sides
at terminal stance a n d s o m e loss of cosmesis
from the increased bulk, especially w h e n t h e
c o p o l y m e r plastic is used. W h e n this is un­
acceptable w e can resort to using a thicker
polypropylene, w i t h little benefit. It is obvious
that if w e are to block a n k l e motion w e m u s t
introduce a stiffener in t h e area from prox­
imal to t h e a n k l e to t h e arch area of t h e foot
on both t h e medial a n d lateral sides.
T h e carbon composite insert has m a d e a
dramatic impact in solving all of t h e above
problems. W i t h o u t c h a n g i n g trim lines or in­
creasing thickness, almost c o m p l e t e a n k l e
rigidity can be achieved b y including a pair
of crescent s h a p e composite inserts in t h e
thermoform. A h y b r i d composite o f glass
a n d carbon fibers in a thermoplastic resin
matrix was c h o s e n as t h e ideal c o m b i n a t i o n
for the stiffness qualities a n d d i m e n s i o n a l
stability desired. A t w o ply 4 0 % carbon fiber
panel 3 / 3 2 " thick is used for light d u t y re­
q u i r e m e n t s a n d a t h r e e ply 4 3 % carbon fiber
panel, 1 / 8 " thick is used for t h e m e d i u m a n d
large size patients.
W h e n t h e a n k l e angle is in t h e n o r m a l
range of about 5 ° to 10° of plantar flexion o n e
Figure 1. Patterns used for carbon composite inserts. Note the beveled edges, which allow the thermoplastic to lock the insert
in during the vaccum forming process.
of three sizes of precut inserts are selected
(Fig. 1). T h e patterns are designed to fit just
posterior to t h e usual trim line, b e g i n n i n g
about three to four i n c h e s proximal to t h e
malleoli, passing posterior to t h e a n k l e
p r o m i n e n c e a n d extending into t h e foot area,
terminating near t h e j u n c t i o n of t h e plantar
surface w i t h t h e medial a n d lateral walls.
T h i s m i n i m i z e s flexing in t h e foot insert a n d
bulk in t h e shoe.
C u s t o m shapes (Fig. 2 ) are required occa­
sionally for other angles a n d these can b e cut
on a metal b a n d saw a n d t h e edges s m o o t h e d
o n a sand c o n e . It is important that t h e i n n e r
edge b e undercut o n a 4 5 ° a n g l e to insure
interlocking w i t h t h e p o l y p r o p y l e n e wall.
Figure 2. Custom carbon composite inserts
can be cut on a metal band saw and smoothed
and beveled with a sander.
T h e r e is n o p r o b l e m if a liner is used; t h e
insert is simply pasted to t h e liner or t h e
plaster model prior to t h e t h e r m o f o r m i n g
procedure.
FABRICATION
T o form t h e carbon composite insert to t h e
model it should b e heated w i t h a heat g u n or
in a n o v e n until it is pliable (approximately
300°F.) a n d t h e n pressed in place w i t h insu­
lated gloves (Fig. 3). Attach t h e molded insert
to t h e liner or to t h e plaster cast w i t h Scotch
M o u n t s 1 / 3 2 " thick (Fig. 4 ) . Scotch M o u n t s
are u r e t h a n e foam pads w i t h a n adhesive
coating on b o t h sides. Place three pieces on
t h e insert, o n e o n each e n d a n d o n e in t h e
center, to provide extra spacing a w a y from
t h e liner w h i c h , along w i t h t h e under-cut
edge, assist in t h e encapsulation of t h e in­
serts. U s e a ventilated foam liner or a n y l o n
hose over t h e cast to assure c o m p l e t e v a c u u m
forming.
T h e drape forming procedure is preferred
since it assures m a x i m u m a n d u n i f o r m m a ­
terial thickness throughout t h e orthosis. O p ­
timally, t h e p o l y p r o p y l e n e s h o w s good
forming a n d definition a r o u n d t h e inserts
(Fig. 5 ) . If t h e inserts are not properly e m ­
bedded in t h e walls of t h e plastic a poor re­
sult can b e expected as t h e y will separate at
terminal stance; w h e n this occurs t h e c o m ­
posite pieces can b e salvaged a n d reused.
Figure 3. The carbon composite insert is
heated to 300° by using an oven or a heat gun;
it is then pliable and is held in place on the
mold until it cools.
Figure 4. The insert is held in place on the
mold with 1/32 inch thick Scotch mounts;
three small pieces above the insert and pro­
vide some space under the insert.
e q u i p m e n t , skill or t i m e c o n s u m i n g labor yet
it adds a n e w d i m e n s i o n to the function of
t h e A F O s . It consists of three steps:
1. Select precut or c u s t o m m a d e inserts.
2. T h e r m o f o r m inserts to model.
3. A d h e r e inserts to model.
Several dozen floor reaction type orthoses
h a v e b e e n in use for over a year w i t h excel­
lent results and n o reported failures. W h e n
t h e inserts of adequate thickness are properly
placed a n d secured i n t h e polypropylene
walls, m i n i m u m deflection and gapping will
occur. W e expect this conformable insert
concept will find widespread use in m a n y
areas of plastic orthoses w h e r e increased
rigidity is required.
Figure 5. Completed Ankle Foot Orthoses
with carbon composite inserts in place.
References
l . V i c e President, Research and Product Development, Durr-Fillauer
SUMMARY
P
resented h e r e is a s i m p l e process added to
an accepted c o n v e n t i o n a l laboratory fab­
rication procedure. It d e m a n d s no n e w
Medical, Inc. Orthopedic Division, 2710 Amnicola Highway, Chattanooga,
Tennessee.
Footnote
1Darrell R
Clark, Thomas R. Lunsford, "Reinforced Lower-Limb
Orthosis-Design Principles", Orthotics and Prosthetics, June, 1978