Carbon Fiber Articulated AFO - An Alternative Design
Steven Hale, M.Sc., C.O.(c)
Background and Theory
In normal walking the ankle joint goes
through two periods of plantarflexion and a
single period of dorsiflexion during the
stance period. The ankle is dorsiflexed immediately after toe-off and this is maintained
through the swing period. After heel-strike
the ankle plantarflexes approximately 15°
until foot-flat is attained. The ground reaction force (GRF) is the main factor causing
plantarflexion and the eccentric contraction
of the dorsiflexors controls the rate of plantarflexion. After foot-flat the tibia rotates
forward over the foot during mid-stance, and
reaches a peak dorsiflexion of 10° The GRF
generates the dorsiflexion moment, and the
amount and rate of dorsiflexion is controlled
by an eccentric contraction of the plantarflexors. The final plantarflexion occurs during "push-off" where a large plantar-flexor
moment, generated from gastrocnemius and
soleus, provides energy to the swing limb.
Currently, there were several different ankle-foot orthoses (AFO) used to provide ankle stability for pathological gait patterns.
Plastic designs were becoming more popular
than the conventional double metal uprights
with mechanical joints attached to the footwear. The advantages of plastic over the conventional metal jointed AFOs are: (1) orthosis weight and bulk are reduced; (2) improved cosmesis; (3) improved control of the
ankle/foot complex through an intimate fit;
and (4) inter-changeability of footwear.
In a study comparing the conventional
metal AFO to the plastic AFO in hemiparetic walking, there were no differences in the
oxygen consumption (Corcoran et al., 1970).
Although the three AFO designs, used frequently by hemiparetics - Klenzak, springwire and plastic shoehornäwere found to
eliminate gait problems (i.e., drop-foot), for
a variety of reasons an AFO did not guarantee improved gait (Hale and Wall, 1988).
Smith et al. (1982) found that rigid plastic
AFOs resulted in initial heel contact, and
demonstrated the normal heel-metatarsal 5- metatarsal 1- greater toe pattern in more patients than the rigid BICAAL AFO.
The disadvantages of the plastic designs
are: (1) amount of skin covered - increased
risk of skin breakdown (diabetics, poor circulation); (2) heat retention and sweating;
(3) less control of ankle motion allowed; (4)
posterior leaf (flexible) designs have the mechanical axis located posterior (and sometimes mal-aligned proximally/distally) to the
anatomical ankle axis of rotation which result
in "pistoning" and undue stresses on the leg.
In rigid plastic AFO designs, dorsiflexion
is eliminated in order to attain optimal mediolateral ankle stability. Thus, functional
dorsiflexion range is compromised to attain
another goal. One stated advantage of a dorsiflexion stop was the simulation of push-off
(Lehmann et al., 1979, 1980). Since dorsiflexion was prevented, the patient pivoted
about the metatarsal break as dorsiflexion
was attempted. This resulted in the heel rising, and hence simulation of the plantarflexors at the beginning of push-off stage. However, this action does not simulate concentric
contraction or positive work done by the
plantarflexors during push-off. A dorsiflexion stop prevents forward progression of the
tibia (tibial advancement) and subsequent
knee flexion, which in turn may affect the
smooth transition from stance to swing, and
require increased energy expenditure to initiate swing.
Several AFO designs have attempted to
incorporate the advantages of the conventional jointed AFO and plastic AFOs in plastic hybrid AFOs. (Bensman and Lossing,
1979; Carlow and Almeida, 1978) These designs utilize metal uprights, stirrups and ankle joints contoured to patient molds and
include plastic calf and foot sections. These
designs maintain an intimate fit and control,
but also are adjustable to allow specific
ranges of ankle motion. The drawbacks that
still exist are weight and bulk.
Several groups recently have been designing different plastic articulated AFOs (Carlson, 1986; McRae, 1986; Miller and Filipovic, 1987; Watanabe et al., 1978). The designs incorporate a plastic joint to coincide
with the anatomical ankle joint and result in
reduced pistoning. Other suggested advantages are: (1) total contact which reduces
pressure; (2) intimate fit and improved control; (3) improved ankle kinematics during
walking which in turn may result in improved
energy efficiency; (4) adjustability to allowable range of motion; and (5) lightweight.
At Gillette Hospital Center (Carlson,
1986) a double-flexure ankle joint was designed. The design has two plastic flexures
co-aligned to the anatomical ankle joint, and
encased in the calf and foot sections of polypropylene. A plantarflexion stop is incorporated in the posterior aspect, in the achilles
tendon region. The stop is made by placing
an extra layer of plastic.
A different design has been used in Canada. (McRae, 1986; Miller and Fillipovic,
1987). This method involves a double lay-up
procedure. The first lay-up is the shank and
joint area, and the second is the foot and
joint area. A positive plantarflexion stop is
provided by the two contacting edges in the
achilles tendon region. A modification to
this, to provide a stronger stop, is to fabri
cate a "lip" for both contacting surfaces.
A new unique articulated plastic AFO has
been designed at the Calgary General Hospital. The orthosis utilized plastic laminates
and carbon fiber. The uniqueness is in the
joint design which includes the plantarflexion and/or dorsiflexion stops within the joint
design. Other advantages found in the articular plastic AFOs are: (1) a more durable
joint; (2) less skin coverage (less heat retention and sweating); (3) lower AFO profile;
and (4) less bulk in the joint design (thickness of 1/4").
The functional criteria of the orthotic design are to: (1) maintain ankle/foot in optimal alignment (stabilization); (2) increase
medio-lateral stability of ankle/foot complex; (3) provide a functional ankle range of
motion, whether it be free motion or limited
motion; and (4) keep the weight and amount
of material of AFO to a minimum.
The patient criteria required for the orthosis are: (1) weak or absent dorsiflexors; (2)
adequate soleus and/or gastrocnemius to
control rate and degree of dorsiflexion during midstance; (3) near normal hip and knee
extensors to maintain joint stability; (4) no
severe fixed deformities; and (5) low degree
of spasticity. The gait pattern may be characterized by: (1) lack of heel-strike or initial
forefoot strike; (2) equinus during swing
(dropfoot); (3) foot slap if heel-strike does
occur; (4) plantarflexion throughout midlate stance; (5) knee hyperextension; and (6)
mild-moderate mediolateral stability of ankle during initial stance, single support, or
during swing.
A selected number of patients from a variety of pathological conditions, i.e., cerebral
palsy, hemiparesis, multiple sclerosis and arthritic patients, may benefit from this articulated AFO.
Fabrication
Casting
Normal casting methods are used, except
particular attention is paid to the anatomical
joint axis location. Optimal correction or positioning of the calcaneus is critical. The ankle joint is passively worked through its
range of motion and then the joint axis is
located and marked. The position of the patella relative to the midline of the foot should
be noted. It is important to observe the individual's gait pattern and look at the affected
and less affected or sound limb, to gain an
understanding of the joint function during
walking. The foot is placed on a foot board
to account for the heel height of the patient's
shoe. Appropriate toe-out is attained and
the leg is positioned over the foot to achieve
the proper amount of dorsi/plantar flexion.
Cast Modification
A 1/4" 20 threaded rod is placed through
the negative cast at the points delineating the
anatomical ankle axis (malleoli) (Figure 1)
.
The direction of the mechanical axis should
be confirmed at this time, and if any adjustments are needed to ensure proper function,
they should be performed. After the cast is
filled, normal modifications are done to obtain optimal fit and control. Two dials, 1/8" x
1-1/8" diameter, are threaded onto the rod.
The dial thickness (1/8") provides some malleoli clearance. The excess rod is cut off and
the area under the dials is filled with plaster
build-ups (Figure 2)
.
Lay-up
A 3 lay-up procedure is used. The first and
last lay-ups are for the calf, uprights and
clevis joints (female). The second lay-up is
for the foot piece and ankle joint (male end).
A nylon stockinette and PVA bag are applied to the whole cast. The first layer is
perlon (Otto Bock Industries), followed by a
layer of Nyglas (Otto Bock - Glas-trikot
623T11). Carbon fiber (Otto Bock), cut into
1-1/4" strips, is applied to the upright and
joint head areas. A layer of Nyglas separates
the first and second layer of carbon fiber.
The final two layers are Nyglas. Acrylic resin
(Otto Bock - Orthocryl 617H19), premixed
at 80% hard and 20% flexible, is laminated
into the lay-up.
After the resin is hard, the trimlines are
marked and the calf section is cut out and the
edges are smoothed. The joint heads are cut
to the dial size, which is large than necessary.
This section is then placed back on the cast.
A nylon and PVA bag are applied over the
cast and calf section. A single perlon is
stretched over the whole leg. The remaining
lay-up is applied distal to the joint region. A
layer of Nyglas is applied. The two layers of
carbon fiber are applied only to the heel and
joint head regions. Between the two carbon
fiber layers a layer of Nyglas and a layer of
Acrylic mat (Daw Industries - FCA-001) are
incorporated. The mat layer is applied only
to the joint heads and provides extra thickness and rigidity. The final two layers are
Nyglas and Perlon.
The trimlines for the foot section were
marked, cut out and finished following lamination. The head regions are left larger to
allow for final adjustment. For free motion
the heads are left rounded, while for plantarflexion stops, the proximal head are cut flat
and squared up.
The first calf section is removed and the
outer surface roughed up. The inner surface
is masked off with tape and a coating of silicon spray is applied to the tape. A nylon and
PVA bag are applied to the cast. The first
calf section is placed onto the cast. A 1/8".
polypropylene joint head is placed over the
first section joint area. The head design is
either for free motion or plantarflexion control. The head is sprayed with silicon and
plasticene modeling clay is used to fill any
voids. A similar lay-up as the first is applied
except the first perlon layer is omitted. After
the resin is hard, the calf section was cut out
and the joint head cleaned.
The foot section is fitted to the calf section. The joint centers are located, drilled
out, and a Chicago screw is used as the axis
of rotation. The plantarflexion stop is
trimmed to the appropriate angle. The joint
heads and uprights are trimmed down to
minimum size. Teflon or nylon washers may
be used to reduce joint friction. The carbon
fiber edges are sealed with zegeilhartz to prevent separation. The completed AFO is seen
in Figures 7A and B
.
Case Study I
A female aged 65 with a long history of
rheumatoid arthritis (RA) was prescribed a
rigid AFO to control moderate-severe valgoplanus, which caused pain in the ankle/foot
region during weight-bearing. The patient
had weakened muscle strength of the major
muscles (grade 3 + + -4 out of grade 5, where
grade 3 was anti-gravity), of the joints of the
ankle/foot (plantar/dorsi, invert/evert). The
patient exhibited limited range of motion in
all planes, going through 5° of dorsiflexion
during midstance to approximately 10° of
plantarflexion during late stance. The patient complained of posterior knee pain after
long periods of walking. Knee hyperextension (approximately 5 - 7°) during early stance
was observed and considered to be the cause
of the knee pain. The knee muscle strength
was slightly weakened, but adequate for ambulation (grade 4) with the flexors slightly
weaker than the extensors.
A rigid AFO prescribed was to maintain
optimal alignment of the ankle/foot complex, to restrict all ankle motion particularly
that motion which may contribute to joint
pain, and to prevent knee hyper-extension.
An articulated AFO was also prescribed.
Our decision to try an articulated plastic
AFO was influenced by the fact that the patient walked stairs several times a day. Stair
ascending and descending requires dorsiflexion. The elimination of the ankle motion
may not be necessary to attain comfort and
pain-free walking.
The two AFO designs, a rigid 3/8" polypropylene and an articulated carbon fiber,
were fabricated from the same cast. The patient tried the rigid AFOs for two weeks,
then was provided with the articulated
AFOs. The patient was asked to try both
designs as often as possible. After four
weeks the patient was questioned with regards to preference. Both designs reduced,
but did not eliminate, the amount of ankle/
foot discomfort associated with walking. The
patient preferred the articulated design, particularly for walking in the house, where she
found stair climbing easier, less tiring and
not as unstable. For long periods of walking
(i.e., shopping) there was no real preference, although at times she did mention that
the rigid AFO felt more comfortable. This
may be related to the restricted motion
which may result in less pain.
The patient's major complaint about the
original articulated design was the bulk of
the joints. Other noted disadvantages of this
design were: (1) fabrication process (3 lay-up
procedure); (2) cost of materials; (3) overall
cost; and (4) alignment of ankle joint which
was very critical especially for the arthritic
patient.
The advantages of the articulated AFO, as
stated by the patient, were: (1) the foot piece
fit into her shoes better (was not as thick as
the rigid AFO); (2) less sweating because
less material covered her leg; and (3) it allowed some dorsiflexion which she felt was
necessary for stair climbing. It was also noted that the weight of the rigid AFO was 306
grams and the articulated AFO was 288
grams. (Figures 8A and 8B)
Addendum
After wearing the orthoses for four
months the patient was seen for a minor adjustment to the foot section of the articulated
AFO. She was then wearing the articulated
AFOs all the time because she was more
comfortable in them.
After two years of wearing the orthoses
the patient returned for an adjustment to the
foot pieces. At that time it was noted that the
plantarflexion stop was disrupted. Several
attempts were made to repair this, but were
unsuccessful.
The reason for the wear of the plantarflexion stop was related to the fabrication process, primarily the use of the dummy joint
head. If the plantarflexion stop of the foot
section was not exactly duplicated in the
dummy head, pressure points would occur,
leading to joint head wear. If the fabrication
process could be reduced to two lay-ups, i.e.,
first doing the foot and male joint head (using a 3/32" spacer between the joint head and
the cast); secondly, doing the inner lay-up
(placing the foot section onto the cast); and
then doing the final lay-up before laminating
the cast, an exact matching of joint surfaces
would be ensured. It would also be advisable
to bulk up the plantarflexion stop area to
increase the total surface area and ultimately
reduce the pressure.
The hole drilled for the bushing must be
straight, not tilted. If the hole is slightly tilted or off centered play will develop and ultimately result in excessive wear, particularly
on the plantarflexion stop.
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