Effect of a Tone-Inhibiting Dynamic
Ankle-Foot Orthosis on the Foot-
Loading Pattern of a Hemiplegic Adult:A Preliminary Study
Karen Mueller, MS, PT
Mark Cornwall, PhD, PT
Thomas McPoil, PhD, PT
David Mueller, PT, CPO
Jane Barnwell, MD
Introduction
Foot-loading patterns are one of the most
direct measures of dynamic foot alignment
during stance. By examining foot-loading
patterns, clinicians can determine the sequence of contact points along the foot as
weight is accepted. For example, a loading
pattern involving initial contact of the forefoot to the floor may give the clinician an
objective indication of spasticity or ankle-joint range-of-motion limitations. This information would then better enable the clinician to make appropriate decisions about the
best course of treatment.
Despite the usefulness of information
about foot-loading patterns, the few published studies documenting neurologic patients' foot-loading patterns involve only the
static examination of weightbearing when
the foot is standing. In one such study it was
noted that hemiplegic subjects bore significantly less weight through the involved leg,
with less weight acceptance through the heel
compared to age-matched normal control
subjects (1). The foot-ground pressure pattern (FGP) used in this study measures the
pattern of weightbearing when the subject is
standing quietly; it does not give information
about how the foot is loaded as the subject
progresses through stance phase.
Ryerson discusses three patterns of foot
loading in hemiplegic subjects as the patient
progresses through stance (2). These different patterns are the result of the patient's
level of muscle tone as well as the range of
motion available at the talocrural and subtalar joints (3). While knowledge of these
three patterns of foot loading is clinically
useful, they have not been experimentally
validated, and therapists working to alter
these loading patterns do not have objective
ways to measure improvement.
Further research in the area of foot-loading patterns in neurologic patients is needed.
Currently, no studies document the effect of
various orthotic devices on these patients'
foot-loading patterns. Data from such research may help validate the effectiveness of
commonly used orthotic management procedures used by clinicians in treating these patients.
Tone-inhibiting orthoses are widely pre
scribed in neurorehabilitation. One of thc
most clinically popular tone-inhibiting orthoses is the dynamic ankle-foot orthosis
(DAFO), a "very thin, flexible supramalleolar orthosis with a custom-contoured sole-plate to include support and stabilization to
the dynamic arches of the foot" (4). The
DAFO was designed based on the concept
that the most important aspect of tone-inhibiting orthotics is obtaining neutral alignment
of the ankle and foot (5). This device provides a supportive total contact exoskeleton
that maintains neutral forefoot and subtalar
joints while allowing graded amounts of ankle eversion, inversion, plantarfiexion and
dorsifiexion.
The DAFO is widely used, particularly in
the pediatric population, and has several
unique features. First, it allows graded foot
motion within the orthosis so normal balance
reactions involving proximal musculature
can occur. Second, by providing support of
the foot's natural arches, weight is more
equally distributed throughout the foot.
Thus, stimulation of foot reflexes better approximates normal function (6-8). Third,
DAFOs provide secure medial-lateral stability and midline positioning, resulting in improved grading of ankle plantar- and dorsiflexion. This stabilization has proven so effective, many clinicians have noted a decrease in abnormal plantarfiexion in patients
wearing DAFOs. In one case, a 15-year-old
spastic diplegic patient with a resistant heel-cord contracture gained 15 degrees of passive dorsiflexion with knee extension as a
result of wearing DAFOs for three months
(4).
The clinical effects of management with
DAFOs are very promising. In one study, a
four-year-old boy with spastic diplegia
showed significant increases in the duration
and efficiency of balanced standing when
wearing DAFOs as compared to his performance without these orthoses (5). In another study, the effects of DAFOs were reported on a 69-year-old male who was 18
months post-CVA, who had no voluntary
movement at the foot or ankle, and who
demonstrated forceful hyperextension of the
knee when wearing a conventional AFO (9).
Within one month of receiving his DAFO,
the patient demonstrated active toe extension and showed a 10-degree increase in
knee flexion during toe-off. The author of
this study attributed these improvements to
the controlled mobility afforded by the
DAFO. The effects of DAFOs on the temporal variables of gait were reported in a
single-subject study by Diamond (10). The
subject, a hemiplegic adult, showed significant increases in velocity, step length and
stance time when wearing a DAFO as compared to his performance when barefoot.
Although all of these studies are single-subject designs, the author of one such study
defended this methodology by stating: "The
single-subject design is particularly appropriate for evaluating treatment effectiveness
in neurologic patients because of the organic
and behavioral variability of this disorder"
(5).
The purpose of this study was to examine
the effect of the DAFO on the foot-loading
patterns of a hemiplegic adult. The authors
believed the application of the DAFO would
result in a significantly better foot-loading
pattern compared to that generated when
wearing a shoe alone.
MethodDesign, Fitting and Fabrication of the DAFO
Before making the plaster mold of the foot
and ankle, a footplate supporting the natural
arches of the foot (i.e., the peroneal, metatarsal, medial longitudinal and toe crease
arches) is constructed. Hylton and Cusick
have described footplate fabrication techniques (11,12).
Nylon stockinette is applied to the foot
and lower leg, and the bony areas of the foot
and ankle are marked with indelible pencil.
The plaster foot plate is then applied to the
plantar aspect of the foot. To obtain an optimal plaster impression, the ankle must be
placed in slight dorsiflexion with the subtalar
joint in neutral position. A cutting tube is
placed anteriorly and fast-setting plaster is
applied, incorporating the footplate. Carefully mold around the calcaneous to keep the
subtalar joint in a neutral position.
The plaster mold usually requires minimal
modifications except for smoothing. Relief
for bony prominences may be provided if
these are extremely prominent. One-eighth-inch of Aliplast(r) and orthopedic-grade polypropylene are vacuum-formed over the cast
and stretched thin (13). A padded circular
calf strap and anterior strap above the ankle
are attached to the orthosis after trimlines
have been cut and smoothed. Figure 1
illustrates the completed DAFO.
Subject
The subject was a 55-year-old male who had
suffered a left side cerebral vascular accident
two years previously, resulting in right hemiplegia. The subject was an independent ambulator who wore a rigid polypropylene ankle-foot orthosis set at zero degrees of ankle
dorsiflexion, which he felt was too restricting.
The subject was evaluated by a registered
physical therapist (KM). He demonstrated
minimal active knee fiexion and full extension in the right lower extremity in standing,
but he had no active ankle dorsiflexion.
Moderate extensor spasticity was present in
the right lower extremity, resulting in calcaneal varus and forefoot supination at heelstrike (2). Moderate genu recurvatum was
also present during stance.
The subject initially displayed a 15-degree
ankle plantar flexion contracture, but this
was managed by a four-week course of serial
casting until 10 degrees of passive dorsiflexion was obtained. The subject was then evaluated by a certified orthotist (DM) and a
plaster impression was taken for the DAFO.
The subject gave written informed consent
to participate in the study, which was approved by the Northern Arizona University
Human Subjects Institutional Review
Board.
Procedure
An A-B-B-A single-subject design was used
for the study. This design allows the comparison of the baseline (A) condition (barefoot)
with the treatment (B) condition (orthosis).
This design also allows the comparison of
pre- and post-treatment baselines (Al and
A2, respectively), making it possible to determine the presence of a carryover effect.
Foot-loading data was collected using the
EMED-SF(r) plantar pressure analysis system
(14). The EMED-SF uses a pressure-sensitive mat embedded with 1 ,300 sensors which
detect pressures as low as one newton/cm2.
The pressure mat is embedded in the center
of a wooden walkway two feet wide, 20 feet
long and one inch high. The EMED-SF is
connected to a computer that provides an
analysis of the progression of foot loading as
well as a graphic representation of the areas
and extent of plantar pressure during the
stance phase of gait.
Data was collected in two sessions. The
first session involved 10 walking trials across
the EMED-SF pressure platform with the
subject wearing his shoes. This was the first
baseline condition (A1), noted as "shoe-only, condition one" (SO-1). Immediately
following the SO-1 condition, the subject
was fitted with the DAFO. The subject then
walked with the DAFO until he felt assured
of his safety and comfort. He then performed 10 walking trials across the EMED-SF pressure platform. This was the first
treatment condition (B1), noted as DAFO-1. Subject fatigue was carefully monitored
during all trials, and brief rest periods were
given as needed.
Following the completion of the SO-I and
DAFO-1 trials, the subject was instructed to
wear the DAFO during all waking hours for
the next 14 days and to walk as much as
possible. The subject was also asked to track
the amount of time spent standing and walking each day.
The second data collection session occurred 14 days later. The subject reported
wearing the DAFO 16 hours a day, spending
about four hours a day standing and/or walking. The subject then performed 10 walking
trials wearing the DAFO. This was the second treatment condition (B2), noted as
DAFO-2. It was immediately followed by 10
return-to-baseline walking trials (A2), noted
as "shoe-only, condition two" (SO-2).
Data Analysis
Three dependent variables related to foot-loading patterns were examined. These included:
- Total Foot Force (TFF,) as measured in
newtons (one Newton = 4.42 lbs). TFF is the
maximum amount of weight borne through
the foot as it comes in contact with the pressure plate during a single stance period.
- Total foot area (TFA) as measured in
square centimeters. That is, the maximum
area of the foot's plantar surface in contact
with the pressure plate during a single stance
period.
- Total foot contact time (TEC) as measured in milliseconds. TFC is the total amount
of time spent on the foot in contact with the
pressure plate as it progresses through a single stance period.
The EMED-SE data analysis system allows a discrete examination of specific sections of the foot. For the purposes of this
study, the foot was divided into thirds to
enable the exploration of these variables as
they pertained to the hindfoot, midfoot and
forefoot.
Data analysis was performed using the
split middle technique to determine statistical differences among the four conditions
(15). The .05 level of significance was used.
Results
The mean and the standard error of the
mean (SEM) for each variable under the
four conditions is represented in Table 1
,
which also shows a summary of the effect of
the DAFO on TFA, TFC and TFF during
stance. Total foot area in contact with the
pressure plate and total force generated
through the foot were both significantly increased as a result of wearing the DAFO.
Total stance duration was significantly decreased as a result of wearing the DAFO.
These changes were noted between the SO-1
and DAFO-1 conditions, suggesting application of the DAFO had an immediate effect.
For each of the three variables, significant
differences between the SO-1 and SO-2 conditions occurred, suggesting a possible carryover effect due to wearing the DAFO for
14 days. For total stance duration and total
force, there were significant differences between DAFO-1 and DAFO-2, indicating
that continuous wear of the DAFO results in
increasing improvements over time.
Discussion
The study indicates that use of a DAFO may
result in greater stability through the foot
during stance as noted by the increases in
TFA and TFF generated when this orthosis
was worn.
The increased TFA seen in this study may
be related to the tone-inhibiting features of
the DAFO design, namely, contoured total
contact support of the foot's natural arches.
These features may hold the foot more securely in the orthosis, perhaps preventing
spasticity-induced foot postures, which result in less complete contact of the plantar
surface with the ground. EMG analysis
would be helpful in future DAFO studies to
determine if support of the natural foot arches promotes firing of proximal lower extremity muscles for optimal foot loading (such as
the anterior tibialis and the peroneals).
The increase in TFF, seen as a result of
wearing the DAFO, may be due to the enhanced stability afforded by the increase in
TEA. Support of the foot's natural arches
may provide more uniform sensory input
throughout the entire foot, resulting in a
greater sense of postural security. This security could in turn manifest itself in the subject's ability to bear more weight through the
hemiplegic leg when wearing the DAFO.
The results of this study indicate that
wearing the DAFO decreases TFC. This
finding may be related to a greater efficiency
of gait, resulting from a smoother forward
progression of pressure through the foot.
This progression of foot loading (which is
analogous to the center of pressure) as it
occurred at the hindfoot, midfoot and forefoot is shown for the SO-1 and DAFO-2 conditions in Figure 2
and Figure 3
, respectively.
In the SO-1 condition, initial contact of
the foot with the pressure plate occurred at
the lateral midfoot, at the base of the fifth
metatarsal. Weight was then shifted backward toward the anterior hindfoot; however,
there was no weight acceptance onto the
hindfoot. This pattern may have been due to
poor foot support within the shoe, resulting
in spasticity-related foot posturing into supination, which was maintained at initial contact. Figure 2
shows a cluster of X's at the
midfoot, indicating the subject spent a considerable portion of stance bearing weight
through this area, possibly to maintain maximal stability. Finally, weight was shifted forward, progressing through the midfoot and
forefoot, culminating in a toe drag. This
loading pattern required a mean of 1,592
milliseconds.
In contrast, the progression of foot loading in the DAFO-2 condition (shown in Figure 3
) indicates initial contact of the foot
with the pressure plate occurred at the posterior hindfoot and progressed forward
through the midfoot and forefoot, bisecting
the foot. In the hindfoot area, the X's are
more spread out, suggesting that acceptance
of weight progressed through the hindfoot in
a smooth manner. In the midfoot and forefoot, the X's are closer together, although
relatively evenly spaced. This pattern of X's
indicates that while the subject spent the majority of the stance period bearing weight
through these areas, the forward progression
of weight acceptance was relatively smooth.
This loading pattern required a mean of
1,252 milliseconds.
The heel-to-toe loading pattern seen in
Figure 3
may be due to the flexibility of the
DAFO, which allows graded movement to
occur at the ankle in the frontal and sagittal
planes, providing unrestricted forward
movement of the tibia over the foot. Such
movement would allow a natural heelstrike
followed by a smooth forward progression
through footflat, heel-off and toe-off.
The decrease in stance time seen in the
hemiplegic leg may also be related to the
faster overall gait velocity allowed by the
DAFO. Future DAFO studies should measure gait velocity to examine relative stance
duration between the hemiplegic and unaffected legs. It is possible that while the
stance duration of the hemiplegic leg is decreased when wearing the DAFO, when
compared to the stance duration of the uninvolved leg, this measure may be more proportional as gait velocity increases.
Finally, the results of this study indicate
the improvements attained from wearing the
DAFO for 14 days may be maintained when
the subject returns to walking with the shoe
only. It is possible that the combined features of foot arch support and the flexibility
afforded by the DAFO result in activation of
foot and ankle musculature, which is maintained as these muscles become stronger. In
this study, the SO-2 condition was measured
immediately after a 14-day trial of DAFO
use, and no attempts were made to measure
the duration of this carryover effect. It may
be enlightening to conduct further studies
exploring this factor.
Conclusion
The results of this study indicate that using
DAFO for 14 days may result in a faster
progression of the center of mass over the
foot during stance. Greater foot stability
may also result as noted by the increases in
total foot force and area generated.
The positive findings of any single-subject
study can certainly be met with optimism;
however, additional studies are needed to
validate findings. Obviously, replications of
this study using larger subject groups would
be valuable.
Karen Mueller, MS, PT, is assistant professor,
Department of Physical Therapy, Northern Arizona University, Flagstaff, Ariz. 86001.
Mark Cornwall, PhD, PT, is assistant professor, Department of Physical Therapy, Northern
Arizona University, Flagstaff, Ariz. 86001.
Thomas McPoil, PhD, PT, is associate professor, Department of Physical Therapy, Northern
Arizona University, Flagstaff, Ariz. 86001.
David Mueller, PT, CPO, is president, David
G. Mueller, PT, CPO Inc., Flagstaff, Ariz. 86001.
Jane Barn well, MD, is medical director of rehabilitation, Flagstaff Medical Center, Flagstaff,
Ariz. 86001.
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