The Effect of Malleolar Prominence on Polypropylene AFO Rigidity and
Buckling
Wesley Golay, C.O.
Thomas Lunsford, M.S.E., C.O.
Brenda Rae Lunsford, M.S.
Jack Greenfield, C.O.
Introduction
A number of catastrophic clinical conditions (e.g., lower extremity fracture, brain injury, stroke,
spinal injury) result in severe impairment of the foot and ankle complex during walking.
Manifestations of these conditions often
include paresis, spasticity, and disturbance of normal phasicmuscle control. Many foot and ankle
disabilities can be improved with the aid of an appropriate orthosis.1-4
The primary goal of a properly fitting orthosis is to realign the foot-ankle complex and maintain
that alignment during all phases of gait. Management of genu recurvatum or excessive knee
flexion is often accomplished by stabilizing
the ankle.5-7 Excessive knee flexion during stance is the most critical to control,
since the patient might fall if the knee buckles.
Polypropylene ankle-foot-orthoses (AFOs) are a popular choice among physicians and
orthotists for treating patients with impaired gait, but there are disadvantages with their use.
Polypropylene AFOs lack adjustability in plantar
and dorsiflexion, and often the AFOs tend to buckle at the ankle during stance phase where
control is needed the most. The ability of the orthosis to restrain
dorsiflexion under the dynamic loading of full body weight is a measure of the appropriateness of
the design and choice of materials.
Generally, it is desired to fit a patient with a rigid polypropylene AFO which will restrain
excessive dorsiflexion during stance phase. Patients whose malleoli are markedly prominent
require polypropylene AFOs with adequate
relief about the ankle to avoid excessive pressure. This relief may have a negative effect on the
rigidity of the polypropylene AFO. It has been observed that plastic AFOs buckle more easily in
the region near the relief, at the ankle.
When a plaster cast is prepared for fabrication of a polypropylene AFO the orthotist builds-up the
plaster where there are bony prominences. The apices of the medial and lateral malleoli on the cast
require the largest build-ups. Unfortunately, this is also the area where the maximum compressive stress (buckling) occurs in the
orthosis when the patient walks.9 The prominence or sharpness of the contours
around the ankle seems to affect the ability
of the orthosis to restrain dorsiflexion in terminal stance.
The purpose of this study, therefore, is to investigate the degree to which plastic AFOs
buckle and lose rigidity as the malleolar prominence is increased. Rigidity is best characterized by
the angle of collapse between the foot and the tibial sections versus applied torque or force. The
maximum rigidity challenge placed
on a plastic AFO occurs in late stance where dorsiflexion restraint is required by a patient with a
weak calf. Therefore, rigidity shall refer to AFO stiffness in the sagittal plane against
biomechanical forces tending to collapse the AFO
into dorsiflexion.
Buckling, conversely, is the tendency of a plastic AFO to expand along an medio-lateral line
through the contours of the malleoli. A convenient characterization of this phenomenon is the
relationship between angle of dorsiflexion
collapse versus the percentage increase in malleolar diameter. The percentage increase in
malleolar diameter is called "diametrical strain." If this bulging at the malleoli disappears after the
applied force is removed, then the diametrical
strain is elastic. However, if the malleolar diameter is slightly larger after the load is removed, then
the diametrical strain is inelastic. In this latter case, internal and irreversible material changes
occur.
Specific Aims
- Fabricate four uniform groups of test AFOs with increasing amounts of malleolar
prominence (no build-up, 1/4" build up, 1/2" build-up, and 3/4" build-up).
- Measure the change in the diameter at the malleoli and the applied force as the AFOs are
forced into 16° of dorsiflexion in 2° increments.
- Compare the characteristics of rigidity and buckling among the four groups of varying
malleolar build-up.
Method
Procedure
Four plaster casts were made from an impression of the below-knee prosthesis portion of the
AFO testing apparatus. 10 The first of the four casts had no malleolar buildups.
The other three casts had 1/4", 1/2", and
3/4" total medial and lateral malleolar buildups.
Four groups of AFOs were fabricated (Figure 1)
, all from the same sheet stock of 3/16"
polypropylene material and all trimmed identically, the only variable being the diameters
between the apices of the malleolar build-ups. Three AFOs were fabricated over each of the four casts, which were created to
allow determination of reliability of fabrication. To ensure what each
of the three AFOs in each of the four build-up groups were similarly drape vacuum formed, the
medial and lateral sagittal wall thicknesses in the area of the malleoli were
measured. These thickness measurements were taken at seven points (Figure 2)
on the medial and
lateral sides near the ankle area using a thickness gauge (Figure
3)
.*
The diameter at the apex of the malleoli was measured on all AFOs before any force was
applied. Malleolar diameter measurements were recorded at eight other
dorsiflexion angles in 2° increments up to 16°. Reference marks were placed on the anterior
edges of the AFOs to ensure that the diameter measurements were taken
at the same place. For every two degrees of additional dorsiflexion the corresponding applied
force value was recorded. Each AFO was tested three times to obtain average
values for force and buckling diameter. One hour elapsed between tests of the AFOs for a given
buildup configuration. This allowed the AFO to return to its original shape
before retesting.
*Oditest, Model 70, The Dyer Box Co., Box 4966, Lancaster, PA.
**Dillon 100 lbs. Forcegauge, W.C. Dillon & Co., Inc., 14620 Keswick St., Van Nuys, CA.
***Nine inch Mitutoyo Vernier Caliper.
****CRUNCH Software Corporation, 2547 22nd Avenue, San Francisco, CA 94116.
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Instrumentation
The testing apparatus 10(Figure 4)
was developed from a below knee prosthesis using a solid foot articulated with the shank of the prosthesis via a free moving hinge (Figure 5)
. A 2" belt
of cotton webbing served as the calf strap to which the tensiometer**
was attached. The tensiometer (Figure 6a)
measured the applied dorsiflexion force on the system
needed to achieve the eight specified dorsiflexion angles. The applied
force was supplied by turning a small winch (Figure 4, bottom right)
mounted to the base of the
apparatus. A cable and pulley system changed the line of pull so that
it was perpendicular to the tibial axis of the below-knee prosthesis and 10" proximal to the
mechanical ankle joint axis. Toe-in/toe-out was referenced on the baseplate
with black tape to maintain a consistent line of pull. The change in the malleolar diameter was
measured with an outside vernier caliper (Figure 7)
.***
The angle of dorsiflexion collapse was indicated by a 1 mm diameter pin attached to the
proximal-lateral edge of the AFO (Figure 8)
. This indicator could be seen
through the stationary transparent angular scale (Figure 8)
. Each mark on the scale represented a
single degree increment.
Data Analysis
The statistical programming package, CRUNCH,**** was used to perform all data analysis. The
data were screened by using univariate summaries. A paired T-TEST
was used to compare medial and lateral sagittal wall thickness. Analysis of variance was used to compare differences between the malleolar
build-up groups for both rigidity and buckling. Plots of a third order regression were used to
demonstrate the relationships of applied force
and diametrical strain to dorsiflexion collapse angle. All testing was done at a .05 significance
level.
Results
The results consist of three parts: sagittal wall thickness, rigidity, and buckling.
Sagittal Wall Thickness
The mean (+/- standard deviation) of the medial and lateral sagittal wall thicknesses are shown in
Table I
for the four groups of AFOs. For Group I (no build-up) the mean (+/-s.d.) medial and lateral
wall thickness was .150" (+/-.006)
and .145" (+/-.009), respectively. For Group II(1/4" build-up) the
mean (+/-s.d.) medial and lateral wall thickness was .153" (+/-.006) and .142" (+/-.009), respectively.
Similarly the thicknesses were .153" (+/-.007) and .142" (+/-.011) for Group III(1/2" build-up), and
.153" (+/-.009) and .148" (+/-.009) for Group
IV (3/4" build-up). The medial wall thickness for all four groups were not significantly different.
The lateral wall thicknesses for all four groups were also not significantly different. However, the
difference between the medial and
lateral wall thicknesses were significantly different for Groups I, II, and III.
Rigidity
As expected, the AFOs with greater malleolar build-up were less rigid (Table II)
. For
a dorsiflexion collapse angle of 6° the AFOs
with no build-up, 1/4" build-up, 1/2" buildup, and 3/4" build-up required 39.2 (+/-4.44),
33.2 (+/-4.76), 26.4 (+/-3.54), and 25.0
(+/- 6.84) pounds, respectively.
An analysis of variance of the mean forces indicated that the AFOs with no build-up were
significantly more rigid than any of the other three versions (Table II
and Table III
). Further, the AFOs
with 1/4" malleolar build-up were
significantly more rigid than the AFOs with either 1/2" or 3/4" malleolar build-up. However, the
rigidity of the AFOs with 1/2" and 3/4" build-up were not significantly different (Table III)
. Four of the rigidity comparisons
(Table II)
were not significantly different at dorsiflexion collapse
angles of 2° and 4°
The non-significance between the AFOs with 1/2" and 3/4" malleolar build-up is easily seen
(Figure 9)
.
Buckling
The measure for buckling is the percentage increase in the diameter of the AFO in the region of
the malleolus. Technically this increase is called "diametrical strain."
The AFOs with greater malleolar build-up had less diametrical strain (Table IV)
. For a
dorsiflexion collapse angle of 6°, the AFOs with no build-up, 1/4" build-up, 1/2" buildup, and
3/4" build-up exhibited 12.6% (+/-.023), 10.7% (+/-.020), 11.8% (+/-.016), and 8.5% (+/-.024)
diametrical strain, respectively.
An analysis of variance of the mean diametrical strains indicated that all of the AFOs were
significantly different except those with 1/4" and 1/2" malleolar build-ups
(Table IV
and Table V
). There were scattered cases at relatively small dorsiflexion angles
(2-6°) where some of the AFO groups were not significantly different.
The non-significance between the AFOs with 1/4" and 1/2" malleolar build-up is obvious (Table V
and Figure 10
).
Discussion
Sagittal Wall Thickness
Many subtle factors can change the thickness and resulting characteristics of a drape vacuum formed plastic AFO. Excessive heat tends to result in "thinned" areas such as the malleoli,
while excessive vacuum and operator vigor can have the same result in areas such as the heel.
This is a possible explanation for the
significant difference between the overall medial and lateral wall thicknesses, .152" (+/-007) versus
.144" (+/-.010). Although this difference is small (5.5%), it was consistent. One possible
explanation is that the plastic tends to be thinner
at the more posterior areas, such as the lateral malleolus. Using multiple thickness measurements
on the medial and lateral sagittal walls of the plastic AFOs was a suitable means for screening out
inconsistently fabricated orthoses.
Rigidity
Sagittal walls without bulging contours to
stantially more rigid AFO. For example, the anteriorly directed force produced by the tibia of a
patient with a weak calf is approximately one third body weight.13 Therefore, for
a patient weighing 150 pounds, the AFO
must restrain a force of 50 pounds. The AFO group with no build-up collapsed 7.5°;
whereas the group with 1/4" and 1/2" buildups collapsed 8.7° and 11.5°, 16% and
52% additional rigidity loss, respectively. This
is a relatively large loss in AFO rigidity, which diminished as the malleoli build-up increased
beyond 1/2". When the build-up was increased from 1/2" to 3/4", the rigidity loss increased from
52% to 60%.
If the biomechanical objective of the prescription is to restrain motion of the ankle joint, then
every effort should be made to avoid bulging contours of the sagittal walls in the region where the
tibial portion of the AFO transitions
to the foot section. The
penalty of flat and parallel sagittal walls is excessive pressure on the malleoli during single limb
stance. Therefore, the design challenge to the orthotist is to produce maximum rigidity without
excessive skin pressure. The greater the
AFO collapses, the greater the demand on the patient's quadriceps. Further, if the collapse is too
great, then the patient will have a smaller window within which to walk.
Buckling
The results obtained for buckling (i.e., increase in malleolar diameter) were surprising and
enlightening. For a dorsiflexion collapse angle of 10°, the group of AFOs with no build-up
buckled 25% (i.e., diametrical strain .25),
whereas the groups with 1/4" build-up and 1/2" build-up each buckled 19%. However, the 1/4"
build-up group required 30% more force to collapse. Statistically, the 1/4" and 1/2" build-up groups were
indistinguishable considering only buckling. The group with 3/4" build-up buckled only 13% for a
10° dorsiflexion angle (Table IV)
. This
result seemed contrary to what was expected.
Furthermore, there are two important considerations. First, the group with no build-up require
65 pounds to collapse into 10° of dorsiflexion, whereas the 3/4" group required only 40 pounds.
Secondly, the group with 3/4" build-up
behaved as though a "natural hinge" existed at the ankle region. Conversely, the group with no
build-up and parallel sagittal walls were the most rigid and when forcefully collapsed into
dorsiflexion, tended to demonstrate more buckling
(diametrical strain).
By creating malleolar contours in the me-
dial and lateral walls, the orthotist is "prebuckling" the AFO. A substantial amount of rigidity is
lost and the AFO easily collapses into dorsiflexion without appreciably increasing the malleolar
diameter. This contrasted to the AFOs
with minimal malleolar build-up which are substantially more rigid and difficult to buckle.
However, when buckling finally occurs, greater geometric distortion is observed.
Clinical Implications
If the treatment objective of the prescription is to protect a weak calf by limiting dorsiflexion in
stance then a rigidly designed 3/16" polypropylene AFO is indicated. Furthermore, if the patient
has prominent malleoli (more than
1/4"), then an attempt to stiffen the AFO should be made (e.g., with metal or carbon composite
inserts). However, if the patient does not have prominent malleoli (1/4" or less), then judicious
modification of the plaster model will suffice
(i.e., build-up of the area around the malleoli until
virtually straight sagittal walls exist). This will reduce the tendency for the AFO to buckle, but will
add conspicuous bulk. Obviously, as either the weight and/or walking velocity of the patient
increases, the easy solutions must be
abandoned in favor of the use of thicker, more anteriorly trimmed plastic, the use of metal insert
reinforcements, or the use of a heavy duty standard metal AFO.
The confusing point in these findings occurs when buckling is observed while evaluating a
stressed orthosis. The buckling can not be used as an indicator of lack of dorsiflexion restraint; in
fact, more buckling often occurs with
greater dorsiflexion restraint. The true indicator of rigidity is the dorsiflexion collapse angle of the
orthosis.
The important observation to be made, therefore, is the dorsiflexion angle throughout stance,
especially at terminal stance. If excessive (greater than 10°) dorsiflexion is observed, then the
orthosis is inadequately rigid. If less
than 10° is observed, then the orthosis has adequate rigidity, no mattcr how much buckling (bulging) is observed.
Summary
Four groups of 3/16" thick polypropylene AFOs with increasing levels of mallcolar prominence were tcsted for rigidity and buckling. Those AFOs with greater mallcolar prominence were less rigid. However.the AFOs with less build-up required more force to buckle, but once buckling occurred there was a greater expansion of the sagittal walls.
Wesley L. Golay is a graduate of the Orthotics and Prosthetics Baccalaureate program at California State University Dominguez Hills and participated in this study as partial fulfillment of his graduation requirements.
Thomas R. Lunsford is Chief Orthotist at Rancho I-os Amigos Medical Center, Orthotics Department, 7450 Leeds Street, Downey, California 90242, and Clinical Director of the Baccalaureate program in Orthotics and Prosthetics at California State University Dominguez Hills.
Brenda Rae Lunsford is Statistician, Physical Therapy Department. Rancho Los Amigos Medical Center.
Jack Greenfield is Assistant Chief of the Orthotic Department at Rancho I-os Amigos Medical Center.
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