Double Short Flexure Type Orthotic
Ankle Joints
J. Martin Carison, M.S.(Engineering), C.P.O.
Bruce Day
Gene Bergiund, C.O.
Abstract
The Habilitation Technology Laboratory
at Gillette Children's Hospital has developed two designs for durable "short" double-flexure type ankle joints. Flexure elements, singly and in multiples, have been
used for centuries to create hinges. Some of
the earliest hinges were of animal hides and
used for doors and lids. In recent decades,
long (flexure length-to-thickness ratios
greater than 5-to-1) plastic flexures have
been used as a bridge between thigh, calf and
foot sections of fracture orthoses. Those designs were unstable when subjected to varus,
valgus, torsional or longitudinal loading,
and, because of that, gave a poor impression
of the potential value of flexure designs in
orthotics. At Gillette, we have been developing and using short (flexure effective
length-to-thickness ratios less than 2-to-1)
double-flexure orthotic joints since 1976.
The short flexure design solves the structural
instability problems but magnifies fatigue
durability problems. We have designed a
testing apparatus which simultaneously subjects four ankle-foot orthoses to service cycle
repetitions. This has helped to determine
more quickly and objectively which designs
and materials are more durable. The double-flexure design approach retains the weight
and cosmetic advantages of plastic orthoses
while providing several advantages over the
posterior leaf design. The flexures can be
located for full congruency at anatomic and
orthotic joint axes. The desired ankle range-of-motion is almost totally free of resistance,
and the degree of motion restraint can be
easily and precisely controlled. Gillette has
provided several thousand ankle-foot and
knee-ankle-foot orthoses utilizing these ankle joints. Utilization has been in a wide
variety of diagnoses among adults as well as
children.
Introduction
The weight, cosmesis and foot control advantages of polypropylene ankle-foot orthoses (AFOs) long ago made them a clear and
appropriate favorite, in most cases, over the
style of AFO which incorporates metal bars,
joints and stirrups. One of the few disadvantages of the polypropylene AFOs has been
that those designs have not provided all of
the motion control characteristics of metal
joints at the ankle. The standard posterior leaf type AFO utilizes a single, long-flexure
linkage between the foot section and calf
section. That flexure has a buckling bias due
to its curved cross section. It buckles to allow
dorsiflexion much more easily than it buckles to allow plantarflexion. This is a characteristic we desire. However, as we narrow
the posterior leaf to allow easier dorsiflexion, the plantarflexion stop becomes less
definite. This "softness" of the plantarflexion stop may be beneficial in some circumstances. However, the point at which motion
is stopped often will creep to greater plantarflexion angles with use over time.
The moderate resistance of the posterior
leaf during dorsiflexion is no problem during
near-normal gait, since body weight is more
than sufficient to power dorsiflexion between mid-stance and heel-off without significant effort by the client. However, that
dorsiflexion resistance is sufficient to cause
problems and fatigue in non-weight bearing
activities. An example is the operation of the
accelerator of a motor vehicle where it is
necessary to vary and hold the ankle through
a range of dorsiflexion angles.
Finally, the axis of flexion of the posterior
leaf is located well posterior to the anatomical ankle axis. This causes pistoning of the
orthosis on the calf of the leg as the ankle
flexes. This is a problem for more active clients.
Of course, many orthotists have devised
ways to overcome some of these shortcomings. Medial and lateral diagonal straps have
been used to create a more definite plantarflexion stop. We, and others1,5 have devised
various types of rotating joint/plastic hybrids
which retain the metal ankle joint features,
but at a significant weight compromise of
fabrication complexity.
Flexure Joint Design Fundamentals
The concept of using multiple flexures to
achieve a hinge action is not new. Some of
the earliest of such inventions were the use
of pieces of animal hides to create hinges for
doors and lids. In modern times, short plastic
hinge designs have been commonly used for
cabinet doors and tool box lids. "Long" flexures (flexure length-to-thickness ratios
greater than 4 to 1) were introduced to the
orthotic field in the `70s as fracture orthosis
joint components. Those designs have fared
well only when use was temporary and when
torsional, compression and sheer loadings
were minimal. The "long" flexures tend to
be unstable when subjected to those loadings.
Watanabe, et al., presented a multitude of
plastic flexure designs in their 1982 article.6
They included inventive variations for intrinsically limiting range-of-motion. Those designs were of the "long" flexure type and
subject to the problems cited above.
The performance of any flexure subjected
to a combination of axial, sheer and bending loads is strongly dependent on the material choice, the proportioning, and the
dimensioning of the design. This paper will
not go into detail regarding the engineering
of flex members but there are a few fundamentals which should be presented to help
orthotists better understand these useful
devices. Some of these fundamentals are
intuitive, such as: If other dimensions are
held constant, increasing the length of a
flexure will increase its tendency to bend,
twist and buckle when subjected to shear,
torsional and compressive loads; however,
shorter flexures will build up greater
stresses when bent through a given flexion
angle.
Some other flexure engineering fundamentals are not so intuitively obvious. As a
flexure is repeatedly bent through its angular
range-of-motion, the bending and unbending (or reverse bending) stresses subject the
flexure to "fatigue." If the peak values of the
fatigue stresses are great enough, a fatigue
crack will be nucleated and will grow until
the flexure fails. When this happens, intuition may tell us we can make the flexure
stronger by making the cross-sectional area
more robust. This is not true. For a given
flexure material, length and angular range-of-motion, peak bending fatigue stresses are
proportional to the thickness (cross-section
dimension in the direction of motion) of the
flexure.4 So, we can actually reduce the peak
bending fatigue stresses, increase the flexure's resistance to fatigue damage, and increase its service life by reducing the thickness of the flexure. In fact, we have seen
many cases where, in a polypropylene flexure, a fatigue crack progressed to the point
where it reduced the effective thickness
(and, therefore, also reduced peak bending
fatigue stresses) to below a critical value and
further fatigue damage was virtually halted.
Of course, thickness cannot be reduced too
much or the cross-sectional area will not be
great enough to withstand torsional, sheer
and tension stresses.
"Gillette" Double Flexure Joint
Designs
At the Habilitation Technology Laboratories of Gillette Children's Hospital, we have
been developing and using "short" flexure
(flexure length-to-thickness ratios of 2-to-1
or less) ankle joint designs since 1976 (Figure
l)
.2,3 We have also occasionally used them
for orthotic knee, elbow and wrist joints
(Figure 2
and Figure 3
). Our work has yielded two
designs which we routinely use in our practice. In one design the flexures are integral
parts of the polypropylene shell (Figure 4)
.
The other design utilizes pre-molded polyurethane flexure units (Figure 5a
and Figure 5b
).
Both of these designs provide the following
characteristics:
- Allow precise and adjustable control of
ankle position and motion;
- Exert minimal resistance throughout
the prescribed range-of-motion;
- Allow the orthotist to align the orthotic
axis of rotation congruent with the anatomical ankle axis, and;
- Accomplish this without significant
weight or cosmetic sacrifices.
Polypropylene is a very interesting material for use in flex members because it seems to
have the ability to realign polymer chains to
resist fatigue damage. Also, procedures such
as cold deformation can be used in the fabrication of polypropylene flexures to create
some preferential micro-structure alignment. The polypropylene flexure design we
have developed involves both material removal and crushing (cold deformation) steps
to form the two in-line flexures into the ankle-foot orthosis shell.
Figure 6
, Figure 7
, and Figure 8
help to explain the
fabrication procedure. When the plaster
model is covered with the hot, flexible 5 mm
thick polypropylene (monopolymer), extra
small pieces of polypropylene (5 mm thick
and heated along with the large sheet) are
added to increase the shell thickness at important locations; one in the area of each
malleolus where the flexures will be formed,
and a third in the area of the Achilles tendon
directly posterior of the malleoli. The surfaces of the polypropylene must be wiped
clean with a solvent before heating. The
polypropylene should be removed from the
oven very soon after it becomes transparent.
If the polypropylene is too hot when removed from the oven, paraffin molecules
will have migrated to, or formed at, the surface and the small pieces will not bond to the
main shell. After the polypropylene shell has
cooled, it is removed from the plaster model
and the desired location of the medial and
lateral flexures is determined and marked. A
6.5 mm diameter drill hole is used to create
the posterior surface of the flexures. The
anterior surface is formed with a hand-held
power tool with a small rotating cutter. Figure 7
shows the desired dimensions. A special long-nosed cylindrical-jaw clamping tool
is then used (Figures 8a, 8b, and 8c)
to cold
form the flexures to the desired thickness (3
mm) and in alignment with each other. The
clamping tool (a modified Vise-Grip?) has a
long extension of one of the cylindrical jaws
so that the extended jaw can be passed
through both medial and lateral holes
(drilled just posterior to the flexures) when
one flexure is cold formed by clamping the
jaws together. The extended jaw is then
passed through both holes from the opposite
side when the other flexure is cold formed.
This special tool and procedure creates an inline formation of the flexures (Figure 9a)
. If
the flexures were formed along separate medial alignments (Figure 9b)
, they would act
like two out-of-line hinges and "binding"
stresses and strains would reduce the free
action and service life of the flexures.
The final step in creating the double flexure joint is to "free" the flexures by cutting
through the solid section of the AFO shell
posterior to the flexures. If the AFO is to
stop plantarflexion, those cut surfaces will be
the "stops" and it is often appropriate to
modify those surfaces to dampen the stopping click noise or to slightly change the angle at which plantarflexion stop occurs (Figure 10). If plantarflexion is to be allowed in
the orthosis, additional material is removed
posterior of the flexures as shown in Figure
11
. Figures 10
andFigure
11
also show a simple
method for limiting dorsiflexion by using a
posterior tether strap when desired. When
fabricated correctly, the result is a very
close-fitting, cosmetic double-flexure joint
with a single-axis action. More detailed fabrication instructions are available from the
authors.
The polypropylene flexures are much
more rugged than they appear to be. During
the course of use they will appear to be undergoing damaging changes (high strain
areas turn milky white) which are actually
the normal non-destructive way polypropylene responds under fatigue cycling. However, no material is indestructible and if they
do break, replacement is very difficult.
To facilitate double-flexure installation
and replacement (if necessary), we have developed an injection-molded polyurethane
flexure unit and corresponding fastening
hardware. Fabrication is simplified. Extra
shell thickness is created in only one location; at the Achilles tendon where the plantarflexion stop will be (if there is to be one).
The pre-manufactured flexure units are positioned in the desired locations under the
snug-fitting hosiery which is pulled over the
plaster model prior to covering with hot
polypropylene sheet (Figure 12)
. Vacuum assist forming pulls the hot polypropylene into
a close-formed shape around the underlying
flexure units. When the polypropylene shell
is cool and rigid, it is removed from the plaster model, the flexure units are pulled out
and a horizontal U-shaped portion of shell
material is removed forward of where the
centerline of the flexure will be (Figure 13)
.
That material removal is to provide clearance needed for dorsiflexion. Holes are
drilled for the flexure fastening screws and
the AFO shell is cut through to create separate foot and calf shell sections. The final
step is to reunite the two shell pieces by installing the flexure members (Figure 14)
.
The polyurethane flexures have another
very important advantage. Polyurethane has
a much lower modulus of elasticity than
polypropylene. This causes the bending-induced stresses to be lower (in this type of
design application), and we can utilize a circular cross-section flexure design rather than
the rectangular one used for polypropylene
(compare Figure 9
and Figure 15
). The circular
cross-section flexure bends equally well in all
transverse directions so no procedure is necessary to co-align the polyurethane flexures.
Durability Testing
In 1986, to facilitate the collection of
quantitative durability data, we designed an
ankle-foot orthosis testing apparatus (Figures 16a and 16b)
. The shaft on which the
foot plates are attached slowly rotates the
AFOs in a backward direction. This causes a
slow cycle into maximum dorsiflexion and
then, as the orthosis rotates over the top of
its circle, the weighted calf section falls,
"banging" the orthosis against its plantarflexion stop. The inertia of the weights (1.0
Kgm mounted on the M-L center of the calf
section and 1.95 Kgm mounted 12.5 cm lateral of center on a rod through the calf section) causes a very significant torsional,
sheer and tension shock to be transmitted to
the flexures when the plantarflexion stops
break their rotating fall. A counter is mounted on the testing machine to count the rotations. The test apparatus does not tell us how
long a given design will serve on a client but
it gives excellent data to evaluate and compare the relative values of various design and
material modifications meant to improve durability.
Both polypropylene and polyurethane
flexure designs are used at Gillette but, because of fabrication ease, the injection molded polyurethane flexures have become the
favorite. The durability testing machine has
helped us to identify the best resin formulation and processing parameters. It is also
used as a quality assurance test apparatus.
Samples from each production batch of polyurethane flexures are tested to ensure that
neither the process nor the material has varied from optimum.
In the durability testing program described, the polypropylene flexures yield a
life of about 400,000 to 500,000 cycles. The
polyurethane flexures last about 600,000 to
700,000 cycles. The child-size version of the
polyurethane design, tested under the same
conditions as the adult-size, will last about
500,000 to 600,000 cycles. To give some basis
for comparison, we tested two traditional
metal AFO designs. In both cases a fatigue
crack nucleated at the anterior lateral edge
of the stainless steel stirrup near the sole
bend. Complete fracture of the stirrups occurred at 168,000 and 125,000 cycles, respectively.
Conclusion
Multiple-flexure hinge joints have been
used for millennia. Optimizing flexure designs
for orthotic use requires some awareness of
design fundamentals and material physical
characteristics. Gillette has developed polypropylene and polyurethane designs which
have been both field and laboratory tested.
They have proven useful in nearly all applications. The polyurethane flexures are easily
replaced if fractured.
Acknowledgements
Design development work at Gillette Children's Hospital always involves significant input
and contributions from many members of the Habilitation Technology Laboratory staff, therapists
and physicians. We appreciate and benefit from a
free and generous exchange of ideas, views and
information. In addition to the authors, Fran Hollerbach, Richard Weber, Kathy Molina, Chuck
Schemitsch, Jim DeCorsey, Scott Espersen, Paul
Quade, Scott Webber, John French, Paula Parker
and Dennis Prescher have been closely associated
with the fabrication and application of double
flexures during the period of their development at
Gillette Children's Hospital. The figures appearing in this article are the work of photographers
Ken Jandl and Brian Benish, and medical illustrator, Lisa Mlazgar. Seemingly endless manuscript
modifications and revisions were processed by
Debbie Day.
J. Martin Carlson, M.S. (Engineering), C.P.O., is former Director of Habilitation Technology Laboratories at Gillette Children's Hospital.
Bruce Day is an orthotist.
Gene Berglund, C.O. is the present Director of Habilitation Technology Laboratories, Gillette Children's Hospital, 200 East University Avenue, St. Paul, MN 55101.
References:
- Bensman, AS. and W.W. Lossing, "A New
Ankle-Foot Orthosis Combining the Advantages
of Metal and Plastics," Orthotics and Prosthetics,
33:1 (March 1979), PP. 3-10.
- Carlson, J. Martin and Bruce Day, "The Gillette Double Flexure Ankle Joint," A presentation at the American Academy of Orthotists and
Prosthetists, Midwest Chapter Continuing Education Seminar, Merrilville, Indiana, June 23,
1984.
- Carlson, J. Martin and Bruce Day, "Double
Flexure Designs for Orthotic Ankle Joints" A
scientific program presentation at the American
Orthotic and Prosthetic Association, Annual
Meeting, San Diego, October, 1985.
- Crandall, Stephen H. and Norman C. Dahl,
An Introduction to the Mechanics of Solids, New
York: McGraw-Hill, 1959, p. 286.
- Hale, Steven; "Carbon Fiber Articulated
AFO - An Alternative Design," Journal of Prosthetics and Orthotics, 1:4, pp. 191-198.
- Watanabe, H., T. Kutsuna, H. Moringa and
T. Okabe, "New Plastic Joints for Plastic Orthoses," Prosthetics and Orthotics International,
1982, 6, pp. 21-23.
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