Designing Foot Orthotics
By Seamus Kennedy, BEng (Mech), CPed  Once again the rule of sociologist Vilfredo Pareto
proves itself to be true. More popularly known as the 80/20 rule,
when adapted to business, it simply states that 80 percent of your
revenue will come from 20 percent of your clients. This certainly
applies in the world of O&P, where practitioners may have to go
from dealing with custom prosthetic patients to dispensing
over-the-counter bunion splints.
I have heard many practitioners speak of the frustrations of
having to cast and fit patients for custom foot orthotics (taking
80 percent of their time), when they would rather focus on what
they do best: fabricating prostheses and helping rehabilitate
patients (generating 80 percent of their revenue). Most practices
today are multidisciplinary, so it is inevitable that you will be
asked to deal with the full range of orthotic and prosthetic
devices.
Casting, ordering, and dispensing custom foot orthotics can be a
tricky business; not least because the patients have preset ideas
about how their feet should feel and about the types of shoes that
they should be able to wear. In order to make the procedure
clearer, I have laid out some guidelines and tips. Hopefully this
will ensure that you can satisfy the patient's needs right the
first time every time, thus optimizing your own time.
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Figure 1 |
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Over the years I have developed a flow chart that I have found
useful in guiding me through the design process (Fig.1).
STEP 1: Although this first step has the least
to do with a doctor's prescription or biomechanical evaluation, the
entire success of the orthotic may well depend on the patient's
weight, shoe style, and lifestyle.
The patient's weight is a critical piece of
information that the lab must have if it is to make the right
material choices. For example, while 4mm Subortholen will be rigid
for a 120-lb. granny, it will be positively flimsy for a 230-lb.
construction worker.
The shoe styles that a person wears will
determine the type of orthotic that will work for him or her. If
the patient is a professional and experiences general foot pain on
a daily basis, then the orthotics will need to fit dressier shoes.
Likewise if the patient is an athlete and is using them strictly to
prevent injuries during training, then the orthotic may only need
to fit into a sneaker. However, don't assume they always wear the
shoes that they bring to your office: some patients will dress up
or dress down for an appointment or on their day off.
The first office visit may also be your best opportunity for
patient shoe education. You can tell them the best types of shoes
for their particular problem. Well-fitting shoes (both in length
and width) with a firm counter, a reasonable heel height (1/2"1"),
and a removable inlay will help to ensure the use and success of
foot orthotics. I always advise patients to wait until they have
their new orthotics before purchasing their next pair of shoes.
STEP 2: Choose the orthotic type based on the
patient's typical shoe style. Sports orthotics are usually
extrinsically posted, providing more control for the rearfoot and
the subtalar joint (STJ). Dress orthotics are intrinsically posted,
allowing them to work in a wider variety of shoes.
In choosing a style, you are trying to find the optimum
compromise between function and fit. For example, the slimmest
designs (Pump Slenders, Cobras, Fashion Plates) will not offer much
midfoot support, but they can be effective if the patient only
requires forefoot corrections such as a met pad or a drop.
STEP 3: In general, foot orthotics fall into
one of two broad categories. They are either
functional: seeking to control the STJ and foot
biomechanics, or they are accommodative:
minimizing changes to foot function while providing relief and/or
protection to specific areas of the foot.
Functional foot orthotics are usually made from thinner, firmer
materials. Subortholen, polypropylene, copolymer, and the graphite
composites are all good choices for functional devices. Usually
they will incorporate a deep heel cup and a good medial
longitudinal arch. Among other diagnoses, functional devices are
used to treat pronation, plantar fasciitis, and heel spur
syndrome.
Accommodative devices tend to be made from less rigid materials,
such as EVA, Thermocork®, Neoprene, Plastazote®, etc.
Although more bulky, they are usually molded to the entire plantar
surface of the foot, providing more comfort. Accommodative
orthotics are a good choice for patients with diabetes, rheumatoid
arthritis, or Charcot neuropathy. In addition, they are often the
better choice for patients who present with a rigid foot structure
or limited range of motion, e.g. cavus foot type, clubfoot, or
post-polio syndrome.
STEP 4: You can choose the degree of control an
orthotic provides by varying heel cup depth, arch height, and
flange height. Higher heel cups (>16mm) will control the range
of motion in the heel and rearfoot. Arch heights can be adjusted at
the lab by adding or subtracting plaster to the positive mold.
Patients with flexible foot types will be better able to tolerate
increased arch heights.
The flange is the medial wall that extends vertically to capture
the midfoot and the navicular. Flanges were very popular when
orthotics were called "arch supports," and most devices were made
from less rigid leather shells or cork. Flanges can still be useful
for patients with extremely flexible pes planus and sometimes for
those with posterior tibial dysfunction.
STEP 5: Drops, pads, cushions, posting, and
lifts can all be included in the orthotic design. In order to
facilitate correct placement, it is a good idea to actually mark
the cast before sending it to the lab. Applying lipstick to the
area of concern or taping felt to the foot will ensure that the
correct location is noted on the negative cast.
There are a couple of points to consider when adding a lift to
an orthotic. (1) Unless the patient is currently
comfortably wearing a lift, it is often best not to add 100 percent
of the lift height immediately. Try building it up gradually over
twothree months, in stages of 50 percent, 75 percent, and then 100
percent. This will allow for a smoother break-in period.
(2) If the lift exceeds 1/2", it may be difficult
to fit the device into regular shoes.
STEP 6: Top cover materials can be used to
improve both the function and aesthetics of foot orthotics. In
general, I prefer to go with thinner materials such as leather or
vinyl, allowing the device to be transferred between more shoe
styles. Many people will look for shock-absorbing characteristics,
and they will favor Microcell, Sky, PPT, Spenco, etc. These work
best if the footwear has removable inlays. Multicolored materials
are often used for pediatric or sports-related applications to
enhance their cosmetic appearance.
Closed-cell foams, such as Plastazote, offer the best protection
for the at-risk diabetic foot. These materials reduce the shear
forces and help prevent ulceration. However, they do tend to wear
down more quickly and should be checked often and replaced before
any bottoming-out occurs.
STEP 7: In choosing a length, I feel that less
is better. This reduces the possibility of cramming the met heads
or toes into the shoe. If the orthotic is primarily for STJ and
midfoot control, there should be no need to extend it beyond the
met heads (3/4 length), unless there is ample room in the shoe both
across the ball of the foot and in the toe box. You can often test
for shoe fit by having patients try on a set of prefab insoles
while they are in your facility.
Casting, designing, and dispensing foot orthotics can be a
tricky business. You are always trying to find the right blend of
science and art to make the best device that will work and be
effective. Clear communication with patients will also help them
set realistic expectations. Hopefully you have picked up some ideas
that will ensure more successful outcomes and reduced headaches.
After all, Signor Pareto's 80/20 rule doesn't stop us from striving
for 100-percent satisfaction! Séamus Kennedy, BEng (Mech), CPed, is president and co-owner of Hersco Orthotic Labs, Long Island, New York. 
Table Of Contents - November 2003
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