Casting for Foot Orthotics—What Works Best?
By Séamus Kennedy, BEng (Mech), CPed  One of the most frequent questions that I am asked
during an inservice on foot orthotics is: "What kind of casting do
you prefer?"
I have given inservice presentations to many different groups:
orthotic and prosthetic labs, podiatrists, physical therapists, and
physiatrists--and yet this same question always comes up.
Originally, sensing a biomechanics ambush, I would diplomatically
begin my answer with: "What do you guys do here?"
Over time I realized that I was not being set up but, in fact,
most practitioners are more than a little confused: caught between
the theory of plaster slipper casts and the practicality of
impression foams. I have been hesitant to write an article on this
topic--as I know there are strong viewpoints--so instead, I will
humbly offer some observations.
Gold Standard
The plaster slipper cast is the gold standard of casting
methods. It was popularized in the 1960s as Root et al. were
developing the field of lower-extremity biomechanics. At that time
there was a refined understanding of foot function and orthotics.
The emphasis was being shifted from general arch support to
subtalar joint control. There were also significant changes in
orthotic fabricating materials: cork, leather, and metal were being
replaced by thermoplastics. All this new science required better
ways to replicate foot shape.
Plaster casting is the optimum method of capturing the foot in
subtalar joint neutral. It is done non-weightbearing with the
patient lying either supine or prone, and thumb and index finger
loading and controlling the fourth and fifth met heads. A good cast
cannot be rushed, and the inside surface should reveal some of the
lines of the skin. Specific anatomical points or areas in need of
relief can be indicated in pencil. Marking calcaneal bisection is
also helpful when forefoot posting is required. There is also a new
synthetic slipper "sock" on the market that mimics plaster without
the delay or mess.
Functional Orthotics: Slipper Cast Best
- Patient is seated comfortably.
- A 5 x 30 casting splint is recommended and used in these
illustrations. Gently dip spling in tepid water and remove excess
water with fingers. (Fold to 5 x 15 before dipping in
water).
- Reinforce this splint by folding over about 3/4 on one edge
as shown.
- This splint is then applied approximately 3/8 beneath the
Malecil with the fold on the outside of the foot.
- The media and lateral flaps are then folded over the
plantar aspect of the foot.
- The remaining excess plaster is then pulled posteriorly and
smoothed into place.
- Take another 5 x 30 splint and fold to 5 x 16 and repeat
step #2.
- Apply the anterior splint overlapping it approximately 1/2
of the posterior splint, medially and lateraly. Allow two fingers
in front as shown for easier removal.
- Fold medial and lateral flap as in step #5.
- Smooth excess plaster on the plantar aspect and blend into
sides. Bring excess splinting into web space as shown.
- Holding toes as shown, gently push foot back to slight
resistance with thumb placed on 4th and 5th metatarsal head as
shown. This will give you neutral position.
- After allowing cast to dry several minutes, gently pull it
away from skin.
Now remove cast by pulling the heel portion off, then the
front portion. Repeat steps 1 through 12 for the other
foot.
The slipper cast is particularly useful when you are designing
functional foot orthotics. Their aim is to improve the
biomechanical function of the foot. It is beneficial in
applications such as sports orthotics, where precision is required
in order to obtain maximum benefits. It is also good for flexible
foot types, where weightbearing casts may only capture a fully
pronated foot structure.
However, after almost a decade of owning a custom foot orthotics
lab, I have witnessed some pretty unusual things come through the
door that are supposed to represent the shape of a foot. Thus, the
first rule of plaster casting is: Make sure that the final cast is
a true image of the patient's foot. Being honest rather than
academic will be more helpful to all involved. If you lack time,
plaster splints, or technique, then it may be best to avoid the
plaster slipper cast.
Impression Foam: Quick, Clean
Impression foam casting has the obvious advantages of being both
quicker and cleaner. It offers a good method of creating a negative
of the patient's foot. Approximately 65 percent of all casts that
we receive are taken with impression foam. I usually recommend that
practitioners take a semi-weightbearing cast when using foam. The
patient should be seated in a chair with both feet plantargrade. It
is important that the cast be taken slowly with the practitioner in
control and guiding the descent of the foot. It is best not to take
impressions on tiled floors as the box may slide halfway through
the process. A good controlled cast will not crack or split the
foam around the perimeter.
Best for Accommodative Molds
Impression foam is the method of choice when you are making
accommodative foot molds. These orthotics are often designed to
alleviate pain in a specific area of the foot. A semi-weightbearing
or even weightbearing cast may often better capture the location of
a bony prominence, callus, or fibrous tissue mass. Many
practitioners use impression foam exclusively when dealing with
restless patients, e.g. young children or people with certain
disabilities.
Once the cast is complete, it is important to take a little time
and care with the shipping. Plaster casts should be left out to
cure and dry for 24 hours before being sent off. Otherwise they
will tend to flatten and even dampen and potentially damage the
shipping box. Although some impression foam is sold in boxes that
are ready to ship, we recommend that you send them inside another
box for better protection. On occasion, if the carrier is having a
rough day, all that may arrive at the lab is a forensic jigsaw
puzzle.
Results Are What Counts!
Using a results-oriented philosophy, you should carefully
evaluate the outcomes and modify your approach accordingly.
Regardless of the casting method employed, the goal is to deliver
the best possible custom orthotics for your patients' foot
conditions. Satisfied clients generate referrals and repeat
business--and that is valuable feedback worth working for. Séamus Kennedy, BEng (Mech), CPed, is president and co-owner of Hersco Orthotic Labs, Long Island, New York. 
Table Of Contents - August 2004
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