The Use of Synthetic Casting Materials
in Obtaining Circumferential
Impressions
Dwain R. Faso, C.O.
Mel Stills, C.O.
Introduction
Plaster of paris bandage, first introduced
by Mathijesen in 1852, has long been used to
immobilize joints and fractures. It was recognized early that these casts represented a
detailed negative impression of the body
part they encompassed. Orthotists have, for
many years, used plaster of paris bandage
and splints to obtain a negative impression as
the first step in creating a positive model to
be used in the fabrication of orthopedic devices.
The delivery of orthotic services has matured and expanded and now includes a significant number of patients who are managed initially in the hospital or clinic. This
necessitates the need for taking an impression away from orthotists' home facilities.
The inconveniences of using plaster of paris
bandage are easily recognized. Much time
and effort go into protecting the patient and
the working area from being soiled. Additional time is needed to clean the area and
the patient, along with the difficulty of disposing of the water saturated with plaster.
An increasing number of devices are fabricated using central fabrication facilities. Significant effort is expended to protect impressions and molds for transportation.
The advent of the new elastic synthetic
casting materials provides an attractive alternative to plaster of paris bandage.
Material
There are many types of synthetic casting
materials on the market today. All are composed of the basic components of a fabric
matrix impregnated with a thermal setting
resin that will set-up at room temperature.
Johnson & Johnson's Delta-Lite Conformable Casting Tape (1) has been used successfully to obtain negative circumferential impressions. It is a knitted elasticized fiberglass
substrate which has been impregnated with a
polyurethane resin. Its elasticity provides excellent conformation to the body. Applied
like Orthoflex (2) or any of the standard
elastic wraps, Delta-Lite Conformable can
be rolled onto the body without excessive
roping, gaping or bulging. Its malleable consistency permits it to be handled with ease
and allows for a smooth inside finish.
Impression Method
Taking an impression of the patient is
done over a single layer of cotton stockinette. All bony landmarks are identified and
marked with an indelible pencil on the stockinette. A cut-off tube or strip is placed as
usual. The procedures for taking a negative
impression are the same as those using plaster of paris with the exception that the Delta-Lite Conformable is substituted. The optimum coverage is two to three layers of material. This will provide sufficient strength and
maximize the coverage. More than three layers of material generally result in increased
rigidity and may cause problems at the time
of removal (Figure 1)
.
The setting temperatures of the synthetic
casting materials pose little danger of burning the skin. This is a potential hazard of
plaster of paris (3,4). Plaster of paris bandage is about 55 percent plaster and 45 percent cloth, whereas synthetic casting tape is
10 percent resin and 90 percent cloth.
The setting time of Delta-Lite Conformable will vary from four to six minutes, as
compared to the plaster of paris' 15 to 20
minutes. The impression can be removed
with bandage scissors or cast saw and will
stay flexible enough to be removed from the
patient while still retaining its shape. Important: (1) Do not delay in removing the impression from the patient. The synthetic material, like plaster, will become more rigid as
it cures with time and thus becomes more
difficult to remove. (2) Close the negative
impression immediately following removal
from the patient and remove the stockinette.
Both points are critical due to the faster curing time of synthetic material.
Delta-Lite does not splatter and drip like
plaster of paris. The water runoff is free of
resin and poses no hazard or difficulties. This
reduces the effort involved in clean-up.
However, as with all resin materials, care
must be taken during application to keep the
sticky resin from contacting skin or clothing.
Preparation of Impression
As mentioned, the stockinette should be
removed from the impression after it is removed from the patient. Closure of the impression can be easily done with a staple gun.
The staples hold the two edges tightly together without pulling out or splitting. The
seam and openings are sealed in the usual
fashion with plaster of paris splints. After the
unit is sealed, another split is cut in the impression directly opposite the original seam.
This new seam need only be three-fourths
the length of the impression to provide a
clam shell effect during removal from the
positive model. For example, on an AFO
impression, the second split would be placed
posterior, from the proximal edge running
distally to the plantar surface of the posterior
calcaneus, leaving the plantar surface of the
foot piece intact.
Attempts were made to remove synthetic
negative impressions with only one seam
from positive models. It proved difficult and
necessitated splitting the cast posteriorly as
stated above. This resulted in saw cuts in the
positive model which had to be filled. Simply
cutting the impression before pouring the
positive model will save time but is not a
necessary step if one wishes to cut the impression carefully after it has been poured.
Seal the new seam as described above.
Dust talcum powder liberally into the impression to act as the parting agent. Due to
the non-absorbent nature of synthetic materials, soap will not work as a parting agent.
Mix and pour plaster of paris as you are
accustomed. We have found that the mixture
sets up approximately 25 percent faster in
the synthetic impression. This may be due to
the porosity of the synthetic material, allowing excessive water to bleed off and/or evaporate more quickly.
During the stripping of the positive mold,
another significant savings in time and effort
is possible. The sealing splints do not become impregnated into the synthetic material of the impression and peel off effortlessly.
The staples are also easily removed and the
impression functions somewhat as a clam
shell hinged by the intact plantar surface.
The impression remains intact after removal
and can be saved or reused if needed. All
indelible markings transfer readily to the
positive model. We have observed that the
surface texture of positive models obtained
from such impressions are smooth with substantially fewer air holes. Again, this is probably due to the porosity of the synthetic materials.
Advantages
Through our experience, the advantages
of using a synthetic casting material in obtaining circumferential impressions are:
- Ease of use - The Delta-Lite Conformable
Casting Tape handles very nicely, much in
the same manner as a standard elastic bandage. The resin is not excessively sticky,
and the roll keeps its integrity throughout
the application. Flaccid soft tissue is easily
contained, with few corrugations noted on
the interior surface of the impression.
Clean-up consists of the disposal of open
packages, gloves and excessive water;
- Thermal effects are significantly reduced;
- The synthetic impressions are 63 percent
lighter in weight than plaster of paris impressions. This factor alone is a significant
benefit when carrying several plaster impressions or sending the impression to a
central fabrication facility and paying
freight by weight. Example: a plaster mold
for a knee orthosis weighs 3 lbs. 6 oz. and
costs $7.50 (UPS Blue Label Dallas to
New York). The same impression of Delta-Lite weighs 1 lb. and costs $5.00, a savings of $2.50; and
- The amount of synthetic material needed
to obtain a good impression varies with the
size of individual being cast. In general, at
least two to three thicknesses of material
are desired, but one thickness in isolated
areas is acceptable. An average adult generally requires one 3-inch roll and one 4inch roll to obtain a good AFO impression.
A single role or any combination of different rolls may be used depending on the size
of the individual. The goal is a finished
impression with two to three thicknesses of
material. Plaster of paris impressions for
an AFO generally require two 4-inch rolls
of elastic plaster and one to two of standard 4-inch plaster. Cost of an AFO impression will vary according to the purchase price of material. Synthetic casting
tapes are more expensive than standard
plaster of paris bandage, and we have experienced an approximate $3.00 increase
per AFO impression.
Conclusion
The use of Delta-Lite Conformable Casting Tape has demonstrated several advantages over plaster of paris in the preparation
of a circumferential impression. These advantages provide the orthotic practitioner
with more efficient use of time (Figure 2
and
Figure 3
) and a reduction in energy. The quality of
the impression obtained with synthetic casting is as good as with plaster bandage, if not
better. The use of these materials does not
require the introduction of any new techniques or procedures. Simply substituting
the synthetic materials for plaster of paris
and the introduction of one added step of
splitting the cast is all that is needed to employ the use of the technique. To date, we
have used this technique for obtaining impressions for AFOs, KOs, KAFOs and
TLSOs. Any impression technique using circumferential wraps would benefit from the
use of these materials.
Dwain R. Faso, C.O., is with Three-D Orthopaedic, division of DeRoyal Industries, 10520
Olympic Drive, Dallas, Texas 75220, telephone
214/620-1393.
Mel Stills, C.O., is with the University of Texas Southwestern Medical Center at Dallas, Department of Orthopaedic Surgery, 5323 Harry Hines Boulevard, Dallas, Texas 75235-8883, telephone 214/688-3874.
References:
- Delta-Lite Conformable Casting Tape -
Johnson & Johnson Products, Inc., Orthopedic
Division, New Brunswick, New Jersey 08903.
- Orthflex Elastic Plaster Bandage - Johnson
& Johnson Products, Inc., Orthopedic Division,
New Brunswick, New Jersey 08903.
- Pope MH, Callahan G, Lavalette R. Setting
temperatures of synthetic casts. Journal of Bone
Joint Surgery 1985;67A:262-264.
- Lavelette R, Pope MH, Dickstein H. Setting
temperatures of plaster casts: the influence of
technical variables. Journal of Bone Joint Surgery
1982;64A:907-91 1.
|
|
