Use of Copolymer for Interfaces in All
Levels of Prosthetic Applications
Richard J. Rosenberg, C.P.O.
Robert Terry, Prosthetic Technician
Thermoplastic
For the past nine years or so, we have been
using thermoplastic technology to fabricate
socket interfaces for all levels of prosthetics.
We have found that copolymer suits our
needs extremely well as it demonstrates
more fracture resistance than polypropylene. In all of the applications, we drape mold
the material rather than use the bubble
molding technique. We find that drape
molding is satisfactory within our system of
fabricating in that a more uniform wall thickness is easier to achieve and the posterior
placed seam is usually not visible. Also with
drape molding, the problem of having to
evacuate a large amount of air during the
initial draw is eliminated. The thickest material we use is 1/4 inch, which keeps the heating time to a minimum.
The Oven
The oven we use is an infrared type which
consists of two banks of four elements each.
Infrared waves heat the material directly
rather than heating the air which must then
heat the material. Therefore, an infrared
oven is not insulated as a convection or conductive oven must be; also, we do not lose
heat when the oven door is opened. The
outer shell of the infrared oven consists of a
single layer of sheet metal which is designed
to reduce drafts through the oven. Different
materials heat better at different wave
lengths in reference to infrared waves.
Fabrication of a Below-Knee
Thermoplastic Interface
A nylon hose is applied over the cast before the Pe-Lite insert is fabricated. We have
found that two layers of Pe-Lite are necessary for the distal end cap; otherwise, the
distal end becomes extremely thin after the
copolymer is formed over the insert. If the
below-knee (BK) is of a modular design, either temporary or definitive, a plaster buildup is fabricated (Figure 1
and Figure 2
). The buildup has a flat surface on the distal end and is
approximately 2 inches square to accommodate Otto Bock componentry. The buildup is
shaped to allow the prosthesis to be assembled in neutral static alignment (Figure 3)
.
Over the insert and plaster buildup, we apply
a heavy cast sock and then a copolymer plate
shaped to the distal end of the buildup. Two
layers of nylon are added under a PVA bag
which is capped distally. Since we are drape
molding and will end up with a posterior
seam, the cast is placed into a horizontal
vacuum manifold and a small piece of stockinette is applied, on top of the PVA bag, from
the proximal area of the cast above the trim
line of the socket, and tucked into the manifold. The stockinette is a wick to make sure
vacuum is not cut off proximally. Small holes
are made through the PVA bag in the area of
the patella tendon and the popliteal. The
PVA bag provides a smooth inner surface to
the socket. The copolymer is heated in the
infrared oven until it turns to a clear glasslike state. We normally use 3/16-inch thick
material, but use 1/4-inch for extremely active patients.
The thermoplastic is draped over the horizontal cast and sealed posteriorly and
around the pipe. The excess material is
trimmed before the plastic has a chance to
cool. We normally use one vacuum pump
but two pumps may be necessary if the cast is
very large. We have not found that a reservoir for the vacuum system is necessary. We
pull as many pounds of vacuum as possible.
Once the plastic has cooled, we trim the
proximal end and either drive the interface
off the cast, or if this is not possible, the cast
is broken out.
What we have designed is a total contact
insert with a void between the soft socket
insert and the rigid outer socket. Depending
on the shape of the socket, the insert might
tend to migrate into the socket during ambulation. To rectify this situation, a pad fabricated from material such as Ensolite or Plastizote can be fitted between the insert and
the socket. It takes a small amount of contact
to keep the insert in position.
If the prosthesis is to be of exoskeletal
design, the fabrication technique is the same
as stated previously with the exclusion of the
plaster buildup. The interface is treated like
a laminated socket in that it is sanded on the
exterior surface and foamed onto an adjustable pylon.
Fabrication of an Above-Knee
Thermoplastic Interface
The distal end of the modified cast is sized,
coated with polyester resin, and a polyurethane foam buildup is fabricated onto it.
Holding the cast in the correct amount of
adduction and flexion, a plumb line is
dropped to indicate the center of the buildup
in the A-P and M-L planes. A mark is drawn
1/2-inch posterior to the lateral alignment
line. The buildup is then shaped to a flat
distal end and decreased in circumference
proximally as dictated by the size and shape
of the cast (Figure 4)
. The surfaces of the
buildup are kept as flat as possible to obtain
the most strength from the copolymer material.
A nylon is pulled over the prepared cast,
which has vacuum holes drilled in the undercut areas and distal plate fabricated from 1/4inch copolymer. A valve dummy is attached
if indicated. A PVA bag is pulled over all
and holes are punched in the bag in the undercut areas as well as the area around the
valve. After the copolymer is heated to its
clear state, it is pulled over the cast while it is
in the horizontal vacuum manifold, and as
much vacuum as possible is used to draw the
plastic to the cast. We use two vacuum
pumps when doing this procedure. After the
plastic has cooled, the socket is removed
from the cast. The urethane distal buildup is
retained and used to provide total contact. A
layer of 1/8-inch or thicker is attached with
Plastizote to the top of the urethane buildup.
In the fabrication of an exoskeletal prosthesis with a thermoplastic socket, the same
procedures are used with the elimination of
the foam buildup. The thermoplastic socket
is treated as if it were a laminated socket,
just as described for the BK laminated socket.
We have been using the copolymer material for socket retainers for flexible wall AK
prostheses from the time flexible wall sockets were first introduced. The process is similar to the above instructions with a few minor
changes. We use plaster as the distal buildup
on the flexible wall interface (Figure 5)
. To
attach the plaster, we first apply some moleskin to the distal end where the plaster must
adhere. After the retainer is fabricated, the
plaster is discarded. We have been able to
provide an anterior and posterior window in
the retainer, as well as allowing for a flexible
proximal area.
At a Symes level, we have fabricated sockets with copolymer, once again treating the
socket as if it were a laminated one.
For a hip disarticulation prosthesis using
an Otto Bock modular hip joint, the attachment plate is shaped and trimmed. The proximal surface of the attachment plate is covered with 3/16-inch Pe-Lite and the Pe-Lite is
sanded thin until the two threaded inserts are
flush with the surface of the Pe-Lite. The PeLite is tapered back, or undercut, from the
edge of the aluminum plate and notches are
ground into the edge of the plate to allow the
thermoplastic to grip it.
Conclusion
We have been using copolymer for virtually all of our interfaces at all levels for endo as well as exoskeletal designs. In an AK situation, we were able to verify that there was a
2-pound weight savings by eliminating the
wooden socket attachment block and the
outer lamination. The Otto Bock endoskeletal AK prosthesis was reduced in weight
from 8 pounds to 6 pounds. Also, by using
thermoplastic in lieu of thermoset technology, we have reduced the volume of dust we
generate in the lab. Thermoplastic techniques also make the lab less environmentally hazardous as well as reducing the fabrication time.
After using thermoplastic technology in
virtually all of our interfaces, we have come
to the conclusion that it produces a lighter
and stronger prosthesis and also takes less
time to fabricate compared to thermoset-plastic techniques which are in wide use today.
The author is with R.J. Rosenberg Orthopedic Lab., Cincinnati, Ohio.
The author is with R.J. Rosenberg Orthopedic Lab., Cincinnati, Ohio.
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