A Technique for Thermoforming Hip
Disarticulation Prosthetic Sockets
Clarence Imler, C.P.
Michael Quigley, C.P.O.
The socket for a hip disarticulation prosthesis has several features that make it
unique in prosthetics and very difficult to
fabricate. A hip disarticulation socket
should have the following features: total
contact weight bearing surface; strong, solid
base for attachment of the hip joint; flexibility to allow easy opening and closing; adjustability to allow for volume changes and pressure relief; suspension provided by waist
groove over contralateral iliac crest.
Historically, hip disarticulation sockets
were fabricated from molded leather to provide flexibility and steel to provide strength;
these materials are time-consuming to use
and difficult to keep clean. After the introduction of thermosetting resins, such as
polyester and acrylic, new techniques were
developed to laminate hip disarticulation
sockets, using combinations of flexible and
rigid resins with fiberglass (or similar) reinforcement. Although thermosetting sockets
were a great improvement over leather and
steel, the lamination procedure took many
hours and often resulted in less than optimum strength and flexibility and a very
heavy bulky socket.
In addition, the use of thermoplastics to
fabricate hip disarticulation sockets has
brought a new set of problems. Although
vacuum forming is faster than laminating, it
is not well suited to large casts with undercuts; using one sheet of plastic that will provide both strength and flexibility is also a
problem.
Described here is a technique that has
been in use for 10 years. The inherent
strength and flexibility of copolymer (90 percent polypropylene, 10 percent polyethylene), combined with specific reinforcement
over the hip joint attachment plate, result, in
a very lightweight, flexible socket with rigidity over the hip joint. A Pe-Lite liner increases the coefficient of friction of the socket, which improves suspension and rotational control.
Fabrication Procedure
One or more test socket fitting procedures
are generally required before the final positive model is made. During the fitting procedures, anterior and lateral plumb lines are
drawn on the socket, and the location and
alignment of the hip joint is determined.
The socket is fabricated over the positive
model in the manner described below:
- Thermoform one sheet of 3mm Pe-Lite
(or similar) over entire model, with the
seam placed in the center of the anterior wall. The seam is placed where the
opening of the socket is planned; if a
side opening is desired, the seam should
be located on the side.
- Transfer and mark the alignment lines
to the Pe-Lite.
- Contour the metal hip joint attachment
plate over the Pe-Lite liner.
- Place a piece of plastic wrap over the
Pe-Lite liner at the location of the hip
attachment plate; this will serve as a
parting agent.
- Use an acrylic or polyester resin filler
material to fill in gaps between the flat
attachment plate and the curved positive model. After the plastic filler has
set, remove it, and sand the edges to
provide a smooth transition from the
plate to the Pe-Lite.
- Cut a piece of 3/16-inch copolymer to
the shape of the hip joint attachment
plate, but approximately 1/2 inch wider
in all dimensions. Smaller or lighter patients need only a 1/8-inch thick copolymer piece.
- Measure the positive model and cut a
piece of 3/16-inch copolymer large
enough to cover the entire model.
- Mark the position of the attachment
plate on the Pe-Lite and fill the screw
holes (not the perforations) in with
plumber's putty.
- Place the large sheet of 3/16-inch copolymer in the oven; when it begins to soften, place the smaller piece in the oven,
along with the metal attachment plate.
- When both pieces of plastic are ready,
remove the small piece and place it over
the premarked position on the model.
- Remove the hip joint attachment plate
from the oven and place it on the plastic
piece on the model. Secure the attachment plate with a large nail through one
of the screw holes. Press the plate into
the hot plastic.
- Rotate the model, so the posterior side
is up (for anterior opening sockets) and
rapidly place the large piece of plastic
over the cast, sandwiching the plate between the two layers of plastic. Use a
piece of teflon fiberglass sheeting to
compress and smooth out bulges at the
edges of the plate. There should be very
little puckering or wrinkling in the finished socket.
- Once the socket is cooled, it is trimmed
and finished in the conventional manner. No rivets or other fasteners are
needed to secure the hip joint attachment plate.
The resulting socket is lightweight and has
a minimum of thickness in the sitting area
which can be a big problem with other techniques. The socket can be easily modified by
adjusting the Pe-Lite liner or by heating and
remolding. Copolymer is a very tough material, and socket breakage has not been a
problem.
Summary
A technique is described for thermoforming a Pe-Lite liner, hip attachment plate and
copolymer socket for hip disarticulation
prostheses. The technique requires less than
half of the fabrication time of conventional
laminations. The advantages of using the
technique are: (1) lightweight, (2) ease of
adjustment, (3) minimum of space and bulk
under the ischial tuberosity and (4) ease and
spread of fabrication.
Both Clarence Imler, C.P., and Michael I.
Quigley, C.P.O., are with Oakbrook Orthopedic
Services, Ltd., 1 South 224 Summit Avenue,
Oakbrook Terrace, Ill. 60181. Quigley is past
president of the American Academy of Orthotists
and Prosthetists and a member of the JPO Journal of Prosthetics and Orthotics editorial board.
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