Direct Forming of Below-Knee PTB Sockets with a Thermoplastic Material
Henry F. Gardner
Prior to forming the socket, a careful
evaluation of the stump must be made. The usual prosthetics data must be noted,
especially any stump characteristics which would require special considerations
for socket comfort.
With the patient seated, a lightweight
cast sock is applied snugly Fig. 1 to maintain tension. The top of the sock is
clamped to a strap encircling the patient's hips. The strap is made of two
halves of mating Velcro for easy adjustment behind the patient's back, and the
two free ends are equipped with Yates clamps, which are placed medially and
laterally at the top of the sock.
A strip of 1/4-in. felt, cut to form a
tib-ial-crest relief, is positioned from the superior border of the tibial
tubercle to and over the end of the stump Fig. 2. The portion of the
pad over the tubercle is made approximately 1 1/4 in. wide, tapering to a
5/8-in. width for the entire length of the tibial crest relief. All edges are
carefully skived. If adhesive-backed felt is not available, medical adhesive may
be used to attach the pad.
A second lightweight cast sock is pulled
snugly over the tibial relief and fastened in the same manner as the first sock
Using the VAPC knee caliper, the
anterior-to-posterior knee measurement at the level of the patellar tendon is
taken Fig. 4. The medial-to-lateral dimensions of the epicondyles of the femur
are measured in the same manner. These dimensions are useful in determining the
accuracy of the socket. The maximum depth of the patellar ledge is determined by
the A-P measurement.
A section of Polysar* X-414
synthetic rubber tubing with a 1/4-in. wall is selected. The diameter of the tubing should
be one-third of the mid-stump circumference. The tube length should be
approximately one and one-half times the distance measured from the top of
the knee to the end of the stump Fig. 5.
A section of Helenca stockinet 36 in.
long is used to pull the heated tube over the stump. One end of the stockinet is
pulled up on the stump as shown in Fig. 6. The other end is passed through the
The inside surface of the tube is
thoroughly cleaned to remove all plastic dust. (When heated, the dust would
cohere to the inner walls, causing undesirable
The dust-free tube is softened by
immersing it in water heated to 180 deg F, or just under the boiling point, for
four to six minutes. Because the inner walls of the tube would cohere instantly
if permitted to touch when heated, the tube
is placed on its end in the water container.
To facilitate slipping the tube over the
knee, the upper half is enlarged by spreading (hands together, palms out). The
end of the stockinet hanging from the stump is pulled through the heated tube.
The tube is pushed on the end of the stump and carried up over the stump by a
continuous pull on the stockinet Fig. 7.
Twists or folds in the stockinet
should be avoided while drawing the stockinet and plastic tube over the stump.
The forming pressures which compress the soft thermoplastic produce a slight
imprint of the stockinet material on the inner surface of the socket, and any
folds or twists in the stockinet will cause undesirable irregularities in the
inner socket wall. The top of the stockinet is then clamped in the same manner
as the cast socks.
The upper socket borders are trimmed with
bandage scissors, leaving the posterior borders approximately 1/2 in.
higher than the required measurement, for later rolling out of the material to
form a relief for the hamstrings Fig. 8. The remainder of the socket border is
cut transversely above the superior edge of the patella. The lower tube end and the stockinet are
trimmed to provide an extension of 3 in. beyond the stump.
The stump is held relaxed in 5 to 10
degrees of flexion. Starting approximately 1/2 in. above the stump end, a
snug wrap of 1-in. elastic pressure-sensitive tape is applied over the tube in a
continuous anterior-to-medial spiral, with increasing
tension approaching the level of the
medial tibial flare and continuing over the knee Fig. 9). The tension is
controlled best if one steadies the socket while the other wraps half of the circumference.
The hands then change functions to wrap the other half of the
The section of soft tubing extending
below the stump will tend to sag. This must be prevented by supporting this
section until it cools while molding the material. Approximately 10 minutes are
required for the material to harden. During this time, the socket is molded to
provide freedom over the anterior end of the tibia by massaging the taped
surface of the socket to define the tibial crest and medial flares of the tibia
Fig. 10. During the molding process, all surface irregularities may be pressed
out of the socket. The socket should not be removed from the stump until the
thermoplastic is no longer deformable by hand. The tape is removed, and with the
knee flexed to at least 90 degrees, the socket is forced from the stump. Later,
pressure-sensitive fiberglass or nylon tape may be put on the socket as a
circumferential (barrel hoop) reinforcement, usually required only around the
The resulting open-end socket will permit
easy attachment of the shank. Once the socket extension has been secured to the
shank, the end of the socket chamber is filled with foam, or another type of
resilient end pad is provided.
To modify the socket, heat is focused
with a heat gun fitted with a cone Fig. 11. With one hand placed inside the
socket against the surface to be modified, heat is directed to the immediate
area from close range until the heat is sensed by the fingers through the
socket wall. Large areas should not be heated, nor should heat be directed
against the socket for a prolonged period of time, because excessive temperature
will cause the plastic to boil and discolor. When molding for a pressure
point, one finger should press from inside the socket, and the surrounding areas
should be supported on the outside of the socket with the fingers of the other
hand. After the molded area has cooled sufficiently to retain its
shape, the socket should be chilled with cold water or refrigerated for a short
interval to reset the plastic. Caution must be exercised to avoid heating the
entire socket. The heat should be concentrated on the one spot until the
pressure applied with the fingers inside the socket causes the material to
A similar procedure is followed to shape
the patellar-tendon ledge. For patients who have previously worn prostheses, the
A-P measurements obtained by caliper are used to determine the depth of the
ledge. For recent amputees, the patellar-tendon ledge is not molded to the
maximum depth in one adjustment. Instead, three or more adjustments should be
made at intervals of one month until the required A-P dimension is
The proximal posterior socket border is
heated and rolled out to form a smooth radius for comfortable knee flexion Fig. 12, the border being maintained at approximately the patellar-ledge
An adjustable pylon is prepared with a
wood socket-attachment block 1 V'2 in. thick and 3 in. in diameter, with a
Vi-in. deep circumferential groove at the midpoint of the block. The block is
tapered to a slightly smaller diameter around the
bottom, then fastened permanently to the pylon with bolts and cement Fig. 13.
The tube end extending distally from the
socket is heated, then fitted over the wood pylon-attachment block, with the
groove helping to make a good bond. A 1-in. space between the stump end and
the attachment block must be maintained. The tube is taped tightly to the
wood block and permitted to cool Fig. 14. Any excess tubing extending below
the wood can be trimmed while the plastic is still soft. When hardened, the tube
is fastened permanently to the wood block with four screws set at 90-degree
angles to one another.
To provide for suspension, the socket can
be trimmed at the regular PTB level and a separate cuff used above the knee. Of
the several kinds of PTB suspension that can be provided with this socket,
suprapatellar-supracondylar suspension is described.
The patient is seated in a chair with his
knee flexed at approximately 45 degrees, and the stump is covered with two cast
socks. The upper socket walls above the level of the upper border of the patella
are softened by holding the socket (bottom up) in hot water. When the socket top
is heated, the stump is pushed into the socket. The plastic is molded against
the thigh over the condyles by wrapping tightly with pressure-sensitive tape and
After the patient has been fitted and the
prosthesis aligned, the bottom of the socket chamber should be foamed to obtain
a total-contact fitting. To avoid difficulty in quickly inserting the stump into
the socket, the stump is covered with a lightweight sock and a powdered PVA bag.
Three 1/8-in. holes are drilled through the lower socket wall at the level at
which the stump begins to taper inwardly, away from the socket wall. A foam
mixture is prepared and poured into the socket Fig. 15. The
patient's stump is inserted into the socket and the patient stands still until
the foam has set. The. foam mixture may vary, depending upon the type of stump
and condition of the distal tissues. Usually a combination of foam and RTV
rubber is used.
Shaping and Finishing
A leg shape can be made from
prefabricated sections of semirigid foam, Koroseal Spongex.*
Beginning at the level of the patella, a paper pattern is cut to fit around the
socket at this level. The pattern is traced upon one foam section Fig. 16.
The foam is carefully sanded to form a hollow for the socket. It is necessary to
obtain a tight, gap-free fitting of the foam to the socket; best results are
obtained from a slight stretch fit. For this, the foam is heated in an oven at
180 deg F before placement over the socket.
To cover the remaining part of the pylon,
a foam block is cut to correspond to the measurement between the bottom of the
foam surrounding the socket and the top of the foot plus 1/4 in. A hole is made
through the length of the block large enough to receive the pylon tube. Since
the foam is semirigid, the areas for the alignment coupling and ankle plug of
the pylon are cut slightly undersize to
permit a snug fit about the pylon Fig. 17.
A 1/2-in. hole is bored transversely
through the foam block to permit entry of a screwdriver to fasten the tube
clamp. The two foam sections are not glued together, in order to
facilitate removal for alignment adjustments. Compression of the foam block
between the socket base and the foot will prevent any movement of the
The blocks are shaped with a band saw or
knife and sanded with a drum or cone sander. For cosmesis, either a flexible
poly-urethane coating over the foam or a stocking cover is recommended Fig. 18.