Evaluation of Synthetic Balata for Fabricating Sockets for Below-Knee Amputation Stumps
A. Bennett Wilson, Jr. *
At the present time, most sockets for
artificial limbs are made of a plastic laminate (usually polyester resin and
Dacron) which has been molded over a modified replica of the stump. A replica of
the stump is required because human tissues cannot withstand the temperatures
generated by the exothermic reaction of the plastic as it cures. The replica is
modified, using general rules established by research groups, in order to
achieve a relationship between the stump and socket that is physiologically
satisfactory, yet permits weight-bearing and provides stability. In addition,
reliefs must be provided to accommodate bony prominences and any tender spots. A
simple plaster-of-paris wrap will usually be too loose for normal use.
Therefore, fabrication of plastic-laminate sockets with presently available
materials involves at least the following steps Fig. 1: (a) development of a
female mold of the stump by wrapping the stump with plaster-of-paris bandages,
(b) casting a male model of the stump by filling the female mold with plaster
of paris, (c) modification of the male model by trimming away plaster in
selected areas and building it up in other areas when necessary, and (d)
lay-up and cure of the plastic laminate. The average time required to make a
hard socket below-knee plastic prosthesis is eight man-hours.
It has been the goal of a number of
research workers to find a simpler and less time-consuming method for
fabricating satisfactory sockets for all levels of amputation. After many
experiments involving a number of casting methods and a variety of materials,
the Veterans Administration Prosthetics Center* by 1961 had developed
a technique for molding a socket of synthetic balata directly over a below-knee
stump. The first successful results were achieved by using an air-pressure
sleeve over a tube of synthetic balata,* which had been softened by
immersion in hot water (160 deg F) and then pulled over the stump Fig. 2.
Upon the recommendations of the CPRD
Subcommittee on Design and Development, the Subcommittee on Evaluation undertook
responsibility for the evaluation of the new technique.
The claims of the development laboratory
were: (a) a substantial decrease in elapsed time between measurement of the
stump and production of a wearable limb, thereby speeding the rehabilitation
process, (b) a substantial reduction in man-hours involved, (c) a capability
for easy adjustment of the prosthesis at any time, and (d) a decrease in
the amount of skill and training required to produce an adequate
A protocol (given at the end of this
article) was developed and five clinics* were asked to participate in
the evaluation. The prosthetists from the clinics were trained as a group at the
Veterans Administration Prosthetics Center on November 6-8, 1968. Each clinic
was requested to fit five new amputees and five amputees who had worn PTB
prostheses before, and provided with
sufficient material and equipment to carry out the fittings.
Follow-up in the spring of 1969 revealed
that all the prosthetists were encountering difficulty in obtaining adequate
fits in nearly all cases except those with long tapered stumps, most of the
sockets being too loose proximally. To overcome this problem, the VAPC devised a
method whereby the air bag was eliminated, and molding pressure was brought about by wrapping the softened
balata tube with one-inch-wide elastic webbing and controlling the shape of the
socket with the hands and fingers as it cooled.
All of the participating prosthetists
were instructed in the revised method, and other prosthetists were instructed in
the new procedure at the same time. Shortly afterwards, plastic
pressure-sensitive tape was substituted for the
elastic webbing Fig. 3.
The results with the revised procedure
were considerably better. The average synthetic balata prosthesis, with pylon
but without cosmetic treatment, weighed 3 1/2 lb, and could be made in 2
1/2 hr. All of the claims of the developer were substantiated with the exception
of the relative amount of skill required, a factor that would be very difficult
to measure at this stage of development. At any rate, it is safe to say that no
more skill is required for the new technique than for older methods.
All prosthetists who used the technique,
with one exception, felt that synthetic balata is quite useful for temporary
prostheses. Some have adopted the method as standard procedures where
procurement practices permit use of temporary prostheses of this
When this technique is used, a
considerable saving in time can be effected, and the patient can be provided with a
prosthesis within a few hours. Furthermore, the use of synthetic balata permits
easier adjustment of the socket later, and the adjustable pylon permits
adjustment in alignment at any time.
It is therefore recommended that use by
federal and state agencies of the VAPC technique for fabricating below-knee
temporary prostheses be encouraged, and that the technique be included in the
curricula of all below-knee prosthetics courses.
- Fleer, Bryson, and A. Bennett Wilson, Jr., Construction of the patellar-tendon-bearing below-knee prosthesis, Artif. Limbs, 6:2:25-73, June 1962.
- The Staff, Veterans Administration Prosthetics Center, Direct forming of below-knee patellar-tendon-bearing sockets with a thermoplastic material, Orth. and Pros., 23:1:36-61, March 1969.
- Staros, Anthony, and Henry F. Gardner, Direct forming of below-knee PTB sockets with a thermoplastic material, Bull. Pros. Res., 10-12:34-47, Fall 1969.