A Material for Direct Forming of Prosthetic Sockets
A. Bennett Wilson, Jr.
For a number of years, prosthetics
research groups have been attempting to develop a method of forming sockets
directly on amputation stumps, in order to reduce the time required to produce a
satisfactory socket and to eliminate the messy working conditions inherent in
the use of plaster of paris.
Direct forming requires a material that:
(a) is plastic at temperatures moderately above ambient, but which
requires reasonably high temperatures for subsequent softening; (b) is
easily handled under conditions found in most limb shops; (c) exhibits
minimum creep or deformation under normal loads, even at temperatures slightly
above body temperature; (d) is nontoxic; and (e) has a reasonable
strength-to-weight ratio.
Recently, research and development groups
in Canada and the United States have developed successful techniques for direct
forming of some types of sockets by using a synthetic balata,
Polysar* X-414.
Polysar X-414 has been found to possess
the properties most essential for direct forming: (a) it becomes plastic at
temperatures between 160 and 180 deg F; (b) it can be applied to the
amputation stump within a minute or two after heating; (c) it remains reasonably
plastic after its surface temperature drops 20 to 30 deg; (d) after it
cools and becomes nonplastic, it maintains its shape, even under stress and
subsequent heating to temperatures of 120 deg F; and (e) it can be reheated and
reformed to permit socket modification after fabrication. In the plastic state,
it exhibits cohesive properties which facilitate fabrication. It yields a
slightly flexible socket which is considered desirable by most patients, and it is practical to use all conventional
components and accessories with Polysar X-414.
Clinical findings indicate that the
sockets remain durable, provided they are not exposed to excessive heat
(e.g., leaving the prosthesis in the sun, in the trunk of a car on a hot
day, or leaning against a house radiator). Also, excessive contact with
perspiration may cause erosion of the material in a year's time; however, stump
socks normally provide an adequate barrier.
The socket-forming procedure is
relatively simple. The need for making a plaster-of-paris wrap cast, pouring a
positive cast, and modifying the positive cast is eliminated. Thus, not only is
fabrication time reduced, but the chance of the errors that are likely to occur
when fabricating a socket with conventional materials also is
lessened.
Lower-Extremity Sockets
A practical method for direct forming of
sockets over the below-knee stump has been developed recently at the Veterans
Administration Prosthetics Center. Early attempts included the use of a
pneumatic bag over a tube of synthetic rubber to provide the pressure necessary
for forming the socket over the stump (Fig. 1) , a procedure which worked
satisfactorily for bony, mature stumps but which often produced sockets that
were too loose when molded over flabby stumps. Further experimentation resulted
in a technique in which pressure is provided by wrapping pressure-sensitive tape
spirally around the tube of Polysar X-414 and molding it with the hands as the
tube cools (Fig. 2).
This method, described in the
article beginning on page 57, has proved to be
successful in a number of clinics, especially for use in temporary, or
preparatory, prostheses. If a pylon is used, the patient can be provided with a
well-fitted prosthesis in a very few hours. If subsequent socket modifications
are required, they can usually be carried out readily, and if one of the
adjustable pylons is used, alignment can be changed easily when required. A
satisfactory cosmetic effect (Fig. 3) can be achieved relatively easily, to
provide a "permanent" prosthesis. Such a prosthesis has proved to be quite
successful as a "permanent" prosthesis for many patients in the old-age
group.
Because of the size of the above-knee
socket and the usual need for rather drastic modification of the socket with
respect to the shape of the stump, a successful method of molding sockets
directly over the above-knee stump has not yet been developed. However, work is
continuing at VAPC.
Upper-Extremity Sockets
A technique for satisfactorily forming
sockets for permanent prostheses directly over below-elbow stumps has been
developed, also at VAPC. Again, extruded tubing of Polysar X-414 is used. All
pressure necessary for forming is provided by the prosthetist's hands. Several
types of cosmetic coverings are available when further cosmetic treatment is
desired. The time required for fabrication of a typical below-elbow prosthesis
can be reduced by half. The VAPC technique is described fully in the article
beginning on page 65.
The Ontario Crippled Children's Centre,
Toronto, Canada, has been routinely using Polysar X-414 in fabrication of the
open-shoulder, above-elbow socket, described in Artificial Limbs for
Autumn 1969. Sockets preformed roughly to the shape required are heated and
applied over the stump.
The Prosthetics Research Center,
Northwestern University, has developed a successful method for forming more
conventional above-elbow sockets directly over the stump. An article describing
this technique is scheduled for publication in the next issue of Artificial
Limbs.
Implications
Forming sockets with synthetic balata
offers the prosthetist and orthotist the opportunity to provide quicker service
to the patient, and also opens up many
possibilities for improving the designs of sockets and orthotic components. The
use of temporary prostheses can now be made routine, giving the clinic team
ample time to determine the optimum prescription for the patient. Errors can be
rectified easily, and new ideas can be tried with a minimum expenditure of time.
Orthotists are already using synthetic balata for cuffs and molded supports. It
is expected that many more uses for this remarkable material will be developed
in the future.
References:
- Committee on Prosthetics Research and Development, Fracture bracing, A report of a workshop, National Academy of Sciences, Washington, D.C., February 1969.
- 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.
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