High Consistency Rubber Silicones Can Be Advantageous for Use in Upper Extremity Prostheses

upper extremity prosthesis construction employs plastics for
interface material and structural components. The use of high
consistency rubber (HCR) silicones offers new ways to fabricate the socket as
well as various upper extremity structural components. According to Jack
Uellendahl, CPO, Hanger Prosthetics and Orthotics, users of HCR silicones have
reported improvement in comfort and suspension of their prosthesis.

  Jack Uellendahl
  Jack Uellendahl

“Comfort is what it is all about,” Uellendahl said. “The
HCR silicone is generally softer and with more flexible and elastic materials,
the range of motion is often improved because the socket can move with the
person as opposed to restricting motion.”

HCR silicones are manufactured using a high pressure two-roll mill and
are clay-like in their forming state. They can be blended to modify the
durometer, generally ranging between 15 and 70 on the Shore A scale.

“With traditional techniques, it is common to use rigid or
semi-rigid materials,” Uellendahl said. “Some of the thermoplastics
that we use are flexible, but they are not elastic.”

Uellendahl said the elasticity of the material allows for donning of
bulbous limbs, such as wrist disarticulations.

“Having an elastic material that can expand when the person pushes
into the prosthesis is important because the area proximal to the bulbous area
expands allowing entry into the residual limb and then contracts when they get
past that point,” Uellendahl said. “Due to the socket’s good
suspension and flexible trimlines and because it can grasp the distal end of
the limb, the patient will retain almost all pronation and supination.”

When Uellendahl began to document experiences with HCR silicones, he
discovered he was using the same interface designs and techniques as he did
with older materials. As he became more comfortable with the materials, he took
greater advantage of the characteristics associated with HCR silicones.

  Pictured is a HCR socket for short transradial with integrated humeral sleeve. The sleeve section is an elastic silicone and the socket section is more rigid.
  Pictured is a HCR socket for
short transradial with integrated humeral sleeve. The sleeve section is an
elastic silicone and the socket section is more rigid.
  Image: Jack Uellendahl

“HCR silicones require specialized equipment for fabrication and
it’s a different technique [from what] is generally used in prosthetics,
so it is a different procedure,” he said.

Since fabrication is so specialized, Uellendahl works closely with a
central fabrication facility specializing in HCR silicone.

Electrode mounts, screw attachments, zippers, battery compartments and
wrist mounts can be incorporated into the silicone during fabrication.

According to Uellendahl, the silicone can be used for the lower
extremity as well. It has been used for transfemoral, transtibial and hip
disarticulation patients in the past.

“Self-suspending sockets are generally a good idea and suction
suspension is often a good way to suspend a prosthesis for both the upper
extremity and lower extremity,” he explained. “Because HCR silicones
have elastic characteristics, we can create a seal on the socket easier than a
practitioner can when they are working with a nonelastic material.”

These different materials can be positioned to control stiff areas as
desired, according to Uellendahl. There are differences between HCR silicones
and other materials, but also differences between HCR silicones and other
silicones that are routinely used in prosthetics.

“The liners made from silicones are generally made with liquid
silicones,” he said. “In their forming state, they are liquid and are
not nearly as tear resistant as HCR silicones.”

HCR silicones have shown the ability to improve the range of motion and
overall functionality of the patient, comfort is still the number one concern
for Uellendahl and his patients. This is perhaps the most important benefit of
these materials.

“You can have a well-functioning prosthesis, but if it is not
comfortable, they are not going to wear it,” he said. — by Anthony


Silicones offer the most customization allowing you to have both dense
and very soft durometers coupled together. This allows you to make the socket
soft and comfortable and rigid where it is needed. In addition to silicones
there is a class of soft and compliant thermoplastics that are also available
to use. The advantage of using a soft thermoplastic is that you can make
changes to it.

  David Rotter
  David Rotter

One of the disadvantages of silicone is once you cast it; you are pretty
much set with what you have. The material itself has elasticity, so it has give
to a certain extent, but you cannot truly change its diameter. With a
thermoplastic material, you really have an almost infinite adjustability as far
as making it larger or smaller. If someone gains or loses volume, you can
reconform it to the desired shape. It offers a lot of flexibility while still
being soft and compliant.

— David Rotter, CPO
Clinical director of
prosthetics, Scheck and Siress

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